GCC Command Options
*******************

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking.  The ``overall options'' allow you to stop this process at an intermediate stage.  For example, the -c option says not to run the linker.  Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing.  Some options control the preprocessor and others the compiler itself.  Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly.  If the description for a particular option does not mention a source language, you can use that option with all supported languages.

See Compiling C++ Programs, for a summary of special options for compiling C++ programs.

The gcc program accepts options and file names as operands.  Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v.

You can mix options and other arguments.  For the most part, the order you use doesn't matter.  Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified.  Also, the placement of the -l option is significant.

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on.  Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo.  This manual documents only one of these two forms, whichever one is not the default.

See Option Index, for an index to GCC's options.

Option Summary
==============

Here is a summary of all the options, grouped by type.  Explanations are in the following sections.

Overall Options
     See Options Controlling the Kind of Output. -c  -S  -E  -o FILE  -no-canonical-prefixes -pipe  -pass-exit-codes -x LANGUAGE  -v  -###  --help[=CLASS[,...]]  --target-help --version -wrapper @FILE -fplugin=FILE -fplugin-arg-NAME=ARG -fdump-ada-spec[-slim] -fada-spec-parent=UNIT -fdump-go-spec=FILE

C Language Options
     See Options Controlling C Dialect. -ansi  -std=STANDARD  -fgnu89-inline -aux-info FILENAME -fallow-parameterless-variadic-functions -fno-asm  -fno-builtin  -fno-builtin-FUNCTION -fhosted  -ffreestanding -fopenacc -fopenmp -fopenmp-simd -fms-extensions -fplan9-extensions -trigraphs -traditional -traditional-cpp -fallow-single-precision  -fcond-mismatch -flax-vector-conversions -fsigned-bitfields  -fsigned-char -funsigned-bitfields  -funsigned-char

C++ Language Options
     See Options Controlling C++ Dialect. -fabi-version=N  -fno-access-control  -fcheck-new -fconstexpr-depth=N  -ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope  -fno-for-scope  -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines  -fms-extensions -fno-nonansi-builtins  -fnothrow-opt  -fno-operator-names -fno-optional-diags  -fpermissive -fno-pretty-templates -frepo  -fno-rtti -fsized-deallocation -fstats  -ftemplate-backtrace-limit=N -ftemplate-depth=N -fno-threadsafe-statics  -fuse-cxa-atexit -fno-weak  -nostdinc++ -fvisibility-inlines-hidden -fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -fvisibility-ms-compat -fext-numeric-literals -Wabi=N  -Wabi-tag  -Wconversion-null  -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wliteral-suffix -Wnarrowing -Wnoexcept -Wnon-virtual-dtor  -Wreorder -Weffc++  -Wstrict-null-sentinel -Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual  -Wno-pmf-conversions -Wsign-promo

Objective-C and Objective-C++ Language Options
     See Options Controlling Objective-C and Objective-C++ Dialects. -fconstant-string-class=CLASS-NAME -fgnu-runtime  -fnext-runtime -fno-nil-receivers -fobjc-abi-version=N -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars -fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol  -Wselector -Wstrict-selector-match -Wundeclared-selector

Diagnostic Message Formatting Options
     See Options to Control Diagnostic Messages Formatting. -fmessage-length=N -fdiagnostics-show-location=[once|every-line] -fdiagnostics-color=[auto|never|always] -fno-diagnostics-show-option -fno-diagnostics-show-caret

Warning Options
     See Options to Request or Suppress Warnings. -fsyntax-only  -fmax-errors=N  -Wpedantic -pedantic-errors -w  -Wextra  -Wall  -Waddress  -Waggregate-return -Waggressive-loop-optimizations -Warray-bounds -Warray-bounds=N -Wbool-compare -Wno-attributes -Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align  -Wcast-qual -Wchar-subscripts -Wclobbered  -Wcomment -Wconditionally-supported -Wconversion -Wcoverage-mismatch -Wdate-time -Wdelete-incomplete -Wno-cpp -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization -Wno-discarded-qualifiers -Wno-discarded-array-qualifiers -Wno-div-by-zero -Wdouble-promotion -Wempty-body  -Wenum-compare -Wno-endif-labels -Werror  -Werror=* -Wfatal-errors  -Wfloat-equal  -Wformat  -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security  -Wformat-signedness  -Wformat-y2k -Wframe-larger-than=LEN -Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers  -Wincompatible-pointer-types -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int -Winit-self  -Winline  -Wno-int-conversion -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=LEN  -Wunsafe-loop-optimizations -Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-transposed-args -Wmisleading-indentation -Wmissing-braces -Wmissing-field-initializers -Wmissing-include-dirs -Wno-multichar  -Wnonnull  -Wnormalized=[none|id|nfc|nfkc] -Wodr  -Wno-overflow  -Wopenmp-simd -Woverride-init-side-effects -Woverlength-strings  -Wpacked  -Wpacked-bitfield-compat  -Wpadded -Wparentheses  -Wpedantic-ms-format -Wno-pedantic-ms-format -Wpointer-arith  -Wno-pointer-to-int-cast -Wredundant-decls  -Wno-return-local-addr -Wreturn-type  -Wsequence-point  -Wshadow  -Wno-shadow-ivar -Wshift-overflow -Wshift-overflow=N -Wshift-count-negative -Wshift-count-overflow -Wshift-negative-value -Wsign-compare  -Wsign-conversion -Wfloat-conversion -Wsizeof-pointer-memaccess  -Wsizeof-array-argument -Wstack-protector -Wstack-usage=LEN -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=N -Wsuggest-attribute=[pure|const|noreturn|format] -Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override -Wmissing-format-attribute -Wswitch  -Wswitch-default  -Wswitch-enum -Wswitch-bool -Wsync-nand -Wsystem-headers  -Wtrampolines  -Wtrigraphs  -Wtype-limits  -Wundef -Wuninitialized  -Wunknown-pragmas  -Wno-pragmas -Wunsuffixed-float-constants  -Wunused  -Wunused-function -Wunused-label  -Wunused-local-typedefs -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-but-set-parameter -Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvolatile-register-var  -Wwrite-strings -Wzero-as-null-pointer-constant

C and Objective-C-only Warning Options
     -Wbad-function-cast  -Wmissing-declarations -Wmissing-parameter-type  -Wmissing-prototypes  -Wnested-externs -Wold-style-declaration  -Wold-style-definition -Wstrict-prototypes  -Wtraditional  -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign

Debugging Options
     See Options for Debugging Your Program or GCC. -dLETTERS  -dumpspecs  -dumpmachine  -dumpversion -fsanitize=STYLE -fsanitize-recover -fsanitize-recover=STYLE -fasan-shadow-offset=NUMBER -fsanitize-sections=S1,S2,... -fsanitize-undefined-trap-on-error -fcheck-pointer-bounds -fchkp-check-incomplete-type -fchkp-first-field-has-own-bounds -fchkp-narrow-bounds -fchkp-narrow-to-innermost-array -fchkp-optimize -fchkp-use-fast-string-functions -fchkp-use-nochk-string-functions -fchkp-use-static-bounds -fchkp-use-static-const-bounds -fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read -fchkp-check-read -fchkp-check-write -fchkp-store-bounds -fchkp-instrument-calls -fchkp-instrument-marked-only -fchkp-use-wrappers -fdbg-cnt-list -fdbg-cnt=COUNTER-VALUE-LIST -fdisable-ipa-PASS_NAME -fdisable-rtl-PASS_NAME -fdisable-rtl-PASS-NAME=RANGE-LIST -fdisable-tree-PASS_NAME -fdisable-tree-PASS-NAME=RANGE-LIST -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-translation-unit[-N] -fdump-class-hierarchy[-N] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes -fdump-statistics -fdump-tree-all -fdump-tree-original[-N] -fdump-tree-optimized[-N] -fdump-tree-cfg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-N] -fdump-tree-pre[-N] -fdump-tree-ccp[-N] -fdump-tree-dce[-N] -fdump-tree-gimple[-raw] -fdump-tree-dom[-N] -fdump-tree-dse[-N] -fdump-tree-phiprop[-N] -fdump-tree-phiopt[-N] -fdump-tree-forwprop[-N] -fdump-tree-copyrename[-N] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-N] -fdump-tree-forwprop[-N] -fdump-tree-fre[-N] -fdump-tree-vtable-verify -fdump-tree-vrp[-N] -fdump-tree-storeccp[-N] -fdump-final-insns=FILE -fcompare-debug[=OPTS]  -fcompare-debug-second -feliminate-dwarf2-dups -fno-eliminate-unused-debug-types -feliminate-unused-debug-symbols -femit-class-debug-always -fenable-KIND-PASS -fenable-KIND-PASS=RANGE-LIST -fdebug-types-section -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs -fopt-info -fopt-info-OPTIONS[=FILE] -frandom-seed=NUMBER -fsched-verbose=N -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstack-usage  -ftest-coverage  -ftime-report -fvar-tracking -fvar-tracking-assignments  -fvar-tracking-assignments-toggle -g  -gLEVEL  -gtoggle  -gcoff  -gdwarf-VERSION -ggdb  -grecord-gcc-switches  -gno-record-gcc-switches -gstabs  -gstabs+  -gstrict-dwarf  -gno-strict-dwarf -gvms  -gxcoff  -gxcoff+ -gz[=TYPE] -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fdebug-prefix-map=OLD=NEW -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=SPEC-LIST] -p  -pg  -print-file-name=LIBRARY  -print-libgcc-file-name -print-multi-directory  -print-multi-lib  -print-multi-os-directory -print-prog-name=PROGRAM  -print-search-dirs  -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=FILE]

Optimization Options
     See Options that Control Optimization. -faggressive-loop-optimizations -falign-functions[=N] -falign-jumps[=N] -falign-labels[=N] -falign-loops[=N] -fassociative-math -fauto-profile -fauto-profile[=PATH] -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=STYLE -fforward-propagate -ffp-contract=STYLE -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=N -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-cp-alignment -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf -fira-algorithm=ALGORITHM -fira-region=REGION -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fira-verbose=N -fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-consts -flive-range-shrinkage -floop-block -floop-interchange -floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat -flto -flto-compression-level -flto-partition=ALG -flto-report -flto-report-wpa -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-report -fprofile-correction -fprofile-dir=PATH -fprofile-generate -fprofile-generate=PATH -fprofile-use -fprofile-use=PATH -fprofile-values -fprofile-reorder-functions -freciprocal-math -free -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks -fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=N] -fsched-stalled-insns[=N] -fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fsemantic-interposition -fshrink-wrap -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-wide-types -fssa-phiopt -fstack-protector -fstack-protector-all -fstack-protector-strong -fstack-protector-explicit -fstdarg-opt -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert -ftree-loop-if-convert-stores -ftree-loop-im -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=N -ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin --param NAME=VALUE -O  -O0  -O1  -O2  -O3  -Os -Ofast -Og

Preprocessor Options
     See Options Controlling the Preprocessor. -AQUESTION=ANSWER -A-QUESTION[=ANSWER] -C  -dD  -dI  -dM  -dN -DMACRO[=DEFN]  -E  -H -idirafter DIR -include FILE  -imacros FILE -iprefix FILE  -iwithprefix DIR -iwithprefixbefore DIR  -isystem DIR -imultilib DIR -isysroot DIR -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc -P  -fdebug-cpp -ftrack-macro-expansion -fworking-directory -remap -trigraphs  -undef  -UMACRO -Wp,OPTION -Xpreprocessor OPTION -no-integrated-cpp

Assembler Option
     See Passing Options to the Assembler. -Wa,OPTION  -Xassembler OPTION

Linker Options
     See Options for Linking. OBJECT-FILE-NAME  -fuse-ld=LINKER -lLIBRARY -nostartfiles  -nodefaultlibs  -nostdlib -pie -rdynamic -s  -static -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -static-liblsan -static-libubsan -static-libmpx -static-libmpxwrappers -shared -shared-libgcc  -symbolic -T SCRIPT  -Wl,OPTION  -Xlinker OPTION -u SYMBOL -z KEYWORD

Directory Options
     See Options for Directory Search. -BPREFIX -IDIR -iplugindir=DIR -iquoteDIR -LDIR -specs=FILE -I- --sysroot=DIR --no-sysroot-suffix

Machine Dependent Options
     See Hardware Models and Configurations.

     AArch64 Options -mabi=NAME  -mbig-endian  -mlittle-endian -mgeneral-regs-only -mcmodel=tiny  -mcmodel=small  -mcmodel=large -mstrict-align -momit-leaf-frame-pointer  -mno-omit-leaf-frame-pointer -mtls-dialect=desc  -mtls-dialect=traditional -mfix-cortex-a53-835769  -mno-fix-cortex-a53-835769 -mfix-cortex-a53-843419  -mno-fix-cortex-a53-843419 -march=NAME  -mcpu=NAME  -mtune=NAME

     Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=NUM -mcmove -mnops=NUM -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=NUM -mround-nearest -mlong-calls -mshort-calls -msmall16 -mfp-mode=MODE -mvect-double -max-vect-align=NUM -msplit-vecmove-early -m1reg-REG

     ARC Options -mbarrel-shifter -mcpu=CPU -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof -mepilogue-cfi -mlong-calls -mmedium-calls -msdata -mucb-mcount -mvolatile-cache -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact mlra-priority-noncompact -mno-millicode -mmixed-code -mq-class -mRcq -mRcw -msize-level=LEVEL -mtune=CPU -mmultcost=NUM -munalign-prob-threshold=PROBABILITY

     ARM Options -mapcs-frame  -mno-apcs-frame -mabi=NAME -mapcs-stack-check  -mno-apcs-stack-check -mapcs-float  -mno-apcs-float -mapcs-reentrant  -mno-apcs-reentrant -msched-prolog  -mno-sched-prolog -mlittle-endian  -mbig-endian -mfloat-abi=NAME -mfp16-format=NAME -mthumb-interwork  -mno-thumb-interwork -mcpu=NAME  -march=NAME  -mfpu=NAME -mtune=NAME -mprint-tune-info -mstructure-size-boundary=N -mabort-on-noreturn -mlong-calls  -mno-long-calls -msingle-pic-base  -mno-single-pic-base -mpic-register=REG -mnop-fun-dllimport -mpoke-function-name -mthumb  -marm -mtpcs-frame  -mtpcs-leaf-frame -mcaller-super-interworking  -mcallee-super-interworking -mtp=NAME -mtls-dialect=DIALECT -mword-relocations -mfix-cortex-m3-ldrd -munaligned-access -mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it

     AVR Options -mmcu=MCU -maccumulate-args -mbranch-cost=COST -mcall-prologues -mint8 -mn_flash=SIZE -mno-interrupts -mrelax -mrmw -mstrict-X -mtiny-stack -nodevicelib -Waddr-space-convert

     Blackfin Options -mcpu=CPU[-SIREVISION] -msim -momit-leaf-frame-pointer  -mno-omit-leaf-frame-pointer -mspecld-anomaly  -mno-specld-anomaly  -mcsync-anomaly  -mno-csync-anomaly -mlow-64k -mno-low64k  -mstack-check-l1  -mid-shared-library -mno-id-shared-library  -mshared-library-id=N -mleaf-id-shared-library  -mno-leaf-id-shared-library -msep-data  -mno-sep-data  -mlong-calls  -mno-long-calls -mfast-fp -minline-plt -mmulticore  -mcorea  -mcoreb  -msdram -micplb

     C6X Options -mbig-endian  -mlittle-endian -march=CPU -msim -msdata=SDATA-TYPE

     CRIS Options -mcpu=CPU  -march=CPU  -mtune=CPU -mmax-stack-frame=N  -melinux-stacksize=N -metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects -mstack-align  -mdata-align  -mconst-align -m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt -melf  -maout  -melinux  -mlinux  -sim  -sim2 -mmul-bug-workaround  -mno-mul-bug-workaround

     CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=MODEL

     Darwin Options -all_load  -allowable_client  -arch  -arch_errors_fatal -arch_only  -bind_at_load  -bundle  -bundle_loader -client_name  -compatibility_version  -current_version -dead_strip -dependency-file  -dylib_file  -dylinker_install_name -dynamic  -dynamiclib  -exported_symbols_list -filelist  -flat_namespace  -force_cpusubtype_ALL -force_flat_namespace  -headerpad_max_install_names -iframework -image_base  -init  -install_name  -keep_private_externs -multi_module  -multiply_defined  -multiply_defined_unused -noall_load   -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs  -noprebind  -noseglinkedit -pagezero_size  -prebind  -prebind_all_twolevel_modules -private_bundle  -read_only_relocs  -sectalign -sectobjectsymbols  -whyload  -seg1addr -sectcreate  -sectobjectsymbols  -sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table  -seg_addr_table_filename  -seglinkedit -segprot  -segs_read_only_addr  -segs_read_write_addr -single_module  -static  -sub_library  -sub_umbrella -twolevel_namespace  -umbrella  -undefined -unexported_symbols_list  -weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=VERSION -mkernel -mone-byte-bool

     DEC Alpha Options -mno-fp-regs  -msoft-float -mieee  -mieee-with-inexact  -mieee-conformant -mfp-trap-mode=MODE  -mfp-rounding-mode=MODE -mtrap-precision=MODE  -mbuild-constants -mcpu=CPU-TYPE  -mtune=CPU-TYPE -mbwx  -mmax  -mfix  -mcix -mfloat-vax  -mfloat-ieee -mexplicit-relocs  -msmall-data  -mlarge-data -msmall-text  -mlarge-text -mmemory-latency=TIME

     FR30 Options -msmall-model -mno-lsim

     FT32 Options -msim -mlra

     FRV Options -mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64 -mhard-float  -msoft-float -malloc-cc  -mfixed-cc  -mdword  -mno-dword -mdouble  -mno-double -mmedia  -mno-media  -mmuladd  -mno-muladd -mfdpic  -minline-plt -mgprel-ro  -multilib-library-pic -mlinked-fp  -mlong-calls  -malign-labels -mlibrary-pic  -macc-4  -macc-8 -mpack  -mno-pack  -mno-eflags  -mcond-move  -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc  -mno-scc  -mcond-exec  -mno-cond-exec -mvliw-branch  -mno-vliw-branch -mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec -mno-nested-cond-exec  -mtomcat-stats -mTLS -mtls -mcpu=CPU

     GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld

     H8/300 Options -mrelax  -mh  -ms  -mn  -mexr -mno-exr  -mint32  -malign-300

     HPPA Options -march=ARCHITECTURE-TYPE -mdisable-fpregs  -mdisable-indexing -mfast-indirect-calls  -mgas  -mgnu-ld   -mhp-ld -mfixed-range=REGISTER-RANGE -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store  -mno-disable-fpregs -mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas -mno-jump-in-delay  -mno-long-load-store -mno-portable-runtime  -mno-soft-float -mno-space-regs  -msoft-float  -mpa-risc-1-0 -mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime -mschedule=CPU-TYPE  -mspace-regs  -msio  -mwsio -munix=UNIX-STD  -nolibdld  -static  -threads

     IA-64 Options -mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic -mvolatile-asm-stop  -mregister-names  -msdata -mno-sdata -mconstant-gp  -mauto-pic  -mfused-madd -minline-float-divide-min-latency -minline-float-divide-max-throughput -mno-inline-float-divide -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits -mfixed-range=REGISTER-RANGE -mtls-size=TLS-SIZE -mtune=CPU-TYPE -milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc -msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit -msched-max-memory-insns=MAX-INSNS

     LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled -muser-enabled

     M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=NUMBER -mbranch-cost=NUMBER -mmodel=CODE-SIZE-MODEL-TYPE -msdata=SDATA-TYPE -mno-flush-func -mflush-func=NAME -mno-flush-trap -mflush-trap=NUMBER -G NUM

     M32C Options -mcpu=CPU -msim -memregs=NUMBER

     M680x0 Options -march=ARCH  -mcpu=CPU  -mtune=TUNE -m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040 -m68060  -mcpu32  -m5200  -m5206e  -m528x  -m5307  -m5407 -mcfv4e  -mbitfield  -mno-bitfield  -mc68000  -mc68020 -mnobitfield  -mrtd  -mno-rtd  -mdiv  -mno-div  -mshort -mno-short  -mhard-float  -m68881  -msoft-float  -mpcrel -malign-int  -mstrict-align  -msep-data  -mno-sep-data -mshared-library-id=n  -mid-shared-library  -mno-id-shared-library -mxgot -mno-xgot

     MCore Options -mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates -mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields -m4byte-functions  -mno-4byte-functions  -mcallgraph-data -mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim -mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment

     MeP Options -mabsdiff -mall-opts -maverage -mbased=N -mbitops -mc=N -mclip -mconfig=NAME -mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf -mtiny=N

     MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=CPU -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-APP-MODEL

     MIPS Options -EL  -EB  -march=ARCH  -mtune=ARCH -mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2  -mips32r3  -mips32r5 -mips32r6  -mips64  -mips64r2  -mips64r3  -mips64r5  -mips64r6 -mips16  -mno-mips16  -mflip-mips16 -minterlink-compressed -mno-interlink-compressed -minterlink-mips16  -mno-interlink-mips16 -mabi=ABI  -mabicalls  -mno-abicalls -mshared  -mno-shared  -mplt  -mno-plt  -mxgot  -mno-xgot -mgp32  -mgp64  -mfp32  -mfpxx  -mfp64  -mhard-float  -msoft-float -mno-float  -msingle-float  -mdouble-float -modd-spreg -mno-odd-spreg -mabs=MODE  -mnan=ENCODING -mdsp  -mno-dsp  -mdspr2  -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mmicromips -mno-micromips -mfpu=FPU-TYPE -msmartmips  -mno-smartmips -mpaired-single  -mno-paired-single  -mdmx  -mno-mdmx -mips3d  -mno-mips3d  -mmt  -mno-mt  -mllsc  -mno-llsc -mlong64  -mlong32  -msym32  -mno-sym32 -GNUM  -mlocal-sdata  -mno-local-sdata -mextern-sdata  -mno-extern-sdata  -mgpopt  -mno-gopt -membedded-data  -mno-embedded-data -muninit-const-in-rodata  -mno-uninit-const-in-rodata -mcode-readable=SETTING -msplit-addresses  -mno-split-addresses -mexplicit-relocs  -mno-explicit-relocs -mcheck-zero-division  -mno-check-zero-division -mdivide-traps  -mdivide-breaks -mmemcpy  -mno-memcpy  -mlong-calls  -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd -mfused-madd  -mno-fused-madd  -nocpp -mfix-24k -mno-fix-24k -mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000  -mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120  -mno-fix-vr4120 -mfix-vr4130  -mno-fix-vr4130  -mfix-sb1  -mno-fix-sb1 -mflush-func=FUNC  -mno-flush-func -mbranch-cost=NUM  -mbranch-likely  -mno-branch-likely -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address

     MMIX Options -mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu -mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols -melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses -mno-base-addresses  -msingle-exit  -mno-single-exit

     MN10300 Options -mmult-bug  -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=CPU-TYPE -mreturn-pointer-on-d0 -mno-crt0  -mrelax -mliw -msetlb

     Moxie Options -meb -mel -mmul.x -mno-crt0

     MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mcode-region= -mdata-region= -mhwmult= -minrt

     NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov -mperf-ext -mno-perf-ext -mv3push -mno-v3push -m16bit -mno-16bit -misr-vector-size=NUM -mcache-block-size=NUM -march=ARCH -mcmodel=CODE-MODEL -mctor-dtor -mrelax

     Nios II Options -G NUM -mgpopt=OPTION -mgpopt -mno-gpopt -mel -meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile -mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div -mcustom-INSN=N -mno-custom-INSN -mcustom-fpu-cfg=NAME -mhal -msmallc -msys-crt0=NAME -msys-lib=NAME -march=ARCH -mbmx -mno-bmx -mcdx -mno-cdx

     Nvidia PTX Options -m32 -m64 -mmainkernel

     PDP-11 Options -mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10 -mbcopy  -mbcopy-builtin  -mint32  -mno-int16 -mint16  -mno-int32  -mfloat32  -mno-float64 -mfloat64  -mno-float32  -mabshi  -mno-abshi -mbranch-expensive  -mbranch-cheap -munix-asm  -mdec-asm

     picoChip Options -mae=AE_TYPE -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings

     PowerPC Options See RS/6000 and PowerPC Options.

     RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles

     RS/6000 and PowerPC Options -mcpu=CPU-TYPE -mtune=CPU-TYPE -mcmodel=CODE-MODEL -mpowerpc64 -maltivec  -mno-altivec -mpowerpc-gpopt  -mno-powerpc-gpopt -mpowerpc-gfxopt  -mno-powerpc-gfxopt -mmfcrf  -mno-mfcrf  -mpopcntb  -mno-popcntb -mpopcntd -mno-popcntd -mfprnd  -mno-fprnd -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc -m64  -m32  -mxl-compat  -mno-xl-compat  -mpe -malign-power  -malign-natural -msoft-float  -mhard-float  -mmultiple  -mno-multiple -msingle-float -mdouble-float -msimple-fpu -mstring  -mno-string  -mupdate  -mno-update -mavoid-indexed-addresses  -mno-avoid-indexed-addresses -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align -mstrict-align  -mno-strict-align  -mrelocatable -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib -mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian -mdynamic-no-pic  -maltivec -mswdiv  -msingle-pic-base -mprioritize-restricted-insns=PRIORITY -msched-costly-dep=DEPENDENCE_TYPE -minsert-sched-nops=SCHEME -mcall-sysv  -mcall-netbsd -maix-struct-return  -msvr4-struct-return -mabi=ABI-TYPE -msecure-plt -mbss-plt -mblock-move-inline-limit=NUM -misel -mno-isel -misel=yes  -misel=no -mspe -mno-spe -mspe=yes  -mspe=no -mpaired -mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes  -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype  -mno-prototype -msim  -mmvme  -mads  -myellowknife  -memb  -msdata -msdata=OPT  -mvxworks  -G NUM  -pthread -mrecip -mrecip=OPT -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=TYPE -mfriz -mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mdirect-move -mno-direct-move -mquad-memory -mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mupper-regs-df -mno-upper-regs-df -mupper-regs-sf -mno-upper-regs-sf -mupper-regs -mno-upper-regs

     RX Options -m64bit-doubles  -m32bit-doubles  -fpu  -nofpu -mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data -msim  -mno-sim -mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size= -mint-register= -mpid -mallow-string-insns -mno-allow-string-insns -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts

     S/390 and zSeries Options -mtune=CPU-TYPE  -march=CPU-TYPE -mhard-float  -msoft-float  -mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain  -mno-backchain -mpacked-stack  -mno-packed-stack -msmall-exec  -mno-small-exec  -mmvcle -mno-mvcle -m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch -mtpf-trace -mno-tpf-trace  -mfused-madd  -mno-fused-madd -mwarn-framesize  -mwarn-dynamicstack  -mstack-size -mstack-guard -mhotpatch=HALFWORDS,HALFWORDS

     Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d

     SH Options -m1  -m2  -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3  -m3e -m4-nofpu  -m4-single-only  -m4-single  -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media  -m5-64media-nofpu -m5-32media  -m5-32media-nofpu -m5-compact  -m5-compact-nofpu -mb  -ml  -mdalign  -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops  -misize  -minline-ic_invalidate -mpadstruct -mspace -mprefergot  -musermode -multcost=NUMBER -mdiv=STRATEGY -mdivsi3_libfunc=NAME -mfixed-range=REGISTER-RANGE -mindexed-addressing -mgettrcost=NUMBER -mpt-fixed -maccumulate-outgoing-args -minvalid-symbols -matomic-model=ATOMIC-MODEL -mbranch-cost=NUM -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas

     Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text  -mno-impure-text -pthreads -pthread

     SPARC Options -mcpu=CPU-TYPE -mtune=CPU-TYPE -mcmodel=CODE-MODEL -mmemory-model=MEM-MODEL -m32  -m64  -mapp-regs  -mno-app-regs -mfaster-structs  -mno-faster-structs  -mflat  -mno-flat -mfpu  -mno-fpu  -mhard-float  -msoft-float -mhard-quad-float  -msoft-quad-float -mstack-bias  -mno-stack-bias -munaligned-doubles  -mno-unaligned-doubles -muser-mode  -mno-user-mode -mv8plus  -mno-v8plus  -mvis  -mno-vis -mvis2  -mno-vis2  -mvis3  -mno-vis3 -mcbcond -mno-cbcond -mfmaf  -mno-fmaf  -mpopc  -mno-popc -mfix-at697f -mfix-ut699

     SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem -mstdmain -mfixed-range=REGISTER-RANGE -mea32 -mea64 -maddress-space-conversion -mno-address-space-conversion -mcache-size=CACHE-SIZE -matomic-updates -mno-atomic-updates

     System V Options -Qy  -Qn  -YP,PATHS  -Ym,DIR

     TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian -mcmodel=CODE-MODEL

     TILEPro Options -mcpu=CPU -m32

     V850 Options -mlong-calls  -mno-long-calls  -mep  -mno-ep -mprolog-function  -mno-prolog-function  -mspace -mtda=N  -msda=N  -mzda=N -mapp-regs  -mno-app-regs -mdisable-callt  -mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch

     VAX Options -mg  -mgnu  -munix

     Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=CPU-TYPE -mtune=CPU-TYPE -msv-mode -muser-mode

     VMS Options -mvms-return-codes -mdebug-main=PREFIX -mmalloc64 -mpointer-size=SIZE

     VxWorks Options -mrtp  -non-static  -Bstatic  -Bdynamic -Xbind-lazy  -Xbind-now

     x86 Options -mtune=CPU-TYPE  -march=CPU-TYPE -mtune-ctrl=FEATURE-LIST -mdump-tune-features -mno-default -mfpmath=UNIT -masm=DIALECT  -mno-fancy-math-387 -mno-fp-ret-in-387  -msoft-float -mno-wide-multiply  -mrtd  -malign-double -mpreferred-stack-boundary=NUM -mincoming-stack-boundary=NUM -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=OPT -mvzeroupper -mprefer-avx128 -mmmx  -msse  -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mprefetchwt1 -mclflushopt -mxsavec -mxsaves -msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -mthreads -mno-align-stringops  -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=ALG -mmemcpy-strategy=STRATEGY -mmemset-strategy=STRATEGY -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=NUM  -msseregparm -mveclibabi=TYPE -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign -momit-leaf-frame-pointer  -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=CODE-MODEL -mabi=NAME -maddress-mode=MODE -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=NUM -msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv -mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=TYPE -mstack-protector-guard=GUARD

     x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -municode -mwin32 -mwindows -fno-set-stack-executable

     Xstormy16 Options -msim

     Xtensa Options -mconst16 -mno-const16 -mfused-madd  -mno-fused-madd -mforce-no-pic -mserialize-volatile  -mno-serialize-volatile -mtext-section-literals  -mno-text-section-literals -mtarget-align  -mno-target-align -mlongcalls  -mno-longcalls

     zSeries Options See S/390 and zSeries Options.

Code Generation Options
     See Options for Code Generation Conventions. -fcall-saved-REG  -fcall-used-REG -ffixed-REG  -fexceptions -fnon-call-exceptions  -fdelete-dead-exceptions  -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive  -finstrument-functions -finstrument-functions-exclude-function-list=SYM,SYM,... -finstrument-functions-exclude-file-list=FILE,FILE,... -fno-common  -fno-ident -fpcc-struct-return  -fpic  -fPIC -fpie -fPIE -fno-plt -fno-jump-tables -frecord-gcc-switches -freg-struct-return  -fshort-enums -fshort-double  -fshort-wchar -fverbose-asm  -fpack-struct[=N]  -fstack-check -fstack-limit-register=REG  -fstack-limit-symbol=SYM -fno-stack-limit -fsplit-stack -fleading-underscore  -ftls-model=MODEL -fstack-reuse=REUSE_LEVEL -ftrapv  -fwrapv  -fbounds-check -fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls

Options Controlling the Kind of Output
======================================

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order.  GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

FILE.c
     C source code that must be preprocessed.

FILE.i
     C source code that should not be preprocessed.

FILE.ii
     C++ source code that should not be preprocessed.

FILE.m
     Objective-C source code.  Note that you must link with the libobjc library to make an Objective-C program work.

FILE.mi
     Objective-C source code that should not be preprocessed.

FILE.mm
FILE.M
     Objective-C++ source code.  Note that you must link with the libobjc library to make an Objective-C++ program work.  Note that .M refers to a literal capital M.

FILE.mii
     Objective-C++ source code that should not be preprocessed.

FILE.h
     C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).

FILE.cc
FILE.cp
FILE.cxx
FILE.cpp
FILE.CPP
FILE.c++
FILE.C
     C++ source code that must be preprocessed.  Note that in .cxx, the last two letters must both be literally x.  Likewise, .C refers to a literal capital C.

FILE.mm
FILE.M
     Objective-C++ source code that must be preprocessed.

FILE.mii
     Objective-C++ source code that should not be preprocessed.

FILE.hh
FILE.H
FILE.hp
FILE.hxx
FILE.hpp
FILE.HPP
FILE.h++
FILE.tcc
     C++ header file to be turned into a precompiled header or Ada spec.

FILE.f
FILE.for
FILE.ftn
     Fixed form Fortran source code that should not be preprocessed.

FILE.F
FILE.FOR
FILE.fpp
FILE.FPP
FILE.FTN
     Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).

FILE.f90
FILE.f95
FILE.f03
FILE.f08
     Free form Fortran source code that should not be preprocessed.

FILE.F90
FILE.F95
FILE.F03
FILE.F08
     Free form Fortran source code that must be preprocessed (with the traditional preprocessor).

FILE.go
     Go source code.

FILE.ads
     Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration).  Such files are also called specs.

FILE.adb
     Ada source code file containing a library unit body (a subprogram or package body).  Such files are also called bodies.

FILE.s
     Assembler code.

FILE.S
FILE.sx
     Assembler code that must be preprocessed.

OTHER
     An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the -x option:

-x LANGUAGE
     Specify explicitly the LANGUAGE for the following input files (rather than letting the compiler choose a default based on the file name suffix).  This option applies to all following input files until the next -x option.  Possible values for LANGUAGE are:

          c  c-header  cpp-output
          c++  c++-header  c++-cpp-output
          objective-c  objective-c-header  objective-c-cpp-output
          objective-c++ objective-c++-header objective-c++-cpp-output
          assembler  assembler-with-cpp
          ada
          f77  f77-cpp-input f95  f95-cpp-input
          go
          java

-x none
     Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

-pass-exit-codes
     Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code.  If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication.  The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop.  Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.

-c
     Compile or assemble the source files, but do not link.  The linking stage simply is not done.  The ultimate output is in the form of an object file for each source file.

     By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

     Unrecognized input files, not requiring compilation or assembly, are ignored.

-S
     Stop after the stage of compilation proper; do not assemble.  The output is in the form of an assembler code file for each non-assembler input file specified.

     By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

     Input files that don't require compilation are ignored.

-E
     Stop after the preprocessing stage; do not run the compiler proper.  The output is in the form of preprocessed source code, which is sent to the standard output.

     Input files that don't require preprocessing are ignored.

-o FILE
     Place output in file FILE.  This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

     If -o is not specified, the default is to put an executable file in a.out, the object file for SOURCE.SUFFIX in SOURCE.o, its assembler file in SOURCE.s, a precompiled header file in SOURCE.SUFFIX.gch, and all preprocessed C source on standard output.

-v
     Print (on standard error output) the commands executed to run the stages of compilation.  Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

-###
     Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines.

-pipe
     Use pipes rather than temporary files for communication between the various stages of compilation.  This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

--help
     Print (on the standard output) a description of the command-line options understood by gcc.  If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept.  If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.

--target-help
     Print (on the standard output) a description of target-specific command-line options for each tool.  For some targets extra target-specific information may also be printed.

--help={CLASS|[^]QUALIFIER}[,...]
     Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers.  These are the supported classes:

     optimizers
          Display all of the optimization options supported by the compiler.

     warnings
          Display all of the options controlling warning messages produced by the compiler.

     target
          Display target-specific options.  Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed.  This is because those tools do not currently support the extended --help= syntax.

     params
          Display the values recognized by the --param option.

     LANGUAGE
          Display the options supported for LANGUAGE, where LANGUAGE is the name of one of the languages supported in this version of GCC.

     common
          Display the options that are common to all languages.

     These are the supported qualifiers:

     undocumented
          Display only those options that are undocumented.

     joined
          Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.

     separate
          Display options taking an argument that appears as a separate word following the original option, such as: -o output-file.

     Thus for example to display all the undocumented target-specific switches supported by the compiler, use:

          --help=target,undocumented

     The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

          --help=warnings,^joined,^undocumented

     The argument to --help= should not consist solely of inverted qualifiers.

     Combining several classes is possible, although this usually restricts the output so much that there is nothing to display.  One case where it does work, however, is when one of the classes is TARGET.  For example, to display all the target-specific optimization options, use:

          --help=target,optimizers

     The --help= option can be repeated on the command line.  Each successive use displays its requested class of options, skipping those that have already been displayed.

     If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed.  Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).

     Here is a truncated example from the ARM port of gcc:

            % gcc -Q -mabi=2 --help=target -c
            The following options are target specific:
            -mabi=                                2
            -mabort-on-noreturn                   [disabled]
            -mapcs                                [disabled]

     The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

          -Q -O2 --help=optimizers

     Alternatively you can discover which binary optimizations are enabled by -O3 by using:

          gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
          gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
          diff /tmp/O2-opts /tmp/O3-opts | grep enabled

-no-canonical-prefixes
     Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.

--version
     Display the version number and copyrights of the invoked GCC.

-wrapper
     Invoke all subcommands under a wrapper program.  The name of the wrapper program and its parameters are passed as a comma separated list.

          gcc -c t.c -wrapper gdb,--args

     This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 ....

-fplugin=NAME.so
     Load the plugin code in file NAME.so, assumed to be a shared object to be dlopen'd by the compiler.  The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-NAME-KEY=VALUE below). Each plugin should define the callback functions specified in the Plugins API.

-fplugin-arg-NAME-KEY=VALUE
     Define an argument called KEY with a value of VALUE for the plugin called NAME.

-fdump-ada-spec[-slim]
     For C and C++ source and include files, generate corresponding Ada specs. See Generating Ada Bindings for C and C++ headers (GNAT User's Guide), which provides detailed documentation on this feature.

-fada-spec-parent=UNIT
     In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent UNIT.

-fdump-go-spec=FILE
     For input files in any language, generate corresponding Go declarations in FILE.  This generates Go const, type, var, and func declarations which may be a useful way to start writing a Go interface to code written in some other language.

          [file at-file.texi not included]

Compiling C++ Programs
======================

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii.  GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library.  It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used.  This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations.  On many systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

Options Controlling C Dialect
=============================

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

-ansi
     In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

     This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using.  It also enables the undesirable and rarely used ISO trigraph feature.  For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

     The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi.  You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi.  Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

     The -ansi option does not cause non-ISO programs to be rejected gratuitously.  For that, -Wpedantic is required in addition to -ansi.  See Warning Options.

     The macro __STRICT_ANSI__ is predefined when the -ansi option is used.  Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

     Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used.  See Other built-in functions provided by GCC, for details of the functions affected.

-std=
     Determine the language standard. See Language Standards Supported by GCC, for details of these standard versions.  This option is currently only supported when compiling C or C++.

     The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98.  When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it.  For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard.  As a result, some strict-conforming programs may be rejected.  The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

     A value for this option must be provided; possible values are

     c90
     c89
     iso9899:1990
          Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.

     iso9899:199409
          ISO C90 as modified in amendment 1.

     c99
     c9x
     iso9899:1999
     iso9899:199x
          ISO C99.  This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G).  See http://gcc.gnu.org/c99status.html for more information.  The names c9x and iso9899:199x are deprecated.

     c11
     c1x
     iso9899:2011
          ISO C11, the 2011 revision of the ISO C standard.  This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability).  The name c1x is deprecated.

     gnu90
     gnu89
          GNU dialect of ISO C90 (including some C99 features).

     gnu99
     gnu9x
          GNU dialect of ISO C99.  The name gnu9x is deprecated.

     gnu11
     gnu1x
          GNU dialect of ISO C11.  This is the default for C code. The name gnu1x is deprecated.

     c++98
     c++03
          The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.

     gnu++98
     gnu++03
          GNU dialect of -std=c++98.  This is the default for C++ code.

     c++11
     c++0x
          The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.

     gnu++11
     gnu++0x
          GNU dialect of -std=c++11. The name gnu++0x is deprecated.

     c++14
     c++1y
          The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.

     gnu++14
     gnu++1y
          GNU dialect of -std=c++14. The name gnu++1y is deprecated.

     c++1z
          The next revision of the ISO C++ standard, tentatively planned for 2017.  Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

     gnu++1z
          GNU dialect of -std=c++1z.  Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

-fgnu89-inline
     The option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See An Inline Function is As Fast As a Macro. Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions (see Function Attributes).

     The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.

     The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions.  See Common Predefined Macros (The C Preprocessor).

-aux-info FILENAME
     Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files.  This option is silently ignored in any language other than C.

     Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character).  In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-fallow-parameterless-variadic-functions
     Accept variadic functions without named parameters.

     Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments.  This is only supported for C as this construct is allowed by C++.

-fno-asm
     Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers.  You can use the keywords __asm__, __inline__ and __typeof__ instead.  -ansi implies -fno-asm.

     In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords.  You may want to use the -fno-gnu-keywords flag instead, which has the same effect.  In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin
-fno-builtin-FUNCTION
     Don't recognize built-in functions that do not begin with __builtin_ as prefix.  See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning.

     GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops.  The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.  In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function.  For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory.

     With the -fno-builtin-FUNCTION option only the built-in function FUNCTION is disabled.  FUNCTION must not begin with __builtin_.  If a function is named that is not built-in in this version of GCC, this option is ignored.  There is no corresponding -fbuiltin-FUNCTION option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

          #define abs(n)          __builtin_abs ((n))
          #define strcpy(d, s)    __builtin_strcpy ((d), (s))

-fhosted

     Assert that compilation targets a hosted environment.  This implies -fbuiltin.  A hosted environment is one in which the entire standard library is available, and in which main has a return type of int.  Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.

-ffreestanding

     Assert that compilation targets a freestanding environment.  This implies -fno-builtin.  A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main.  The most obvious example is an OS kernel. This is equivalent to -fno-hosted.

     See Language Standards Supported by GCC, for details of freestanding and hosted environments.

-fopenacc
     Enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in Fortran.  When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0 http://www.openacc.org/.  This option implies -pthread, and thus is only supported on targets that have support for -pthread.

     Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC.  See https://gcc.gnu.org/wiki/OpenACC for more information.

-fopenmp
     Enable handling of OpenMP directives #pragma omp in C/C++ and !$omp in Fortran.  When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.0 http://www.openmp.org/.  This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.

-fopenmp-simd
     Enable handling of OpenMP's SIMD directives with #pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored.

-fcilkplus
     Enable the usage of Cilk Plus language extension features for C/C++. When the option -fcilkplus is specified, enable the usage of the Cilk Plus Language extension features for C/C++.  The present implementation follows ABI version 1.2.  This is an experimental feature that is only partially complete, and whose interface may change in future versions of GCC as the official specification changes.  Currently, all features but _Cilk_for have been implemented.

-fgnu-tm
     When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009).  This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes.  Please note that not all architectures are supported for this feature.

     For more information on GCC's support for transactional memory, see The GNU Transactional Memory Library (GNU Transactional Memory Library).

     Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

-fms-extensions
     Accept some non-standard constructs used in Microsoft header files.

     In C++ code, this allows member names in structures to be similar to previous types declarations.

          typedef int UOW;
          struct ABC {
            UOW UOW;
          };

     Some cases of unnamed fields in structures and unions are only accepted with this option.  See Unnamed struct/union fields within structs/unions, for details.

     Note that this option is off for all targets but x86 targets using ms-abi.

-fplan9-extensions
     Accept some non-standard constructs used in Plan 9 code.

     This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef.  See Unnamed struct/union fields within structs/unions, for details.  This is only supported for C, not C++.

-trigraphs
     Support ISO C trigraphs.  The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-traditional
-traditional-cpp
     Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler.  They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode.  See the GNU CPP manual for details.

-fcond-mismatch
     Allow conditional expressions with mismatched types in the second and third arguments.  The value of such an expression is void.  This option is not supported for C++.

-flax-vector-conversions
     Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types.  This option should not be used for new code.

-funsigned-char
     Let the type char be unsigned, like unsigned char.

     Each kind of machine has a default for what char should be.  It is either like unsigned char by default or like signed char by default.

     Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for.  This option, and its inverse, let you make such a program work with the opposite default.

     The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char
     Let the type char be signed, like signed char.

     Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.  Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
     These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned.  By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.

Options Controlling C++ Dialect
===============================

This section describes the command-line options that are only meaningful for C++ programs.  You can also use most of the GNU compiler options regardless of what language your program is in.  For example, you might compile a file firstClass.C like this:

     g++ -g -frepo -O -c firstClass.C

In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=N
     Use version N of the C++ ABI.  The default is version 0.

     Version 0 refers to the version conforming most closely to the C++ ABI specification.  Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

     Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

     Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

     Version 3 corrects an error in mangling a constant address as a template argument.

     Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

     Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

     Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.

     Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

     Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

     See also -Wabi.

-fabi-compat-version=N
     On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

     With -fabi-version=0 (the default), this defaults to 2.  If another ABI version is explicitly selected, this defaults to 0.

     The compatibility version is also set by -Wabi=N.

-fno-access-control
     Turn off all access checking.  This switch is mainly useful for working around bugs in the access control code.

-fcheck-new
     Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated.  This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option.  In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc.  See also new (nothrow).

-fconstexpr-depth=N
     Set the maximum nested evaluation depth for C++11 constexpr functions to N.  A limit is needed to detect endless recursion during constant expression evaluation.  The minimum specified by the standard is 512.

-fdeduce-init-list
     Enable deduction of a template type parameter as std::initializer_list from a brace-enclosed initializer list, i.e.

          template <class T> auto forward(T t) -> decltype (realfn (t))
          {
            return realfn (t);
          }

          void f()
          {
            forward({1,2}); // call forward<std::initializer_list<int>>
          }

     This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default.  This option is deprecated, and may be removed in a future version of G++.

-ffriend-injection
     Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual. However, in ISO C++ a friend function that is not declared in an enclosing scope can only be found using argument dependent lookup.  GCC defaults to the standard behavior.

     This option is for compatibility, and may be removed in a future release of G++.

-fno-elide-constructors
     The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

-fno-enforce-eh-specs
     Don't generate code to check for violation of exception specifications at run time.  This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.  This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

-fextern-tls-init
-fno-extern-tls-init
     The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization.  To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization.  If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option.

     On targets that support symbol aliases, the default is -fextern-tls-init.  On targets that do not support symbol aliases, the default is -fno-extern-tls-init.

-ffor-scope
-fno-for-scope
     If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.

     If neither flag is given, the default is to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-fno-gnu-keywords
     Do not recognize typeof as a keyword, so that code can use this word as an identifier.  You can use the keyword __typeof__ instead. -ansi implies -fno-gnu-keywords.

-fno-implicit-templates
     Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.

-fno-implicit-inline-templates
     Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.

-fno-implement-inlines
     To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation.  This causes linker errors if these functions are not inlined everywhere they are called.

-fms-extensions
     Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

-fno-nonansi-builtins
     Disable built-in declarations of functions that are not mandated by ANSI/ISO C.  These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.

-fnothrow-opt
     Treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification.  If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away.  The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected.

-fno-operator-names
     Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.

-fno-optional-diags
     Disable diagnostics that the standard says a compiler does not need to issue.  Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

-fpermissive
     Downgrade some diagnostics about nonconformant code from errors to warnings.  Thus, using -fpermissive allows some nonconforming code to compile.

-fno-pretty-templates
     When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved.  When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template.  If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.

-frepo
     Enable automatic template instantiation at link time.  This option also implies -fno-implicit-templates.  See Template Instantiation, for more information.

-fno-rtti
     Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid).  If you don't use those parts of the language, you can save some space by using this flag.  Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

-fsized-deallocation
     Enable the built-in global declarations

          void operator delete (void *, std::size_t) noexcept;
          void operator delete[] (void *, std::size_t) noexcept;

     as introduced in C++14.  This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster.  Enabled by default under -std=c++14 and above.  The flag -Wsized-deallocation warns about places that might want to add a definition.

-fstats
     Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-fstrict-enums
     Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators).  This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.

-ftemplate-backtrace-limit=N
     Set the maximum number of template instantiation notes for a single warning or error to N.  The default value is 10.

-ftemplate-depth=N
     Set the maximum instantiation depth for template classes to N. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation.  ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11).  The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.

-fno-threadsafe-statics
     Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics.  You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.

-fuse-cxa-atexit
     Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit.

-fno-use-cxa-get-exception-ptr
     Don't use the __cxa_get_exception_ptr runtime routine.  This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available.

-fvisibility-inlines-hidden
     This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

     The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.  Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

     The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

     You may mark a method as having a visibility explicitly to negate the effect of the switch for that method.  For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility.  Marking the enclosing class with explicit visibility has no effect.

     Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See Template Instantiation.

-fvisibility-ms-compat
     This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.

     The flag makes these changes to GCC's linkage model:

      1. It sets the default visibility to hidden, like -fvisibility=hidden.

      2. Types, but not their members, are not hidden by default.

      3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

     In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

     Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal.  When this flag is given, it is a violation of the ODR to define types with the same name differently.

-fvtable-verify=[std|preinit|none]
     Turn on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten.  If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted.

     This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built.  In both cases the data structures are built before execution reaches main.  Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.

     If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std.

-fvtv-debug
     When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise.

     Note:  This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.

-fvtv-counts
     This is a debugging flag.  When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts.  It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise.  It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory.

     Note:  This feature appends data to the log files.  To get fresh log files, be sure to delete any existing ones.

-fno-weak
     Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available.  This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits.  This option may be removed in a future release of G++.

-nostdinc++
     Do not search for header files in the standard directories specific to C++, but do still search the other standard directories.  (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

-Wabi (C, Objective-C, C++ and Objective-C++ only)
     When an explicit -fabi-version=N option is used, causes G++ to warn when it generates code that is probably not compatible with the vendor-neutral C++ ABI.  Since G++ now defaults to -fabi-version=0, -Wabi has no effect unless either an older ABI version is selected (with -fabi-version=N) or an older compatibility version is selected (with -Wabi=N or -fabi-compat-version=N).

     Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code.  There may also be cases where warnings are emitted even though the code that is generated is compatible.

     You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

     -Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.  Specifying a version number also sets -fabi-compat-version=N.

     The known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

       * A template with a non-type template parameter of reference type was mangled incorrectly:

               extern int N;
               template <int &> struct S {};
               void n (S<N>) {2}

          This was fixed in -fabi-version=3.

       * SIMD vector types declared using __attribute ((vector_size)) were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

          The mangling was changed in -fabi-version=4.

       * __attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.

          These mangling issues were fixed in -fabi-version=5.

       * Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int.

          Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

          These issues were corrected in -fabi-version=6.

       * Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t.

          These issues were corrected in -fabi-version=7.

       * When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.

          This was fixed in -fabi-version=8.

     It also warns about psABI-related changes.  The known psABI changes at this point include:

       * For SysV/x86-64, unions with long double members are passed in memory as specified in psABI.  For example:

               union U {
                 long double ld;
                 int i;
               };

          union U is always passed in memory.

-Wabi-tag (C++ and Objective-C++ only)
     Warn when a type with an ABI tag is used in a context that does not have that ABI tag.  See C++ Attributes for more information about ABI tags.

-Wctor-dtor-privacy (C++ and Objective-C++ only)
     Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.  Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor.

-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
     Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor.  This warning is enabled by -Wall.

-Wliteral-suffix (C++ and Objective-C++ only)
     Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore.  As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example:

          #define __STDC_FORMAT_MACROS
          #include <inttypes.h>
          #include <stdio.h>

          int main() {
            int64_t i64 = 123;
            printf("My int64: %" PRId64"\n", i64);
          }

     In this case, PRId64 is treated as a separate preprocessing token.

     This warning is enabled by default.

-Wlto-type-mismatch

     During the link-time optimization warn about type mismatches in between global declarations from different compilation units. Requires -flto to be enabled.  Enabled by default.

-Wnarrowing (C++ and Objective-C++ only)
     Warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.

          int i = { 2.2 }; // error: narrowing from double to int

     This flag is included in -Wall and -Wc++11-compat.

     With -std=c++11, -Wno-narrowing suppresses the diagnostic required by the standard.  Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE context.

-Wnoexcept (C++ and Objective-C++ only)
     Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.

-Wnon-virtual-dtor (C++ and Objective-C++ only)
     Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class.  This warning is automatically enabled if -Weffc++ is specified.

-Wreorder (C++ and Objective-C++ only)
     Warn when the order of member initializers given in the code does not match the order in which they must be executed.  For instance:

          struct A {
            int i;
            int j;
            A(): j (0), i (1) { }
          };

     The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect.  This warning is enabled by -Wall.

-fext-numeric-literals (C++ and Objective-C++ only)
     Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).

The following -W... options are not affected by -Wall.

-Weffc++ (C++ and Objective-C++ only)
     Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:

       * Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.

       * Prefer initialization to assignment in constructors.

       * Have operator= return a reference to *this.

       * Don't try to return a reference when you must return an object.

       * Distinguish between prefix and postfix forms of increment and decrement operators.

       * Never overload &&, ||, or ,.

     This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations.  However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.

     When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.

-Wstrict-null-sentinel (C++ and Objective-C++ only)
     Warn about the use of an uncasted NULL as sentinel.  When compiling only with GCC this is a valid sentinel, as NULL is defined to __null.  Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.

-Wno-non-template-friend (C++ and Objective-C++ only)
     Disable warnings when non-templatized friend functions are declared within a template.  Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function.  (Section 14.5.3).  Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function.  Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend, which keeps the conformant compiler code but disables the helpful warning.

-Wold-style-cast (C++ and Objective-C++ only)
     Warn if an old-style (C-style) cast to a non-void type is used within a C++ program.  The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.

-Woverloaded-virtual (C++ and Objective-C++ only)
     Warn when a function declaration hides virtual functions from a base class.  For example, in:

          struct A {
            virtual void f();
          };

          struct B: public A {
            void f(int);
          };

     the A class version of f is hidden in B, and code like:

          B* b;
          b->f();

     fails to compile.

-Wno-pmf-conversions (C++ and Objective-C++ only)
     Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

-Wsign-promo (C++ and Objective-C++ only)
     Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size.  Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.

-Wno-terminate (C++ and Objective-C++ only)
     Disable the warning about a throw-expression that will immediately result in a call to terminate.

Options Controlling Objective-C and Objective-C++ Dialects
==========================================================

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.  See Language Standards Supported by GCC, for references.)

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs.  You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

     gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional).  Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

-fconstant-string-class=CLASS-NAME
     Use CLASS-NAME as the name of the class to instantiate for each literal string specified with the syntax @"...".  The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below).  The -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.

-fgnu-runtime
     Generate object code compatible with the standard GNU Objective-C runtime.  This is the default for most types of systems.

-fnext-runtime
     Generate output compatible with the NeXT runtime.  This is the default for NeXT-based systems, including Darwin and Mac OS X.  The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.

-fno-nil-receivers
     Assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil.  This allows for more efficient entry points in the runtime to be used.  This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.

-fobjc-abi-version=N
     Use version N of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime.  In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.  Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.  Version 2 is the modern (64-bit) ABI.  If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.

-fobjc-call-cxx-cdtors
     For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor.  If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self.  Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

     The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses.  It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy.  The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.

     As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.

-fobjc-direct-dispatch
     Allow fast jumps to the message dispatcher.  On Darwin this is accomplished via the comm page.

-fobjc-exceptions
     Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java.  This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized.  This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).

-fobjc-gc
     Enable garbage collection (GC) in Objective-C and Objective-C++ programs.  This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.

-fobjc-nilcheck
     For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck.  Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.

-fobjc-std=objc1
     Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0.  This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags.  When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.

-freplace-objc-classes
     Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead.  This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself.  Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fzero-link
     When compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance.  Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained.  This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.

-fno-local-ivars
     By default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in.  This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name.  Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues.

-fivar-visibility=[public|protected|private|package]
     Set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.

-gen-decls
     Dump interface declarations for all classes seen in the source file to a file named SOURCENAME.decl.

-Wassign-intercept (Objective-C and Objective-C++ only)
     Warn whenever an Objective-C assignment is being intercepted by the garbage collector.

-Wno-protocol (Objective-C and Objective-C++ only)
     If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class.  The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass.  If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.

-Wselector (Objective-C and Objective-C++ only)
     Warn if multiple methods of different types for the same selector are found during compilation.  The check is performed on the list of methods in the final stage of compilation.  Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation.  Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.

-Wstrict-selector-match (Objective-C and Objective-C++ only)
     Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class.  When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.

-Wundeclared-selector (Objective-C and Objective-C++ only)
     Warn if a @selector(...) expression referring to an undeclared selector is found.  A selector is considered undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section.  This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation.  This also enforces the coding style convention that methods and selectors must be declared before being used.

-print-objc-runtime-info
     Generate C header describing the largest structure that is passed by value, if any.

Options to Control Diagnostic Messages Formatting
=================================================

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...).  You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported.  Note that some language front ends may not honor these options.

-fmessage-length=N
     Try to format error messages so that they fit on lines of about N characters.  If N is zero, then no line-wrapping is done; each error message appears on a single line.  This is the default for all front ends.

-fdiagnostics-show-location=once
     Only meaningful in line-wrapping mode.  Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines.  This is the default behavior.

-fdiagnostics-show-location=every-line
     Only meaningful in line-wrapping mode.  Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.

-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
     Use color in diagnostics.  WHEN is never, always, or auto.  The default depends on how the compiler has been configured, it can be any of the above WHEN options or also never if GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto means to use color only when the standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.

     The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator.  (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.)  These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

     The default GCC_COLORS is

          error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01

     where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 01;32 is bold green and 01 is bold. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.

     error=
          SGR substring for error: markers.

     warning=
          SGR substring for warning: markers.

     note=
          SGR substring for note: markers.

     caret=
          SGR substring for caret line.

     locus=
          SGR substring for location information, file:line or file:line:column etc.

     quote=
          SGR substring for information printed within quotes.

-fno-diagnostics-show-option
     By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery).  Specifying the -fno-diagnostics-show-option flag suppresses that behavior.

-fno-diagnostics-show-caret
     By default, each diagnostic emitted includes the original source line and a caret '^' indicating the column.  This option suppresses this information.  The source line is truncated to N characters, if the -fmessage-length=n option is given.  When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.

Options to Request or Suppress Warnings
=======================================

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

-fsyntax-only
     Check the code for syntax errors, but don't do anything beyond that.

-fmax-errors=N
     Limits the maximum number of error messages to N, at which point GCC bails out rather than attempting to continue processing the source code.  If N is 0 (the default), there is no limit on the number of error messages produced.  If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.

-w
     Inhibit all warning messages.

-Werror
     Make all warnings into errors.

-Werror=
     Make the specified warning into an error.  The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors.  This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

     The warning message for each controllable warning includes the option that controls the warning.  That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

     Note that specifying -Werror=FOO automatically implies -WFOO.  However, -Wno-error=FOO does not imply anything.

-Wfatal-errors
     This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.

You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations.  Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit.  This manual lists only one of the two forms, whichever is not the default.  For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas (see Diagnostic Pragmas) take effect as if they appeared at the end of the command-line.

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized.  However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced.  This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.

-Wpedantic
-pedantic
     Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++.  For ISO C, follows the version of the ISO C standard specified by any -std option used.

     Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C).  However, without this option, certain GNU extensions and traditional C and C++ features are supported as well.  With this option, they are rejected.

     -Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __.  Pedantic warnings are also disabled in the expression that follows __extension__.  However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

     Some users try to use -Wpedantic to check programs for strict ISO C conformance.  They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

     A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic.  We don't have plans to support such a feature in the near future.

     Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.  Warnings from -Wpedantic are given where they are required by the base standard.  (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors
     Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa.

-Wall
     This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros.  This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

     -Wall turns on the following warning flags:

     -Waddress -Warray-bounds=1 (only with -O2) -Wc++11-compat  -Wc++14-compat -Wchar-subscripts -Wenum-compare (in C/ObjC; this is on by default in C++) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Wcomment -Wformat -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmissing-braces (only for C/ObjC) -Wnonnull -Wopenmp-simd -Wparentheses -Wpointer-sign -Wreorder -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var

     Note that some warning flags are not implied by -Wall.  Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.

-Wextra
     This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W.  The older name is still supported, but the newer name is more descriptive.)

     -Wclobbered -Wempty-body -Wignored-qualifiers -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare -Wtype-limits -Wuninitialized -Wshift-negative-value -Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)

     The option -Wextra also prints warning messages for the following cases:

       * A pointer is compared against integer zero with <, <=, >, or >=.

       * (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.

       * (C++ only) Ambiguous virtual bases.

       * (C++ only) Subscripting an array that has been declared register.

       * (C++ only) Taking the address of a variable that has been declared register.

       * (C++ only) A base class is not initialized in a derived class's copy constructor.

-Wchar-subscripts
     Warn if an array subscript has type char.  This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall.

-Wcomment
     Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. This warning is enabled by -Wall.

-Wno-coverage-mismatch
     Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-gen and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information.  By default, this warning is enabled and is treated as an error.  -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error.  Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

-Wno-cpp
     (C, Objective-C, C++, Objective-C++ and Fortran only)

     Suppress warning messages emitted by #warning directives.

-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
     Give a warning when a value of type float is implicitly promoted to double.  CPUs with a 32-bit ``single-precision'' floating-point unit implement float in hardware, but emulate double in software.  On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation.

     It is easy to accidentally do computations with double because floating-point literals are implicitly of type double.  For example, in:

          float area(float radius)
          {
             return 3.14159 * radius * radius;
          }

     the compiler performs the entire computation with double because the floating-point literal is a double.

-Wformat
-Wformat=N
     Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense.  This includes standard functions, and others specified by format attributes (see Function Attributes), in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

     The formats are checked against the format features supported by GNU libc version 2.2.  These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions.  Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations.  However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).  See Options Controlling C Dialect.

     -Wformat=1
     -Wformat
          Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0.  Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.  Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.

     -Wno-format-contains-nul
          If -Wformat is specified, do not warn about format strings that contain NUL bytes.

     -Wno-format-extra-args
          If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function.  The C standard specifies that such arguments are ignored.

          Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments.  However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

     -Wno-format-zero-length
          If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

     -Wformat=2
          Enable -Wformat plus additional format checks.  Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

     -Wformat-nonliteral
          If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list.

     -Wformat-security
          If -Wformat is specified, also warn about uses of format functions that represent possible security problems.  At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);.  This may be a security hole if the format string came from untrusted input and contains %n.  (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

     -Wformat-signedness
          If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.

     -Wformat-y2k
          If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year.

-Wnonnull
     Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

     -Wnonnull is included in -Wall and -Wformat.  It can be disabled with the -Wno-nonnull option.

-Winit-self (C, C++, Objective-C and Objective-C++ only)
     Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

     For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

          int f()
          {
            int i = i;
            return i;
          }

     This warning is enabled by -Wall in C++.

-Wimplicit-int (C and Objective-C only)
     Warn when a declaration does not specify a type. This warning is enabled by -Wall.

-Wimplicit-function-declaration (C and Objective-C only)
     Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall.

-Wimplicit (C and Objective-C only)
     Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.

-Wignored-qualifiers (C and C++ only)
     Warn if the return type of a function has a type qualifier such as const.  For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.

     This warning is also enabled by -Wextra.

-Wmain
     Warn if the type of main is suspicious.  main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types.  This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.

-Wmisleading-indentation (C and C++ only)
     Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.

     This warning is disabled by default.

     In the following example, the call to ``bar'' is misleadingly indented as if it were guarded by the ``if'' conditional.

            if (some_condition ())
              foo ();
              bar ();  /* Gotcha: this is not guarded by the "if".  */

     In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

     The warning is not issued for code involving multiline preprocessor logic such as the following example.

            if (flagA)
              foo (0);
          #if SOME_CONDITION_THAT_DOES_NOT_HOLD
            if (flagB)
          #endif
              foo (1);

     The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

-Wmissing-braces
     Warn if an aggregate or union initializer is not fully bracketed.  In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.  This warning is enabled by -Wall in C.

          int a[2][2] = { 0, 1, 2, 3 };
          int b[2][2] = { { 0, 1 }, { 2, 3 } };

     This warning is enabled by -Wall.

-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
     Warn if a user-supplied include directory does not exist.

-Wparentheses
     Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

     Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.

     Also warn about constructions where there may be confusion to which if statement an else branch belongs.  Here is an example of such a case:

          {
            if (a)
              if (b)
                foo ();
            else
              bar ();
          }

     In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b).  This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose.  When there is the potential for this confusion, GCC issues a warning when this flag is specified.  To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if.  The resulting code looks like this:

          {
            if (a)
              {
                if (b)
                  foo ();
                else
                  bar ();
              }
          }

     Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1.  Often programmers expect it to be a value computed inside the conditional expression instead.

     This warning is enabled by -Wall.

-Wsequence-point
     Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.

     The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it.  These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified.  All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified.  However, the standards committee have ruled that function calls do not overlap.

     It is not specified when between sequence points modifications to the values of objects take effect.  Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that ``Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.''.  If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

     Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;.  Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

     The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

     This warning is enabled by -Wall for C and C++.

-Wno-return-local-addr
     Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.

-Wreturn-type
     Warn whenever a function is defined with a return type that defaults to int.  Also warn about any return statement with no return value in a function whose return type is not void (falling off the end of the function body is considered returning without a value), and about a return statement with an expression in a function whose return type is void.

     For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified.  The only exceptions are main and functions defined in system headers.

     This warning is enabled by -Wall.

-Wshift-count-negative
     Warn if shift count is negative. This warning is enabled by default.

-Wshift-count-overflow
     Warn if shift count >= width of type. This warning is enabled by default.

-Wshift-negative-value
     Warn if left shifting a negative value.  This warning is enabled by -Wextra in C99 and C++11 modes (and newer).

-Wshift-overflow
-Wshift-overflow=N
     Warn about left shift overflows.  This warning is enabled by default in C99 and C++11 modes (and newer).

     -Wshift-overflow=1
          This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer).  This warning level does not warn about left-shifting 1 into the sign bit.  (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.)

     -Wshift-overflow=2
          This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode is active.

-Wswitch
     Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration.  (The presence of a default label prevents this warning.)  case labels outside the enumeration range also provoke warnings when this option is used (even if there is a default label). This warning is enabled by -Wall.

-Wswitch-default
     Warn whenever a switch statement does not have a default case.

-Wswitch-enum
     Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration.  case labels outside the enumeration range also provoke warnings when this option is used.  The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.

-Wswitch-bool
     Warn whenever a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than bool.  For example:

          switch ((int) (a == 4))
            {
            ...
            }

     This warning is enabled by default for C and C++ programs.

-Wsync-nand (C and C++ only)
     Warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used.  These functions changed semantics in GCC 4.4.

-Wtrigraphs
     Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by -Wall.

-Wunused-but-set-parameter
     Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

     To suppress this warning use the unused attribute (see Variable Attributes).

     This warning is also enabled by -Wunused together with -Wextra.

-Wunused-but-set-variable
     Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall.

     To suppress this warning use the unused attribute (see Variable Attributes).

     This warning is also enabled by -Wunused, which is enabled by -Wall.

-Wunused-function
     Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall.

-Wunused-label
     Warn whenever a label is declared but not used. This warning is enabled by -Wall.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
     Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.

-Wunused-parameter
     Warn whenever a function parameter is unused aside from its declaration.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wno-unused-result
     Do not warn if a caller of a function marked with attribute warn_unused_result (see Function Attributes) does not use its return value. The default is -Wunused-result.

-Wunused-variable
     Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-value
     Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not.

     This warning is enabled by -Wall.

-Wunused
     All the above -Wunused options combined.

     In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.

-Wuninitialized
     Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call. In C++, warn if a non-static reference or non-static const member appears in a class without constructors.

     If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

     These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered as a whole.  They do not occur for variables or elements declared volatile.  Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used.

     Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

-Wmaybe-uninitialized
     For an automatic variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct in spite of appearing to have an error.  Here is one example of how this can happen:

          {
            int x;
            switch (y)
              {
              case 1: x = 1;
                break;
              case 2: x = 4;
                break;
              case 3: x = 5;
              }
            foo (x);
          }

     If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.

     This option also warns when a non-volatile automatic variable might be changed by a call to longjmp.  These warnings as well are possible only in optimizing compilation.

     The compiler sees only the calls to setjmp.  It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code.  As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place that would cause a problem.

     Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.  See Function Attributes.

     This warning is enabled by -Wall or -Wextra.

-Wunknown-pragmas
     Warn when a #pragma directive is encountered that is not understood by GCC.  If this command-line option is used, warnings are even issued for unknown pragmas in system header files.  This is not the case if the warnings are only enabled by the -Wall command-line option.

-Wno-pragmas
     Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas.  See also -Wunknown-pragmas.

-Wstrict-aliasing
     This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization.  The warning does not catch all cases, but does attempt to catch the more common pitfalls.  It is included in -Wall. It is equivalent to -Wstrict-aliasing=3

-Wstrict-aliasing=n
     This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

     Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives.  However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced.  Runs in the front end only.

     Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken.  Warns about incomplete types.  Runs in the front end only.

     Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives.  Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

-Wstrict-overflow
-Wstrict-overflow=N
     This option is only active when -fstrict-overflow is active. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur.  Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization.  Thus this warning depends on the optimization level.

     An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur.  Therefore this warning can easily give a false positive: a warning about code that is not actually a problem.  To help focus on important issues, several warning levels are defined.  No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

     -Wstrict-overflow=1
          Warn about cases that are both questionable and easy to avoid.  For example,  with -fstrict-overflow, the compiler simplifies x + 1 > x to 1.  This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.

     -Wstrict-overflow=2
          Also warn about other cases where a comparison is simplified to a constant.  For example: abs (x) >= 0.  This can only be simplified when -fstrict-overflow is in effect, because abs (INT_MIN) overflows to INT_MIN, which is less than zero.  -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.

     -Wstrict-overflow=3
          Also warn about other cases where a comparison is simplified.  For example: x + 1 > 1 is simplified to x > 0.

     -Wstrict-overflow=4
          Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.

     -Wstrict-overflow=5
          Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison.  For example: x + 2 > y is simplified to x + 1 >= y.  This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

-Wsuggest-attribute=[pure|const|noreturn|format]
     Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.

     -Wsuggest-attribute=pure
     -Wsuggest-attribute=const
     -Wsuggest-attribute=noreturn

          Warn about functions that might be candidates for attributes pure, const or noreturn.  The compiler only warns for functions visible in other compilation units or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping.  This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher.  Higher optimization levels improve the accuracy of the analysis.

     -Wsuggest-attribute=format
     -Wmissing-format-attribute

          Warn about function pointers that might be candidates for format attributes.  Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type.  I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.

          GCC also warns about function definitions that might be candidates for format attributes.  Again, these are only possible candidates.  GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.

-Wsuggest-final-types
     Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete.

-Wsuggest-final-methods
     Warn about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations.

-Wsuggest-override
     Warn about overriding virtual functions that are not marked with the override keyword.

-Warray-bounds
-Warray-bounds=N
     This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.

     -Warray-bounds=1
          This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.

     -Warray-bounds=2
          This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default.

-Wbool-compare
     Warn about boolean expression compared with an integer value different from true/false.  For instance, the following comparison is always false:

          int n = 5;
          ...
          if ((n > 1) == 2) { ... }

     This warning is enabled by -Wall.

-Wno-discarded-qualifiers (C and Objective-C only)
     Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter.  This option can be used to suppress such a warning.

-Wno-discarded-array-qualifiers (C and Objective-C only)
     Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter.  This option can be used to suppress such a warning.

-Wno-incompatible-pointer-types (C and Objective-C only)
     Do not warn when there is a conversion between pointers that have incompatible types.  This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.

-Wno-int-conversion (C and Objective-C only)
     Do not warn about incompatible integer to pointer and pointer to integer conversions.  This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.

-Wno-div-by-zero
     Do not warn about compile-time integer division by zero.  Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

-Wsystem-headers
     Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read.  Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code.  However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-Wtrampolines
     Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly.  For some targets, it is made up of data only and thus requires no special treatment.  But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly.

-Wfloat-equal
     Warn if floating-point values are used in equality comparisons.

     The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers.  If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem).  In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C and Objective-C only)
     Warn about certain constructs that behave differently in traditional and ISO C.  Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.

       * Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.

       * In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line.  Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line.  It also suggests you hide directives like #pragma not understood by traditional C by indenting them.  Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.

       * A function-like macro that appears without arguments.

       * The unary plus operator.

       * The U integer constant suffix, or the F or L floating-point constant suffixes.  (Traditional C does support the L suffix on integer constants.)  Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.

       * A function declared external in one block and then used after the end of the block.

       * A switch statement has an operand of type long.

       * A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.

       * The ISO type of an integer constant has a different width or signedness from its traditional type.  This warning is only issued if the base of the constant is ten.  I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.

       * Usage of ISO string concatenation is detected.

       * Initialization of automatic aggregates.

       * Identifier conflicts with labels.  Traditional C lacks a separate namespace for labels.

       * Initialization of unions.  If the initializer is zero, the warning is omitted.  This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.

       * Conversions by prototypes between fixed/floating-point values and vice versa.  The absence of these prototypes when compiling with traditional C causes serious problems.  This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.

       * Use of ISO C style function definitions.  This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS.  This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

-Wtraditional-conversion (C and Objective-C only)
     Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype.  This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.

-Wdeclaration-after-statement (C and Objective-C only)
     Warn when a declaration is found after a statement in a block.  This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC.  It is not supported by ISO C90.  See Mixed Declarations.

-Wundef
     Warn if an undefined identifier is evaluated in an #if directive.

-Wno-endif-labels
     Do not warn whenever an #else or an #endif are followed by text.

-Wshadow
     Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.

-Wno-shadow-ivar (Objective-C only)
     Do not warn whenever a local variable shadows an instance variable in an Objective-C method.

-Wlarger-than=LEN
     Warn whenever an object of larger than LEN bytes is defined.

-Wframe-larger-than=LEN
     Warn if the size of a function frame is larger than LEN bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than LEN even if you do not get a warning.  In addition, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.

-Wno-free-nonheap-object
     Do not warn when attempting to free an object that was not allocated on the heap.

-Wstack-usage=LEN
     Warn if the stack usage of a function might be larger than LEN bytes. The computation done to determine the stack usage is conservative. Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

     The message is in keeping with the output of -fstack-usage.

       * If the stack usage is fully static but exceeds the specified amount, it's:

                 warning: stack usage is 1120 bytes

       * If the stack usage is (partly) dynamic but bounded, it's:

                 warning: stack usage might be 1648 bytes

       * If the stack usage is (partly) dynamic and not bounded, it's:

                 warning: stack usage might be unbounded

-Wunsafe-loop-optimizations
     Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices.  With -funsafe-loop-optimizations warn if the compiler makes such assumptions.

-Wno-pedantic-ms-format (MinGW targets only)
     When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime.

-Wpointer-arith
     Warn about anything that depends on the ``size of'' a function type or of void.  GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions.  In C++, warn also when an arithmetic operation involves NULL.  This warning is also enabled by -Wpedantic.

-Wtype-limits
     Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions.  For example, warn if an unsigned variable is compared against zero with < or >=.  This warning is also enabled by -Wextra.

-Wbad-function-cast (C and Objective-C only)
     Warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type.

-Wc90-c99-compat (C and Objective-C only)
     Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on.  This option is independent of the standards mode.  Warnings are disabled in the expression that follows __extension__.

-Wc99-c11-compat (C and Objective-C only)
     Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on.  This option is independent of the standards mode.  Warnings are disabled in the expression that follows __extension__.

-Wc++-compat (C and Objective-C only)
     Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from void * to a pointer to non-void type.

-Wc++11-compat (C++ and Objective-C++ only)
     Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011.  This warning turns on -Wnarrowing and is enabled by -Wall.

-Wc++14-compat (C++ and Objective-C++ only)
     Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014.  This warning is enabled by -Wall.

-Wcast-qual
     Warn whenever a pointer is cast so as to remove a type qualifier from the target type.  For example, warn if a const char * is cast to an ordinary char *.

     Also warn when making a cast that introduces a type qualifier in an unsafe way.  For example, casting char ** to const char ** is unsafe, as in this example:

            /* p is char ** value.  */
            const char **q = (const char **) p;
            /* Assignment of readonly string to const char * is OK.  */
            *q = "string";
            /* Now char** pointer points to read-only memory.  */
            **p = 'b';

-Wcast-align
     Warn whenever a pointer is cast such that the required alignment of the target is increased.  For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

-Wwrite-strings
     When compiling C, give string constants the type const char[LENGTH] so that copying the address of one into a non-const char * pointer produces a warning.  These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes.  Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.

     When compiling C++, warn about the deprecated conversion from string literals to char *.  This warning is enabled by default for C++ programs.

-Wclobbered
     Warn for variables that might be changed by longjmp or vfork.  This warning is also enabled by -Wextra.

-Wconditionally-supported (C++ and Objective-C++ only)
     Warn for conditionally-supported (C++11 [intro.defs]) constructs.

-Wconversion
     Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double; conversions between signed and unsigned, like unsigned ui = -1; and conversions to smaller types, like sqrtf (M_PI). Do not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0).  Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

     For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.

-Wno-conversion-null (C++ and Objective-C++ only)
     Do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default.

-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
     Warn when a literal '0' is used as null pointer constant.  This can be useful to facilitate the conversion to nullptr in C++11.

-Wdate-time
     Warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered as they might prevent bit-wise-identical reproducible compilations.

-Wdelete-incomplete (C++ and Objective-C++ only)
     Warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime.  This warning is enabled by default.

-Wuseless-cast (C++ and Objective-C++ only)
     Warn when an expression is casted to its own type.

-Wempty-body
     Warn if an empty body occurs in an if, else or do while statement.  This warning is also enabled by -Wextra.

-Wenum-compare
     Warn about a comparison between values of different enumerated types. In C++ enumeral mismatches in conditional expressions are also diagnosed and the warning is enabled by default.  In C this warning is enabled by -Wall.

-Wjump-misses-init (C, Objective-C only)
     Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized.  This only warns about variables that are initialized when they are declared.  This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.

     -Wjump-misses-init is included in -Wc++-compat.  It can be disabled with the -Wno-jump-misses-init option.

-Wsign-compare
     Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.

-Wsign-conversion
     Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion.

-Wfloat-conversion
     Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values.  This option is also enabled by -Wconversion.

-Wsized-deallocation (C++ and Objective-C++ only)
     Warn about a definition of an unsized deallocation function

          void operator delete (void *) noexcept;
          void operator delete[] (void *) noexcept;

     without a definition of the corresponding sized deallocation function

          void operator delete (void *, std::size_t) noexcept;
          void operator delete[] (void *, std::size_t) noexcept;

     or vice versa.  Enabled by -Wextra along with -fsized-deallocation.

-Wsizeof-pointer-memaccess
     Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof.  This warning warns e.g. about memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo));.  This warning is enabled by -Wall.

-Wsizeof-array-argument
     Warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition.  This warning is enabled by default for C and C++ programs.

-Wmemset-transposed-args
     Warn for suspicious calls to the memset built-in function, if the second argument is not zero and the third argument is zero.  This warns e.g. about memset (buf, sizeof buf, 0) where most probably memset (buf, 0, sizeof buf) was meant instead.  The diagnostics is only emitted if the third argument is literal zero.  If it is some expression that is folded to zero, a cast of zero to some type, etc., it is far less likely that the user has mistakenly exchanged the arguments and no warning is emitted.  This warning is enabled by -Wall.

-Waddress
     Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as void func(void); if (func), and comparisons against the memory address of a string literal, such as if (x == "abc").  Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use strcmp.  This warning is enabled by -Wall.

-Wlogical-op
     Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected.  Also warns when the operands of a logical operator are the same:

          extern int a;
          if (a < 0 && a < 0) { ... }

-Wlogical-not-parentheses
     Warn about logical not used on the left hand side operand of a comparison. This option does not warn if the RHS operand is of a boolean type.  Its purpose is to detect suspicious code like the following:

          int a;
          ...
          if (!a > 1) { ... }

     It is possible to suppress the warning by wrapping the LHS into parentheses:

          if ((!a) > 1) { ... }

     This warning is enabled by -Wall.

-Waggregate-return
     Warn if any functions that return structures or unions are defined or called.  (In languages where you can return an array, this also elicits a warning.)

-Wno-aggressive-loop-optimizations
     Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.

-Wno-attributes
     Do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc.  This does not stop errors for incorrect use of supported attributes.

-Wno-builtin-macro-redefined
     Do not warn if certain built-in macros are redefined.  This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.

-Wstrict-prototypes (C and Objective-C only)
     Warn if a function is declared or defined without specifying the argument types.  (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)

-Wold-style-declaration (C and Objective-C only)
     Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration.  This warning is also enabled by -Wextra.

-Wold-style-definition (C and Objective-C only)
     Warn if an old-style function definition is used.  A warning is given even if there is a previous prototype.

-Wmissing-parameter-type (C and Objective-C only)
     A function parameter is declared without a type specifier in K&R-style functions:

          void foo(bar) { }

     This warning is also enabled by -Wextra.

-Wmissing-prototypes (C and Objective-C only)
     Warn if a global function is defined without a previous prototype declaration.  This warning is issued even if the definition itself provides a prototype.  Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.

-Wmissing-declarations
     Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.  In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes.  In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.

-Wmissing-field-initializers
     Warn if a structure's initializer has some fields missing.  For example, the following code causes such a warning, because x.h is implicitly zero:

          struct s { int f, g, h; };
          struct s x = { 3, 4 };

     This option does not warn about designated initializers, so the following modification does not trigger a warning:

          struct s { int f, g, h; };
          struct s x = { .f = 3, .g = 4 };

     In C++ this option does not warn either about the empty { } initializer, for example:

          struct s { int f, g, h; };
          s x = { };

     This warning is included in -Wextra.  To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.

-Wno-multichar
     Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.

-Wnormalized[=<none|id|nfc|nfkc>]
     In ISO C and ISO C++, two identifiers are different if they are different sequences of characters.  However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same.  To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence.  GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.

     There are four levels of warning supported by GCC.  The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 ``C'' normalized form, NFC.  NFC is the recommended form for most uses.  It is equivalent to -Wnormalized.

     Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers.  That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.

     You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized.  You should only do this if you are using some other normalization scheme (like ``D''), because otherwise you can easily create bugs that are literally impossible to see.

     Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied.  For instance \u207F, ``SUPERSCRIPT LATIN SMALL LETTER N'', displays just like a regular n that has been placed in a superscript.  ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc.  This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly.

-Wno-deprecated
     Do not warn about usage of deprecated features.  See Deprecated Features.

-Wno-deprecated-declarations
     Do not warn about uses of functions (see Function Attributes), variables (see Variable Attributes), and types (see Type Attributes) marked as deprecated by using the deprecated attribute.

-Wno-overflow
     Do not warn about compile-time overflow in constant expressions.

-Wno-odr
     Warn about One Definition Rule violations during link-time optimization. Requires -flto-odr-type-merging to be enabled.  Enabled by default.

-Wopenmp-simd
     Warn if the vectorizer cost model overrides the OpenMP or the Cilk Plus simd directive set by user.  The -fsimd-cost-model=unlimited option can be used to relax the cost model.

-Woverride-init (C and Objective-C only)
     Warn if an initialized field without side effects is overridden when using designated initializers (see Designated Initializers).

     This warning is included in -Wextra.  To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.

-Woverride-init-side-effects (C and Objective-C only)
     Warn if an initialized field with side effects is overridden when using designated initializers (see Designated Initializers).  This warning is enabled by default.

-Wpacked
     Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit.  For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

          struct foo {
            int x;
            char a, b, c, d;
          } __attribute__((packed));
          struct bar {
            char z;
            struct foo f;
          };

-Wpacked-bitfield-compat
     The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char.  This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout.  GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure:

          struct foo
          {
            char a:4;
            char b:8;
          } __attribute__ ((packed));

     This warning is enabled by default.  Use -Wno-packed-bitfield-compat to disable this warning.

-Wpadded
     Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure.  Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.

-Wredundant-decls
     Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.

-Wnested-externs (C and Objective-C only)
     Warn if an extern declaration is encountered within a function.

-Wno-inherited-variadic-ctor
     Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.

-Winline
     Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

     The compiler uses a variety of heuristics to determine whether or not to inline a function.  For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function.  Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

-Wno-invalid-offsetof (C++ and Objective-C++ only)
     Suppress warnings from applying the offsetof macro to a non-POD type.  According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined.  In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

     The restrictions on offsetof may be relaxed in a future version of the C++ standard.

-Wno-int-to-pointer-cast
     Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.

-Wno-pointer-to-int-cast (C and Objective-C only)
     Suppress warnings from casts from a pointer to an integer type of a different size.

-Winvalid-pch
     Warn if a precompiled header (see Precompiled Headers) is found in the search path but can't be used.

-Wlong-long
     Warn if long long type is used.  This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes.  To inhibit the warning messages, use -Wno-long-long.

-Wvariadic-macros
     Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode.  This is enabled by either -Wpedantic or -Wtraditional.  To inhibit the warning messages, use -Wno-variadic-macros.

-Wvarargs
     Warn upon questionable usage of the macros used to handle variable arguments like va_start.  This is default.  To inhibit the warning messages, use -Wno-varargs.

-Wvector-operation-performance
     Warn if vector operation is not implemented via SIMD capabilities of the architecture.  Mainly useful for the performance tuning. Vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type.

-Wno-virtual-move-assign
     Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator.  This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state.  If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.

-Wvla
     Warn if variable length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable length array.

-Wvolatile-register-var
     Warn if a register variable is declared volatile.  The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables.  This warning is enabled by -Wall.

-Wdisabled-optimization
     Warn if a requested optimization pass is disabled.  This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively.  Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.

-Wpointer-sign (C and Objective-C only)
     Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C.  It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.

-Wstack-protector
     This option is only active when -fstack-protector is active.  It warns about functions that are not protected against stack smashing.

-Woverlength-strings
     Warn about string constants that are longer than the ``minimum maximum'' length specified in the C standard.  Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.

     The limit applies after string constant concatenation, and does not count the trailing NUL.  In C90, the limit was 509 characters; in C99, it was raised to 4095.  C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

     This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

-Wunsuffixed-float-constants (C and Objective-C only)

     Issue a warning for any floating constant that does not have a suffix.  When used together with -Wsystem-headers it warns about such constants in system header files.  This can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.

-Wno-designated-init (C and Objective-C only)
     Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute.

Options for Debugging Your Program or GCC
=========================================

GCC has various special options that are used for debugging either your program or GCC:

-g
     Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2).  GDB can work with this debugging information.

     On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program.  If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

     GCC allows you to use -g with -O.  The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops.

     Nevertheless it proves possible to debug optimized output.  This makes it reasonable to use the optimizer for programs that might have bugs.

     The following options are useful when GCC is generated with the capability for more than one debugging format.

-gsplit-dwarf
     Separate as much dwarf debugging information as possible into a separate output file with the extension .dwo.  This option allows the build system to avoid linking files with debug information.  To be useful, this option requires a debugger capable of reading .dwo files.

-ggdb
     Produce debugging information for use by GDB.  This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-gpubnames
     Generate dwarf .debug_pubnames and .debug_pubtypes sections.

-ggnu-pubnames
     Generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index.  This option is only useful with a linker that can produce GDB index version 7.

-gstabs
     Produce debugging information in stabs format (if that is supported), without GDB extensions.  This is the format used by DBX on most BSD systems.  On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-feliminate-unused-debug-symbols
     Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.

-femit-class-debug-always
     Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class.  This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.

-fdebug-types-section
     When using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section.  It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support .debug_types sections yet and on some objects .debug_types produces larger instead of smaller debugging information.

-gstabs+
     Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB).  The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff
     Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff
     Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+
     Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB).  The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf-VERSION
     Produce debugging information in DWARF format (if that is supported). The value of VERSION may be either 2, 3, 4 or 5; the default version for most targets is 4.  DWARF Version 5 is only experimental.

     Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

     Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.

-grecord-gcc-switches
     This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information.  The options are concatenated with spaces separating them from each other and from the compiler version.  See also -frecord-gcc-switches for another way of storing compiler options into the object file.  This is the default.

-gno-record-gcc-switches
     Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.

-gstrict-dwarf
     Disallow using extensions of later DWARF standard version than selected with -gdwarf-VERSION.  On most targets using non-conflicting DWARF extensions from later standard versions is allowed.

-gno-strict-dwarf
     Allow using extensions of later DWARF standard version than selected with -gdwarf-VERSION.

-gz[=TYPE]
     Produce compressed debug sections in DWARF format, if that is supported. If TYPE is not given, the default type depends on the capabilities of the assembler and linker used.  TYPE may be one of none (don't compress debug sections), zlib (use zlib compression in ELF gABI format), or zlib-gnu (use zlib compression in traditional GNU format).  If the linker doesn't support writing compressed debug sections, the option is rejected.  Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files.

-gvms
     Produce debugging information in Alpha/VMS debug format (if that is supported).  This is the format used by DEBUG on Alpha/VMS systems.

-gLEVEL
-ggdbLEVEL
-gstabsLEVEL
-gcoffLEVEL
-gxcoffLEVEL
-gvmsLEVEL
     Request debugging information and also use LEVEL to specify how much information.  The default level is 2.

     Level 0 produces no debug information at all.  Thus, -g0 negates -g.

     Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug.  This includes descriptions of functions and external variables, and line number tables, but no information about local variables.

     Level 3 includes extra information, such as all the macro definitions present in the program.  Some debuggers support macro expansion when you use -g3.

     -gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing.  That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -gLEVEL option to change the debug level for DWARF.

-gtoggle
     Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise.  The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given.  This is mainly intended to be used with -fcompare-debug.

-fsanitize=address
     Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. See http://code.google.com/p/address-sanitizer/ for more details.  The run-time behavior can be influenced using the ASAN_OPTIONS environment variable; see https://code.google.com/p/address-sanitizer/wiki/Flags#Run-time_flags for a list of supported options.

-fsanitize=kernel-address
     Enable AddressSanitizer for Linux kernel. See http://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel for more details.

-fsanitize=thread
     Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to detect data race bugs.  See http://code.google.com/p/thread-sanitizer/ for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://code.google.com/p/thread-sanitizer/wiki/Flags for a list of supported options.

-fsanitize=leak
     Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and if neither -fsanitize=address nor -fsanitize=thread is used.  In that case the executable is linked against a library that overrides malloc and other allocator functions.  See https://code.google.com/p/address-sanitizer/wiki/LeakSanitizer for more details.  The run-time behavior can be influenced using the LSAN_OPTIONS environment variable.

-fsanitize=undefined
     Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime.  Current suboptions are:

     -fsanitize=shift
          This option enables checking that the result of a shift operation is not undefined.  Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.

     -fsanitize=integer-divide-by-zero
          Detect integer division by zero as well as INT_MIN / -1 division.

     -fsanitize=unreachable
          With this option, the compiler turns the __builtin_unreachable call into a diagnostics message call instead.  When reaching the __builtin_unreachable call, the behavior is undefined.

     -fsanitize=vla-bound
          This option instructs the compiler to check that the size of a variable length array is positive.

     -fsanitize=null
          This option enables pointer checking.  Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.

     -fsanitize=return
          This option enables return statement checking.  Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value.  This option works in C++ only.

     -fsanitize=signed-integer-overflow
          This option enables signed integer overflow checking.  We check that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics.  Note, integer promotion rules must be taken into account.  That is, the following is not an overflow:

               signed char a = SCHAR_MAX;
               a++;

     -fsanitize=bounds
          This option enables instrumentation of array bounds.  Various out of bounds accesses are detected.  Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.

     -fsanitize=bounds-strict
          This option enables strict instrumentation of array bounds.  Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays.  Initializers of variables with static storage are not instrumented.

     -fsanitize=alignment

          This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.

     -fsanitize=object-size
          This option enables instrumentation of memory references using the __builtin_object_size function.  Various out of bounds pointer accesses are detected.

     -fsanitize=float-divide-by-zero
          Detect floating-point division by zero.  Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.

     -fsanitize=float-cast-overflow
          This option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work well with FE_INVALID exceptions enabled.

     -fsanitize=nonnull-attribute

          This option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute.

     -fsanitize=returns-nonnull-attribute

          This option enables instrumentation of return statements in functions marked with returns_nonnull function attribute, to detect returning of null values from such functions.

     -fsanitize=bool

          This option enables instrumentation of loads from bool.  If a value other than 0/1 is loaded, a run-time error is issued.

     -fsanitize=enum

          This option enables instrumentation of loads from an enum type.  If a value outside the range of values for the enum type is loaded, a run-time error is issued.

     -fsanitize=vptr

          This option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.

     While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.

-fno-sanitize=all

     This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some sanitizers cannot be used together.

-fasan-shadow-offset=NUMBER
     This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.

-fsanitize-sections=S1,S2,...
     Sanitize global variables in selected user-defined sections.  SI may contain wildcards.

-fsanitize-recover[=OPTS]
     -fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-separated list of OPTS.  Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened.  This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported.  The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.

     Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=kernel-address.  For these sanitizers error recovery is turned on by default. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

     Syntax without explicit OPTS parameter is deprecated.  It is equivalent to

          -fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero

     Similarly -fno-sanitize-recover is equivalent to

          -fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero

-fsanitize-undefined-trap-on-error
     The -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine.  The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments.

-fcheck-pointer-bounds
     Enable Pointer Bounds Checker instrumentation.  Each memory reference is instrumented with checks of the pointer used for memory access against bounds associated with that pointer.

     Currently there is only an implementation for Intel MPX available, thus x86 target and -mmpx are required to enable this feature. MPX-based instrumentation requires a runtime library to enable MPX in hardware and handle bounds violation signals.  By default when -fcheck-pointer-bounds and -mmpx options are used to link a program, the GCC driver links against the libmpx runtime library and libmpxwrappers library.  It also passes '-z bndplt' to a linker in case it supports this option (which is checked on libmpx configuration).  Note that old versions of linker may ignore option.  Gold linker doesn't support '-z bndplt' option.  With no '-z bndplt' support in linker all calls to dynamic libraries lose passed bounds reducing overall protection level.  It's highly recommended to use linker with '-z bndplt' support.  In case such linker is not available it is adviced to always use -static-libmpxwrappers for better protection level or use -static to completely avoid external calls to dynamic libraries.  MPX-based instrumentation may be used for debugging and also may be included in production code to increase program security.  Depending on usage, you may have different requirements for the runtime library.  The current version of the MPX runtime library is more oriented for use as a debugging tool.  MPX runtime library usage implies -lpthread.  See also -static-libmpx.  The runtime library  behavior can be influenced using various CHKP_RT_* environment variables.  See https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler for more details.

     Generated instrumentation may be controlled by various -fchkp-* options and by the bnd_variable_size structure field attribute (see Type Attributes) and bnd_legacy, and bnd_instrument function attributes (see Function Attributes).  GCC also provides a number of built-in functions for controlling the Pointer Bounds Checker.  See Pointer Bounds Checker builtins, for more information.

-fchkp-check-incomplete-type
     Generate pointer bounds checks for variables with incomplete type. Enabled by default.

-fchkp-narrow-bounds
     Controls bounds used by Pointer Bounds Checker for pointers to object fields.  If narrowing is enabled then field bounds are used.  Otherwise object bounds are used.  See also -fchkp-narrow-to-innermost-array and -fchkp-first-field-has-own-bounds.  Enabled by default.

-fchkp-first-field-has-own-bounds
     Forces Pointer Bounds Checker to use narrowed bounds for the address of the first field in the structure.  By default a pointer to the first field has the same bounds as a pointer to the whole structure.

-fchkp-narrow-to-innermost-array
     Forces Pointer Bounds Checker to use bounds of the innermost arrays in case of nested static array access.  By default this option is disabled and bounds of the outermost array are used.

-fchkp-optimize
     Enables Pointer Bounds Checker optimizations.  Enabled by default at optimization levels -O, -O2, -O3.

-fchkp-use-fast-string-functions
     Enables use of *_nobnd versions of string functions (not copying bounds) by Pointer Bounds Checker.  Disabled by default.

-fchkp-use-nochk-string-functions
     Enables use of *_nochk versions of string functions (not checking bounds) by Pointer Bounds Checker.  Disabled by default.

-fchkp-use-static-bounds
     Allow Pointer Bounds Checker to generate static bounds holding bounds of static variables.  Enabled by default.

-fchkp-use-static-const-bounds
     Use statically-initialized bounds for constant bounds instead of generating them each time they are required.  By default enabled when -fchkp-use-static-bounds is enabled.

-fchkp-treat-zero-dynamic-size-as-infinite
     With this option, objects with incomplete type whose dynamically-obtained size is zero are treated as having infinite size instead by Pointer Bounds Checker.  This option may be helpful if a program is linked with a library missing size information for some symbols.  Disabled by default.

-fchkp-check-read
     Instructs Pointer Bounds Checker to generate checks for all read accesses to memory.  Enabled by default.

-fchkp-check-write
     Instructs Pointer Bounds Checker to generate checks for all write accesses to memory.  Enabled by default.

-fchkp-store-bounds
     Instructs Pointer Bounds Checker to generate bounds stores for pointer writes.  Enabled by default.

-fchkp-instrument-calls
     Instructs Pointer Bounds Checker to pass pointer bounds to calls. Enabled by default.

-fchkp-instrument-marked-only
     Instructs Pointer Bounds Checker to instrument only functions marked with the bnd_instrument attribute (see Function Attributes).  Disabled by default.

-fchkp-use-wrappers
     Allows Pointer Bounds Checker to replace calls to built-in functions with calls to wrapper functions.  When -fchkp-use-wrappers is used to link a program, the GCC driver automatically links against libmpxwrappers.  See also -static-libmpxwrappers. Enabled by default.

-fdump-final-insns[=FILE]
     Dump the final internal representation (RTL) to FILE.  If the optional argument is omitted (or if FILE is .), the name of the dump file is determined by appending .gkd to the compilation output file name.

-fcompare-debug[=OPTS]
     If no error occurs during compilation, run the compiler a second time, adding OPTS and -fcompare-debug-second to the arguments passed to the second compilation.  Dump the final internal representation in both compilations, and print an error if they differ.

     If the equal sign is omitted, the default -gtoggle is used.

     The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug.  If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for OPTS, otherwise the default -gtoggle is used.

     -fcompare-debug=, with the equal sign but without OPTS, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

     To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking).  To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.

-fcompare-debug-second
     This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output.  Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first.

     When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

-feliminate-dwarf2-dups
     Compress DWARF 2 debugging information by eliminating duplicated information about each symbol.  This option only makes sense when generating DWARF 2 debugging information with -gdwarf-2.

-femit-struct-debug-baseonly
     Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.

     This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

     This option works only with DWARF 2.

-femit-struct-debug-reduced
     Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.

     This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

     This option works only with DWARF 2.

-femit-struct-debug-detailed[=SPEC-LIST]
     Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

     This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

     A specification has the syntax

     [dir:|ind:][ord:|gen:](any|sys|base|none)

     The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

     The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

     The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

     You may need to experiment to determine the best settings for your application.

     The default is -femit-struct-debug-detailed=all.

     This option works only with DWARF 2.

-fno-merge-debug-strings
     Direct the linker to not merge together strings in the debugging information that are identical in different object files.  Merging is not supported by all assemblers or linkers.  Merging decreases the size of the debug information in the output file at the cost of increasing link processing time.  Merging is enabled by default.

-fdebug-prefix-map=OLD=NEW
     When compiling files in directory OLD, record debugging information describing them as in NEW instead.

-fno-dwarf2-cfi-asm
     Emit DWARF 2 unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.

-p
     Generate extra code to write profile information suitable for the analysis program prof.  You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg
     Generate extra code to write profile information suitable for the analysis program gprof.  You must use this option when compiling the source files you want data about, and you must also use it when linking.

-Q
     Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report
     Makes the compiler print some statistics about the time consumed by each pass when it finishes.

-fmem-report
     Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fmem-report-wpa
     Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

-fpre-ipa-mem-report

-fpost-ipa-mem-report
     Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

-fprofile-report
     Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.

-fstack-usage
     Makes the compiler output stack usage information for the program, on a per-function basis.  The filename for the dump is made by appending .su to the AUXNAME.  AUXNAME is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file.  An entry is made up of three fields:

       * The name of the function.

       * A number of bytes.

       * One or more qualifiers: static, dynamic, bounded.

     The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function.  The second field is this fixed number of bytes.

     The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls.  If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function.  If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

-fprofile-arcs
     Add code so that program flow arcs are instrumented.  During execution the program records how many times each branch and call is executed and how many times it is taken or returns.  When the compiled program exits it saves this data to a file called AUXNAME.gcda for each source file.  The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage).  Each object file's AUXNAME is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file.  In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). See Cross-profiling.

--coverage

     This option is used to compile and link code instrumented for coverage analysis.  The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking).  See the documentation for those options for more details.

       * Compile the source files with -fprofile-arcs plus optimization and code generation options.  For test coverage analysis, use the additional -ftest-coverage option.  You do not need to profile every source file in a program.

       * Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).

       * Run the program on a representative workload to generate the arc profile information.  This may be repeated any number of times.  You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated.  Also fork calls are detected and correctly handled (double counting will not happen).

       * For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization).

       * For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files.  Refer to the gcov documentation for further information.

     With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed.  When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftest-coverage
     Produce a notes file that the gcov code-coverage utility (see gcov---a Test Coverage Program) can use to show program coverage.  Each source file's note file is called AUXNAME.gcno.  Refer to the -fprofile-arcs option above for a description of AUXNAME and instructions on how to generate test coverage data.  Coverage data matches the source files more closely if you do not optimize.

-fdbg-cnt-list
     Print the name and the counter upper bound for all debug counters.

-fdbg-cnt=COUNTER-VALUE-LIST
     Set the internal debug counter upper bound.  COUNTER-VALUE-LIST is a comma-separated list of NAME:VALUE pairs which sets the upper bound of each debug counter NAME to VALUE. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0, dbg_cnt(dce) returns true only for first 10 invocations.

-fenable-KIND-PASS
-fdisable-KIND-PASS=RANGE-LIST

     This is a set of options that are used to explicitly disable/enable optimization passes.  These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.

     -fdisable-ipa-PASS
          Disable IPA pass PASS. PASS is the pass name.  If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

     -fdisable-rtl-PASS
     -fdisable-rtl-PASS=RANGE-LIST
          Disable RTL pass PASS.  PASS is the pass name.  If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.  RANGE-LIST is a comma-separated list of function ranges or assembler names.  Each range is a number pair separated by a colon.  The range is inclusive in both ends.  If the range is trivial, the number pair can be simplified as a single number.  If the function's call graph node's UID falls within one of the specified ranges, the PASS is disabled for that function.  The UID is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.

     -fdisable-tree-PASS
     -fdisable-tree-PASS=RANGE-LIST
          Disable tree pass PASS.  See -fdisable-rtl for the description of option arguments.

     -fenable-ipa-PASS
          Enable IPA pass PASS.  PASS is the pass name.  If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

     -fenable-rtl-PASS
     -fenable-rtl-PASS=RANGE-LIST
          Enable RTL pass PASS.  See -fdisable-rtl for option argument description and examples.

     -fenable-tree-PASS
     -fenable-tree-PASS=RANGE-LIST
          Enable tree pass PASS.  See -fdisable-rtl for the description of option arguments.

     Here are some examples showing uses of these options.

          # disable ccp1 for all functions
             -fdisable-tree-ccp1
          # disable complete unroll for function whose cgraph node uid is 1
             -fenable-tree-cunroll=1
          # disable gcse2 for functions at the following ranges [1,1],
          # [300,400], and [400,1000]
          # disable gcse2 for functions foo and foo2
             -fdisable-rtl-gcse2=foo,foo2
          # disable early inlining
             -fdisable-tree-einline
          # disable ipa inlining
             -fdisable-ipa-inline
          # enable tree full unroll
             -fenable-tree-unroll

-dLETTERS
-fdump-rtl-PASS
-fdump-rtl-PASS=FILENAME
     Says to make debugging dumps during compilation at times specified by LETTERS.  This is used for debugging the RTL-based passes of the compiler.  The file names for most of the dumps are made by appending a pass number and a word to the DUMPNAME, and the files are created in the directory of the output file. In case of =FILENAME option, the dump is output on the given file instead of the pass numbered dump files. Note that the pass number is computed statically as passes get registered into the pass manager. Thus the numbering is not related to the dynamic order of execution of passes.  In particular, a pass installed by a plugin could have a number over 200 even if it executed quite early.  DUMPNAME is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. These switches may have different effects when -E is used for preprocessing.

     Debug dumps can be enabled with a -fdump-rtl switch or some -d option LETTERS.  Here are the possible letters for use in PASS and LETTERS, and their meanings:

     -fdump-rtl-alignments
          Dump after branch alignments have been computed.

     -fdump-rtl-asmcons
          Dump after fixing rtl statements that have unsatisfied in/out constraints.

     -fdump-rtl-auto_inc_dec
          Dump after auto-inc-dec discovery.  This pass is only run on architectures that have auto inc or auto dec instructions.

     -fdump-rtl-barriers
          Dump after cleaning up the barrier instructions.

     -fdump-rtl-bbpart
          Dump after partitioning hot and cold basic blocks.

     -fdump-rtl-bbro
          Dump after block reordering.

     -fdump-rtl-btl1
     -fdump-rtl-btl2
          -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.

     -fdump-rtl-bypass
          Dump after jump bypassing and control flow optimizations.

     -fdump-rtl-combine
          Dump after the RTL instruction combination pass.

     -fdump-rtl-compgotos
          Dump after duplicating the computed gotos.

     -fdump-rtl-ce1
     -fdump-rtl-ce2
     -fdump-rtl-ce3
          -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.

     -fdump-rtl-cprop_hardreg
          Dump after hard register copy propagation.

     -fdump-rtl-csa
          Dump after combining stack adjustments.

     -fdump-rtl-cse1
     -fdump-rtl-cse2
          -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.

     -fdump-rtl-dce
          Dump after the standalone dead code elimination passes.

     -fdump-rtl-dbr
          Dump after delayed branch scheduling.

     -fdump-rtl-dce1
     -fdump-rtl-dce2
          -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.

     -fdump-rtl-eh
          Dump after finalization of EH handling code.

     -fdump-rtl-eh_ranges
          Dump after conversion of EH handling range regions.

     -fdump-rtl-expand
          Dump after RTL generation.

     -fdump-rtl-fwprop1
     -fdump-rtl-fwprop2
          -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.

     -fdump-rtl-gcse1
     -fdump-rtl-gcse2
          -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.

     -fdump-rtl-init-regs
          Dump after the initialization of the registers.

     -fdump-rtl-initvals
          Dump after the computation of the initial value sets.

     -fdump-rtl-into_cfglayout
          Dump after converting to cfglayout mode.

     -fdump-rtl-ira
          Dump after iterated register allocation.

     -fdump-rtl-jump
          Dump after the second jump optimization.

     -fdump-rtl-loop2
          -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.

     -fdump-rtl-mach
          Dump after performing the machine dependent reorganization pass, if that pass exists.

     -fdump-rtl-mode_sw
          Dump after removing redundant mode switches.

     -fdump-rtl-rnreg
          Dump after register renumbering.

     -fdump-rtl-outof_cfglayout
          Dump after converting from cfglayout mode.

     -fdump-rtl-peephole2
          Dump after the peephole pass.

     -fdump-rtl-postreload
          Dump after post-reload optimizations.

     -fdump-rtl-pro_and_epilogue
          Dump after generating the function prologues and epilogues.

     -fdump-rtl-sched1
     -fdump-rtl-sched2
          -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.

     -fdump-rtl-ree
          Dump after sign/zero extension elimination.

     -fdump-rtl-seqabstr
          Dump after common sequence discovery.

     -fdump-rtl-shorten
          Dump after shortening branches.

     -fdump-rtl-sibling
          Dump after sibling call optimizations.

     -fdump-rtl-split1
     -fdump-rtl-split2
     -fdump-rtl-split3
     -fdump-rtl-split4
     -fdump-rtl-split5
          These options enable dumping after five rounds of instruction splitting.

     -fdump-rtl-sms
          Dump after modulo scheduling.  This pass is only run on some architectures.

     -fdump-rtl-stack
          Dump after conversion from GCC's ``flat register file'' registers to the x87's stack-like registers.  This pass is only run on x86 variants.

     -fdump-rtl-subreg1
     -fdump-rtl-subreg2
          -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.

     -fdump-rtl-unshare
          Dump after all rtl has been unshared.

     -fdump-rtl-vartrack
          Dump after variable tracking.

     -fdump-rtl-vregs
          Dump after converting virtual registers to hard registers.

     -fdump-rtl-web
          Dump after live range splitting.

     -fdump-rtl-regclass
     -fdump-rtl-subregs_of_mode_init
     -fdump-rtl-subregs_of_mode_finish
     -fdump-rtl-dfinit
     -fdump-rtl-dfinish
          These dumps are defined but always produce empty files.

     -da
     -fdump-rtl-all
          Produce all the dumps listed above.

     -dA
          Annotate the assembler output with miscellaneous debugging information.

     -dD
          Dump all macro definitions, at the end of preprocessing, in addition to normal output.

     -dH
          Produce a core dump whenever an error occurs.

     -dp
          Annotate the assembler output with a comment indicating which pattern and alternative is used.  The length of each instruction is also printed.

     -dP
          Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.

     -dx
          Just generate RTL for a function instead of compiling it.  Usually used with -fdump-rtl-expand.

-fdump-noaddr
     When doing debugging dumps, suppress address output.  This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.

-freport-bug
     Collect and dump debug information into temporary file if ICE in C/C++ compiler occured.

-fdump-unnumbered
     When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.

-fdump-unnumbered-links
     When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

-fdump-translation-unit (C++ only)
-fdump-translation-unit-OPTIONS (C++ only)
     Dump a representation of the tree structure for the entire translation unit to a file.  The file name is made by appending .tu to the source file name, and the file is created in the same directory as the output file.  If the -OPTIONS form is used, OPTIONS controls the details of the dump as described for the -fdump-tree options.

-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-OPTIONS (C++ only)
     Dump a representation of each class's hierarchy and virtual function table layout to a file.  The file name is made by appending .class to the source file name, and the file is created in the same directory as the output file.  If the -OPTIONS form is used, OPTIONS controls the details of the dump as described for the -fdump-tree options.

-fdump-ipa-SWITCH
     Control the dumping at various stages of inter-procedural analysis language tree to a file.  The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file.  The following dumps are possible:

     all
          Enables all inter-procedural analysis dumps.

     cgraph
          Dumps information about call-graph optimization, unused function removal, and inlining decisions.

     inline
          Dump after function inlining.

-fdump-passes
     Dump the list of optimization passes that are turned on and off by the current command-line options.

-fdump-statistics-OPTION
     Enable and control dumping of pass statistics in a separate file.  The file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file.  If the -OPTION form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them.  The default with no option is to sum counters for each function compiled.

-fdump-tree-SWITCH
-fdump-tree-SWITCH-OPTIONS
-fdump-tree-SWITCH-OPTIONS=FILENAME
     Control the dumping at various stages of processing the intermediate language tree to a file.  The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. In case of =FILENAME option, the dump is output on the given file instead of the auto named dump files.  If the -OPTIONS form is used, OPTIONS is a list of - separated options which control the details of the dump.  Not all options are applicable to all dumps; those that are not meaningful are ignored.  The following options are available

     address
          Print the address of each node.  Usually this is not meaningful as it changes according to the environment and source file.  Its primary use is for tying up a dump file with a debug environment.

     asmname
          If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME.  Its primary use is ease of use working backward from mangled names in the assembly file.

     slim
          When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached.  Only dump such items when they are directly reachable by some other path.

          When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

          When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

     raw
          Print a raw representation of the tree.  By default, trees are pretty-printed into a C-like representation.

     details
          Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.

     stats
          Enable dumping various statistics about the pass (not honored by every dump option).

     blocks
          Enable showing basic block boundaries (disabled in raw dumps).

     graph
          For each of the other indicated dump files (-fdump-rtl-PASS), dump a representation of the control flow graph suitable for viewing with GraphViz to FILE.PASSID.PASS.dot.  Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

          This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

     vops
          Enable showing virtual operands for every statement.

     lineno
          Enable showing line numbers for statements.

     uid
          Enable showing the unique ID (DECL_UID) for each variable.

     verbose
          Enable showing the tree dump for each statement.

     eh
          Enable showing the EH region number holding each statement.

     scev
          Enable showing scalar evolution analysis details.

     optimized
          Enable showing optimization information (only available in certain passes).

     missed
          Enable showing missed optimization information (only available in certain passes).

     note
          Enable other detailed optimization information (only available in certain passes).

     =FILENAME
          Instead of an auto named dump file, output into the given file name. The file names stdout and stderr are treated specially and are considered already open standard streams. For example,

               gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
                    -fdump-tree-pre=stderr file.c

          outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr. If two conflicting dump filenames are given for the same pass, then the latter option overrides the earlier one.

     all
          Turn on all options, except raw, slim, verbose and lineno.

     optall
          Turn on all optimization options, i.e., optimized, missed, and note.

     The following tree dumps are possible:

     original
          Dump before any tree based optimization, to FILE.original.

     optimized
          Dump after all tree based optimization, to FILE.optimized.

     gimple
          Dump each function before and after the gimplification pass to a file.  The file name is made by appending .gimple to the source file name.

     cfg
          Dump the control flow graph of each function to a file.  The file name is made by appending .cfg to the source file name.

     ch
          Dump each function after copying loop headers.  The file name is made by appending .ch to the source file name.

     ssa
          Dump SSA related information to a file.  The file name is made by appending .ssa to the source file name.

     alias
          Dump aliasing information for each function.  The file name is made by appending .alias to the source file name.

     ccp
          Dump each function after CCP.  The file name is made by appending .ccp to the source file name.

     storeccp
          Dump each function after STORE-CCP.  The file name is made by appending .storeccp to the source file name.

     pre
          Dump trees after partial redundancy elimination.  The file name is made by appending .pre to the source file name.

     fre
          Dump trees after full redundancy elimination.  The file name is made by appending .fre to the source file name.

     copyprop
          Dump trees after copy propagation.  The file name is made by appending .copyprop to the source file name.

     store_copyprop
          Dump trees after store copy-propagation.  The file name is made by appending .store_copyprop to the source file name.

     dce
          Dump each function after dead code elimination.  The file name is made by appending .dce to the source file name.

     sra
          Dump each function after performing scalar replacement of aggregates.  The file name is made by appending .sra to the source file name.

     sink
          Dump each function after performing code sinking.  The file name is made by appending .sink to the source file name.

     dom
          Dump each function after applying dominator tree optimizations.  The file name is made by appending .dom to the source file name.

     dse
          Dump each function after applying dead store elimination.  The file name is made by appending .dse to the source file name.

     phiopt
          Dump each function after optimizing PHI nodes into straightline code.  The file name is made by appending .phiopt to the source file name.

     forwprop
          Dump each function after forward propagating single use variables.  The file name is made by appending .forwprop to the source file name.

     copyrename
          Dump each function after applying the copy rename optimization.  The file name is made by appending .copyrename to the source file name.

     nrv
          Dump each function after applying the named return value optimization on generic trees.  The file name is made by appending .nrv to the source file name.

     vect
          Dump each function after applying vectorization of loops.  The file name is made by appending .vect to the source file name.

     slp
          Dump each function after applying vectorization of basic blocks.  The file name is made by appending .slp to the source file name.

     vrp
          Dump each function after Value Range Propagation (VRP).  The file name is made by appending .vrp to the source file name.

     all
          Enable all the available tree dumps with the flags provided in this option.

-fopt-info
-fopt-info-OPTIONS
-fopt-info-OPTIONS=FILENAME
     Controls optimization dumps from various optimization passes. If the -OPTIONS form is used, OPTIONS is a list of - separated option keywords to select the dump details and optimizations.

     The OPTIONS can be divided into two groups: options describing the verbosity of the dump, and options describing which optimizations should be included. The options from both the groups can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.

     The following options control the dump verbosity:

     optimized
          Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.

     missed
          Print information about missed optimizations. Individual passes control which information to include in the output.

     note
          Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.

     all
          Print detailed optimization information. This includes optimized, missed, and note.

     One or more of the following option keywords can be used to describe a group of optimizations:

     ipa
          Enable dumps from all interprocedural optimizations.

     loop
          Enable dumps from all loop optimizations.

     inline
          Enable dumps from all inlining optimizations.

     vec
          Enable dumps from all vectorization optimizations.

     optall
          Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

     If OPTIONS is omitted, it defaults to optimized-optall, which means to dump all info about successful optimizations from all the passes.

     If the FILENAME is provided, then the dumps from all the applicable optimizations are concatenated into the FILENAME. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a FILENAME. If other filenames are provided then all but the first such option are ignored.

     Note that the output FILENAME is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.

     In the following example, the optimization info is output to stderr:

          gcc -O3 -fopt-info

     This example:

          gcc -O3 -fopt-info-missed=missed.all

     outputs missed optimization report from all the passes into missed.all, and this one:

          gcc -O2 -ftree-vectorize -fopt-info-vec-missed

     prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec.

     As another example,

          gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

     outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

     Finally, consider:

          gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

     Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.

-frandom-seed=NUMBER
     This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file.  It is also used to place unique stamps in coverage data files and the object files that produce them.  You can use the -frandom-seed option to produce reproducibly identical object files.

     The NUMBER should be different for every file you compile.

-fsched-verbose=N
     On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints.  This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched1 or .sched2 respectively.  However for N greater than nine, the output is always printed to standard error.

     For N greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For N greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info.  For N greater than two, it includes RTL at abort point, control-flow and regions info. And for N over four, -fsched-verbose also includes dependence info.

-save-temps
-save-temps=cwd
     Store the usual ``temporary'' intermediate files permanently; place them in the current directory and name them based on the source file.  Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o.  This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.

     When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.

     If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files.  For instance:

          gcc -save-temps -o outdir1/foo.o indir1/foo.c&
          gcc -save-temps -o outdir2/foo.o indir2/foo.c&

     may result in foo.i and foo.o being written to simultaneously by both compilers.

-save-temps=obj
     Store the usual ``temporary'' intermediate files permanently.  If the -o option is used, the temporary files are based on the object file.  If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.

     For example:

          gcc -save-temps=obj -c foo.c
          gcc -save-temps=obj -c bar.c -o dir/xbar.o
          gcc -save-temps=obj foobar.c -o dir2/yfoobar

     creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.

-time[=FILE]
     Report the CPU time taken by each subprocess in the compilation sequence.  For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

     Without the specification of an output file, the output looks like this:

          # cc1 0.12 0.01
          # as 0.00 0.01

     The first number on each line is the ``user time'', that is time spent executing the program itself.  The second number is ``system time'', time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

     With the specification of an output file, the output is appended to the named file, and it looks like this:

          0.12 0.01 cc1 OPTIONS
          0.00 0.01 as OPTIONS

     The ``user time'' and the ``system time'' are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.

-fvar-tracking
     Run variable tracking pass.  It computes where variables are stored at each position in code.  Better debugging information is then generated (if the debugging information format supports this information).

     It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.

-fvar-tracking-assignments
     Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing.  Use of -gdwarf-4 is recommended along with it.

     It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

-fvar-tracking-assignments-toggle
     Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.

-print-file-name=LIBRARY
     Print the full absolute name of the library file LIBRARY that would be used when linking---and don't do anything else.  With this option, GCC does not compile or link anything; it just prints the file name.

-print-multi-directory
     Print the directory name corresponding to the multilib selected by any other switches present in the command line.  This directory is supposed to exist in GCC_EXEC_PREFIX.

-print-multi-lib
     Print the mapping from multilib directory names to compiler switches that enable them.  The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches.  This is supposed to ease shell processing.

-print-multi-os-directory
     Print the path to OS libraries for the selected multilib, relative to some lib subdirectory.  If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libSUFFIX sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/SUBDIR subdirectories it prints e.g. amd64, sparcv9 or ev6.

-print-multiarch
     Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.

-print-prog-name=PROGRAM
     Like -print-file-name, but searches for a program such as cpp.

-print-libgcc-file-name
     Same as -print-file-name=libgcc.a.

     This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a.  You can do:

          gcc -nostdlib FILES... `gcc -print-libgcc-file-name`

-print-search-dirs
     Print the name of the configured installation directory and a list of program and library directories gcc searches---and don't do anything else.

     This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. See Environment Variables.

-print-sysroot
     Print the target sysroot directory that is used during compilation.  This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options.  If no target sysroot is specified, the option prints nothing.

-print-sysroot-headers-suffix
     Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix---and don't do anything else.

-dumpmachine
     Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.

-dumpversion
     Print the compiler version (for example, 3.0)---and don't do anything else.

-dumpspecs
     Print the compiler's built-in specs---and don't do anything else.  (This is used when GCC itself is being built.)  See Spec Files.

-fno-eliminate-unused-debug-types
     Normally, when producing DWARF 2 output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared).  More often, however, this results in a significant amount of wasted space.

Options That Control Optimization
=================================

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results.  Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program.  Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag.  Only optimizations that have a flag are listed in this section.

Most optimizations are only enabled if an -O level is set on the command line.  Otherwise they are disabled, even if individual optimization flags are specified.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here.  You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. See Overall Options, for examples.

-O
-O1
     Optimize.  Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

     With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

     -O turns on the following optimization flags:

     -fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion2 -fif-conversion -finline-functions-called-once -fipa-pure-const -fipa-profile -fipa-reference -fmerge-constants -fmove-loop-invariants -fshrink-wrap -fsplit-wide-types -ftree-bit-ccp -ftree-ccp -fssa-phiopt -ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-sink -ftree-slsr -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time

     -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-O2
     Optimize even more.  GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code.

     -O2 turns on all optimization flags specified by -O.  It also turns on the following optimization flags: -fthread-jumps -falign-functions  -falign-jumps -falign-loops  -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps  -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fexpensive-optimizations -fgcse  -fgcse-lm -fhoist-adjacent-loads -finline-small-functions -findirect-inlining -fipa-cp -fipa-cp-alignment -fipa-sra -fipa-icf -fisolate-erroneous-paths-dereference -flra-remat -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fsched-interblock  -fsched-spec -fschedule-insns  -fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-builtin-call-dce -ftree-switch-conversion -ftree-tail-merge -ftree-pre -ftree-vrp -fipa-ra

     Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.

-O3
     Optimize yet more.  -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-loop-vectorize, -ftree-loop-distribute-patterns, -ftree-slp-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone options.

-O0
     Reduce compilation time and make debugging produce the expected results.  This is the default.

-Os
     Optimize for size.  -Os enables all -O2 optimizations that do not typically increase code size.  It also performs further optimizations designed to reduce code size.

     -Os disables the following optimization flags: -falign-functions  -falign-jumps  -falign-loops -falign-labels  -freorder-blocks  -freorder-blocks-and-partition -fprefetch-loop-arrays

-Ofast
     Disregard strict standards compliance.  -Ofast enables all -O3 optimizations.  It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.

-Og
     Optimize debugging experience.  -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

     If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Options of the form -fFLAG specify machine-independent flags.  Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo.  In the table below, only one of the forms is listed---the one you typically use.  You can figure out the other form by either removing no- or adding it.

The following options control specific optimizations.  They are either activated by -O options or are related to ones that are.  You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.

-fno-defer-pop
     Always pop the arguments to each function call as soon as that function returns.  For machines that must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

     Disabled at levels -O, -O2, -O3, -Os.

-fforward-propagate
     Perform a forward propagation pass on RTL.  The pass tries to combine two instructions and checks if the result can be simplified.  If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

     This option is enabled by default at optimization levels -O, -O2, -O3, -Os.

-ffp-contract=STYLE
     -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard.  This is currently not implemented and treated equal to -ffp-contract=off.

     The default is -ffp-contract=fast.

-fomit-frame-pointer
     Don't keep the frame pointer in a register for functions that don't need one.  This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions.  It also makes debugging impossible on some machines.

     On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist.  The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag.  See Register Usage (GNU Compiler Collection (GCC) Internals).

     The default setting (when not optimizing for size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is -fomit-frame-pointer.  You can configure GCC with the --enable-frame-pointer configure option to change the default.

     Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls
     Optimize sibling and tail recursive calls.

     Enabled at levels -O2, -O3, -Os.

-foptimize-strlen
     Optimize various standard C string functions (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives.

     Enabled at levels -O2, -O3.

-fno-inline
     Do not expand any functions inline apart from those marked with the always_inline attribute.  This is the default when not optimizing.

     Single functions can be exempted from inlining by marking them with the noinline attribute.

-finline-small-functions
     Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller).  The compiler heuristically decides which functions are simple enough to be worth integrating in this way.  This inlining applies to all functions, even those not declared inline.

     Enabled at level -O2.

-findirect-inlining
     Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining.  This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

     Enabled at level -O2.

-finline-functions
     Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way.

     If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

     Enabled at level -O3.

-finline-functions-called-once
     Consider all static functions called once for inlining into their caller even if they are not marked inline.  If a call to a given function is integrated, then the function is not output as assembler code in its own right.

     Enabled at levels -O1, -O2, -O3 and -Os.

-fearly-inlining
     Inline functions marked by always_inline and functions whose body seems smaller than the function call overhead early before doing -fprofile-generate instrumentation and real inlining pass.  Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.

     Enabled by default.

-fipa-sra
     Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.

     Enabled at levels -O2, -O3 and -Os.

-finline-limit=N
     By default, GCC limits the size of functions that can be inlined.  This flag allows coarse control of this limit.  N is the size of functions that can be inlined in number of pseudo instructions.

     Inlining is actually controlled by a number of parameters, which may be specified individually by using --param NAME=VALUE. The -finline-limit=N option sets some of these parameters as follows:

     max-inline-insns-single
          is set to N/2.

     max-inline-insns-auto
          is set to N/2.

     See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.

     Note: there may be no value to -finline-limit that results in default behavior.

     Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size.  In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.

-fno-keep-inline-dllexport
     This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec (see Declaring Attributes of Functions.)

-fkeep-inline-functions
     In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers.  This switch does not affect functions using the extern inline extension in GNU C90.  In C++, emit any and all inline functions into the object file.

-fkeep-static-consts
     Emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

     GCC enables this option by default.  If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

-fmerge-constants
     Attempt to merge identical constants (string constants and floating-point constants) across compilation units.

     This option is the default for optimized compilation if the assembler and linker support it.  Use -fno-merge-constants to inhibit this behavior.

     Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants
     Attempt to merge identical constants and identical variables.

     This option implies -fmerge-constants.  In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types.  Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior.

-fmodulo-sched
     Perform swing modulo scheduling immediately before the first scheduling pass.  This pass looks at innermost loops and reorders their instructions by overlapping different iterations.

-fmodulo-sched-allow-regmoves
     Perform more aggressive SMS-based modulo scheduling with register moves allowed.  By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis.  This option is effective only with -fmodulo-sched enabled.

-fno-branch-count-reg
     Do not use ``decrement and branch'' instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

     Enabled by default at -O1 and higher.

     The default is -fbranch-count-reg.

-fno-function-cse
     Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

     This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

     The default is -ffunction-cse

-fno-zero-initialized-in-bss
     If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS.  This can save space in the resulting code.

     This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

     The default is -fzero-initialized-in-bss.

-fthread-jumps
     Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found.  If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

     Enabled at levels -O2, -O3, -Os.

-fsplit-wide-types
     When using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently.  This normally generates better code for those types, but may make debugging more difficult.

     Enabled at levels -O, -O2, -O3, -Os.

-fcse-follow-jumps
     In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path.  For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

     Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks
     This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks.  When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

     Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop
     Re-run common subexpression elimination after loop optimizations are performed.

     Enabled at levels -O2, -O3, -Os.

-fgcse
     Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

     Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

     Enabled at levels -O2, -O3, -Os.

-fgcse-lm
     When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves.  This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.

     Enabled by default when -fgcse is enabled.

-fgcse-sm
     When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination.  This pass attempts to move stores out of loops.  When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

     Not enabled at any optimization level.

-fgcse-las
     When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

     Not enabled at any optimization level.

-fgcse-after-reload
     When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload.  The purpose of this pass is to clean up redundant spilling.

-faggressive-loop-optimizations
     This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop.  This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses.  The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.

-funsafe-loop-optimizations
     This option tells the loop optimizer to assume that loop indices do not overflow, and that loops with nontrivial exit condition are not infinite.  This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. If you use -Wunsafe-loop-optimizations, the compiler warns you if it finds this kind of loop.

-fcrossjumping
     Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size.  The resulting code may or may not perform better than without cross-jumping.

     Enabled at levels -O2, -O3, -Os.

-fauto-inc-dec
     Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this.  Enabled by default at -O and higher on architectures that support this.

-fdce
     Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.

-fdse
     Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.

-fif-conversion
     Attempt to transform conditional jumps into branch-less equivalents.  This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics.  The use of conditional execution on chips where it is available is controlled by -fif-conversion2.

     Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion2
     Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

     Enabled at levels -O, -O2, -O3, -Os.

-fdeclone-ctor-dtor
     The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function.  With this option, the base and complete variants are changed to be thunks that call a common implementation.

     Enabled by -Os.

-fdelete-null-pointer-checks
     Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels.  In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.

     Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

     This option is enabled by default on most targets.  On Nios II ELF, it defaults to off.  On AVR and CR16, this option is completely disabled.

     Passes that use the dataflow information are enabled independently at different optimization levels.

-fdevirtualize
     Attempt to convert calls to virtual functions to direct calls.  This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.

-fdevirtualize-speculatively
     Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls.  The speculative calls enable more optimizations, such as inlining.  When they seem useless after further optimization, they are converted back into original form.

-fdevirtualize-at-ltrans
     Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.

-fexpensive-optimizations
     Perform a number of minor optimizations that are relatively expensive.

     Enabled at levels -O2, -O3, -Os.

-free
     Attempt to remove redundant extension instructions.  This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.

     Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.

-fno-lifetime-dse
     In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed.  Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization.

-flive-range-shrinkage
     Attempt to decrease register pressure through register live range shrinkage.  This is helpful for fast processors with small or moderate size register sets.

-fira-algorithm=ALGORITHM
     Use the specified coloring algorithm for the integrated register allocator.  The ALGORITHM argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring.  Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.

-fira-region=REGION
     Use specified regions for the integrated register allocator.  The REGION argument should be one of the following:

     all
          Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.

     mixed
          Use all loops except for loops with small register pressure as the regions.  This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).

     one
          Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.

-fira-hoist-pressure
     Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions.  This option usually results in smaller code, but it can slow the compiler down.

     This option is enabled at level -Os for all targets.

-fira-loop-pressure
     Use IRA to evaluate register pressure in loops for decisions to move loop invariants.  This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.

     This option is enabled at level -O3 for some targets.

-fno-ira-share-save-slots
     Disable sharing of stack slots used for saving call-used hard registers living through a call.  Each hard register gets a separate stack slot, and as a result function stack frames are larger.

-fno-ira-share-spill-slots
     Disable sharing of stack slots allocated for pseudo-registers.  Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger.

-fira-verbose=N
     Control the verbosity of the dump file for the integrated register allocator. The default value is 5.  If the value N is greater or equal to 10, the dump output is sent to stderr using the same format as N minus 10.

-flra-remat
     Enable CFG-sensitive rematerialization in LRA.  Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable.

     Enabled at levels -O2, -O3, -Os.

-fdelayed-branch
     If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

     Enabled at levels -O, -O2, -O3, -Os.

-fschedule-insns
     If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable.  This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required.

     Enabled at levels -O2, -O3.

-fschedule-insns2
     Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done.  This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

     Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock
     Don't schedule instructions across basic blocks.  This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec
     Don't allow speculative motion of non-load instructions.  This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-pressure
     Enable register pressure sensitive insn scheduling before register allocation.  This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher.  Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation.

-fsched-spec-load
     Allow speculative motion of some load instructions.  This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous
     Allow speculative motion of more load instructions.  This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-stalled-insns
-fsched-stalled-insns=N
     Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.

-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=N
     Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns.  This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.

-fsched2-use-superblocks
     When scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules.  This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

     This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsched-group-heuristic
     Enable the group heuristic in the scheduler.  This heuristic favors the instruction that belongs to a schedule group.  This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-critical-path-heuristic
     Enable the critical-path heuristic in the scheduler.  This heuristic favors instructions on the critical path.  This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-spec-insn-heuristic
     Enable the speculative instruction heuristic in the scheduler.  This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-rank-heuristic
     Enable the rank heuristic in the scheduler.  This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-last-insn-heuristic
     Enable the last-instruction heuristic in the scheduler.  This heuristic favors the instruction that is less dependent on the last instruction scheduled.  This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-dep-count-heuristic
     Enable the dependent-count heuristic in the scheduler.  This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-freschedule-modulo-scheduled-loops
     Modulo scheduling is performed before traditional scheduling.  If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.

-fselective-scheduling
     Schedule instructions using selective scheduling algorithm.  Selective scheduling runs instead of the first scheduler pass.

-fselective-scheduling2
     Schedule instructions using selective scheduling algorithm.  Selective scheduling runs instead of the second scheduler pass.

-fsel-sched-pipelining
     Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.

-fsel-sched-pipelining-outer-loops
     When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on.

-fsemantic-interposition
     Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations in anticipation that the function or variable in question may change. While this feature is useful, for example, to rewrite memory allocation functions by a debugging implementation, it is expensive in the terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions the overwriting function will have precisely the same semantics (and side effects). Similarly if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak.

-fshrink-wrap
     Emit function prologues only before parts of the function that need it, rather than at the top of the function.  This flag is enabled by default at -O and higher.

-fcaller-saves
     Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls.  Such allocation is done only when it seems to result in better code.

     This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

     Enabled at levels -O2, -O3, -Os.

-fcombine-stack-adjustments
     Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them.

     Enabled by default at -O1 and higher.

-fipa-ra
     Use caller save registers for allocation if those registers are not used by any called function.  In that case it is not necessary to save and restore them around calls.  This is only possible if called functions are part of same compilation unit as current function and they are compiled before it.

     Enabled at levels -O2, -O3, -Os.

-fconserve-stack
     Attempt to minimize stack usage.  The compiler attempts to use less stack space, even if that makes the program slower.  This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.

-ftree-reassoc
     Perform reassociation on trees.  This flag is enabled by default at -O and higher.

-ftree-pre
     Perform partial redundancy elimination (PRE) on trees.  This flag is enabled by default at -O2 and -O3.

-ftree-partial-pre
     Make partial redundancy elimination (PRE) more aggressive.  This flag is enabled by default at -O3.

-ftree-forwprop
     Perform forward propagation on trees.  This flag is enabled by default at -O and higher.

-ftree-fre
     Perform full redundancy elimination (FRE) on trees.  The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.

-ftree-phiprop
     Perform hoisting of loads from conditional pointers on trees.  This pass is enabled by default at -O and higher.

-fhoist-adjacent-loads
     Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction.  This flag is enabled by default at -O2 and higher.

-ftree-copy-prop
     Perform copy propagation on trees.  This pass eliminates unnecessary copy operations.  This flag is enabled by default at -O and higher.

-fipa-pure-const
     Discover which functions are pure or constant. Enabled by default at -O and higher.

-fipa-reference
     Discover which static variables do not escape the compilation unit. Enabled by default at -O and higher.

-fipa-pta
     Perform interprocedural pointer analysis and interprocedural modification and reference analysis.  This option can cause excessive memory and compile-time usage on large compilation units.  It is not enabled by default at any optimization level.

-fipa-profile
     Perform interprocedural profile propagation.  The functions called only from cold functions are marked as cold. Also functions executed once (such as cold, noreturn, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.

-fipa-cp
     Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.

-fipa-cp-clone
     Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=VALUE). This flag is enabled by default at -O3.

-fipa-cp-alignment
     When enabled, this optimization propagates alignment of function parameters to support better vectorization and string operations.

     This flag is enabled by default at -O2 and -Os.  It requires that -fipa-cp is enabled.

-fipa-icf
     Perform Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link time optimization enabled.

     Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF works on different levels and thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found only by Gold.

     This flag is enabled by default at -O2 and -Os.

-fisolate-erroneous-paths-dereference
     Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer.  Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled.

-fisolate-erroneous-paths-attribute
     Detect paths that trigger erroneous or undefined behavior due a null value being used in a way forbidden by a returns_nonnull or nonnull attribute.  Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap.  This is not currently enabled, but may be enabled by -O2 in the future.

-ftree-sink
     Perform forward store motion  on trees.  This flag is enabled by default at -O and higher.

-ftree-bit-ccp
     Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O and higher.  It requires that -ftree-ccp is enabled.

-ftree-ccp
     Perform sparse conditional constant propagation (CCP) on trees.  This pass only operates on local scalar variables and is enabled by default at -O and higher.

-fssa-phiopt
     Perform pattern matching on SSA PHI nodes to optimize conditional code.  This pass is enabled by default at -O and higher.

-ftree-switch-conversion
     Perform conversion of simple initializations in a switch to initializations from a scalar array.  This flag is enabled by default at -O2 and higher.

-ftree-tail-merge
     Look for identical code sequences.  When found, replace one with a jump to the other.  This optimization is known as tail merging or cross jumping.  This flag is enabled by default at -O2 and higher.  The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.

-ftree-dce
     Perform dead code elimination (DCE) on trees.  This flag is enabled by default at -O and higher.

-ftree-builtin-call-dce
     Perform conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise side-effect free.  This flag is enabled by default at -O2 and higher if -Os is not also specified.

-ftree-dominator-opts
     Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal.  This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.

-ftree-dse
     Perform dead store elimination (DSE) on trees.  A dead store is a store into a memory location that is later overwritten by another store without any intervening loads.  In this case the earlier store can be deleted.  This flag is enabled by default at -O and higher.

-ftree-ch
     Perform loop header copying on trees.  This is beneficial since it increases effectiveness of code motion optimizations.  It also saves one jump.  This flag is enabled by default at -O and higher.  It is not enabled for -Os, since it usually increases code size.

-ftree-loop-optimize
     Perform loop optimizations on trees.  This flag is enabled by default at -O and higher.

-ftree-loop-linear
     Perform loop interchange transformations on tree.  Same as -floop-interchange.  To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.

-floop-interchange
     Perform loop interchange transformations on loops.  Interchanging two nested loops switches the inner and outer loops.  For example, given a loop like:

          DO J = 1, M
            DO I = 1, N
              A(J, I) = A(J, I) * C
            ENDDO
          ENDDO

     loop interchange transforms the loop as if it were written:

          DO I = 1, N
            DO J = 1, M
              A(J, I) = A(J, I) * C
            ENDDO
          ENDDO

     which can be beneficial when N is larger than the caches, because in Fortran, the elements of an array are stored in memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache miss.  This optimization applies to all the languages supported by GCC and is not limited to Fortran.  To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.

-floop-strip-mine
     Perform loop strip mining transformations on loops.  Strip mining splits a loop into two nested loops.  The outer loop has strides equal to the strip size and the inner loop has strides of the original loop within a strip.  The strip length can be changed using the loop-block-tile-size parameter.  For example, given a loop like:

          DO I = 1, N
            A(I) = A(I) + C
          ENDDO

     loop strip mining transforms the loop as if it were written:

          DO II = 1, N, 51
            DO I = II, min (II + 50, N)
              A(I) = A(I) + C
            ENDDO
          ENDDO

     This optimization applies to all the languages supported by GCC and is not limited to Fortran.  To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.

-floop-block
     Perform loop blocking transformations on loops.  Blocking strip mines each loop in the loop nest such that the memory accesses of the element loops fit inside caches.  The strip length can be changed using the loop-block-tile-size parameter.  For example, given a loop like:

          DO I = 1, N
            DO J = 1, M
              A(J, I) = B(I) + C(J)
            ENDDO
          ENDDO

     loop blocking transforms the loop as if it were written:

          DO II = 1, N, 51
            DO JJ = 1, M, 51
              DO I = II, min (II + 50, N)
                DO J = JJ, min (JJ + 50, M)
                  A(J, I) = B(I) + C(J)
                ENDDO
              ENDDO
            ENDDO
          ENDDO

     which can be beneficial when M is larger than the caches, because the innermost loop iterates over a smaller amount of data which can be kept in the caches.  This optimization applies to all the languages supported by GCC and is not limited to Fortran.  To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.

-fgraphite-identity
     Enable the identity transformation for graphite.  For every SCoP we generate the polyhedral representation and transform it back to gimple.  Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation.  Some minimal optimizations are also performed by the code generator ISL, like index splitting and dead code elimination in loops.

-floop-nest-optimize
     Enable the ISL based loop nest optimizer.  This is a generic loop nest optimizer based on the Pluto optimization algorithms.  It calculates a loop structure optimized for data-locality and parallelism.  This option is experimental.

-floop-unroll-and-jam
     Enable unroll and jam for the ISL based loop nest optimizer.  The unroll factor can be changed using the loop-unroll-jam-size parameter. The unrolled dimension (counting from the most inner one) can be changed using the loop-unroll-jam-depth parameter.                 .

-floop-parallelize-all
     Use the Graphite data dependence analysis to identify loops that can be parallelized.  Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.

-ftree-loop-if-convert
     Attempt to transform conditional jumps in the innermost loops to branch-less equivalents.  The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops.  This is enabled by default if vectorization is enabled.

-ftree-loop-if-convert-stores
     Attempt to also if-convert conditional jumps containing memory writes. This transformation can be unsafe for multi-threaded programs as it transforms conditional memory writes into unconditional memory writes. For example,

          for (i = 0; i < N; i++)
            if (cond)
              A[i] = expr;

     is transformed to

          for (i = 0; i < N; i++)
            A[i] = cond ? expr : A[i];

     potentially producing data races.

-ftree-loop-distribution
     Perform loop distribution.  This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place.  For example, the loop

          DO I = 1, N
            A(I) = B(I) + C
            D(I) = E(I) * F
          ENDDO

     is transformed to

          DO I = 1, N
             A(I) = B(I) + C
          ENDDO
          DO I = 1, N
             D(I) = E(I) * F
          ENDDO

-ftree-loop-distribute-patterns
     Perform loop distribution of patterns that can be code generated with calls to a library.  This flag is enabled by default at -O3.

     This pass distributes the initialization loops and generates a call to memset zero.  For example, the loop

          DO I = 1, N
            A(I) = 0
            B(I) = A(I) + I
          ENDDO

     is transformed to

          DO I = 1, N
             A(I) = 0
          ENDDO
          DO I = 1, N
             B(I) = A(I) + I
          ENDDO

     and the initialization loop is transformed into a call to memset zero.

-ftree-loop-im
     Perform loop invariant motion on trees.  This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns).  With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching.  The pass also includes store motion.

-ftree-loop-ivcanon
     Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis.  Later optimizations then may determine the number easily.  Useful especially in connection with unrolling.

-fivopts
     Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.

-ftree-parallelize-loops=n
     Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered.  The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth.  This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-ftree-pta
     Perform function-local points-to analysis on trees.  This flag is enabled by default at -O and higher.

-ftree-sra
     Perform scalar replacement of aggregates.  This pass replaces structure references with scalars to prevent committing structures to memory too early.  This flag is enabled by default at -O and higher.

-ftree-copyrename
     Perform copy renaming on trees.  This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables.  This flag is enabled by default at -O and higher.

-ftree-coalesce-inlined-vars
     Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, but only if they are inlined from other functions.  It is a more limited form of -ftree-coalesce-vars.  This may harm debug information of such inlined variables, but it keeps variables of the inlined-into function apart from each other, such that they are more likely to contain the expected values in a debugging session.

-ftree-coalesce-vars
     Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, instead of just compiler temporaries.  This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments.  In the negated form, this flag prevents SSA coalescing of user variables, including inlined ones.  This option is enabled by default.

-ftree-ter
     Perform temporary expression replacement during the SSA->normal phase.  Single use/single def temporaries are replaced at their use location with their defining expression.  This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation.  This is enabled by default at -O and higher.

-ftree-slsr
     Perform straight-line strength reduction on trees.  This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible.  This is enabled by default at -O and higher.

-ftree-vectorize
     Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.

-ftree-loop-vectorize
     Perform loop vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.

-ftree-slp-vectorize
     Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.

-fvect-cost-model=MODEL
     Alter the cost model used for vectorization.  The MODEL argument should be one of unlimited, dynamic or cheap. With the unlimited model the vectorized code-path is assumed to be profitable while with the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop.  The cheap model disables vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the dynamic model. The default cost model depends on other optimization flags and is either dynamic or cheap.

-fsimd-cost-model=MODEL
     Alter the cost model used for vectorization of loops marked with the OpenMP or Cilk Plus simd directive.  The MODEL argument should be one of unlimited, dynamic, cheap.  All values of MODEL have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.

-ftree-vrp
     Perform Value Range Propagation on trees.  This is similar to the constant propagation pass, but instead of values, ranges of values are propagated.  This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks.  This is enabled by default at -O2 and higher.  Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.

-fsplit-ivs-in-unroller
     Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration.  This breaks long dependency chains, thus improving efficiency of the scheduling passes.

     A combination of -fweb and CSE is often sufficient to obtain the same effect.  However, that is not reliable in cases where the loop body is more complicated than a single basic block.  It also does not work at all on some architectures due to restrictions in the CSE pass.

     This optimization is enabled by default.

-fvariable-expansion-in-unroller
     With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.

-fpartial-inlining
     Inline parts of functions.  This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

     Enabled at level -O2.

-fpredictive-commoning
     Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops.

     This option is enabled at level -O3.

-fprefetch-loop-arrays
     If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

     This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.

     Disabled at level -Os.

-fno-peephole
-fno-peephole2
     Disable any machine-specific peephole optimizations.  The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

     -fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

-fno-guess-branch-probability
     Do not guess branch probabilities using heuristics.

     GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs).  These heuristics are based on the control flow graph.  If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account.  The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.

     The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

-freorder-blocks
     Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

     Enabled at levels -O2, -O3.

-freorder-blocks-and-partition
     In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.

     This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

     Enabled for x86 at levels -O2, -O3.

-freorder-functions
     Reorder functions in the object file in order to improve code locality.  This is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions.  Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

     Also profile feedback must be available to make this option effective.  See -fprofile-arcs for details.

     Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing
     Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled.  For C (and C++), this activates optimizations based on the type of expressions.  In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same.  For example, an unsigned int can alias an int, but not a void* or a double.  A character type may alias any other type.

     Pay special attention to code like this:

          union a_union {
            int i;
            double d;
          };

          int f() {
            union a_union t;
            t.d = 3.0;
            return t.i;
          }

     The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common.  Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.  So, the code above works as expected.  See Structures unions enumerations and bit-fields implementation.  However, this code might not:

          int f() {
            union a_union t;
            int* ip;
            t.d = 3.0;
            ip = &t.i;
            return *ip;
          }

     Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

          int f() {
            double d = 3.0;
            return ((union a_union *) &d)->i;
          }

     The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

-fstrict-overflow
     Allow the compiler to assume strict signed overflow rules, depending on the language being compiled.  For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it does not happen.  This permits various optimizations.  For example, the compiler assumes that an expression like i + 10 > i is always true for signed i.  This assumption is only valid if signed overflow is undefined, as the expression is false if i + 10 overflows when using twos complement arithmetic.  When this option is in effect any attempt to determine whether an operation on signed numbers overflows must be written carefully to not actually involve overflow.

     This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined.  This permits the compiler to conclude that p + u > p is always true for a pointer p and unsigned integer u.  This assumption is only valid because pointer wraparound is undefined, as the expression is false if p + u overflows using twos complement arithmetic.

     See also the -fwrapv option.  Using -fwrapv means that integer signed overflow is fully defined: it wraps.  When -fwrapv is used, there is no difference between -fstrict-overflow and -fno-strict-overflow for integers.  With -fwrapv certain types of overflow are permitted.  For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with -fwrapv, but not otherwise.

     The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.

-falign-functions
-falign-functions=N
     Align the start of functions to the next power-of-two greater than N, skipping up to N bytes.  For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

     -fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.

     Some assemblers only support this flag when N is a power of two; in that case, it is rounded up.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-falign-labels
-falign-labels=N
     Align all branch targets to a power-of-two boundary, skipping up to N bytes like -falign-functions.  This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.

     -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

     If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

     If N is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.

     Enabled at levels -O2, -O3.

-falign-loops
-falign-loops=N
     Align loops to a power-of-two boundary, skipping up to N bytes like -falign-functions.  If the loops are executed many times, this makes up for any execution of the dummy operations.

     -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-falign-jumps
-falign-jumps=N
     Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to N bytes like -falign-functions.  In this case, no dummy operations need be executed.

     -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-funit-at-a-time
     This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

     Enabled by default.

-fno-toplevel-reorder
     Do not reorder top-level functions, variables, and asm statements.  Output them in the same order that they appear in the input file.  When this option is used, unreferenced static variables are not removed.  This option is intended to support existing code that relies on a particular ordering.  For new code, it is better to use attributes when possible.

     Enabled at level -O0.  When disabled explicitly, it also implies -fno-section-anchors, which is otherwise enabled at -O0 on some targets.

-fweb
     Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register.  This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover.  It can, however, make debugging impossible, since variables no longer stay in a ``home register''.

     Enabled by default with -funroll-loops.

-fwhole-program
     Assume that the current compilation unit represents the whole program being compiled.  All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.

     This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information.

-flto[=N]
     This option runs the standard link-time optimizer.  When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file.  When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.

     To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. For example:

          gcc -c -O2 -flto foo.c
          gcc -c -O2 -flto bar.c
          gcc -o myprog -flto -O2 foo.o bar.o

     The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o.  The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual.  Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one.  This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.

     Another (simpler) way to enable link-time optimization is:

          gcc -o myprog -flto -O2 foo.c bar.c

     The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

     The only important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link-step. GCC then automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You generally should specify the optimization options to be used for link-time optimization though GCC tries to be clever at guessing an optimization level to use from the options used at compile-time if you fail to specify one at link-time.  You can always override the automatic decision to do link-time optimization at link-time by passing -fno-lto to the link command.

     To make whole program optimization effective, it is necessary to make certain whole program assumptions.  The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit.  When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols.  When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.

     When -fuse-linker-plugin is not enabled then, when a file is compiled with -flto, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects.  This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied.  Note that when -fno-fat-lto-objects is enabled the compile-stage is faster but you cannot perform a regular, non-LTO link on them.

     Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link time.  For instance,

          gcc -c -O0 -ffat-lto-objects -flto foo.c
          gcc -c -O0 -ffat-lto-objects -flto bar.c
          gcc -o myprog -O3 foo.o bar.o

     This produces individual object files with unoptimized assembler code, but the resulting binary myprog is optimized at -O3.  If, instead, the final binary is generated with -fno-lto, then myprog is not optimized.

     When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code.  GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.

     There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link stage.  Generally options specified at link-time override those specified at compile-time.

     If you do not specify an optimization level option -O at link-time then GCC computes one based on the optimization levels used when compiling the object files.  The highest optimization level wins here.

     Currently, the following options and their setting are take from the first object file that explicitely specified it: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

     Certain ABI changing flags are required to match in all compilation-units and trying to override this at link-time with a conflicting value is ignored.  This includes options such as -freg-struct-return and -fpcc-struct-return.

     Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units.  Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence and for example -ffp-contract=off takes precedence over -ffp-contract=fast.  You can override them at linke-time.

     It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time.

     If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued.  The behavior is still undefined at run time.  Similar diagnostics may be raised for other languages.

     Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

          gcc -c -flto foo.c
          g++ -c -flto bar.cc
          gfortran -c -flto baz.f90
          g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

     Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries.  In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.

     If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support.  To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm.  Those commands require that ar, ranlib and nm have been compiled with plugin support.  At link time, use the the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:

          gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

     With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.

     If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process.  In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.

     Link-time optimizations do not require the presence of the whole program to operate.  If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).

     The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts.  The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.

     Link-time optimization does not work well with generation of debugging information.  Combining -flto with -g is currently experimental and expected to produce unexpected results.

     If you specify the optional N, the optimization and code generation done at link time is executed in parallel using N parallel jobs by utilizing an installed make program.  The environment variable MAKE may be used to override the program used.  The default value for N is 1.

     You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work.  This option likely only works if MAKE is GNU make.

-flto-partition=ALG
     Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files or balanced to specify partitioning into equally sized chunks (whenever possible) or max to create new partition for every symbol where possible.  Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as an workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.

-flto-odr-type-merging
     Enable streaming of mangled types names of C++ types and their unification at linktime.  This increases size of LTO object files, but enable diagnostics about One Definition Rule violations.

-flto-compression-level=N
     This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto).  Valid values are 0 (no compression) to 9 (maximum compression).  Values outside this range are clamped to either 0 or 9.  If the option is not given, a default balanced compression setting is used.

-flto-report
     Prints a report with internal details on the workings of the link-time optimizer.  The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).

     Disabled by default.

-flto-report-wpa
     Like -flto-report, but only print for the WPA phase of Link Time Optimization.

-fuse-linker-plugin
     Enables the use of a linker plugin during link-time optimization.  This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

     This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer.  This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking).  Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.

     This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

-ffat-lto-objects
     Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

     -fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality.  Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc).  GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.

     The default is -fno-fat-lto-objects on targets with linker plugin support.

-fcompare-elim
     After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic.  If possible, eliminate the explicit comparison operation.

     This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

     Enabled at levels -O, -O2, -O3, -Os.

-fcprop-registers
     After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

     Enabled at levels -O, -O2, -O3, -Os.

-fprofile-correction
     Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.

-fprofile-dir=PATH

     Set the directory to search for the profile data files in to PATH. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options.  Both absolute and relative paths can be used. By default, GCC uses the current directory as PATH, thus the profile data file appears in the same directory as the object file.

-fprofile-generate
-fprofile-generate=PATH

     Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization.  You must use -fprofile-generate both when compiling and when linking your program.

     The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.

     If PATH is specified, GCC looks at the PATH to find the profile feedback data files. See -fprofile-dir.

-fprofile-use
-fprofile-use=PATH
     Enable profile feedback-directed optimizations, and the following optimizations which are generally profitable only with profile feedback available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize, and ftree-loop-distribute-patterns.

     By default, GCC emits an error message if the feedback profiles do not match the source code.  This error can be turned into a warning by using -Wcoverage-mismatch.  Note this may result in poorly optimized code.

     If PATH is specified, GCC looks at the PATH to find the profile feedback data files. See -fprofile-dir.

-fauto-profile
-fauto-profile=PATH
     Enable sampling-based feedback-directed optimizations, and the following optimizations which are generally profitable only with profile feedback available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize, -finline-functions, -fipa-cp, -fipa-cp-clone, -fpredictive-commoning, -funswitch-loops, -fgcse-after-reload, and -ftree-loop-distribute-patterns.

     PATH is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.

     Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/.

     E.g.

          perf record -e br_inst_retired:near_taken -b -o perf.data \
              -- your_program

     Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC.  You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo.

     E.g.

          create_gcov --binary=your_program.unstripped --profile=perf.data \
              --gcov=profile.afdo

The following options control compiler behavior regarding floating-point arithmetic.  These options trade off between speed and correctness.  All must be specifically enabled.

-ffloat-store
     Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory.

     This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have.  Similarly for the x86 architecture.  For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point.  Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

-fexcess-precision=STYLE
     This option allows further control over excess precision on machines where floating-point registers have more precision than the IEEE float and double types and the processor does not support operations rounding to those types.  By default, -fexcess-precision=fast is in effect; this means that operations are carried out in the precision of the registers and that it is unpredictable when rounding to the types specified in the source code takes place.  When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments).  This option is enabled by default for C if a strict conformance option such as -std=c99 is used.

     -fexcess-precision=standard is not implemented for languages other than C, and has no effect if -funsafe-math-optimizations or -ffast-math is specified.  On the x86, it also has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.

-ffast-math
     Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range.

     This option causes the preprocessor macro __FAST_MATH__ to be defined.

     This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

-fno-math-errno
     Do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt.  A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

     This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

     The default is -fmath-errno.

     On Darwin systems, the math library never sets errno.  There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default.

-funsafe-math-optimizations

     Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards.  When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.

     This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

     The default is -fno-unsafe-math-optimizations.

-fassociative-math

     Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result.  NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52.  May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect.  Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

     The default is -fno-associative-math.

-freciprocal-math

     Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations.  For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination.  Note that this loses precision and increases the number of flops operating on the value.

     The default is -fno-reciprocal-math.

-ffinite-math-only
     Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

     This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

     The default is -fno-finite-math-only.

-fno-signed-zeros
     Allow optimizations for floating-point arithmetic that ignore the signedness of zero.  IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.

     The default is -fsigned-zeros.

-fno-trapping-math
     Compile code assuming that floating-point operations cannot generate user-visible traps.  These traps include division by zero, overflow, underflow, inexact result and invalid operation.  This option requires that -fno-signaling-nans be in effect.  Setting this option may allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.

     This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -ftrapping-math.

-frounding-math
     Disable transformations and optimizations that assume default floating-point rounding behavior.  This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations.  This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode.  This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

     The default is -fno-rounding-math.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma.  This command-line option will be used to specify the default state for FENV_ACCESS.

-fsignaling-nans
     Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations.  Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs.  This option implies -ftrapping-math.

     This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

     The default is -fno-signaling-nans.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant
     Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.

-fcx-limited-range
     When enabled, this option states that a range reduction step is not needed when performing complex division.  Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.  The default is -fno-cx-limited-range, but is enabled by -ffast-math.

     This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma.  Nevertheless, the option applies to all languages.

-fcx-fortran-rules
     Complex multiplication and division follow Fortran rules.  Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.

     The default is -fno-cx-fortran-rules.

The following options control optimizations that may improve performance, but are not enabled by any -O options.  This section includes experimental options that may produce broken code.

-fbranch-probabilities
     After running a program compiled with -fprofile-arcs (see Options for Debugging Your Program or gcc), you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken.  When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called SOURCENAME.gcda for each source file.  The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.

     With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization.  Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

-fprofile-values
     If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

     With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

     Enabled with -fprofile-generate and -fprofile-use.

-fprofile-reorder-functions
     Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.

     Enabled with -fprofile-use.

-fvpt
     If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.

     With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.

-frename-registers
     Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation.  This optimization most benefits processors with lots of registers.  Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a ``home register''.

     Enabled by default with -funroll-loops and -fpeel-loops.

-fschedule-fusion
     Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.

     Enabled at levels -O2, -O3, -Os.

-ftracer
     Perform tail duplication to enlarge superblock size.  This transformation simplifies the control flow of the function allowing other optimizations to do a better job.

     Enabled with -fprofile-use.

-funroll-loops
     Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.  -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations).  This option makes code larger, and may or may not make it run faster.

     Enabled with -fprofile-use.

-funroll-all-loops
     Unroll all loops, even if their number of iterations is uncertain when the loop is entered.  This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.

-fpeel-loops
     Peels loops for which there is enough information that they do not roll much (from profile feedback).  It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

     Enabled with -fprofile-use.

-fmove-loop-invariants
     Enables the loop invariant motion pass in the RTL loop optimizer.  Enabled at level -O1

-funswitch-loops
     Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

-ffunction-sections
-fdata-sections
     Place each function or data item into its own section in the output file if the target supports arbitrary sections.  The name of the function or the name of the data item determines the section's name in the output file.

     Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space.  Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations.  AIX may have these optimizations in the future.

     Only use these options when there are significant benefits from doing so.  When you specify these options, the assembler and linker create larger object and executable files and are also slower. You cannot use gprof on all systems if you specify this option, and you may have problems with debugging if you specify both this option and -g.

-fbranch-target-load-optimize
     Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.

-fbranch-target-load-optimize2
     Perform branch target register load optimization after prologue / epilogue threading.

-fbtr-bb-exclusive
     When performing branch target register load optimization, don't reuse branch target registers within any basic block.

-fstack-protector
     Emit extra code to check for buffer overflows, such as stack smashing attacks.  This is done by adding a guard variable to functions with vulnerable objects.  This includes functions that call alloca, and functions with buffers larger than 8 bytes.  The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.

-fstack-protector-all
     Like -fstack-protector except that all functions are protected.

-fstack-protector-strong
     Like -fstack-protector but includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses.

-fstack-protector-explicit
     Like -fstack-protector but only protects those functions which have the stack_protect attribute

-fstdarg-opt
     Optimize the prologue of variadic argument functions with respect to usage of those arguments.

-fsection-anchors
     Try to reduce the number of symbolic address calculations by using shared ``anchor'' symbols to address nearby objects.  This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.

     For example, the implementation of the following function foo:

          static int a, b, c;
          int foo (void) { return a + b + c; }

     usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead.  The effect is similar to the following pseudocode (which isn't valid C):

          int foo (void)
          {
            register int *xr = &x;
            return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
          }

     Not all targets support this option.

--param NAME=VALUE
     In some places, GCC uses various constants to control the amount of optimization that is done.  For example, GCC does not inline functions that contain more than a certain number of instructions.  You can control some of these constants on the command line using the --param option.

     The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

     In each case, the VALUE is an integer.  The allowable choices for NAME are:

     predictable-branch-outcome
          When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. The default is 10.

     max-crossjump-edges
          The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block.  Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.

     min-crossjump-insns
          The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them.  This value is ignored in the case where all instructions in the block being cross-jumped from are matched.  The default value is 5.

     max-grow-copy-bb-insns
          The maximum code size expansion factor when copying basic blocks instead of jumping.  The expansion is relative to a jump instruction. The default value is 8.

     max-goto-duplication-insns
          The maximum number of instructions to duplicate to a block that jumps to a computed goto.  To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible.  Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.  The default value is 8.

     max-delay-slot-insn-search
          The maximum number of instructions to consider when looking for an instruction to fill a delay slot.  If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching.  Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.

     max-delay-slot-live-search
          When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information.  Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time.  This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.

     max-gcse-memory
          The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization.  If more memory than specified is required, the optimization is not done.

     max-gcse-insertion-ratio
          If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.  The default value is 20.

     max-pending-list-length
          The maximum number of pending dependencies scheduling allows before flushing the current state and starting over.  Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.

     max-modulo-backtrack-attempts
          The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop.  Larger values can exponentially increase compilation time.

     max-inline-insns-single
          Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining.  This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 400.

     max-inline-insns-auto
          When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated.  To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 40.

     inline-min-speedup
          When estimated performance improvement of caller + callee runtime exceeds this threshold (in precent), the function can be inlined regardless the limit on --param max-inline-insns-single and --param max-inline-insns-auto.

     large-function-insns
          The limit specifying really large functions.  For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth.  This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. The default value is 2700.

     large-function-growth
          Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits large function growth to 2.0 times the original size.

     large-unit-insns
          The limit specifying large translation unit.  Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times.  If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane.  For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size.  Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth.  The default is 10000.

     inline-unit-growth
          Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 20 which limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.

     ipcp-unit-growth
          Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation.  The default value is 10 which limits unit growth to 1.1 times the original size.

     large-stack-frame
          The limit specifying large stack frames.  While inlining the algorithm is trying to not grow past this limit too much.  The default value is 256 bytes.

     large-stack-frame-growth
          Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which limits large stack frame growth to 11 times the original size.

     max-inline-insns-recursive
     max-inline-insns-recursive-auto
          Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.

          --param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead.  The default value is 450.

     max-inline-recursive-depth
     max-inline-recursive-depth-auto
          Specifies the maximum recursion depth used for recursive inlining.

          --param max-inline-recursive-depth applies to functions declared inline.  For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead.  The default value is 8.

     min-inline-recursive-probability
          Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers.

          When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from probability that function recurses via a given call expression.  This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents). The default value is 10.

     early-inlining-insns
          Specify growth that the early inliner can make.  In effect it increases the amount of inlining for code having a large abstraction penalty. The default value is 14.

     max-early-inliner-iterations
          Limit of iterations of the early inliner.  This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.

     comdat-sharing-probability
          Probability (in percent) that C++ inline function with comdat visibility are shared across multiple compilation units.  The default value is 20.

     profile-func-internal-id
          A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. The default value is 0.

     min-vect-loop-bound
          The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used.  The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.  The default value is 0.

     gcse-cost-distance-ratio
          Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations.  This is currently supported only in the code hoisting pass.  The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost.  Specifying 0 disables hoisting of simple expressions.  The default value is 10.

     gcse-unrestricted-cost
          Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel.  This is currently supported only in the code hoisting pass.  The lesser the cost, the more aggressive code hoisting is.  Specifying 0 allows all expressions to travel unrestricted distances. The default value is 3.

     max-hoist-depth
          The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions.  The default value is 30.

     max-tail-merge-comparisons
          The maximum amount of similar bbs to compare a bb with.  This is used to avoid quadratic behavior in tree tail merging.  The default value is 10.

     max-tail-merge-iterations
          The maximum amount of iterations of the pass over the function.  This is used to limit compilation time in tree tail merging.  The default value is 2.

     max-unrolled-insns
          The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.

     max-average-unrolled-insns
          The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled.  If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.

     max-unroll-times
          The maximum number of unrollings of a single loop.

     max-peeled-insns
          The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.

     max-peel-times
          The maximum number of peelings of a single loop.

     max-peel-branches
          The maximum number of branches on the hot path through the peeled sequence.

     max-completely-peeled-insns
          The maximum number of insns of a completely peeled loop.

     max-completely-peel-times
          The maximum number of iterations of a loop to be suitable for complete peeling.

     max-completely-peel-loop-nest-depth
          The maximum depth of a loop nest suitable for complete peeling.

     max-unswitch-insns
          The maximum number of insns of an unswitched loop.

     max-unswitch-level
          The maximum number of branches unswitched in a single loop.

     lim-expensive
          The minimum cost of an expensive expression in the loop invariant motion.

     iv-consider-all-candidates-bound
          Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations.  If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.

     iv-max-considered-uses
          The induction variable optimizations give up on loops that contain more induction variable uses.

     iv-always-prune-cand-set-bound
          If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.

     scev-max-expr-size
          Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.

     scev-max-expr-complexity
          Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.

     vect-max-version-for-alignment-checks
          The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.

     vect-max-version-for-alias-checks
          The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.

     vect-max-peeling-for-alignment
          The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means 'no limit'.

     max-iterations-to-track
          The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.

     hot-bb-count-ws-permille
          A basic block profile count is considered hot if it contributes to the given permillage (i.e. 0...1000) of the entire profiled execution.

     hot-bb-frequency-fraction
          Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have to be considered hot.

     max-predicted-iterations
          The maximum number of loop iterations we predict statically.  This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10.  This means that the loop without bounds appears artificially cold relative to the other one.

     builtin-expect-probability
          Control the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input. The default probability of 90 is obtained empirically.

     align-threshold

          Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.

     align-loop-iterations

          A loop expected to iterate at least the selected number of iterations is aligned.

     tracer-dynamic-coverage
     tracer-dynamic-coverage-feedback

          This value is used to limit superblock formation once the given percentage of executed instructions is covered.  This limits unnecessary code size expansion.

          The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available.  The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.

     tracer-max-code-growth
          Stop tail duplication once code growth has reached given percentage.  This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.

     tracer-min-branch-ratio

          Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).

     tracer-min-branch-ratio
     tracer-min-branch-ratio-feedback

          Stop forward growth if the best edge has probability lower than this threshold.

          Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without.  The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

     max-cse-path-length

          The maximum number of basic blocks on path that CSE considers. The default is 10.

     max-cse-insns
          The maximum number of instructions CSE processes before flushing. The default is 1000.

     ggc-min-expand

          GCC uses a garbage collector to manage its own memory allocation.  This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.

          The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB.  If getrlimit is available, the notion of ``RAM'' is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS.  If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used.  Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity.  This is extremely slow, but can be useful for debugging.

     ggc-min-heapsize

          Minimum size of the garbage collector's heap before it begins bothering to collect garbage.  The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize.  Again, tuning this may improve compilation speed, and has no effect on code generation.

          The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes).  If GCC is not able to calculate RAM on a particular platform, the lower bound is used.  Setting this parameter very large effectively disables garbage collection.  Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

     max-reload-search-insns
          The maximum number of instruction reload should look backward for equivalent register.  Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. The default value is 100.

     max-cselib-memory-locations
          The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance.  The default value is 500.

     reorder-blocks-duplicate
     reorder-blocks-duplicate-feedback

          Used by the basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination.  Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.

          The reorder-block-duplicate-feedback parameter is used only when profile feedback is available.  It may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.

     max-sched-ready-insns
          The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass.  Increasing values mean more thorough searches, making the compilation time increase with probably little benefit.  The default value is 100.

     max-sched-region-blocks
          The maximum number of blocks in a region to be considered for interblock scheduling.  The default value is 10.

     max-pipeline-region-blocks
          The maximum number of blocks in a region to be considered for pipelining in the selective scheduler.  The default value is 15.

     max-sched-region-insns
          The maximum number of insns in a region to be considered for interblock scheduling.  The default value is 100.

     max-pipeline-region-insns
          The maximum number of insns in a region to be considered for pipelining in the selective scheduler.  The default value is 200.

     min-spec-prob
          The minimum probability (in percents) of reaching a source block for interblock speculative scheduling.  The default value is 40.

     max-sched-extend-regions-iters
          The maximum number of iterations through CFG to extend regions. A value of 0 (the default) disables region extensions.

     max-sched-insn-conflict-delay
          The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.

     sched-spec-prob-cutoff
          The minimal probability of speculation success (in percents), so that speculative insns are scheduled. The default value is 40.

     sched-spec-state-edge-prob-cutoff
          The minimum probability an edge must have for the scheduler to save its state across it. The default value is 10.

     sched-mem-true-dep-cost
          Minimal distance (in CPU cycles) between store and load targeting same memory locations.  The default value is 1.

     selsched-max-lookahead
          The maximum size of the lookahead window of selective scheduling.  It is a depth of search for available instructions. The default value is 50.

     selsched-max-sched-times
          The maximum number of times that an instruction is scheduled during selective scheduling.  This is the limit on the number of iterations through which the instruction may be pipelined.  The default value is 2.

     selsched-max-insns-to-rename
          The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler.  The default value is 2.

     sms-min-sc
          The minimum value of stage count that swing modulo scheduler generates.  The default value is 2.

     max-last-value-rtl
          The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register.  The default is 10000.

     max-combine-insns
          The maximum number of instructions the RTL combiner tries to combine. The default value is 2 at -Og and 4 otherwise.

     integer-share-limit
          Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed.  This sets the maximum value of a shared integer constant.  The default value is 256.

     ssp-buffer-size
          The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used.

     min-size-for-stack-sharing
          The minimum size of variables taking part in stack slot sharing when not optimizing. The default value is 32.

     max-jump-thread-duplication-stmts
          Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.

     max-fields-for-field-sensitive
          Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.  The default is zero for -O0 and -O1, and 100 for -Os, -O2, and -O3.

     prefetch-latency
          Estimate on average number of instructions that are executed before prefetch finishes.  The distance prefetched ahead is proportional to this constant.  Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches).

     simultaneous-prefetches
          Maximum number of prefetches that can run at the same time.

     l1-cache-line-size
          The size of cache line in L1 cache, in bytes.

     l1-cache-size
          The size of L1 cache, in kilobytes.

     l2-cache-size
          The size of L2 cache, in kilobytes.

     min-insn-to-prefetch-ratio
          The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.

     prefetch-min-insn-to-mem-ratio
          The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.

     use-canonical-types
          Whether the compiler should use the ``canonical'' type system.  By default, this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++.  However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.

     switch-conversion-max-branch-ratio
          Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.

     max-partial-antic-length
          Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above.  For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine.  This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior.  Setting a value of 0 for this parameter allows an unlimited set length.

     sccvn-max-scc-size
          Maximum size of a strongly connected component (SCC) during SCCVN processing.  If this limit is hit, SCCVN processing for the whole function is not done and optimizations depending on it are disabled.  The default maximum SCC size is 10000.

     sccvn-max-alias-queries-per-access
          Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores.  If this limit is hit the search is aborted and the load or store is not considered redundant.  The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. The default maxmimum number of queries is 1000.

     ira-max-loops-num
          IRA uses regional register allocation by default.  If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation.  The default value of the parameter is 100.

     ira-max-conflict-table-size
          Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions.  If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. The default value of the parameter is 2000.

     ira-loop-reserved-regs
          IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3).  The number of available registers reserved for some other purposes is given by this parameter.  The default value of the parameter is 2, which is the minimal number of registers needed by typical instructions. This value is the best found from numerous experiments.

     lra-inheritance-ebb-probability-cutoff
          LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance.  EBB is used as a region to do this optimization.  The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA.  The default value of the parameter is 40.  The value was chosen from numerous runs of SPEC2000 on x86-64.

     loop-invariant-max-bbs-in-loop
          Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops.  Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them.  The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.

     loop-max-datarefs-for-datadeps
          Building data dapendencies is expensive for very large loops.  This parameter limits the number of data references in loops that are considered for data dependence analysis.  These large loops are no handled by the optimizations using loop data dependencies. The default value is 1000.

     max-vartrack-size
          Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function.  If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function.  If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function.  Setting the parameter to zero makes it unlimited.

     max-vartrack-expr-depth
          Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions.  This trades compilation time for more complete debug information.  If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow.  The default is 12.

     min-nondebug-insn-uid
          Use uids starting at this parameter for nondebug insns.  The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.

     ipa-sra-ptr-growth-factor
          IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.

     sra-max-scalarization-size-Ospeed

     sra-max-scalarization-size-Osize
          The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables.  These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively.

     tm-max-aggregate-size
          When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs.  This option only applies when using -fgnu-tm.

     graphite-max-nb-scop-params
          To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded.  The default value is 10 parameters.  A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.

     graphite-max-bbs-per-function
          To avoid exponential effects in the detection of SCoPs, the size of the functions analyzed by Graphite is bounded.  The default value is 100 basic blocks.

     loop-block-tile-size
          Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations.  The strip length can be changed using the loop-block-tile-size parameter.  The default value is 51 iterations.

     loop-unroll-jam-size
          Specify the unroll factor for the -floop-unroll-and-jam option.  The default value is 4.

     loop-unroll-jam-depth
          Specify the dimension to be unrolled (counting from the most inner loop) for the  -floop-unroll-and-jam.  The default value is 2.

     ipa-cp-value-list-size
          IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function.

     ipa-cp-eval-threshold
          IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold.

     ipa-cp-recursion-penalty
          Percentage penalty the recursive functions will receive when they are evaluated for cloning.

     ipa-cp-single-call-penalty
          Percentage penalty functions containg a single call to another function will receive when they are evaluated for cloning.

     ipa-max-agg-items
          IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter.

     ipa-cp-loop-hint-bonus
          When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate.

     ipa-cp-array-index-hint-bonus
          When IPA-CP determines that a cloning candidate would make the index of an array access known, it adds a bonus of ipa-cp-array-index-hint-bonus to the profitability score of the candidate.

     ipa-max-aa-steps
          During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters.  In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examining ipa-max-aa-steps statements modifying memory.

     lto-partitions
          Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. The default value is 32.

     lto-minpartition
          Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions.

     cxx-max-namespaces-for-diagnostic-help
          The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier.  The default is 1000.

     sink-frequency-threshold
          The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement.  Larger numbers result in more aggressive statement sinking. The default value is 75.  A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink.

     max-stores-to-sink
          The maximum number of conditional stores paires that can be sunk.  Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.  The default is 2.

     allow-store-data-races
          Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0.  This option is enabled by default at optimization level -Ofast.

     case-values-threshold
          The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches.  If the value is 0, use the default for the machine.  The default is 0.

     tree-reassoc-width
          Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value.

     sched-pressure-algorithm
          Choose between the two available implementations of -fsched-pressure.  Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler.  It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.c in the GCC sources for more details.

          The default choice depends on the target.

     max-slsr-cand-scan
          Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate.

     asan-globals
          Enable buffer overflow detection for global objects.  This kind of protection is enabled by default if you are using -fsanitize=address option. To disable global objects protection use --param asan-globals=0.

     asan-stack
          Enable buffer overflow detection for stack objects.  This kind of protection is enabled by default when using-fsanitize=address. To disable stack protection use --param asan-stack=0 option.

     asan-instrument-reads
          Enable buffer overflow detection for memory reads.  This kind of protection is enabled by default when using -fsanitize=address. To disable memory reads protection use --param asan-instrument-reads=0.

     asan-instrument-writes
          Enable buffer overflow detection for memory writes.  This kind of protection is enabled by default when using -fsanitize=address. To disable memory writes protection use --param asan-instrument-writes=0 option.

     asan-memintrin
          Enable detection for built-in functions.  This kind of protection is enabled by default when using -fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0.

     asan-use-after-return
          Enable detection of use-after-return.  This kind of protection is enabled by default when using -fsanitize=address option. To disable use-after-return detection use --param asan-use-after-return=0.

     asan-instrumentation-with-call-threshold
          If number of memory accesses in function being instrumented is greater or equal to this number, use callbacks instead of inline checks. E.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0.

     chkp-max-ctor-size
          Static constructors generated by Pointer Bounds Checker may become very large and significantly increase compile time at optimization level -O1 and higher.  This parameter is a maximum nubmer of statements in a single generated constructor.  Default value is 5000.

     max-fsm-thread-path-insns
          Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.  The default is 100.

     max-fsm-thread-length
          Maximum number of basic blocks on a finite state automaton jump thread path.  The default is 10.

     max-fsm-thread-paths
          Maximum number of new jump thread paths to create for a finite state automaton.  The default is 50.

Options Controlling the Preprocessor
====================================

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.

-Wp,OPTION
     You can use -Wp,OPTION to bypass the compiler driver and pass OPTION directly through to the preprocessor.  If OPTION contains commas, it is split into multiple options at the commas.  However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase.  The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.

-Xpreprocessor OPTION
     Pass OPTION as an option to the preprocessor.  You can use this to supply system-specific preprocessor options that GCC does not recognize.

     If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.

-no-integrated-cpp
     Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.

     [file cppopts.texi not included]

Passing Options to the Assembler
================================

You can pass options to the assembler.

-Wa,OPTION
     Pass OPTION as an option to the assembler.  If OPTION contains commas, it is split into multiple options at the commas.

-Xassembler OPTION
     Pass OPTION as an option to the assembler.  You can use this to supply system-specific assembler options that GCC does not recognize.

     If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.

Options for Linking
===================

These options come into play when the compiler links object files into an executable output file.  They are meaningless if the compiler is not doing a link step.

OBJECT-FILE-NAME
     A file name that does not end in a special recognized suffix is considered to name an object file or library.  (Object files are distinguished from libraries by the linker according to the file contents.)  If linking is done, these object files are used as input to the linker.

-c
-S
-E
     If any of these options is used, then the linker is not run, and object file names should not be used as arguments.  See Overall Options.

-fuse-ld=bfd
     Use the bfd linker instead of the default linker.

-fuse-ld=gold
     Use the gold linker instead of the default linker.

-lLIBRARY
-l LIBRARY
     Search the library named LIBRARY when linking.  (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

     It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified.  Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o.  If bar.o refers to functions in z, those functions may not be loaded.

     The linker searches a standard list of directories for the library, which is actually a file named libLIBRARY.a.  The linker then uses this file as if it had been specified precisely by name.

     The directories searched include several standard system directories plus any that you specify with -L.

     Normally the files found this way are library files---archive files whose members are object files.  The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined.  But if the file that is found is an ordinary object file, it is linked in the usual fashion.  The only difference between using an -l option and specifying a file name is that -l surrounds LIBRARY with lib and .a and searches several directories.

-lobjc
     You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.

-nostartfiles
     Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.

-nodefaultlibs
     Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

     The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

-nostdlib
     Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

     The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

     One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (see Interfacing to GCC Output (GNU Compiler Collection (GCC) Internals), for more discussion of libgcc.a.) In most cases, you need libgcc.a even when you want to avoid other standard libraries.  In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called; see collect2 (GNU Compiler Collection (GCC) Internals).)

-pie
     Produce a position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.

-no-pie
     Don't produce a position independent executable.

-rdynamic
     Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.

-s
     Remove all symbol table and relocation information from the executable.

-static
     On systems that support dynamic linking, this prevents linking with the shared libraries.  On other systems, this option has no effect.

-shared
     Produce a shared object which can then be linked with other objects to form an executable.  Not all systems support this option.  For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option. (On some systems, gcc -shared needs to build supplementary stub code for constructors to work.  On multi-libbed systems, gcc -shared must select the correct support libraries to link against.  Failing to supply the correct flags may lead to subtle defects.  Supplying them in cases where they are not necessary is innocuous.)

-shared-libgcc
-static-libgcc
     On systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

     There are several situations in which an application should use the shared libgcc instead of the static version.  The most common of these is when the application wishes to throw and catch exceptions across different shared libraries.  In that case, each of the libraries as well as the application itself should use the shared libgcc.

     Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

     If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default.  Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default.  This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

     However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

-static-libasan
     When the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan.  If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan.  The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically.

-static-libtsan
     When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan.  If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan.  The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.

-static-liblsan
     When the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan.  If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan.  The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically.

-static-libubsan
     When the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan.  If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan.  The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically.

-static-libmpx
     When the -fcheck-pointer bounds and -mmpx options are used to link a program, the GCC driver automatically links against libmpx.  If libmpx is available as a shared library, and the -static option is not used, then this links against the shared version of libmpx.  The -static-libmpx option directs the GCC driver to link libmpx statically, without necessarily linking other libraries statically.

-static-libmpxwrappers
     When the -fcheck-pointer bounds and -mmpx options are used to link a program without also using -fno-chkp-use-wrappers, the GCC driver automatically links against libmpxwrappers.  If libmpxwrappers is available as a shared library, and the -static option is not used, then this links against the shared version of libmpxwrappers.  The -static-libmpxwrappers option directs the GCC driver to link libmpxwrappers statically, without necessarily linking other libraries statically.

-static-libstdc++
     When the g++ program is used to link a C++ program, it normally automatically links against libstdc++.  If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++.  That is normally fine.  However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link.  The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.

-symbolic
     Bind references to global symbols when building a shared object.  Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs).  Only a few systems support this option.

-T SCRIPT
     Use SCRIPT as the linker script.  This option is supported by most systems using the GNU linker.  On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols.

-Xlinker OPTION
     Pass OPTION as an option to the linker.  You can use this to supply system-specific linker options that GCC does not recognize.

     If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions.  It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.

     When using the GNU linker, it is usually more convenient to pass arguments to linker options using the OPTION=VALUE syntax than as separate arguments.  For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map.  Other linkers may not support this syntax for command-line options.

-Wl,OPTION
     Pass OPTION as an option to the linker.  If OPTION contains commas, it is split into multiple options at the commas.  You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker.  When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.

-u SYMBOL
     Pretend the symbol SYMBOL is undefined, to force linking of library modules to define it.  You can use -u multiple times with different symbols to force loading of additional library modules.

-z KEYWORD
     -z is passed directly on to the linker along with the keyword KEYWORD. See the section in the documentation of your linker for permitted values and their meanings.

Options for Directory Search
============================

These options specify directories to search for header files, for libraries and for parts of the compiler:

-IDIR
     Add the directory DIR to the head of the list of directories to be searched for header files.  This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories.  However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that).  If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.

     If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored.  The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-iplugindir=DIR
     Set the directory to search for plugins that are passed by -fplugin=NAME instead of -fplugin=PATH/NAME.so.  This option is not meant to be used by the user, but only passed by the driver.

-iquoteDIR
     Add the directory DIR to the head of the list of directories to be searched for header files only for the case of #include "FILE"; they are not searched for #include <FILE>, otherwise just like -I.

-LDIR
     Add directory DIR to the list of directories to be searched for -l.

-BPREFIX
     This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

     The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld.  It tries PREFIX as a prefix for each program it tries to run, both with and without MACHINE/VERSION/ (see Target Options).

     For each subprogram to be run, the compiler driver first tries the -B prefix, if any.  If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/.  If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

     The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

     -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker.  They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor.  In this case, the compiler appends include to the prefix.

     The runtime support file libgcc.a can also be searched for using the -B prefix, if needed.  If it is not found there, the two standard prefixes above are tried, and that is all.  The file is left out of the link if it is not found by those means.

     Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.  See Environment Variables.

     As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include.  This is to help with boot-strapping the compiler.

-specs=FILE
     Process FILE after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc.  More than one -specs=FILE can be specified on the command line, and they are processed in order, from left to right.

--sysroot=DIR
     Use DIR as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches DIR/usr/include and DIR/usr/lib.

     If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

     The GNU linker (beginning with version 2.16) has the necessary support for this option.  If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not.

--no-sysroot-suffix
     For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in DIR/SUFFIX/usr/include instead of DIR/usr/include.  This option disables the addition of such a suffix.

-I-
     This option has been deprecated.  Please use -iquote instead for -I directories before the -I- and remove the -I- option. Any directories you specify with -I options before the -I- option are searched only for the case of #include "FILE"; they are not searched for #include <FILE>.

     If additional directories are specified with -I options after the -I- option, these directories are searched for all #include directives.  (Ordinarily all -I directories are used this way.)

     In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the first search directory for #include "FILE".  There is no way to override this effect of -I-.  With -I. you can specify searching the directory that is current when the compiler is invoked.  That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

     -I- does not inhibit the use of the standard system directories for header files.  Thus, -I- and -nostdinc are independent.

Specifying Subprocesses and the Switches to Pass to Them
========================================================

gcc is a driver program.  It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking.  GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines.  This behavior is controlled by spec strings.  In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior.  The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings.  They consist of a sequence of directives separated by blank lines.  The type of directive is determined by the first non-whitespace character on the line, which can be one of the following:

%COMMAND
     Issues a COMMAND to the spec file processor.  The commands that can appear here are:

     %include <FILE>
          Search for FILE and insert its text at the current point in the specs file.

     %include_noerr <FILE>
          Just like %include, but do not generate an error message if the include file cannot be found.

     %rename OLD_NAME NEW_NAME
          Rename the spec string OLD_NAME to NEW_NAME.

*[SPEC_NAME]:
     This tells the compiler to create, override or delete the named spec string.  All lines after this directive up to the next directive or blank line are considered to be the text for the spec string.  If this results in an empty string then the spec is deleted.  (Or, if the spec did not exist, then nothing happens.)  Otherwise, if the spec does not currently exist a new spec is created.  If the spec does exist then its contents are overridden by the text of this directive, unless the first character of that text is the + character, in which case the text is appended to the spec.

[SUFFIX]:
     Creates a new [SUFFIX] spec pair.  All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix.  When the compiler encounters an input file with the named suffix, it processes the spec string in order to work out how to compile that file.  For example:

          .ZZ:
          z-compile -input %i

     This says that any input file whose name ends in .ZZ should be passed to the program z-compile, which should be invoked with the command-line switch -input and with the result of performing the %i substitution.  (See below.)

     As an alternative to providing a spec string, the text following a suffix directive can be one of the following:

     @LANGUAGE
          This says that the suffix is an alias for a known LANGUAGE.  This is similar to using the -x command-line switch to GCC to specify a language explicitly.  For example:

               .ZZ:
               @c++

          Says that .ZZ files are, in fact, C++ source files.

     #NAME
          This causes an error messages saying:

               NAME compiler not installed on this system.

     GCC already has an extensive list of suffixes built into it. This directive adds an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it.  Spec files can override these strings or create their own.  Note that individual targets can also add their own spec strings to this list.

     asm          Options to pass to the assembler
     asm_final    Options to pass to the assembler post-processor
     cpp          Options to pass to the C preprocessor
     cc1          Options to pass to the C compiler
     cc1plus      Options to pass to the C++ compiler
     endfile      Object files to include at the end of the link
     link         Options to pass to the linker
     lib          Libraries to include on the command line to the linker
     libgcc       Decides which GCC support library to pass to the linker
     linker       Sets the name of the linker
     predefines   Defines to be passed to the C preprocessor
     signed_char  Defines to pass to CPP to say whether char is signed
                  by default
     startfile    Object files to include at the start of the link

Here is a small example of a spec file:

     %rename lib                 old_lib

     *lib:
     --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called lib to old_lib and then overrides the previous definition of lib with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program.  In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally insert text into the command line.  Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined %-sequences for spec strings.  Note that spaces are not generated automatically around the results of expanding these sequences.  Therefore you can concatenate them together or combine them with constant text in a single argument.

%%
     Substitute one % into the program name or argument.

%i
     Substitute the name of the input file being processed.

%b
     Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.

%B
     This is the same as %b, but include the file suffix (text after the last period).

%d
     Marks the argument containing or following the %d as a temporary file name, so that that file is deleted if GCC exits successfully.  Unlike %g, this contributes no text to the argument.

%gSUFFIX
     Substitute a file name that has suffix SUFFIX and is chosen once per compilation, and mark the argument in the same way as %d.  To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known.  For example, %g.s ... %g.o ... %g.s might turn into ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s.  SUFFIX matches the regexp [.A-Za-z]* or the special string %O, which is treated exactly as if %O had been preprocessed.  Previously, %g was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.

%uSUFFIX
     Like %g, but generates a new temporary file name each time it appears instead of once per compilation.

%USUFFIX
     Substitutes the last file name generated with %uSUFFIX, generating a new one if there is no such last file name.  In the absence of any %uSUFFIX, this is just like %gSUFFIX, except they don't share the same suffix space, so %g.s ... %U.s ... %g.s ... %U.s involves the generation of two distinct file names, one for each %g.s and another for each %U.s.  Previously, %U was simply substituted with a file name chosen for the previous %u, without regard to any appended suffix.

%jSUFFIX
     Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if -save-temps is not used; otherwise, substitute the name of a temporary file, just like %u.  This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.

%|SUFFIX
%mSUFFIX
     Like %g, except if -pipe is in effect.  In that case %| substitutes a single dash and %m substitutes nothing at all.  These are the two most common ways to instruct a program that it should read from standard input or write to standard output.  If you need something more elaborate you can use an %{pipe:X} construct: see for example f/lang-specs.h.

%.SUFFIX
     Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with %*.  SUFFIX is terminated by the next space or %.

%w
     Marks the argument containing or following the %w as the designated output file of this compilation.  This puts the argument into the sequence of arguments that %o substitutes.

%o
     Substitutes the names of all the output files, with spaces automatically placed around them.  You should write spaces around the %o as well or the results are undefined. %o is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they are linked.

%O
     Substitutes the suffix for object files.  Note that this is handled specially when it immediately follows %g, %u, or %U, because of the need for those to form complete file names.  The handling is such that %O is treated exactly as if it had already been substituted, except that %g, %u, and %U do not currently support additional SUFFIX characters following %O as they do following, for example, .o.

%p
     Substitutes the standard macro predefinitions for the current target machine.  Use this when running cpp.

%P
     Like %p, but puts __ before and after the name of each predefined macro, except for macros that start with __ or with _L, where L is an uppercase letter.  This is for ISO C.

%I
     Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from TARGET_SYSTEM_ROOT), -isystem (made from COMPILER_PATH and -B options) and -imultilib as necessary.

%s
     Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.  The current working directory is included in the list of directories scanned.

%T
     Current argument is the name of a linker script.  Search for that file in the current list of directories to scan for libraries. If the file is located insert a --script option into the command line followed by the full path name found.  If the file is not found then generate an error message.  Note: the current working directory is not searched.

%eSTR
     Print STR as an error message.  STR is terminated by a newline. Use this when inconsistent options are detected.

%(NAME)
     Substitute the contents of spec string NAME at this point.

%x{OPTION}
     Accumulate an option for %X.

%X
     Output the accumulated linker options specified by -Wl or a %x spec string.

%Y
     Output the accumulated assembler options specified by -Wa.

%Z
     Output the accumulated preprocessor options specified by -Wp.

%a
     Process the asm spec.  This is used to compute the switches to be passed to the assembler.

%A
     Process the asm_final spec.  This is a spec string for passing switches to an assembler post-processor, if such a program is needed.

%l
     Process the link spec.  This is the spec for computing the command line passed to the linker.  Typically it makes use of the %L %G %S %D and %E sequences.

%D
     Dump out a -L option for each directory that GCC believes might contain startup files.  If the target supports multilibs then the current multilib directory is prepended to each of these paths.

%L
     Process the lib spec.  This is a spec string for deciding which libraries are included on the command line to the linker.

%G
     Process the libgcc spec.  This is a spec string for deciding which GCC support library is included on the command line to the linker.

%S
     Process the startfile spec.  This is a spec for deciding which object files are the first ones passed to the linker.  Typically this might be a file named crt0.o.

%E
     Process the endfile spec.  This is a spec string that specifies the last object files that are passed to the linker.

%C
     Process the cpp spec.  This is used to construct the arguments to be passed to the C preprocessor.

%1
     Process the cc1 spec.  This is used to construct the options to be passed to the actual C compiler (cc1).

%2
     Process the cc1plus spec.  This is used to construct the options to be passed to the actual C++ compiler (cc1plus).

%*
     Substitute the variable part of a matched option.  See below. Note that each comma in the substituted string is replaced by a single space.

%<S
     Remove all occurrences of -S from the command line.  Note---this command is position dependent.  % commands in the spec string before this one see -S, % commands in the spec string after this one do not.

%:FUNCTION(ARGS)
     Call the named function FUNCTION, passing it ARGS. ARGS is first processed as a nested spec string, then split into an argument vector in the usual fashion.  The function returns a string which is processed as if it had appeared literally as part of the current spec.

     The following built-in spec functions are provided:

     getenv
          The getenv spec function takes two arguments: an environment variable name and a string.  If the environment variable is not defined, a fatal error is issued.  Otherwise, the return value is the value of the environment variable concatenated with the string.  For example, if TOPDIR is defined as /path/to/top, then:

               %:getenv(TOPDIR /include)

          expands to /path/to/top/include.

     if-exists
          The if-exists spec function takes one argument, an absolute pathname to a file.  If the file exists, if-exists returns the pathname.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s

     if-exists-else
          The if-exists-else spec function is similar to the if-exists spec function, except that it takes two arguments.  The first argument is an absolute pathname to a file.  If the file exists, if-exists-else returns the pathname.  If it does not exist, it returns the second argument. This way, if-exists-else can be used to select one file or another, based on the existence of the first.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) \
               %:if-exists-else(crtbeginT%O%s crtbegin%O%s)

     replace-outfile
          The replace-outfile spec function takes two arguments.  It looks for the first argument in the outfiles array and replaces it with the second argument.  Here is a small example of its usage:

               %{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}

     remove-outfile
          The remove-outfile spec function takes one argument.  It looks for the first argument in the outfiles array and removes it.  Here is a small example its usage:

               %:remove-outfile(-lm)

     pass-through-libs
          The pass-through-libs spec function takes any number of arguments.  It finds any -l options and any non-options ending in .a (which it assumes are the names of linker input library archive files) and returns a result containing all the found arguments each prepended by -plugin-opt=-pass-through= and joined by spaces.  This list is intended to be passed to the LTO linker plugin.

               %:pass-through-libs(%G %L %G)

     print-asm-header
          The print-asm-header function takes no arguments and simply prints a banner like:

               Assembler options
               =================

               Use "-Wa,OPTION" to pass "OPTION" to the assembler.

          It is used to separate compiler options from assembler options in the --target-help output.

%{S}
     Substitutes the -S switch, if that switch is given to GCC. If that switch is not specified, this substitutes nothing.  Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed.  Thus the spec string %{foo} matches the command-line option -foo and outputs the command-line option -foo.

%W{S}
     Like %{S} but mark last argument supplied within as a file to be deleted on failure.

%{S*}
     Substitutes all the switches specified to GCC whose names start with -S, but which also take an argument.  This is used for switches like -o, -D, -I, etc. GCC considers -o foo as being one switch whose name starts with o.  %{o*} substitutes this text, including the space.  Thus two arguments are generated.

%{S*&T*}
     Like %{S*}, but preserve order of S and T options (the order of S and T in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional.  Useful for CPP as %{D*&U*&A*}.

%{S:X}
     Substitutes X, if the -S switch is given to GCC.

%{!S:X}
     Substitutes X, if the -S switch is not given to GCC.

%{S*:X}
     Substitutes X if one or more switches whose names start with -S are specified to GCC.  Normally X is substituted only once, no matter how many such switches appeared.  However, if %* appears somewhere in X, then X is substituted once for each matching switch, with the %* replaced by the part of that switch matching the *.

     If %* appears as the last part of a spec sequence then a space is added after the end of the last substitution.  If there is more text in the sequence, however, then a space is not generated.  This allows the %* substitution to be used as part of a larger string.  For example, a spec string like this:

          %{mcu=*:--script=%*/memory.ld}

     when matching an option like -mcu=newchip produces:

          --script=newchip/memory.ld

%{.S:X}
     Substitutes X, if processing a file with suffix S.

%{!.S:X}
     Substitutes X, if not processing a file with suffix S.

%{,S:X}
     Substitutes X, if processing a file for language S.

%{!,S:X}
     Substitutes X, if not processing a file for language S.

%{S|P:X}
     Substitutes X if either -S or -P is given to GCC.  This may be combined with !, ., ,, and * sequences as well, although they have a stronger binding than the |.  If %* appears in X, all of the alternatives must be starred, and only the first matching alternative is substituted.

     For example, a spec string like this:

          %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}

     outputs the following command-line options from the following input command-line options:

          fred.c        -foo -baz
          jim.d         -bar -boggle
          -d fred.c     -foo -baz -boggle
          -d jim.d      -bar -baz -boggle

%{S:X; T:Y; :D}

     If S is given to GCC, substitutes X; else if T is given to GCC, substitutes Y; else substitutes D.  There can be as many clauses as you need.  This may be combined with ., ,, !, |, and * as needed.

The conditional text X in a %{S:X} or similar construct may contain other nested % constructs or spaces, or even newlines.  They are processed as usual, as described above. Trailing white space in X is ignored.  White space may also appear anywhere on the left side of the colon in these constructs, except between . or * and the corresponding word.

The -O, -f, -m, and -W switches are handled specifically in these constructs.  If another value of -O or the negated form of a -f, -m, or -W switch is found later in the command line, the earlier switch value is ignored, except with {S*} where S is just one letter, which passes all matching options.

The character | at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if -pipe is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments.  But this cannot be done in a consistent fashion.  GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.

Specifying Target Machine and Compiler Version
==============================================

The usual way to run GCC is to run the executable called gcc, or MACHINE-gcc when cross-compiling, or MACHINE-gcc-VERSION to run a version other than the one that was installed last.

Hardware Models and Configurations
==================================

Each target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none.  A single installed version of the compiler can compile for any model or configuration, according to the options specified.

Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.

AArch64 Options
---------------

These options are defined for AArch64 implementations:

-mabi=NAME
     Generate code for the specified data model.  Permissible values are ilp32 for SysV-like data model where int, long int and pointer are 32-bit, and lp64 for SysV-like data model where int is 32-bit, but long int and pointer are 64-bit.

     The default depends on the specific target configuration.  Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

-mbig-endian
     Generate big-endian code.  This is the default when GCC is configured for an aarch64_be-*-* target.

-mgeneral-regs-only
     Generate code which uses only the general-purpose registers.  This is equivalent to feature modifier nofp of -march or -mcpu, except that -mgeneral-regs-only takes precedence over any conflicting feature modifier regardless of sequence.

-mlittle-endian
     Generate little-endian code.  This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.

-mcmodel=tiny
     Generate code for the tiny code model.  The program and its statically defined symbols must be within 1GB of each other.  Pointers are 64 bits.  Programs can be statically or dynamically linked.  This model is not fully implemented and mostly treated as small.

-mcmodel=small
     Generate code for the small code model.  The program and its statically defined symbols must be within 4GB of each other.  Pointers are 64 bits.  Programs can be statically or dynamically linked.  This is the default code model.

-mcmodel=large
     Generate code for the large code model.  This makes no assumptions about addresses and sizes of sections.  Pointers are 64 bits.  Programs can be statically linked only.

-mstrict-align
     Do not assume that unaligned memory references are handled by the system.

-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
     Omit or keep the frame pointer in leaf functions.  The former behaviour is the default.

-mtls-dialect=desc
     Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.  This is the default.

-mtls-dialect=traditional
     Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.

-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
     Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.

-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
     Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.

-march=NAME
     Specify the name of the target architecture, optionally suffixed by one or more feature modifiers.  This option has the form -march=ARCH{+[no]FEATURE}*.

     The permissible values for ARCH are armv8-a or armv8.1-a.

     For the permissible values for FEATURE, see the sub-section on -march and -mcpu Feature Modifiers.  Where conflicting feature modifiers are specified, the right-most feature is used.

     Additionally on native AArch64 GNU/Linux systems the value native is available.  This option causes the compiler to pick the architecture of the host system.  If the compiler is unable to recognize the architecture of the host system this option has no effect.

     GCC uses NAME to determine what kind of instructions it can emit when generating assembly code.  If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.

-mtune=NAME
     Specify the name of the target processor for which GCC should tune the performance of the code.  Permissible values for this option are: generic, cortex-a53, cortex-a57, cortex-a72, exynos-m1, thunderx, xgene1.

     Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system.  Permissible values for this option are: cortex-a57.cortex-a53, cortex-a72.cortex-a53.

     Additionally on native AArch64 GNU/Linux systems the value native is available.  This option causes the compiler to pick the architecture of and tune the performance of the code for the processor of the host system.  If the compiler is unable to recognize the processor of the host system this option has no effect.

     Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

     This option cannot be suffixed by feature modifiers.

-mcpu=NAME
     Specify the name of the target processor, optionally suffixed by one or more feature modifiers.  This option has the form -mcpu=CPU{+[no]FEATURE}*, where the permissible values for CPU are the same as those available for -mtune.  The permissible values for FEATURE are documented in the sub-section on -march and -mcpu Feature Modifiers.  Where conflicting feature modifiers are specified, the right-most feature is used.

     Additionally on native AArch64 GNU/Linux systems the value native is available.  This option causes the compiler to tune the performance of the code for the processor of the host system.  If the compiler is unable to recognize the processor of the host system this option has no effect.

     GCC uses NAME to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune).  Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-moverride=STRING
     Override tuning decisions made by the back-end in response to a -mtune= switch.  The syntax, semantics, and accepted values for STRING in this option are not guaranteed to be consistent across releases.

     This option is only intended to be useful when developing GCC.

-march and -mcpu Feature Modifiers
..................................

Feature modifiers used with -march and -mcpu can be any of the following and their inverses noFEATURE:

crc
     Enable CRC extension.

crypto
     Enable Crypto extension.  This also enables Advanced SIMD and floating-point instructions.

fp
     Enable floating-point instructions.  This is on by default for all possible values for options -march and -mcpu.

simd
     Enable Advanced SIMD instructions.  This also enables floating-point instructions.  This is on by default for all possible values for options -march and -mcpu.

lse
     Enable Large System Extension instructions.

pan
     Enable Privileged Access Never support.

lor
     Enable Limited Ordering Regions support.

rdma
     Enable ARMv8.1 Advanced SIMD instructions.  This implies Advanced SIMD is enabled.

That is, crypto implies simd implies fp. Conversely, nofp (or equivalently, -mgeneral-regs-only) implies nosimd implies nocrypto.

Adapteva Epiphany Options
-------------------------

These -m options are defined for Adapteva Epiphany:

-mhalf-reg-file
     Don't allocate any register in the range r32...r63. That allows code to run on hardware variants that lack these registers.

-mprefer-short-insn-regs
     Preferrentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.

-mbranch-cost=NUM
     Set the cost of branches to roughly NUM ``simple'' instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.

-mcmove
     Enable the generation of conditional moves.

-mnops=NUM
     Emit NUM NOPs before every other generated instruction.

-mno-soft-cmpsf
     For single-precision floating-point comparisons, emit an fsub instruction and test the flags.  This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.

-mstack-offset=NUM
     Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=NUM option.

-mno-round-nearest
     Make the scheduler assume that the rounding mode has been set to truncating.  The default is -mround-nearest.

-mlong-calls
     If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.

-mshort-calls
     If not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls.  The default is -mlong-calls.

-msmall16
     Assume addresses can be loaded as 16-bit unsigned values.  This does not apply to function addresses for which -mlong-calls semantics are in effect.

-mfp-mode=MODE
     Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time.  Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches.

     MODE can be set to one the following values:

     caller
          Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.

     truncate
          This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode.  That includes conversion from floating point to integer.

     round-nearest
          This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.

     int
          This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

     The default is -mfp-mode=caller

-mnosplit-lohi
-mno-postinc
-mno-postmodify
     Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses.  The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.

-mnovect-double
     Change the preferred SIMD mode to SImode.  The default is -mvect-double, which uses DImode as preferred SIMD mode.

-max-vect-align=NUM
     The maximum alignment for SIMD vector mode types. NUM may be 4 or 8.  The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.

-msplit-vecmove-early
     Split vector moves into single word moves before reload.  In theory this can give better register allocation, but so far the reverse seems to be generally the case.

-m1reg-REG
     Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for REG are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose.  The default is -m1reg-none.

ARC Options
-----------

The following options control the architecture variant for which code is being compiled:

-mbarrel-shifter
     Generate instructions supported by barrel shifter.  This is the default unless -mcpu=ARC601 is in effect.

-mcpu=CPU
     Set architecture type, register usage, and instruction scheduling parameters for CPU.  There are also shortcut alias options available for backward compatibility and convenience.  Supported values for CPU are

     ARC600
          Compile for ARC600.  Aliases: -mA6, -mARC600.

     ARC601
          Compile for ARC601.  Alias: -mARC601.

     ARC700
          Compile for ARC700.  Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.

-mdpfp
-mdpfp-compact
     FPX: Generate Double Precision FPX instructions, tuned for the compact implementation.

-mdpfp-fast
     FPX: Generate Double Precision FPX instructions, tuned for the fast implementation.

-mno-dpfp-lrsr
     Disable LR and SR instructions from using FPX extension aux registers.

-mea
     Generate Extended arithmetic instructions.  Currently only divaw, adds, subs, and sat16 are supported.  This is always enabled for -mcpu=ARC700.

-mno-mpy
     Do not generate mpy instructions for ARC700.

-mmul32x16
     Generate 32x16 bit multiply and mac instructions.

-mmul64
     Generate mul64 and mulu64 instructions.  Only valid for -mcpu=ARC600.

-mnorm
     Generate norm instruction.  This is the default if -mcpu=ARC700 is in effect.

-mspfp
-mspfp-compact
     FPX: Generate Single Precision FPX instructions, tuned for the compact implementation.

-mspfp-fast
     FPX: Generate Single Precision FPX instructions, tuned for the fast implementation.

-msimd
     Enable generation of ARC SIMD instructions via target-specific builtins.  Only valid for -mcpu=ARC700.

-msoft-float
     This option ignored; it is provided for compatibility purposes only. Software floating point code is emitted by default, and this default can overridden by FPX options; mspfp, mspfp-compact, or mspfp-fast for single precision, and mdpfp, mdpfp-compact, or mdpfp-fast for double precision.

-mswap
     Generate swap instructions.

The following options are passed through to the assembler, and also define preprocessor macro symbols.

-mdsp-packa
     Passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol __Xdsp_packa.

-mdvbf
     Passed down to the assembler to enable the dual viterbi butterfly extension.  Also sets the preprocessor symbol __Xdvbf.

-mlock
     Passed down to the assembler to enable the Locked Load/Store Conditional extension.  Also sets the preprocessor symbol __Xlock.

-mmac-d16
     Passed down to the assembler.  Also sets the preprocessor symbol __Xxmac_d16.

-mmac-24
     Passed down to the assembler.  Also sets the preprocessor symbol __Xxmac_24.

-mrtsc
     Passed down to the assembler to enable the 64-bit Time-Stamp Counter extension instruction.  Also sets the preprocessor symbol __Xrtsc.

-mswape
     Passed down to the assembler to enable the swap byte ordering extension instruction.  Also sets the preprocessor symbol __Xswape.

-mtelephony
     Passed down to the assembler to enable dual and single operand instructions for telephony.  Also sets the preprocessor symbol __Xtelephony.

-mxy
     Passed down to the assembler to enable the XY Memory extension.  Also sets the preprocessor symbol __Xxy.

The following options control how the assembly code is annotated:

-misize
     Annotate assembler instructions with estimated addresses.

-mannotate-align
     Explain what alignment considerations lead to the decision to make an instruction short or long.

The following options are passed through to the linker:

-marclinux
     Passed through to the linker, to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.

-marclinux_prof
     Passed through to the linker, to specify use of the arclinux_prof emulation.  This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.

The following options control the semantics of generated code:

-mepilogue-cfi
     Enable generation of call frame information for epilogues.

-mno-epilogue-cfi
     Disable generation of call frame information for epilogues.

-mlong-calls
     Generate call insns as register indirect calls, thus providing access to the full 32-bit address range.

-mmedium-calls
     Don't use less than 25 bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction.  Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link.  This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.

-mno-sdata
     Do not generate sdata references.  This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.

-mucb-mcount
     Instrument with mcount calls as used in UCB code.  I.e. do the counting in the callee, not the caller.  By default ARC instrumentation counts in the caller.

-mvolatile-cache
     Use ordinarily cached memory accesses for volatile references.  This is the default.

-mno-volatile-cache
     Enable cache bypass for volatile references.

The following options fine tune code generation:

-malign-call
     Do alignment optimizations for call instructions.

-mauto-modify-reg
     Enable the use of pre/post modify with register displacement.

-mbbit-peephole
     Enable bbit peephole2.

-mno-brcc
     This option disables a target-specific pass in arc_reorg to generate BRcc instructions.  It has no effect on BRcc generation driven by the combiner pass.

-mcase-vector-pcrel
     Use pc-relative switch case tables - this enables case table shortening. This is the default for -Os.

-mcompact-casesi
     Enable compact casesi pattern. This is the default for -Os.

-mno-cond-exec
     Disable ARCompact specific pass to generate conditional execution instructions. Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generating opportunities after register allocation, branch shortening, and delay slot scheduling have been done.  This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead.

-mearly-cbranchsi
     Enable pre-reload use of the cbranchsi pattern.

-mexpand-adddi
     Expand adddi3 and subdi3 at rtl generation time into add.f, adc etc.

-mindexed-loads
     Enable the use of indexed loads.  This can be problematic because some optimizers then assume that indexed stores exist, which is not the case.

-mlra
     Enable Local Register Allocation.  This is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra).

-mlra-priority-none
     Don't indicate any priority for target registers.

-mlra-priority-compact
     Indicate target register priority for r0..r3 / r12..r15.

-mlra-priority-noncompact
     Reduce target regsiter priority for r0..r3 / r12..r15.

-mno-millicode
     When optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function in libgcc; this is referred to as a millicode call.  As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn off millicode call generation.

-mmixed-code
     Tweak register allocation to help 16-bit instruction generation. This generally has the effect of decreasing the average instruction size while increasing the instruction count.

-mq-class
     Enable 'q' instruction alternatives. This is the default for -Os.

-mRcq
     Enable Rcq constraint handling - most short code generation depends on this. This is the default.

-mRcw
     Enable Rcw constraint handling - ccfsm condexec mostly depends on this. This is the default.

-msize-level=LEVEL
     Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for LEVEL are:

     0
          No size optimization.  This level is deprecated and treated like 1.

     1
          Short instructions are used opportunistically.

     2
          In addition, alignment of loops and of code after barriers are dropped.

     3
          In addition, optional data alignment is dropped, and the option Os is enabled.

     This defaults to 3 when -Os is in effect.  Otherwise, the behavior when this is not set is equivalent to level 1.

-mtune=CPU
     Set instruction scheduling parameters for CPU, overriding any implied by -mcpu=.

     Supported values for CPU are

     ARC600
          Tune for ARC600 cpu.

     ARC601
          Tune for ARC601 cpu.

     ARC700
          Tune for ARC700 cpu with standard multiplier block.

     ARC700-xmac
          Tune for ARC700 cpu with XMAC block.

     ARC725D
          Tune for ARC725D cpu.

     ARC750D
          Tune for ARC750D cpu.

-mmultcost=NUM
     Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.

-munalign-prob-threshold=PROBABILITY
     Set probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability for the branch to be taken is below PROBABILITY.  See Cross-profiling. The default is (REG_BR_PROB_BASE/2), i.e. 5000.

The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release:

-margonaut
     Obsolete FPX.

-mbig-endian
-EB
     Compile code for big endian targets.  Use of these options is now deprecated.  Users wanting big-endian code, should use the arceb-elf32 and arceb-linux-uclibc targets when building the tool chain, for which big-endian is the default.

-mlittle-endian
-EL
     Compile code for little endian targets.  Use of these options is now deprecated.  Users wanting little-endian code should use the arc-elf32 and arc-linux-uclibc targets when building the tool chain, for which little-endian is the default.

-mbarrel_shifter
     Replaced by -mbarrel-shifter.

-mdpfp_compact
     Replaced by -mdpfp-compact.

-mdpfp_fast
     Replaced by -mdpfp-fast.

-mdsp_packa
     Replaced by -mdsp-packa.

-mEA
     Replaced by -mea.

-mmac_24
     Replaced by -mmac-24.

-mmac_d16
     Replaced by -mmac-d16.

-mspfp_compact
     Replaced by -mspfp-compact.

-mspfp_fast
     Replaced by -mspfp-fast.

-mtune=CPU
     Values arc600, arc601, arc700 and arc700-xmac for CPU are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively

-multcost=NUM
     Replaced by -mmultcost.

ARM Options
-----------

These -m options are defined for the ARM port:

-mabi=NAME
     Generate code for the specified ABI.  Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.

-mapcs-frame
     Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code.  Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions.  The default is -mno-apcs-frame. This option is deprecated.

-mapcs
     This is a synonym for -mapcs-frame and is deprecated.

-mthumb-interwork
     Generate code that supports calling between the ARM and Thumb instruction sets.  Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program.  The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified.  In AAPCS configurations this option is meaningless.

-mno-sched-prolog
     Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body.  This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code.  The default is -msched-prolog.

-mfloat-abi=NAME
     Specifies which floating-point ABI to use.  Permissible values are: soft, softfp and hard.

     Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

     The default depends on the specific target configuration.  Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

-mlittle-endian
     Generate code for a processor running in little-endian mode.  This is the default for all standard configurations.

-mbig-endian
     Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

-march=NAME
     This specifies the name of the target ARM architecture.  GCC uses this name to determine what kind of instructions it can emit when generating assembly code.  This option can be used in conjunction with or instead of the -mcpu= option.  Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5e, armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a, armv7-r, armv7-m, armv7e-m, armv7ve, armv8-a, armv8-a+crc, iwmmxt, iwmmxt2, ep9312.

     -march=armv7ve is the armv7-a architecture with virtualization extensions.

     -march=armv8-a+crc enables code generation for the ARMv8-A architecture together with the optional CRC32 extensions.

     -march=native causes the compiler to auto-detect the architecture of the build computer.  At present, this feature is only supported on GNU/Linux, and not all architectures are recognized.  If the auto-detect is unsuccessful the option has no effect.

-mtune=NAME
     This option specifies the name of the target ARM processor for which GCC should tune the performance of the code. For some ARM implementations better performance can be obtained by using this option. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a53, cortex-a57, cortex-a72, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-m7, cortex-m4, cortex-m3, cortex-m1, cortex-m0, cortex-m0plus, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.

     Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system.  Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53.

     -mtune=generic-ARCH specifies that GCC should tune the performance for a blend of processors within architecture ARCH. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs.  The effects of this option may change in future GCC versions as CPU models come and go.

     -mtune=native causes the compiler to auto-detect the CPU of the build computer.  At present, this feature is only supported on GNU/Linux, and not all architectures are recognized.  If the auto-detect is unsuccessful the option has no effect.

-mcpu=NAME
     This specifies the name of the target ARM processor.  GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune).  Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

     Permissible names for this option are the same as those for -mtune.

     -mcpu=generic-ARCH is also permissible, and is equivalent to -march=ARCH -mtune=generic-ARCH. See -mtune for more information.

     -mcpu=native causes the compiler to auto-detect the CPU of the build computer.  At present, this feature is only supported on GNU/Linux, and not all architectures are recognized.  If the auto-detect is unsuccessful the option has no effect.

-mfpu=NAME
     This specifies what floating-point hardware (or hardware emulation) is available on the target.  Permissible names are: vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8, and crypto-neon-fp-armv8.

     If -msoft-float is specified this specifies the format of floating-point values.

     If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified.  This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.

-mfp16-format=NAME
     Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined.  See Half-Precision, for more information.

-mstructure-size-boundary=N
     The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option.  Permissible values are 8, 32 and 64.  The default value varies for different toolchains.  For the COFF targeted toolchain the default value is 8.  A value of 64 is only allowed if the underlying ABI supports it.

     Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program.  Different values are potentially incompatible.  Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

-mabort-on-noreturn
     Generate a call to the function abort at the end of a noreturn function.  It is executed if the function tries to return.

-mlong-calls
-mno-long-calls
     Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register.  This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.

     Even if this switch is enabled, not all function calls are turned into long calls.  The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls.  The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.

     This feature is not enabled by default.  Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive.  Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

-msingle-pic-base
     Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function.  The runtime system is responsible for initializing this register with an appropriate value before execution begins.

-mpic-register=REG
     Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler.  For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.

-mpic-data-is-text-relative
     Assume that each data segments are relative to text segment at load time. Therefore, it permits addressing data using PC-relative operations. This option is on by default for targets other than VxWorks RTP.

-mpoke-function-name
     Write the name of each function into the text section, directly preceding the function prologue.  The generated code is similar to this:

               t0
                   .ascii "arm_poke_function_name", 0
                   .align
               t1
                   .word 0xff000000 + (t1 - t0)
               arm_poke_function_name
                   mov     ip, sp
                   stmfd   sp!, {fp, ip, lr, pc}
                   sub     fp, ip, #4

     When performing a stack backtrace, code can inspect the value of pc stored at fp + 0.  If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000).

-mthumb
-marm

     Select between generating code that executes in ARM and Thumb states.  The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=STATE configure option.

     You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes (see ARM Function Attributes) or pragmas (see Function Specific Option Pragmas).

-mtpcs-frame
     Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions.  (A leaf function is one that does not call any other functions.)  The default is -mno-tpcs-frame.

-mtpcs-leaf-frame
     Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions.  (A leaf function is one that does not call any other functions.)  The default is -mno-apcs-leaf-frame.

-mcallee-super-interworking
     Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function.  This allows these functions to be called from non-interworking code.  This option is not valid in AAPCS configurations because interworking is enabled by default.

-mcaller-super-interworking
     Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not.  There is a small overhead in the cost of executing a function pointer if this option is enabled.  This option is not valid in AAPCS configurations because interworking is enabled by default.

-mtp=NAME
     Specify the access model for the thread local storage pointer.  The valid models are soft, which generates calls to __aeabi_read_tp, cp15, which fetches the thread pointer from cp15 directly (supported in the arm6k architecture), and auto, which uses the best available method for the selected processor.  The default setting is auto.

-mtls-dialect=DIALECT
     Specify the dialect to use for accessing thread local storage.  Two DIALECTs are supported---gnu and gnu2.  The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models.  The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries.  The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support.  Initial and local exec TLS models are unaffected by this option and always use the original scheme.

-mword-relocations
     Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified.

-mfix-cortex-m3-ldrd
     Some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used.  This option avoids generating these instructions.  This option is enabled by default when -mcpu=cortex-m3 is specified.

-munaligned-access
-mno-unaligned-access
     Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned.  By default unaligned access is disabled for all pre-ARMv6 and all ARMv6-M architectures, and enabled for all other architectures.  If unaligned access is not enabled then words in packed data structures are accessed a byte at a time.

     The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option.  If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined.

-mneon-for-64bits
     Enables using Neon to handle scalar 64-bits operations. This is disabled by default since the cost of moving data from core registers to Neon is high.

-mslow-flash-data
     Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default.

-masm-syntax-unified
     Assume inline assembler is using unified asm syntax.  The default is currently off which implies divided syntax.  Currently this option is available only for Thumb1 and has no effect on ARM state and Thumb2. However, this may change in future releases of GCC.  Divided syntax should be considered deprecated.

-mrestrict-it
     Restricts generation of IT blocks to conform to the rules of ARMv8. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8 Thumb mode.

-mprint-tune-info
     Print CPU tuning information as comment in assembler file.  This is an option used only for regression testing of the compiler and not intended for ordinary use in compiling code.  This option is disabled by default.

AVR Options
-----------

These options are defined for AVR implementations:

-mmcu=MCU
     Specify Atmel AVR instruction set architectures (ISA) or MCU type.

     The default for this option is avr2.

     GCC supports the following AVR devices and ISAs:

          [file avr-mmcu.texi not included]

-maccumulate-args
     Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue.  Without this option, outgoing arguments are pushed before calling a function and popped afterwards.

     Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to smaller executables because arguments need not to be removed from the stack after such a function call.

     This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack like calls to printf-like functions.

-mbranch-cost=COST
     Set the branch costs for conditional branch instructions to COST.  Reasonable values for COST are small, non-negative integers. The default branch cost is 0.

-mcall-prologues
     Functions prologues/epilogues are expanded as calls to appropriate subroutines.  Code size is smaller.

-mint8
     Assume int to be 8-bit integer.  This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes.  Please note that this option does not conform to the C standards, but it results in smaller code size.

-mn-flash=NUM
     Assume that the flash memory has a size of NUM times 64 KiB.

-mno-interrupts
     Generated code is not compatible with hardware interrupts. Code size is smaller.

-mrelax
     Try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

     Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.

     Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.

-mrmw
     Assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.

-msp8
     Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.

     This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not.

-mstrict-X
     Use address register X in a way proposed by the hardware.  This means that X is only used in indirect, post-increment or pre-decrement addressing.

     Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register RN is performed as

          adiw r26, const   ; X += const
          ld   RN, X        ; RN = *X
          sbiw r26, const   ; X -= const

-mtiny-stack
     Only change the lower 8 bits of the stack pointer.

-nodevicelib
     Don't link against AVR-LibC's device specific library libdev.a.

-Waddr-space-convert
     Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.

EIND and Devices with More Than 128 Ki Bytes of Flash
.....................................................

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

  * The compiler never sets EIND.

  * The compiler uses EIND implicitely in EICALL/EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.

  * The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue.

  * For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.

  * Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message.

  * The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.

  * The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/).

  * It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

          #include <avr/io.h>

          static void
          __attribute__((section(".init3"),naked,used,no_instrument_function))
          init3_set_eind (void)
          {
            __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                            "out %i0,r24" :: "n" (&EIND) : "r24","memory");
          }

     The __trampolines_start symbol is defined in the linker script.

  * Stubs are generated automatically by the linker if the following two conditions are met:

       - The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

               LDI r24, lo8(gs(FUNC))
               LDI r25, hi8(gs(FUNC))

       - The final location of that label is in a code segment outside the segment where the stubs are located.

  * The compiler emits such gs modifiers for code labels in the following situations:

       - Taking address of a function or code label.

       - Computed goto.

       - If prologue-save function is used, see -mcall-prologues command-line option.

       - Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.

       - C and C++ constructors/destructors called during startup/shutdown.

       - If the tools hit a gs() modifier explained above.

  * Jumping to non-symbolic addresses like so is not supported:

          int main (void)
          {
              /* Call function at word address 0x2 */
              return ((int(*)(void)) 0x2)();
          }

     Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

          int main (void)
          {
              extern int func_4 (void);

              /* Call function at byte address 0x4 */
              return func_4();
          }

     and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script.

Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers
........................................................................

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers.  To access memory locations outside this 64 KiB range, the contentent of a RAMP register is used as high part of the address: The X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing.

  * The startup code initializes the RAMP special function registers with zero.

  * If a named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation.

  * If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.

  * If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.

  * RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access.

AVR Built-in Macros
...................

GCC defines several built-in macros so that the user code can test for the presence or absence of features.  Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

__AVR_ARCH__
     Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=MCU option. Possible values are:

     2, 25, 3, 31, 35, 4, 5, 51, 6

     for MCU=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6,

     respectively and

     100, 102, 104, 105, 106, 107

     for MCU=avrtiny, avrxmega2, avrxmega4, avrxmega5, avrxmega6, avrxmega7, respectively. If MCU specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.

__AVR_DEVICE__
     Setting -mmcu=DEVICE defines this built-in macro which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

     The built-in macros' names follow the scheme __AVR_DEVICE__ where DEVICE is the device name as from the AVR user manual. The difference between DEVICE in the built-in macro and DEVICE in -mmcu=DEVICE is that the latter is always lowercase.

     If DEVICE is not a device but only a core architecture like avr51, this macro is not defined.

__AVR_DEVICE_NAME__
     Setting -mmcu=DEVICE defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

     If DEVICE is not a device but only a core architecture like avr51, this macro is not defined.

__AVR_XMEGA__
     The device / architecture belongs to the XMEGA family of devices.

__AVR_HAVE_ELPM__
     The device has the ELPM instruction.

__AVR_HAVE_ELPMX__
     The device has the ELPM RN,Z and ELPM RN,Z+ instructions.

__AVR_HAVE_MOVW__
     The device has the MOVW instruction to perform 16-bit register-register moves.

__AVR_HAVE_LPMX__
     The device has the LPM RN,Z and LPM RN,Z+ instructions.

__AVR_HAVE_MUL__
     The device has a hardware multiplier.

__AVR_HAVE_JMP_CALL__
     The device has the JMP and CALL instructions. This is the case for devices with at least 16 KiB of program memory.

__AVR_HAVE_EIJMP_EICALL__
__AVR_3_BYTE_PC__
     The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.

__AVR_2_BYTE_PC__
     The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.

__AVR_HAVE_8BIT_SP__
__AVR_HAVE_16BIT_SP__
     The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.

__AVR_HAVE_SPH__
__AVR_SP8__
     The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.

__AVR_HAVE_RAMPD__
__AVR_HAVE_RAMPX__
__AVR_HAVE_RAMPY__
__AVR_HAVE_RAMPZ__
     The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.

__NO_INTERRUPTS__
     This macro reflects the -mno-interrupts command-line option.

__AVR_ERRATA_SKIP__
__AVR_ERRATA_SKIP_JMP_CALL__
     Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum.  Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.

__AVR_ISA_RMW__
     The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).

__AVR_SFR_OFFSET__=OFFSET
     Instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.

__WITH_AVRLIBC__
     The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.

Blackfin Options
----------------

-mcpu=CPU[-SIREVISION]
     Specifies the name of the target Blackfin processor.  Currently, CPU can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

     The optional SIREVISION specifies the silicon revision of the target Blackfin processor.  Any workarounds available for the targeted silicon revision are enabled.  If SIREVISION is none, no workarounds are enabled. If SIREVISION is any, all workarounds for the targeted processor are enabled.  The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision.  If SIREVISION is none, the __SILICON_REVISION__ is not defined.  If SIREVISION is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional SIREVISION is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.

     GCC defines a preprocessor macro for the specified CPU. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

     Without this option, bf532 is used as the processor by default.

     Note that support for bf561 is incomplete.  For bf561, only the preprocessor macro is defined.

-msim
     Specifies that the program will be run on the simulator.  This causes the simulator BSP provided by libgloss to be linked in.  This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.

-momit-leaf-frame-pointer
     Don't keep the frame pointer in a register for leaf functions.  This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions.  The option -fomit-frame-pointer removes the frame pointer for all functions, which might make debugging harder.

-mspecld-anomaly
     When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined.

-mno-specld-anomaly
     Don't generate extra code to prevent speculative loads from occurring.

-mcsync-anomaly
     When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.

-mno-csync-anomaly
     Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.

-mlow-64k
     When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.

-mno-low-64k
     Assume that the program is arbitrarily large.  This is the default.

-mstack-check-l1
     Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.

-mid-shared-library
     Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management.  This option implies -fPIC. With a bfin-elf target, this option implies -msim.

-mno-id-shared-library
     Generate code that doesn't assume ID-based shared libraries are being used. This is the default.

-mleaf-id-shared-library
     Generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries.  That allows the compiler to use faster code for jumps and calls.

-mno-leaf-id-shared-library
     Do not assume that the code being compiled won't link against any ID shared libraries.  Slower code is generated for jump and call insns.

-mshared-library-id=n
     Specifies the identification number of the ID-based shared library being compiled.  Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.

-msep-data
     Generate code that allows the data segment to be located in a different area of memory from the text segment.  This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.

-mno-sep-data
     Generate code that assumes that the data segment follows the text segment. This is the default.

-mlong-calls
-mno-long-calls
     Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register.  This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

     This feature is not enabled by default.  Specifying -mno-long-calls restores the default behavior.  Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

-mfast-fp
     Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.

-minline-plt
     Enable inlining of PLT entries in function calls to functions that are not known to bind locally.  It has no effect without -mfdpic.

-mmulticore
     Build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-SIREVISION].

     This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model.  Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main.

     If this option is not used, the single-core application programming model is used.

-mcorea
     Build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore.

-mcoreb
     Build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. This option can only be used in conjunction with -mmulticore.

-msdram
     Build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application.

-micplb
     Assume that ICPLBs are enabled at run time.  This has an effect on certain anomaly workarounds.  For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

C6X Options
-----------

-march=NAME
     This specifies the name of the target architecture.  GCC uses this name to determine what kind of instructions it can emit when generating assembly code.  Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.

-mbig-endian
     Generate code for a big-endian target.

-mlittle-endian
     Generate code for a little-endian target.  This is the default.

-msim
     Choose startup files and linker script suitable for the simulator.

-msdata=default
     Put small global and static data in the .neardata section, which is pointed to by register B14.  Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section.  Put small read-only data into the .rodata section.  The corresponding sections used for large pieces of data are .fardata, .far and .const.

-msdata=all
     Put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them.

-msdata=none
     Make no use of the sections reserved for small data, and use absolute addresses to access all data.  Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section.  Put all constant data into the .const section.

CRIS Options
------------

These options are defined specifically for the CRIS ports.

-march=ARCHITECTURE-TYPE
-mcpu=ARCHITECTURE-TYPE
     Generate code for the specified architecture.  The choices for ARCHITECTURE-TYPE are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.

-mtune=ARCHITECTURE-TYPE
     Tune to ARCHITECTURE-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions.  The choices for ARCHITECTURE-TYPE are the same as for -march=ARCHITECTURE-TYPE.

-mmax-stack-frame=N
     Warn when the stack frame of a function exceeds N bytes.

-metrax4
-metrax100
     The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.

-mmul-bug-workaround
-mno-mul-bug-workaround
     Work around a bug in the muls and mulu instructions for CPU models where it applies.  This option is active by default.

-mpdebug
     Enable CRIS-specific verbose debug-related information in the assembly code.  This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.

-mcc-init
     Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.

-mno-side-effects
     Do not emit instructions with side effects in addressing modes other than post-increment.

-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
     These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model.  The default is to arrange for 32-bit alignment.  ABI details such as structure layout are not affected by these options.

-m32-bit
-m16-bit
-m8-bit
     Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned.  The default is 32-bit alignment.

-mno-prologue-epilogue
-mprologue-epilogue
     With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code.  Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated.

-mno-gotplt
-mgotplt
     With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT.  The default is -mgotplt.

-melf
     Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.

-mlinux
     Legacy no-op option only recognized with the cris-axis-linux-gnu target.

-sim
     This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library.  Code, initialized data and zero-initialized data are allocated consecutively.

-sim2
     Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

CR16 Options
------------

These options are defined specifically for the CR16 ports.

-mmac
     Enable the use of multiply-accumulate instructions. Disabled by default.

-mcr16cplus
-mcr16c
     Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.

-msim
     Links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.

-mint32
     Choose integer type as 32-bit wide.

-mbit-ops
     Generates sbit/cbit instructions for bit manipulations.

-mdata-model=MODEL
     Choose a data model. The choices for MODEL are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model.

Darwin Options
--------------

These options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create ``fat'' object files; it creates an object file for the single architecture that GCC was built to target.  Apple's GCC on Darwin does create ``fat'' files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march.  The -force_cpusubtype_ALL option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch.  The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library).  The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.

-FDIR
     Add the framework directory DIR to the head of the list of directories to be searched for header files.  These directories are interleaved with those specified by -I options and are scanned in a left-to-right order.

     A framework directory is a directory with frameworks in it.  A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework.  The name of a framework is the name of this directory excluding the .framework.  Headers associated with the framework are found in one of those two directories, with Headers being searched first.  A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header.  Two subframeworks are siblings if they occur in the same framework.  A subframework should not have the same name as a framework; a warning is issued if this is violated.  Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this.  The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks.  An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.

-iframeworkDIR
     Like -F except the directory is a treated as a system directory.  The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via DIR.  This option is valid only for the C family of languages.

-gused
     Emit debugging information for symbols that are used.  For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.

-gfull
     Emit debugging information for all symbols and types.

-mmacosx-version-min=VERSION
     The earliest version of MacOS X that this executable will run on is VERSION.  Typical values of VERSION include 10.1, 10.2, and 10.3.9.

     If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.

-mkernel
     Enable kernel development mode.  The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable.  This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.

-mone-byte-bool
     Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.

     Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Using this switch may require recompiling all other modules in a program, including system libraries.  Use this switch to conform to a non-default data model.

-mfix-and-continue
-ffix-and-continue
-findirect-data
     Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs.  -findirect-data and -ffix-and-continue are provided for backwards compatibility.

-all_load
     Loads all members of static archive libraries. See man ld(1) for more information.

-arch_errors_fatal
     Cause the errors having to do with files that have the wrong architecture to be fatal.

-bind_at_load
     Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.

-bundle
     Produce a Mach-o bundle format file. See man ld(1) for more information.

-bundle_loader EXECUTABLE
     This option specifies the EXECUTABLE that will load the build output file being linked.  See man ld(1) for more information.

-dynamiclib
     When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.

-force_cpusubtype_ALL
     This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.

-allowable_client  CLIENT_NAME
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
     These options are passed to the Darwin linker.  The Darwin linker man page describes them in detail.

DEC Alpha Options
-----------------

These -m options are defined for the DEC Alpha implementations:

-mno-soft-float
-msoft-float
     Use (do not use) the hardware floating-point instructions for floating-point operations.  When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations.  Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations.   If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

     Note that Alpha implementations without floating-point operations are required to have floating-point registers.

-mfp-reg
-mno-fp-regs
     Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float.  If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0.  This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

     A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

-mieee
     The Alpha architecture implements floating-point hardware optimized for maximum performance.  It is mostly compliant with the IEEE floating-point standard.  However, for full compliance, software assistance is required.  This option generates code fully IEEE-compliant code except that the INEXACT-FLAG is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation.  The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity.  Other Alpha compilers call this option -ieee_with_no_inexact.

-mieee-with-inexact
     This is like -mieee except the generated code also maintains the IEEE INEXACT-FLAG.  Turning on this option causes the generated code to implement fully-compliant IEEE math.  In addition to _IEEE_FP, _IEEE_FP_EXACT is defined as a preprocessor macro.  On some Alpha implementations the resulting code may execute significantly slower than the code generated by default.  Since there is very little code that depends on the INEXACT-FLAG, you should normally not specify this option.  Other Alpha compilers call this option -ieee_with_inexact.

-mfp-trap-mode=TRAP-MODE
     This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm TRAP-MODE. The trap mode can be set to one of four values:

     n
          This is the default (normal) setting.  The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).

     u
          In addition to the traps enabled by n, underflow traps are enabled as well.

     su
          Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).

     sui
          Like su, but inexact traps are enabled as well.

-mfp-rounding-mode=ROUNDING-MODE
     Selects the IEEE rounding mode.  Other Alpha compilers call this option -fprm ROUNDING-MODE.  The ROUNDING-MODE can be one of:

     n
          Normal IEEE rounding mode.  Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.

     m
          Round towards minus infinity.

     c
          Chopped rounding mode.  Floating-point numbers are rounded towards zero.

     d
          Dynamic rounding mode.  A field in the floating-point control register (FPCR, see Alpha architecture reference manual) controls the rounding mode in effect.  The C library initializes this register for rounding towards plus infinity.  Thus, unless your program modifies the FPCR, d corresponds to round towards plus infinity.

-mtrap-precision=TRAP-PRECISION
     In the Alpha architecture, floating-point traps are imprecise.  This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:

     p
          Program precision.  This option is the default and means a trap handler can only identify which program caused a floating-point exception.

     f
          Function precision.  The trap handler can determine the function that caused a floating-point exception.

     i
          Instruction precision.  The trap handler can determine the exact instruction that caused a floating-point exception.

     Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

-mieee-conformant
     This option marks the generated code as IEEE conformant.  You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui.  Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.

-mbuild-constants
     Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions.  If it cannot, it outputs the constant as a literal and generates code to load it from the data segment at run time.

     Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

     You typically use this option to build a shared library dynamic loader.  Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
     Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets.  The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none is specified.

-mfloat-vax
-mfloat-ieee
     Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.

-mexplicit-relocs
-mno-explicit-relocs
     Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros.  Use of these macros does not allow optimal instruction scheduling.  GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions.  This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.

-msmall-data
-mlarge-data
     When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations.  When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register.  This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

     The default is -mlarge-data.  With this option the data area is limited to just below 2GB.  Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

     When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

-msmall-text
-mlarge-text
     When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction.  When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.

     The default is -mlarge-text.

-mcpu=CPU_TYPE
     Set the instruction set and instruction scheduling parameters for machine type CPU_TYPE.  You can specify either the EV style name or the corresponding chip number.  GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify.  If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.

     Supported values for CPU_TYPE are

     ev4
     ev45
     21064
          Schedules as an EV4 and has no instruction set extensions.

     ev5
     21164
          Schedules as an EV5 and has no instruction set extensions.

     ev56
     21164a
          Schedules as an EV5 and supports the BWX extension.

     pca56
     21164pc
     21164PC
          Schedules as an EV5 and supports the BWX and MAX extensions.

     ev6
     21264
          Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.

     ev67
     21264a
          Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

     Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

-mtune=CPU_TYPE
     Set only the instruction scheduling parameters for machine type CPU_TYPE.  The instruction set is not changed.

     Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

-mmemory-latency=TIME
     Sets the latency the scheduler should assume for typical memory references as seen by the application.  This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

     Valid options for TIME are

     NUMBER
          A decimal number representing clock cycles.

     L1
     L2
     L3
     main
          The compiler contains estimates of the number of clock cycles for ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

FR30 Options
------------

These options are defined specifically for the FR30 port.

-msmall-model
     Use the small address space model.  This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.

-mno-lsim
     Assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

FT32 Options
------------

These options are defined specifically for the FT32 port.

-msim
     Specifies that the program will be run on the simulator.  This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.

-mlra
     Enable Local Register Allocation.  This is still experimental for FT32, so by default the compiler uses standard reload.

FRV Options
-----------

-mgpr-32

     Only use the first 32 general-purpose registers.

-mgpr-64

     Use all 64 general-purpose registers.

-mfpr-32

     Use only the first 32 floating-point registers.

-mfpr-64

     Use all 64 floating-point registers.

-mhard-float

     Use hardware instructions for floating-point operations.

-msoft-float

     Use library routines for floating-point operations.

-malloc-cc

     Dynamically allocate condition code registers.

-mfixed-cc

     Do not try to dynamically allocate condition code registers, only use icc0 and fcc0.

-mdword

     Change ABI to use double word insns.

-mno-dword

     Do not use double word instructions.

-mdouble

     Use floating-point double instructions.

-mno-double

     Do not use floating-point double instructions.

-mmedia

     Use media instructions.

-mno-media

     Do not use media instructions.

-mmuladd

     Use multiply and add/subtract instructions.

-mno-muladd

     Do not use multiply and add/subtract instructions.

-mfdpic

     Select the FDPIC ABI, which uses function descriptors to represent pointers to functions.  Without any PIC/PIE-related options, it implies -fPIE.  With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.

-minline-plt

     Enable inlining of PLT entries in function calls to functions that are not known to bind locally.  It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.

-mTLS

     Assume a large TLS segment when generating thread-local code.

-mtls

     Do not assume a large TLS segment when generating thread-local code.

-mgprel-ro

     Enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections.  It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win.  If it is not, -mno-gprel-ro can be used to disable it.

-multilib-library-pic

     Link with the (library, not FD) pic libraries.  It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic.  You should never have to use it explicitly.

-mlinked-fp

     Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated.  This option is enabled by default and can be disabled with -mno-linked-fp.

-mlong-calls

     Use indirect addressing to call functions outside the current compilation unit.  This allows the functions to be placed anywhere within the 32-bit address space.

-malign-labels

     Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet.  This option only has an effect when VLIW packing is enabled.  It doesn't create new packets; it merely adds NOPs to existing ones.

-mlibrary-pic

     Generate position-independent EABI code.

-macc-4

     Use only the first four media accumulator registers.

-macc-8

     Use all eight media accumulator registers.

-mpack

     Pack VLIW instructions.

-mno-pack

     Do not pack VLIW instructions.

-mno-eflags

     Do not mark ABI switches in e_flags.

-mcond-move

     Enable the use of conditional-move instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-cond-move

     Disable the use of conditional-move instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mscc

     Enable the use of conditional set instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-scc

     Disable the use of conditional set instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mcond-exec

     Enable the use of conditional execution (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-cond-exec

     Disable the use of conditional execution.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mvliw-branch

     Run a pass to pack branches into VLIW instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-vliw-branch

     Do not run a pass to pack branches into VLIW instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mmulti-cond-exec

     Enable optimization of && and || in conditional execution (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-multi-cond-exec

     Disable optimization of && and || in conditional execution.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mnested-cond-exec

     Enable nested conditional execution optimizations (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-nested-cond-exec

     Disable nested conditional execution optimizations.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-moptimize-membar

     This switch removes redundant membar instructions from the compiler-generated code.  It is enabled by default.

-mno-optimize-membar

     This switch disables the automatic removal of redundant membar instructions from the generated code.

-mtomcat-stats

     Cause gas to print out tomcat statistics.

-mcpu=CPU

     Select the processor type for which to generate code.  Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

GNU/Linux Options
-----------------

These -m options are defined for GNU/Linux targets:

-mglibc
     Use the GNU C library.  This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.

-muclibc
     Use uClibc C library.  This is the default on *-*-linux-*uclibc* targets.

-mmusl
     Use the musl C library.  This is the default on *-*-linux-*musl* targets.

-mbionic
     Use Bionic C library.  This is the default on *-*-linux-*android* targets.

-mandroid
     Compile code compatible with Android platform.  This is the default on *-*-linux-*android* targets.

     When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.  When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined.

-tno-android-cc
     Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.

-tno-android-ld
     Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

H8/300 Options
--------------

These -m options are defined for the H8/300 implementations:

-mrelax
     Shorten some address references at link time, when possible; uses the linker option -relax.  See ld and the H8/300 (Using ld), for a fuller description.

-mh
     Generate code for the H8/300H.

-ms
     Generate code for the H8S.

-mn
     Generate code for the H8S and H8/300H in the normal mode.  This switch must be used either with -mh or -ms.

-ms2600
     Generate code for the H8S/2600.  This switch must be used with -ms.

-mexr
     Extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.

-mno-exr
     Extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.

-mint32
     Make int data 32 bits by default.

-malign-300
     On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

HPPA Options
------------

These -m options are defined for the HPPA family of computers:

-march=ARCHITECTURE-TYPE
     Generate code for the specified architecture.  The choices for ARCHITECTURE-TYPE are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors.  Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine.  Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around.

-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
     Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

-mjump-in-delay
     This option is ignored and provided for compatibility purposes only.

-mdisable-fpregs
     Prevent floating-point registers from being used in any manner.  This is necessary for compiling kernels that perform lazy context switching of floating-point registers.  If you use this option and attempt to perform floating-point operations, the compiler aborts.

-mdisable-indexing
     Prevent the compiler from using indexing address modes.  This avoids some rather obscure problems when compiling MIG generated code under MACH.

-mno-space-regs
     Generate code that assumes the target has no space registers.  This allows GCC to generate faster indirect calls and use unscaled index address modes.

     Such code is suitable for level 0 PA systems and kernels.

-mfast-indirect-calls
     Generate code that assumes calls never cross space boundaries.  This allows GCC to emit code that performs faster indirect calls.

     This option does not work in the presence of shared libraries or nested functions.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mlong-load-store
     Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker.  This is equivalent to the +k option to the HP compilers.

-mportable-runtime
     Use the portable calling conventions proposed by HP for ELF systems.

-mgas
     Enable the use of assembler directives only GAS understands.

-mschedule=CPU-TYPE
     Schedule code according to the constraints for the machine type CPU-TYPE.  The choices for CPU-TYPE are 700 7100, 7100LC, 7200, 7300 and 8000.  Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine.  The default scheduling is 8000.

-mlinker-opt
     Enable the optimization pass in the HP-UX linker.  Note this makes symbolic debugging impossible.  It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets.  Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-msio
     Generate the predefine, _SIO, for server IO.  The default is -mwsio.  This generates the predefines, __hp9000s700, __hp9000s700__ and _WSIO, for workstation IO.  These options are available under HP-UX and HI-UX.

-mgnu-ld
     Use options specific to GNU ld. This passes -shared to ld when building a shared library.  It is the default when GCC is configured, explicitly or implicitly, with the GNU linker.  This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH.  The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.  This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.

-mhp-ld
     Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links.  It is the default when GCC is configured, explicitly or implicitly, with the HP linker.  This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH.  The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.  This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.

-mlong-calls
     Generate code that uses long call sequences.  This ensures that a call is always able to reach linker generated stubs.  The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used.  The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures.  Sibcalls are always limited at 240,000 bytes.

     Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

     It is normally not desirable to use this option as it degrades performance.  However, it may be useful in large applications, particularly when partial linking is used to build the application.

     The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated.  The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

-munix=UNIX-STD
     Generate compiler predefines and select a startfile for the specified UNIX standard.  The choices for UNIX-STD are 93, 95 and 98.  93 is supported on all HP-UX versions.  95 is available on HP-UX 10.10 and later.  98 is available on HP-UX 11.11 and later.  The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.

     -munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED, and the startfile unix95.o. -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o.

     It is important to note that this option changes the interfaces for various library routines.  It also affects the operational behavior of the C library.  Thus, extreme care is needed in using this option.

     Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate.  Most GNU software doesn't provide this capability.

-nolibdld
     Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.

-static
     The HP-UX implementation of setlocale in libc has a dependency on libdld.sl.  There isn't an archive version of libdld.sl.  Thus, when the -static option is specified, special link options are needed to resolve this dependency.

     On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic.  On the 64-bit port, the linkers generate dynamic binaries by default in any case.  The -nolibdld option can be used to prevent the GCC driver from adding these link options.

-threads
     Add support for multithreading with the dce thread library under HP-UX.  This option sets flags for both the preprocessor and linker.

IA-64 Options
-------------

These are the -m options defined for the Intel IA-64 architecture.

-mbig-endian
     Generate code for a big-endian target.  This is the default for HP-UX.

-mlittle-endian
     Generate code for a little-endian target.  This is the default for AIX5 and GNU/Linux.

-mgnu-as
-mno-gnu-as
     Generate (or don't) code for the GNU assembler.  This is the default.

-mgnu-ld
-mno-gnu-ld
     Generate (or don't) code for the GNU linker.  This is the default.

-mno-pic
     Generate code that does not use a global pointer register.  The result is not position independent code, and violates the IA-64 ABI.

-mvolatile-asm-stop
-mno-volatile-asm-stop
     Generate (or don't) a stop bit immediately before and after volatile asm statements.

-mregister-names
-mno-register-names
     Generate (or don't) in, loc, and out register names for the stacked registers.  This may make assembler output more readable.

-mno-sdata
-msdata
     Disable (or enable) optimizations that use the small data section.  This may be useful for working around optimizer bugs.

-mconstant-gp
     Generate code that uses a single constant global pointer value.  This is useful when compiling kernel code.

-mauto-pic
     Generate code that is self-relocatable.  This implies -mconstant-gp. This is useful when compiling firmware code.

-minline-float-divide-min-latency
     Generate code for inline divides of floating-point values using the minimum latency algorithm.

-minline-float-divide-max-throughput
     Generate code for inline divides of floating-point values using the maximum throughput algorithm.

-mno-inline-float-divide
     Do not generate inline code for divides of floating-point values.

-minline-int-divide-min-latency
     Generate code for inline divides of integer values using the minimum latency algorithm.

-minline-int-divide-max-throughput
     Generate code for inline divides of integer values using the maximum throughput algorithm.

-mno-inline-int-divide
     Do not generate inline code for divides of integer values.

-minline-sqrt-min-latency
     Generate code for inline square roots using the minimum latency algorithm.

-minline-sqrt-max-throughput
     Generate code for inline square roots using the maximum throughput algorithm.

-mno-inline-sqrt
     Do not generate inline code for sqrt.

-mfused-madd
-mno-fused-madd
     Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions.  The default is to use these instructions.

-mno-dwarf2-asm
-mdwarf2-asm
     Don't (or do) generate assembler code for the DWARF 2 line number debugging info.  This may be useful when not using the GNU assembler.

-mearly-stop-bits
-mno-early-stop-bits
     Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit.  This can improve instruction scheduling, but does not always do so.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mtls-size=TLS-SIZE
     Specify bit size of immediate TLS offsets.  Valid values are 14, 22, and 64.

-mtune=CPU-TYPE
     Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.

-milp32
-mlp64
     Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.  These are HP-UX specific flags.

-mno-sched-br-data-spec
-msched-br-data-spec
     (Dis/En)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default is 'disable'.

-msched-ar-data-spec
-mno-sched-ar-data-spec
     (En/Dis)able data speculative scheduling after reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default is 'enable'.

-mno-sched-control-spec
-msched-control-spec
     (Dis/En)able control speculative scheduling.  This feature is available only during region scheduling (i.e. before reload). This results in generation of the ld.s instructions and the corresponding check instructions chk.s. The default is 'disable'.

-msched-br-in-data-spec
-mno-sched-br-in-data-spec
     (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default is 'enable'.

-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
     (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default is 'enable'.

-msched-in-control-spec
-mno-sched-in-control-spec
     (En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default is 'enable'.

-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
     If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment.  This makes the use of the data speculation much more conservative. The default is 'disable'.

-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
     If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment.  This makes the use of the control speculation much more conservative. The default is 'disable'.

-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
     If enabled, speculative dependencies are considered during computation of the instructions priorities.  This makes the use of the speculation a bit more conservative. The default is 'disable'.

-msched-spec-ldc
     Use a simple data speculation check.  This option is on by default.

-msched-control-spec-ldc
     Use a simple check for control speculation.  This option is on by default.

-msched-stop-bits-after-every-cycle
     Place a stop bit after every cycle when scheduling.  This option is on by default.

-msched-fp-mem-deps-zero-cost
     Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group.  This option is disabled by default.

-msel-sched-dont-check-control-spec
     Generate checks for control speculation in selective scheduling. This flag is disabled by default.

-msched-max-memory-insns=MAX-INSNS
     Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.

-msched-max-memory-insns-hard-limit
     Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is ``soft'', meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.

LM32 Options
------------

These -m options are defined for the LatticeMico32 architecture:

-mbarrel-shift-enabled
     Enable barrel-shift instructions.

-mdivide-enabled
     Enable divide and modulus instructions.

-mmultiply-enabled
     Enable multiply instructions.

-msign-extend-enabled
     Enable sign extend instructions.

-muser-enabled
     Enable user-defined instructions.

M32C Options
------------

-mcpu=NAME
     Select the CPU for which code is generated.  NAME may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.

-msim
     Specifies that the program will be run on the simulator.  This causes an alternate runtime library to be linked in which supports, for example, file I/O.  You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.

-memregs=NUMBER
     Specifies the number of memory-based pseudo-registers GCC uses during code generation.  These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers.  Note that all modules in a program must be compiled with the same value for this option.  Because of that, you must not use this option with GCC's default runtime libraries.

M32R/D Options
--------------

These -m options are defined for Renesas M32R/D architectures:

-m32r2
     Generate code for the M32R/2.

-m32rx
     Generate code for the M32R/X.

-m32r
     Generate code for the M32R.  This is the default.

-mmodel=small
     Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

     The addressability of a particular object can be set with the model attribute.

-mmodel=medium
     Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.

-mmodel=large
     Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence).

-msdata=none
     Disable use of the small data area.  Variables are put into one of .data, .bss, or .rodata (unless the section attribute has been specified). This is the default.

     The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

-msdata=sdata
     Put small global and static data in the small data area, but do not generate special code to reference them.

-msdata=use
     Put small global and static data in the small data area, and generate special instructions to reference them.

-G NUM
     Put global and static objects less than or equal to NUM bytes into the small data or BSS sections instead of the normal data or BSS sections.  The default value of NUM is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

     All modules should be compiled with the same -G NUM value. Compiling with different values of NUM may or may not work; if it doesn't the linker gives an error message---incorrect code is not generated.

-mdebug
     Makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.

-malign-loops
     Align all loops to a 32-byte boundary.

-mno-align-loops
     Do not enforce a 32-byte alignment for loops.  This is the default.

-missue-rate=NUMBER
     Issue NUMBER instructions per cycle.  NUMBER can only be 1 or 2.

-mbranch-cost=NUMBER
     NUMBER can only be 1 or 2.  If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies.

-mflush-trap=NUMBER
     Specifies the trap number to use to flush the cache.  The default is 12.  Valid numbers are between 0 and 15 inclusive.

-mno-flush-trap
     Specifies that the cache cannot be flushed by using a trap.

-mflush-func=NAME
     Specifies the name of the operating system function to call to flush the cache.  The default is _flush_cache, but a function call is only used if a trap is not available.

-mno-flush-func
     Indicates that there is no OS function for flushing the cache.

M680x0 Options
--------------

These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.

-march=ARCH
     Generate code for a specific M680x0 or ColdFire instruction set architecture.  Permissible values of ARCH for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32.  ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.

     GCC defines a macro __mcfARCH__ whenever it is generating code for a ColdFire target.  The ARCH in this macro is one of the -march arguments given above.

     When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture.

-mcpu=CPU
     Generate code for a specific M680x0 or ColdFire processor. The M680x0 CPUs are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32.  The ColdFire CPUs are given by the table below, which also classifies the CPUs into families:

     Family   -mcpu arguments
     51   51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
     5206   5202 5204 5206
     5206e   5206e
     5208   5207 5208
     5211a   5210a 5211a
     5213   5211 5212 5213
     5216   5214 5216
     52235   52230 52231 52232 52233 52234 52235
     5225   5224 5225
     52259   52252 52254 52255 52256 52258 52259
     5235   5232 5233 5234 5235 523x
     5249   5249
     5250   5250
     5271   5270 5271
     5272   5272
     5275   5274 5275
     5282   5280 5281 5282 528x
     53017   53011 53012 53013 53014 53015 53016 53017
     5307   5307
     5329   5327 5328 5329 532x
     5373   5372 5373 537x
     5407   5407
     5475   5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

     -mcpu=CPU overrides -march=ARCH if ARCH is compatible with CPU.  Other combinations of -mcpu and -march are rejected.

     GCC defines the macro __mcf_cpu_CPU when ColdFire target CPU is selected.  It also defines __mcf_family_FAMILY, where the value of FAMILY is given by the table above.

-mtune=TUNE
     Tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32.  The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

     You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well.  These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.

     GCC defines the macros __mcARCH and __mcARCH__ when tuning for 680x0 architecture ARCH.  It also defines mcARCH unless either -ansi or a non-GNU -std option is used.  If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.

     GCC also defines the macro __mUARCH__ when tuning for ColdFire microarchitecture UARCH, where UARCH is one of the arguments given above.

-m68000
-mc68000
     Generate output for a 68000.  This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000.

     Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68010
     Generate output for a 68010.  This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010.

-m68020
-mc68020
     Generate output for a 68020.  This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020.

-m68030
     Generate output for a 68030.  This is the default when the compiler is configured for 68030-based systems.  It is equivalent to -march=68030.

-m68040
     Generate output for a 68040.  This is the default when the compiler is configured for 68040-based systems.  It is equivalent to -march=68040.

     This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040.  Use this option if your 68040 does not have code to emulate those instructions.

-m68060
     Generate output for a 68060.  This is the default when the compiler is configured for 68060-based systems.  It is equivalent to -march=68060.

     This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060.  Use this option if your 68060 does not have code to emulate those instructions.

-mcpu32
     Generate output for a CPU32.  This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32.

     Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200
     Generate output for a 520X ColdFire CPU.  This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.

     Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

-m5206e
     Generate output for a 5206e ColdFire CPU.  The option is now deprecated in favor of the equivalent -mcpu=5206e.

-m528x
     Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.

-m5307
     Generate output for a ColdFire 5307 CPU.  The option is now deprecated in favor of the equivalent -mcpu=5307.

-m5407
     Generate output for a ColdFire 5407 CPU.  The option is now deprecated in favor of the equivalent -mcpu=5407.

-mcfv4e
     Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.

-m68020-40
     Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.  The generated code does use the 68881 instructions that are emulated on the 68040.

     The option is equivalent to -march=68020 -mtune=68020-40.

-m68020-60
     Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.  The generated code does use the 68881 instructions that are emulated on the 68060.

     The option is equivalent to -march=68020 -mtune=68020-60.

-mhard-float
-m68881
     Generate floating-point instructions.  This is the default for 68020 and above, and for ColdFire devices that have an FPU.  It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets.

-msoft-float
     Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets.  It is also the default for ColdFire devices that have no FPU.

-mdiv
-mno-div
     Generate (do not generate) ColdFire hardware divide and remainder instructions.  If -march is used without -mcpu, the default is ``on'' for ColdFire architectures and ``off'' for M680x0 architectures.  Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu).  For example, the default is ``off'' for -mcpu=5206 and ``on'' for -mcpu=5206e.

     GCC defines the macro __mcfhwdiv__ when this option is enabled.

-mshort
     Consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.

-mno-short
     Do not consider type int to be 16 bits wide.  This is the default.

-mnobitfield
-mno-bitfield
     Do not use the bit-field instructions.  The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.

-mbitfield
     Do use the bit-field instructions.  The -m68020 option implies -mbitfield.  This is the default if you use a configuration designed for a 68020.

-mrtd
     Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning.  This saves one instruction in the caller since there is no need to pop the arguments there.

     This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

     In addition, seriously incorrect code results if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

     The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

-mno-rtd
     Do not use the calling conventions selected by -mrtd. This is the default.

-malign-int
-mno-align-int
     Control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

     Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k.

-mpcrel
     Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table.  At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing.  -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

-mno-strict-align
-mstrict-align
     Do not (do) assume that unaligned memory references are handled by the system.

-msep-data
     Generate code that allows the data segment to be located in a different area of memory from the text segment.  This allows for execute-in-place in an environment without virtual memory management.  This option implies -fPIC.

-mno-sep-data
     Generate code that assumes that the data segment follows the text segment. This is the default.

-mid-shared-library
     Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management.  This option implies -fPIC.

-mno-id-shared-library
     Generate code that doesn't assume ID-based shared libraries are being used. This is the default.

-mshared-library-id=n
     Specifies the identification number of the ID-based shared library being compiled.  Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.

-mxgot
-mno-xgot
     When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries.  This code is larger and slower than code generated without this option.  On M680x0 processors, this option is not needed; -fPIC suffices.

     GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k.  Anything larger causes the linker to report an error such as:

          relocation truncated to fit: R_68K_GOT16O foobar

     If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs.  However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol.

     Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries.  If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries.  Very few do.

     These options have no effect unless GCC is generating position-independent code.

MCore Options
-------------

These are the -m options defined for the Motorola M*Core processors.

-mhardlit
-mno-hardlit
     Inline constants into the code stream if it can be done in two instructions or less.

-mdiv
-mno-div
     Use the divide instruction.  (Enabled by default).

-mrelax-immediate
-mno-relax-immediate
     Allow arbitrary-sized immediates in bit operations.

-mwide-bitfields
-mno-wide-bitfields
     Always treat bit-fields as int-sized.

-m4byte-functions
-mno-4byte-functions
     Force all functions to be aligned to a 4-byte boundary.

-mcallgraph-data
-mno-callgraph-data
     Emit callgraph information.

-mslow-bytes
-mno-slow-bytes
     Prefer word access when reading byte quantities.

-mlittle-endian
-mbig-endian
     Generate code for a little-endian target.

-m210
-m340
     Generate code for the 210 processor.

-mno-lsim
     Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.

-mstack-increment=SIZE
     Set the maximum amount for a single stack increment operation.  Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much.  The default value is 0x1000.

MeP Options
-----------

-mabsdiff
     Enables the abs instruction, which is the absolute difference between two registers.

-mall-opts
     Enables all the optional instructions---average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation.

-maverage
     Enables the ave instruction, which computes the average of two registers.

-mbased=N
     Variables of size N bytes or smaller are placed in the .based section by default.  Based variables use the $tp register as a base register, and there is a 128-byte limit to the .based section.

-mbitops
     Enables the bit operation instructions---bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas).

-mc=NAME
     Selects which section constant data is placed in.  NAME may be tiny, near, or far.

-mclip
     Enables the clip instruction.  Note that -mclip is not useful unless you also provide -mminmax.

-mconfig=NAME
     Selects one of the built-in core configurations.  Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals.  The MeP-Integrator tool, not part of GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options.  The default configuration is default.

-mcop
     Enables the coprocessor instructions.  By default, this is a 32-bit coprocessor.  Note that the coprocessor is normally enabled via the -mconfig= option.

-mcop32
     Enables the 32-bit coprocessor's instructions.

-mcop64
     Enables the 64-bit coprocessor's instructions.

-mivc2
     Enables IVC2 scheduling.  IVC2 is a 64-bit VLIW coprocessor.

-mdc
     Causes constant variables to be placed in the .near section.

-mdiv
     Enables the div and divu instructions.

-meb
     Generate big-endian code.

-mel
     Generate little-endian code.

-mio-volatile
     Tells the compiler that any variable marked with the io attribute is to be considered volatile.

-ml
     Causes variables to be assigned to the .far section by default.

-mleadz
     Enables the leadz (leading zero) instruction.

-mm
     Causes variables to be assigned to the .near section by default.

-mminmax
     Enables the min and max instructions.

-mmult
     Enables the multiplication and multiply-accumulate instructions.

-mno-opts
     Disables all the optional instructions enabled by -mall-opts.

-mrepeat
     Enables the repeat and erepeat instructions, used for low-overhead looping.

-ms
     Causes all variables to default to the .tiny section.  Note that there is a 65536-byte limit to this section.  Accesses to these variables use the %gp base register.

-msatur
     Enables the saturation instructions.  Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like as.

-msdram
     Link the SDRAM-based runtime instead of the default ROM-based runtime.

-msim
     Link the simulator run-time libraries.

-msimnovec
     Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.

-mtf
     Causes all functions to default to the .far section.  Without this option, functions default to the .near section.

-mtiny=N
     Variables that are N bytes or smaller are allocated to the .tiny section.  These variables use the $gp base register.  The default for this option is 4, but note that there's a 65536-byte limit to the .tiny section.

MicroBlaze Options
------------------

-msoft-float
     Use software emulation for floating point (default).

-mhard-float
     Use hardware floating-point instructions.

-mmemcpy
     Do not optimize block moves, use memcpy.

-mno-clearbss
     This option is deprecated.  Use -fno-zero-initialized-in-bss instead.

-mcpu=CPU-TYPE
     Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code.  Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.

-mxl-soft-mul
     Use software multiply emulation (default).

-mxl-soft-div
     Use software emulation for divides (default).

-mxl-barrel-shift
     Use the hardware barrel shifter.

-mxl-pattern-compare
     Use pattern compare instructions.

-msmall-divides
     Use table lookup optimization for small signed integer divisions.

-mxl-stack-check
     This option is deprecated.  Use -fstack-check instead.

-mxl-gp-opt
     Use GP-relative .sdata/.sbss sections.

-mxl-multiply-high
     Use multiply high instructions for high part of 32x32 multiply.

-mxl-float-convert
     Use hardware floating-point conversion instructions.

-mxl-float-sqrt
     Use hardware floating-point square root instruction.

-mbig-endian
     Generate code for a big-endian target.

-mlittle-endian
     Generate code for a little-endian target.

-mxl-reorder
     Use reorder instructions (swap and byte reversed load/store).

-mxl-mode-APP-MODEL
     Select application model APP-MODEL.  Valid models are

     executable
          normal executable (default), uses startup code crt0.o.

     xmdstub
          for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.

     bootstrap
          for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.

     novectors
          for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.

     Option -xl-mode-APP-MODEL is a deprecated alias for -mxl-mode-APP-MODEL.

MIPS Options
------------

-EB
     Generate big-endian code.

-EL
     Generate little-endian code.  This is the default for mips*el-*-* configurations.

-march=ARCH
     Generate code that runs on ARCH, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, interaptiv, loongson2e, loongson2f, loongson3a, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

     The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor.

     In processor names, a final 000 can be abbreviated as k (for example, -march=r2k).  Prefixes are optional, and vr may be written r.

     Names of the form Nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form Nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form Nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, Nf is accepted as a synonym for Nf2_1 while Nx and Bfx are accepted as synonyms for Nf1_1.

     GCC defines two macros based on the value of this option.  The first is _MIPS_ARCH, which gives the name of target architecture, as a string.  The second has the form _MIPS_ARCH_FOO, where FOO is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.

     Note that the _MIPS_ARCH macro uses the processor names given above.  In other words, it has the full prefix and does not abbreviate 000 as k.  In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3").  It names the default architecture when no -march option is given.

-mtune=ARCH
     Optimize for ARCH.  Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations.  The list of ARCH values is the same as for -march.

     When this option is not used, GCC optimizes for the processor specified by -march.  By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family.

     -mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_FOO, which work in the same way as the -march ones described above.

-mips1
     Equivalent to -march=mips1.

-mips2
     Equivalent to -march=mips2.

-mips3
     Equivalent to -march=mips3.

-mips4
     Equivalent to -march=mips4.

-mips32
     Equivalent to -march=mips32.

-mips32r3
     Equivalent to -march=mips32r3.

-mips32r5
     Equivalent to -march=mips32r5.

-mips32r6
     Equivalent to -march=mips32r6.

-mips64
     Equivalent to -march=mips64.

-mips64r2
     Equivalent to -march=mips64r2.

-mips64r3
     Equivalent to -march=mips64r3.

-mips64r5
     Equivalent to -march=mips64r5.

-mips64r6
     Equivalent to -march=mips64r6.

-mips16
-mno-mips16
     Generate (do not generate) MIPS16 code.  If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.

     MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes. See Function Attributes, for more information.

-mflip-mips16
     Generate MIPS16 code on alternating functions.  This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.

-minterlink-compressed

-mno-interlink-compressed
     Require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.

     For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

-minterlink-mips16
-mno-interlink-mips16
     Aliases of -minterlink-compressed and -mno-interlink-compressed.  These options predate the microMIPS ASE and are retained for backwards compatibility.

-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
     Generate code for the given ABI.

     Note that the EABI has a 32-bit and a 64-bit variant.  GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

     For information about the O64 ABI, see http://gcc.gnu.org/projects/mipso64-abi.html.

     GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide.  You can select this combination with -mabi=32 -mfp64.  This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.

     The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers.  For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair.  The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved.

     Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers.  These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers.  The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers.  This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes.

-mabicalls
-mno-abicalls
     Generate (do not generate) code that is suitable for SVR4-style dynamic objects.  -mabicalls is the default for SVR4-based systems.

-mshared
-mno-shared
     Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries.  This option only affects -mabicalls.

     All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic.  However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols.  It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions.  This mode is selected by -mno-shared.

     -mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker.  However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects.  Using -mno-shared generally makes executables both smaller and quicker.

     -mshared is the default.

-mplt
-mno-plt
     Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations.  This option only affects -mno-shared -mabicalls.  For the n64 ABI, this option has no effect without -msym32.

     You can make -mplt the default by configuring GCC with --with-mips-plt.  The default is -mno-plt otherwise.

-mxgot
-mno-xgot
     Lift (do not lift) the usual restrictions on the size of the global offset table.

     GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k.  Anything larger causes the linker to report an error such as:

          relocation truncated to fit: R_MIPS_GOT16 foobar

     If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol.

     Note that some linkers can create multiple GOTs.  If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries.  Very few do.

     These options have no effect unless GCC is generating position independent code.

-mgp32
     Assume that general-purpose registers are 32 bits wide.

-mgp64
     Assume that general-purpose registers are 64 bits wide.

-mfp32
     Assume that floating-point registers are 32 bits wide.

-mfp64
     Assume that floating-point registers are 64 bits wide.

-mfpxx
     Do not assume the width of floating-point registers.

-mhard-float
     Use floating-point coprocessor instructions.

-msoft-float
     Do not use floating-point coprocessor instructions.  Implement floating-point calculations using library calls instead.

-mno-float
     Equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point printf formats). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.

-msingle-float
     Assume that the floating-point coprocessor only supports single-precision operations.

-mdouble-float
     Assume that the floating-point coprocessor supports double-precision operations.  This is the default.

-modd-spreg
-mno-odd-spreg
     Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI.  This is the default for processors that are known to support these registers.  When using the o32 FPXX ABI, -mno-odd-spreg is set by default.

-mabs=2008
-mabs=legacy
     These options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the abs.fmt and neg.fmt machine instructions.

     By default or when -mabs=legacy is used the legacy treatment is selected.  In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN.  A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the -ffinite-math-only option has also been specified.

     The -mabs=2008 option selects the IEEE 754-2008 treatment.  In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN.  These instructions are therefore always used for the respective operations.

-mnan=2008
-mnan=legacy
     These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

     The -mnan=legacy option selects the legacy encoding.  In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signalling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

     The -mnan=2008 option selects the IEEE 754-2008 encoding.  In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.

     The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

-mllsc
-mno-llsc
     Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions.  When neither option is specified, GCC uses the instructions if the target architecture supports them.

     -mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when compiling for nonstandard ISAs.  You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively.  --with-llsc is the default for some configurations; see the installation documentation for details.

-mdsp
-mno-dsp
     Use (do not use) revision 1 of the MIPS DSP ASE. See MIPS DSP Built-in Functions.  This option defines the preprocessor macro __mips_dsp.  It also defines __mips_dsp_rev to 1.

-mdspr2
-mno-dspr2
     Use (do not use) revision 2 of the MIPS DSP ASE. See MIPS DSP Built-in Functions.  This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2.

-msmartmips
-mno-smartmips
     Use (do not use) the MIPS SmartMIPS ASE.

-mpaired-single
-mno-paired-single
     Use (do not use) paired-single floating-point instructions. See MIPS Paired-Single Support.  This option requires hardware floating-point support to be enabled.

-mdmx
-mno-mdmx
     Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.

-mips3d
-mno-mips3d
     Use (do not use) the MIPS-3D ASE.  See MIPS-3D Built-in Functions. The option -mips3d implies -mpaired-single.

-mmicromips
-mno-micromips
     Generate (do not generate) microMIPS code.

     MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes. See Function Attributes, for more information.

-mmt
-mno-mt
     Use (do not use) MT Multithreading instructions.

-mmcu
-mno-mcu
     Use (do not use) the MIPS MCU ASE instructions.

-meva
-mno-eva
     Use (do not use) the MIPS Enhanced Virtual Addressing instructions.

-mvirt
-mno-virt
     Use (do not use) the MIPS Virtualization Application Specific instructions.

-mxpa
-mno-xpa
     Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.

-mlong64
     Force long types to be 64 bits wide.  See -mlong32 for an explanation of the default and the way that the pointer size is determined.

-mlong32
     Force long, int, and pointer types to be 32 bits wide.

     The default size of ints, longs and pointers depends on the ABI.  All the supported ABIs use 32-bit ints.  The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs.  Pointers are the same size as longs, or the same size as integer registers, whichever is smaller.

-msym32
-mno-sym32
     Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI.  This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses.

-G NUM
     Put definitions of externally-visible data in a small data section if that data is no bigger than NUM bytes.  GCC can then generate more efficient accesses to the data; see -mgpopt for details.

     The default -G option depends on the configuration.

-mlocal-sdata
-mno-local-sdata
     Extend (do not extend) the -G behavior to local data too, such as to static variables in C.  -mlocal-sdata is the default for all configurations.

     If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata.  You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program.

-mextern-sdata
-mno-extern-sdata
     Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.

     If you compile a module MOD with -mextern-sdata -G NUM -mgpopt, and MOD references a variable VAR that is no bigger than NUM bytes, you must make sure that VAR is placed in a small data section.  If VAR is defined by another module, you must either compile that module with a high-enough -G setting or attach a section attribute to VAR's definition.  If VAR is common, you must link the application with a high-enough -G setting.

     The easiest way of satisfying these restrictions is to compile and link every module with the same -G option.  However, you may wish to build a library that supports several different small data limits.  You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data.

-mgpopt
-mno-gpopt
     Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata.  -mgpopt is the default for all configurations.

     -mno-gpopt is useful for cases where the $gp register might not hold the value of _gp.  For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

     -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.

-membedded-data
-mno-embedded-data
     Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data.  This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.

-muninit-const-in-rodata
-mno-uninit-const-in-rodata
     Put uninitialized const variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data.

-mcode-readable=SETTING
     Specify whether GCC may generate code that reads from executable sections. There are three possible settings:

     -mcode-readable=yes
          Instructions may freely access executable sections.  This is the default setting.

     -mcode-readable=pcrel
          MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so.  This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.

     -mcode-readable=no
          Instructions must not access executable sections.  This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.

-msplit-addresses
-mno-split-addresses
     Enable (disable) use of the %hi() and %lo() assembler relocation operators.  This option has been superseded by -mexplicit-relocs but is retained for backwards compatibility.

-mexplicit-relocs
-mno-explicit-relocs
     Use (do not use) assembler relocation operators when dealing with symbolic addresses.  The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

     -mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators.

-mcheck-zero-division
-mno-check-zero-division
     Trap (do not trap) on integer division by zero.

     The default is -mcheck-zero-division.

-mdivide-traps
-mdivide-breaks
     MIPS systems check for division by zero by generating either a conditional trap or a break instruction.  Using traps results in smaller code, but is only supported on MIPS II and later.  Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (SIGFPE).  Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

     The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.

-mmemcpy
-mno-memcpy
     Force (do not force) the use of memcpy for non-trivial block moves.  The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.

-mlong-calls
-mno-long-calls
     Disable (do not disable) use of the jal instruction.  Calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment.

     This option has no effect on abicalls code.  The default is -mno-long-calls.

-mmad
-mno-mad
     Enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA.

-mimadd
-mno-imadd
     Enable (disable) use of the madd and msub integer instructions.  The default is -mimadd on architectures that support madd and msub except for the 74k architecture where it was found to generate slower code.

-mfused-madd
-mno-fused-madd
     Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available.  The default is -mfused-madd.

     On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit.  This may be undesirable in some circumstances.  On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

-nocpp
     Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.

-mfix-24k

-mno-fix-24k
     Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC.

-mfix-r4000
-mno-fix-r4000
     Work around certain R4000 CPU errata:

       - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.

       - A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.

       - An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.

-mfix-r4400
-mno-fix-r4400
     Work around certain R4400 CPU errata:

       - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.

-mfix-r10000
-mno-fix-r10000
     Work around certain R10000 errata:

       - ll/sc sequences may not behave atomically on revisions prior to 3.0.  They may deadlock on revisions 2.6 and earlier.

     This option can only be used if the target architecture supports branch-likely instructions.  -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

-mfix-rm7000
-mno-fix-rm7000
     Work around the RM7000 dmult/dmultu errata.  The workarounds are implemented by the assembler rather than by GCC.

-mfix-vr4120
-mno-fix-vr4120
     Work around certain VR4120 errata:

       - dmultu does not always produce the correct result.

       - div and ddiv do not always produce the correct result if one of the operands is negative.

     The workarounds for the division errata rely on special functions in libgcc.a.  At present, these functions are only provided by the mips64vr*-elf configurations.

     Other VR4120 errata require a NOP to be inserted between certain pairs of instructions.  These errata are handled by the assembler, not by GCC itself.

-mfix-vr4130
     Work around the VR4130 mflo/mfhi errata.  The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead.

-mfix-sb1
-mno-fix-sb1
     Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 ``F1'' and ``F2'' floating-point errata.)

-mr10k-cache-barrier=SETTING
     Specify whether GCC should insert cache barriers to avoid the side-effects of speculation on R10K processors.

     In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the ``taken'' branch.  It later aborts these instructions if the predicted outcome is wrong.  However, on the R10K, even aborted instructions can have side effects.

     This problem only affects kernel stores and, depending on the system, kernel loads.  As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted.  If a DMA operation writes to the same area of memory before the ``dirty'' line is flushed, the cached data overwrites the DMA-ed data.  See the R10K processor manual for a full description, including other potential problems.

     One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted.  -mr10k-cache-barrier=SETTING controls GCC's implementation of this workaround.  It assumes that aborted accesses to any byte in the following regions does not have side effects:

      1. the memory occupied by the current function's stack frame;

      2. the memory occupied by an incoming stack argument;

      3. the memory occupied by an object with a link-time-constant address.

     It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe.

     If the input program contains a function declaration such as:

          void foo (void);

     then the implementation of foo must allow j foo and jal foo to be executed speculatively.  GCC honors this restriction for functions it compiles itself.  It expects non-GCC functions (such as hand-written assembly code) to do the same.

     The option has three forms:

     -mr10k-cache-barrier=load-store
          Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.

     -mr10k-cache-barrier=store
          Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.

     -mr10k-cache-barrier=none
          Disable the insertion of cache barriers.  This is the default setting.

-mflush-func=FUNC
-mno-flush-func
     Specifies the function to call to flush the I and D caches, or to not call any such function.  If called, the function must take the same arguments as the common _flush_func, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches).  The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush.

mbranch-cost=NUM
     Set the cost of branches to roughly NUM ``simple'' instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.  A zero cost redundantly selects the default, which is based on the -mtune setting.

-mbranch-likely
-mno-branch-likely
     Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture.  By default, Branch Likely instructions may be generated if they are supported by the selected architecture.  An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.

-mfp-exceptions
-mno-fp-exceptions
     Specifies whether FP exceptions are enabled.  This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.

     For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes.  Otherwise, we can only use one FP pipe.

-mvr4130-align
-mno-vr4130-align
     The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned.  When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.

     This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3.

-msynci
-mno-synci
     Enable (disable) generation of synci instructions on architectures that support it.  The synci instructions (if enabled) are generated when __builtin___clear_cache is compiled.

     This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.

     When compiling code for single processor systems, it is generally safe to use synci.  However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior.

-mrelax-pic-calls
-mno-relax-pic-calls
     Try to turn PIC calls that are normally dispatched via register $25 into direct calls.  This is only possible if the linker can resolve the destination at link-time and if the destination is within range for a direct call.

     -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect.  With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler.

-mmcount-ra-address
-mno-mcount-ra-address
     Emit (do not emit) code that allows _mcount to modify the calling function's return address.  When enabled, this option extends the usual _mcount interface with a new RA-ADDRESS parameter, which has type intptr_t * and is passed in register $12.  _mcount can then modify the return address by doing both of the following:

       * Returning the new address in register $31.

       * Storing the new address in *RA-ADDRESS, if RA-ADDRESS is nonnull.

     The default is -mno-mcount-ra-address.

MMIX Options
------------

These options are defined for the MMIX:

-mlibfuncs
-mno-libfuncs
     Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.

-mepsilon
-mno-epsilon
     Generate floating-point comparison instructions that compare with respect to the rE epsilon register.

-mabi=mmixware
-mabi=gnu
     Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.

-mzero-extend
-mno-zero-extend
     When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.

-mknuthdiv
-mno-knuthdiv
     Make the result of a division yielding a remainder have the same sign as the divisor.  With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend.  Both methods are arithmetically valid, the latter being almost exclusively used.

-mtoplevel-symbols
-mno-toplevel-symbols
     Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.

-melf
     Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.

-mbranch-predict
-mno-branch-predict
     Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.

-mbase-addresses
-mno-base-addresses
     Generate (do not generate) code that uses base addresses.  Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register.  The register is used for one or more base address requests within the range 0 to 255 from the value held in the register.  The generally leads to short and fast code, but the number of different data items that can be addressed is limited.  This means that a program that uses lots of static data may require -mno-base-addresses.

-msingle-exit
-mno-single-exit
     Force (do not force) generated code to have a single exit point in each function.

MN10300 Options
---------------

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug
     Generate code to avoid bugs in the multiply instructions for the MN10300 processors.  This is the default.

-mno-mult-bug
     Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.

-mam33
     Generate code using features specific to the AM33 processor.

-mno-am33
     Do not generate code using features specific to the AM33 processor.  This is the default.

-mam33-2
     Generate code using features specific to the AM33/2.0 processor.

-mam34
     Generate code using features specific to the AM34 processor.

-mtune=CPU-TYPE
     Use the timing characteristics of the indicated CPU type when scheduling instructions.  This does not change the targeted processor type.  The CPU type must be one of mn10300, am33, am33-2 or am34.

-mreturn-pointer-on-d0
     When generating a function that returns a pointer, return the pointer in both a0 and d0.  Otherwise, the pointer is returned only in a0, and attempts to call such functions without a prototype result in errors.  Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.

-mno-crt0
     Do not link in the C run-time initialization object file.

-mrelax
     Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses.  This option only has an effect when used on the command line for the final link step.

     This option makes symbolic debugging impossible.

-mliw
     Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later.  This is the default.  This option defines the preprocessor macro __LIW__.

-mnoliw
     Do not allow the compiler to generate Long Instruction Word instructions.  This option defines the preprocessor macro __NO_LIW__.

-msetlb
     Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later.  This is the default.  This option defines the preprocessor macro __SETLB__.

-mnosetlb
     Do not allow the compiler to generate SETLB or Lcc instructions.  This option defines the preprocessor macro __NO_SETLB__.

Moxie Options
-------------

-meb
     Generate big-endian code.  This is the default for moxie-*-* configurations.

-mel
     Generate little-endian code.

-mmul.x
     Generate mul.x and umul.x instructions.  This is the default for moxiebox-*-* configurations.

-mno-crt0
     Do not link in the C run-time initialization object file.

MSP430 Options
--------------

These options are defined for the MSP430:

-masm-hex
     Force assembly output to always use hex constants.  Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes.

-mmcu=
     Select the MCU to target.  This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with __.  This in turn is used by the msp430.h header file to select an MCU-specific supplementary header file.

     The option also sets the ISA to use.  If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected.  A generic MCU name of msp430 can also be used to select the 430 ISA.  Similarly the generic msp430x MCU name selects the 430X ISA.

     In addition an MCU-specific linker script is added to the linker command line.  The script's name is the name of the MCU with .ld appended.  Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and cause the linker to search for a script called xxx.ld.

     This option is also passed on to the assembler.

-mcpu=
     Specifies the ISA to use.  Accepted values are msp430, msp430x and msp430xv2.  This option is deprecated.  The -mmcu= option should be used to select the ISA.

-msim
     Link to the simulator runtime libraries and linker script.  Overrides any scripts that would be selected by the -mmcu= option.

-mlarge
     Use large-model addressing (20-bit pointers, 32-bit size_t).

-msmall
     Use small-model addressing (16-bit pointers, 16-bit size_t).

-mrelax
     This option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link.

mhwmult=
     Describes the type of hardware multiply supported by the target. Accepted values are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs. 32bit for the 16/32-bit multiply supported by later MCUs and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given.  This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option.  If no -mmcu option is specified then 32bit hardware multiply support is assumed.  auto is the default setting.

     Hardware multiplies are normally performed by calling a library routine.  This saves space in the generated code.  When compiling at -O3 or higher however the hardware multiplier is invoked inline.  This makes for bigger, but faster code.

     The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish.  This makes them safe to use inside interrupt handlers as well as in normal code.

-minrt
     Enable the use of a minimum runtime environment - no static initializers or constructors.  This is intended for memory-constrained devices.  The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.

-mcode-region=
-mdata-region=
     These options tell the compiler where to place functions and data that do not have one of the lower, upper, either or section attributes.  Possible values are lower, upper, either or any.  The first three behave like the corresponding attribute.  The fourth possible value - any - is the default.  It leaves placement entirely up to the linker script and how it assigns the standard sections (.text, .data etc) to the memory regions.

NDS32 Options
-------------

These options are defined for NDS32 implementations:

-mbig-endian
     Generate code in big-endian mode.

-mlittle-endian
     Generate code in little-endian mode.

-mreduced-regs
     Use reduced-set registers for register allocation.

-mfull-regs
     Use full-set registers for register allocation.

-mcmov
     Generate conditional move instructions.

-mno-cmov
     Do not generate conditional move instructions.

-mperf-ext
     Generate performance extension instructions.

-mno-perf-ext
     Do not generate performance extension instructions.

-mv3push
     Generate v3 push25/pop25 instructions.

-mno-v3push
     Do not generate v3 push25/pop25 instructions.

-m16-bit
     Generate 16-bit instructions.

-mno-16-bit
     Do not generate 16-bit instructions.

-misr-vector-size=NUM
     Specify the size of each interrupt vector, which must be 4 or 16.

-mcache-block-size=NUM
     Specify the size of each cache block, which must be a power of 2 between 4 and 512.

-march=ARCH
     Specify the name of the target architecture.

-mcmodel=CODE-MODEL
     Set the code model to one of

     small
          All the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.

     medium
          The data segment must be within 512KB while the read-only data segment can be within 4GB addressing space.  The text segment should be still within 16MB addressing space.

     large
          All the text and data segments can be within 4GB addressing space.

-mctor-dtor
     Enable constructor/destructor feature.

-mrelax
     Guide linker to relax instructions.

Nios II Options
---------------

These are the options defined for the Altera Nios II processor.

-G NUM
     Put global and static objects less than or equal to NUM bytes into the small data or BSS sections instead of the normal data or BSS sections.  The default value of NUM is 8.

-mgpopt=OPTION

-mgpopt
-mno-gpopt
     Generate (do not generate) GP-relative accesses.  The following OPTION names are recognized:

     none
          Do not generate GP-relative accesses.

     local
          Generate GP-relative accesses for small data objects that are not external or weak.  Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a section attribute.

     global
          As for local, but also generate GP-relative accesses for small data objects that are external or weak.  If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same -G setting.

     data
          Generate GP-relative accesses for all data objects in the program.  If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressible range of the global pointer.

     all
          Generate GP-relative addresses for function pointers as well as data pointers.  If you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressible range of the global pointer.

     -mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

     The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries.

     You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections.  In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section.

-mel
-meb
     Generate little-endian (default) or big-endian (experimental) code, respectively.

-march=ARCH
     This specifies the name of the target Nios II architecture.  GCC uses this name to determine what kind of instructions it can emit when generating assembly code.  Permissible names are: r1, r2.

     The preprocessor macro __nios2_arch__ is available to programs, with value 1 or 2, indicating the targeted ISA level.

-mbypass-cache
-mno-bypass-cache
     Force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache.

-mno-cache-volatile
-mcache-volatile
     Volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.

-mno-fast-sw-div
-mfast-sw-div
     Do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3 and above.

-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
     Enable or disable emitting mul, mulx and div family of instructions by the compiler. The default is to emit mul and not emit div and mulx.

-mbmx
-mno-bmx
-mcdx
-mno-cdx
     Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions.  Enabling these instructions also requires -march=r2.  Since these instructions are optional extensions to the R2 architecture, the default is not to emit them.

-mcustom-INSN=N
-mno-custom-INSN
     Each -mcustom-INSN=N option enables use of a custom instruction with encoding N when generating code that uses INSN.  For example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call.

     The following values of INSN are supported.  Except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions (see Other Builtins).

     Single-precision floating point:

     fadds, fsubs, fdivs, fmuls
          Binary arithmetic operations.

     fnegs
          Unary negation.

     fabss
          Unary absolute value.

     fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
          Comparison operations.

     fmins, fmaxs
          Floating-point minimum and maximum.  These instructions are only generated if -ffinite-math-only is specified.

     fsqrts
          Unary square root operation.

     fcoss, fsins, ftans, fatans, fexps, flogs
          Floating-point trigonometric and exponential functions.  These instructions are only generated if -funsafe-math-optimizations is also specified.

     Double-precision floating point:

     faddd, fsubd, fdivd, fmuld
          Binary arithmetic operations.

     fnegd
          Unary negation.

     fabsd
          Unary absolute value.

     fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
          Comparison operations.

     fmind, fmaxd
          Double-precision minimum and maximum.  These instructions are only generated if -ffinite-math-only is specified.

     fsqrtd
          Unary square root operation.

     fcosd, fsind, ftand, fatand, fexpd, flogd
          Double-precision trigonometric and exponential functions.  These instructions are only generated if -funsafe-math-optimizations is also specified.

     Conversions:

     fextsd
          Conversion from single precision to double precision.

     ftruncds
          Conversion from double precision to single precision.

     fixsi, fixsu, fixdi, fixdu
          Conversion from floating point to signed or unsigned integer types, with truncation towards zero.

     round
          Conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the __builtin_lroundf function when -fno-math-errno is used.

     floatis, floatus, floatid, floatud
          Conversion from signed or unsigned integer types to floating-point types.

     In addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions.  Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X.  The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register SRC1 and the most significant half in SRC2.  A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used.

     fwrx
          Write SRC1 into the least significant half of X and SRC2 into the most significant half of X.

     fwry
          Write SRC1 into Y.

     frdxhi, frdxlo
          Read the most or least (respectively) significant half of X and store it in DEST.

     frdy
          Read the value of Y and store it into DEST.

     Note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-INSN=N") and target("no-custom-INSN") function attributes (see Function Attributes) or pragmas (see Function Specific Option Pragmas).

-mcustom-fpu-cfg=NAME

     This option enables a predefined, named set of custom instruction encodings (see -mcustom-INSN above). Currently, the following sets are defined:

     -mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

     -mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant

     -mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant

     Custom instruction assignments given by individual -mcustom-INSN= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line.

     Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=NAME") function attribute (see Function Attributes) or pragma (see Function Specific Option Pragmas).

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

-mhal
     Link with HAL BSP.  This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP.

-msmallc
     Link with a limited version of the C library, -lsmallc, rather than Newlib.

-msys-crt0=STARTFILE
     STARTFILE is the file name of the startfile (crt0) to use when linking.  This option is only useful in conjunction with -mhal.

-msys-lib=SYSTEMLIB
     SYSTEMLIB is the library name of the library that provides low-level system calls required by the C library, e.g. read and write. This option is typically used to link with a library provided by a HAL BSP.

Nvidia PTX Options
------------------

These options are defined for Nvidia PTX:

-m32
-m64
     Generate code for 32-bit or 64-bit ABI.

-mmainkernel
     Link in code for a __main kernel.  This is for stand-alone instead of offloading execution.

PDP-11 Options
--------------

These options are defined for the PDP-11:

-mfpu
     Use hardware FPP floating point.  This is the default.  (FIS floating point on the PDP-11/40 is not supported.)

-msoft-float
     Do not use hardware floating point.

-mac0
     Return floating-point results in ac0 (fr0 in Unix assembler syntax).

-mno-ac0
     Return floating-point results in memory.  This is the default.

-m40
     Generate code for a PDP-11/40.

-m45
     Generate code for a PDP-11/45.  This is the default.

-m10
     Generate code for a PDP-11/10.

-mbcopy-builtin
     Use inline movmemhi patterns for copying memory.  This is the default.

-mbcopy
     Do not use inline movmemhi patterns for copying memory.

-mint16
-mno-int32
     Use 16-bit int.  This is the default.

-mint32
-mno-int16
     Use 32-bit int.

-mfloat64
-mno-float32
     Use 64-bit float.  This is the default.

-mfloat32
-mno-float64
     Use 32-bit float.

-mabshi
     Use abshi2 pattern.  This is the default.

-mno-abshi
     Do not use abshi2 pattern.

-mbranch-expensive
     Pretend that branches are expensive.  This is for experimenting with code generation only.

-mbranch-cheap
     Do not pretend that branches are expensive.  This is the default.

-munix-asm
     Use Unix assembler syntax.  This is the default when configured for pdp11-*-bsd.

-mdec-asm
     Use DEC assembler syntax.  This is the default when configured for any PDP-11 target other than pdp11-*-bsd.

picoChip Options
----------------

These -m options are defined for picoChip implementations:

-mae=AE_TYPE
     Set the instruction set, register set, and instruction scheduling parameters for array element type AE_TYPE.  Supported values for AE_TYPE are ANY, MUL, and MAC.

     -mae=ANY selects a completely generic AE type.  Code generated with this option runs on any of the other AE types.  The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.

     -mae=MUL selects a MUL AE type.  This is the most useful AE type for compiled code, and is the default.

     -mae=MAC selects a DSP-style MAC AE.  Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.

-msymbol-as-address
     Enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register.  Typically, the use of this option generates larger programs, which run faster than when the option isn't used.  However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled.

-mno-inefficient-warnings
     Disables warnings about the generation of inefficient code.  These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type.  The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations.  This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support.  This option disables these warnings.

PowerPC Options
---------------

These are listed under see RS/6000 and PowerPC Options.

RL78 Options
------------

-msim
     Links in additional target libraries to support operation within a simulator.

-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
     Specifies the type of hardware multiplication and division support to be used.  The simplest is none, which uses software for both multiplication and division.  This is the default.  The g13 value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.  The g14 value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.  The value rl78 is an alias for g14 and the value mg10 is an alias for none.

     In addition a C preprocessor macro is defined, based upon the setting of this option.  Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__.

-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
     Specifies the RL78 core to target.  The default is the G14 core, also known as an S3 core or just RL78.  The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations.  The G10 or S1 core does not have register banks, so it uses a different calling convention.

     If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line.  Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multipications altogether.

     Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behaviour of the toolchain since it also enables G14 hardware multiply support.  If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core.  This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support.

     In addition a C preprocessor macro is defined, based upon the setting of this option.  Possible values are: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

-mg10
-mg13
-mg14
-mrl78
     These are aliases for the corresponding -mcpu= option.  They are provided for backwards compatibility.

-mallregs
     Allow the compiler to use all of the available registers.  By default registers r24..r31 are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well.

-m64bit-doubles
-m32bit-doubles
     Make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size.  The default is -m32bit-doubles.

IBM RS/6000 and PowerPC Options
-------------------------------

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
     You use these options to specify which instructions are available on the processor you are using.  The default value of these options is determined when configuring GCC.  Specifying the -mcpu=CPU_TYPE overrides the specification of these options.  We recommend you use the -mcpu=CPU_TYPE option rather than the options listed above.

     Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root.  Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

     The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The -mmfpgpr option allows GCC to generate the FP move to/from general-purpose register instructions implemented on the POWER6X processor and other processors that support the extended PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.

     The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities.  GCC defaults to -mno-powerpc64.

-mcpu=CPU_TYPE
     Set architecture type, register usage, and instruction scheduling parameters for machine type CPU_TYPE. Supported values for CPU_TYPE are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, powerpc, powerpc64, powerpc64le, and rs64.

     -mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

     The other options specify a specific processor.  Code generated under those options runs best on that processor, and may not run at all on others.

     The -mcpu options automatically enable or disable the following options:

     -maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb -mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt  -msingle-float -mdouble-float -msimple-fpu -mstring  -mmulhw  -mdlmzb  -mmfpgpr -mvsx -mcrypto -mdirect-move -mpower8-fusion -mpower8-vector -mquad-memory -mquad-memory-atomic

     The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities.  If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.

     On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present because AIX does not have full support for these options.  You may still enable or disable them individually if you're sure it'll work in your environment.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the architecture type or register usage, as -mcpu=CPU_TYPE does.  The same values for CPU_TYPE are used for -mtune as for -mcpu.  If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune.

-mcmodel=small
     Generate PowerPC64 code for the small model: The TOC is limited to 64k.

-mcmodel=medium
     Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in size.

-mcmodel=large
     Generate PowerPC64 code for the large model: The TOC may be up to 4G in size.  Other data and code is only limited by the 64-bit address space.

-maltivec
-mno-altivec
     Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set.  You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

     When -maltivec is used, rather than -maltivec=le or -maltivec=be, the element order for Altivec intrinsics such as vec_splat, vec_extract, and vec_insert match array element order corresponding to the endianness of the target.  That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform.

-maltivec=be
     Generate Altivec instructions using big-endian element order, regardless of whether the target is big- or little-endian.  This is the default when targeting a big-endian platform.

     The element order is used to interpret element numbers in Altivec intrinsics such as vec_splat, vec_extract, and vec_insert.  By default, these match array element order corresponding to the endianness for the target.

-maltivec=le
     Generate Altivec instructions using little-endian element order, regardless of whether the target is big- or little-endian.  This is the default when targeting a little-endian platform.  This option is currently ignored when targeting a big-endian platform.

     The element order is used to interpret element numbers in Altivec intrinsics such as vec_splat, vec_extract, and vec_insert.  By default, these match array element order corresponding to the endianness for the target.

-mvrsave
-mno-vrsave
     Generate VRSAVE instructions when generating AltiVec code.

-mgen-cell-microcode
     Generate Cell microcode instructions.

-mwarn-cell-microcode
     Warn when a Cell microcode instruction is emitted.  An example of a Cell microcode instruction is a variable shift.

-msecure-plt
     Generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option.

-mbss-plt
     Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.

-misel
-mno-isel
     This switch enables or disables the generation of ISEL instructions.

-misel=YES/NO
     This switch has been deprecated.  Use -misel and -mno-isel instead.

-mspe
-mno-spe
     This switch enables or disables the generation of SPE simd instructions.

-mpaired
-mno-paired
     This switch enables or disables the generation of PAIRED simd instructions.

-mspe=YES/NO
     This option has been deprecated.  Use -mspe and -mno-spe instead.

-mvsx
-mno-vsx
     Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set.

-mcrypto
-mno-crypto
     Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA.

-mdirect-move
-mno-direct-move
     Generate code that uses (does not use) the instructions to move data between the general purpose registers and the vector/scalar (VSX) registers that were added in version 2.07 of the PowerPC ISA.

-mpower8-fusion
-mno-power8-fusion
     Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors.

-mpower8-vector
-mno-power8-vector
     Generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA.  Also enable the use of built-in functions that allow more direct access to the vector instructions.

-mquad-memory
-mno-quad-memory
     Generate code that uses (does not use) the non-atomic quad word memory instructions.  The -mquad-memory option requires use of 64-bit mode.

-mquad-memory-atomic
-mno-quad-memory-atomic
     Generate code that uses (does not use) the atomic quad word memory instructions.  The -mquad-memory-atomic option requires use of 64-bit mode.

-mupper-regs-df
-mno-upper-regs-df
     Generate code that uses (does not use) the scalar double precision instructions that target all 64 registers in the vector/scalar floating point register set that were added in version 2.06 of the PowerPC ISA.  -mupper-regs-df is turned on by default if you use any of the -mcpu=power7, -mcpu=power8, or -mvsx options.

-mupper-regs-sf
-mno-upper-regs-sf
     Generate code that uses (does not use) the scalar single precision instructions that target all 64 registers in the vector/scalar floating point register set that were added in version 2.07 of the PowerPC ISA.  -mupper-regs-sf is turned on by default if you use either of the -mcpu=power8 or -mpower8-vector options.

-mupper-regs
-mno-upper-regs
     Generate code that uses (does not use) the scalar instructions that target all 64 registers in the vector/scalar floating point register set, depending on the model of the machine.

     If the -mno-upper-regs option is used, it turns off both -mupper-regs-sf and -mupper-regs-df options.

-mfloat-gprs=YES/SINGLE/DOUBLE/NO
-mfloat-gprs
     This switch enables or disables the generation of floating-point operations on the general-purpose registers for architectures that support it.

     The argument yes or single enables the use of single-precision floating-point operations.

     The argument double enables the use of single and double-precision floating-point operations.

     The argument no disables floating-point operations on the general-purpose registers.

     This option is currently only available on the MPC854x.

-m32
-m64
     Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux).  The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant.  The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.

-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
     Modify generation of the TOC (Table Of Contents), which is created for every executable file.  The -mfull-toc option is selected by default.  In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program.  GCC also places floating-point constants in the TOC.  However, only 16,384 entries are available in the TOC.

     If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC.  You may specify one or both of these options.  Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

     If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead.  This option causes GCC to make only one TOC entry for every file.  When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space.  You may wish to use this option only on files that contain less frequently-executed code.

-maix64
-maix32
     Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64.  GCC defaults to -maix32.

-mxl-compat
-mno-xl-compat
     Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI.  Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs.  Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double.  Use XL symbol names for long double support routines.

     The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared.  IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization.  Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization.

-mpe
     Support IBM RS/6000 SP Parallel Environment (PE).  Link an application written to use message passing with special startup code to enable the application to run.  The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location.  The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible.

-malign-natural
-malign-power
     On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules.  GCC defaults to the standard alignment defined in the ABI.

     On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.

-msoft-float
-mhard-float
     Generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.

-msingle-float
-mdouble-float
     Generate code for single- or double-precision floating-point operations. -mdouble-float implies -msingle-float.

-msimple-fpu
     Do not generate sqrt and div instructions for hardware floating-point unit.

-mfpu=NAME
     Specify type of floating-point unit.  Valid values for NAME are sp_lite (equivalent to -msingle-float -msimple-fpu), dp_lite (equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent to -msingle-float), and dp_full (equivalent to -mdouble-float).

-mxilinx-fpu
     Perform optimizations for the floating-point unit on Xilinx PPC 405/440.

-mmultiple
-mno-multiple
     Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions.  These instructions are generated by default on POWER systems, and not generated on PowerPC systems.  Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode.  The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.

-mstring
-mno-string
     Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves.  These instructions are generated by default on POWER systems, and not generated on PowerPC systems.  Do not use -mstring on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.

-mupdate
-mno-update
     Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location.  These instructions are generated by default.  If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
     Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary.  This option is enabled by default when targeting Power6 and disabled otherwise.

-mfused-madd
-mno-fused-madd
     Generate code that uses (does not use) the floating-point multiply and accumulate instructions.  These instructions are generated by default if hardware floating point is used.  The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.

-mmulhw
-mno-mulhw
     Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors.

-mdlmzb
-mno-dlmzb
     Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors.  This instruction is generated by default when targeting those processors.

-mno-bit-align
-mbit-align
     On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

     For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes.  By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size.

-mno-strict-align
-mstrict-align
     On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system.

-mrelocatable
-mno-relocatable
     Generate code that allows (does not allow) a static executable to be relocated to a different address at run time.  A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section, a table of 32-bit addresses generated by this option.  For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary.

-mrelocatable-lib
-mno-relocatable-lib
     Like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable.  Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options.

-mno-toc
-mtoc
     On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

-mlittle
-mlittle-endian
     On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode.  The -mlittle-endian option is the same as -mlittle.

-mbig
-mbig-endian
     On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode.  The -mbig-endian option is the same as -mbig.

-mdynamic-no-pic
     On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable.  The resulting code is suitable for applications, but not shared libraries.

-msingle-pic-base
     Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function.  The runtime system is responsible for initializing this register with an appropriate value before execution begins.

-mprioritize-restricted-insns=PRIORITY
     This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass.  The argument PRIORITY takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.

-msched-costly-dep=DEPENDENCE_TYPE
     This option controls which dependences are considered costly by the target during instruction scheduling.  The argument DEPENDENCE_TYPE takes one of the following values:

     no
          No dependence is costly.

     all
          All dependences are costly.

     true_store_to_load
          A true dependence from store to load is costly.

     store_to_load
          Any dependence from store to load is costly.

     NUMBER
          Any dependence for which the latency is greater than or equal to NUMBER is costly.

-minsert-sched-nops=SCHEME
     This option controls which NOP insertion scheme is used during the second scheduling pass.  The argument SCHEME takes one of the following values:

     no
          Don't insert NOPs.

     pad
          Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.

     regroup_exact
          Insert NOPs to force costly dependent insns into separate groups.  Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.

     NUMBER
          Insert NOPs to force costly dependent insns into separate groups.  Insert NUMBER NOPs to force an insn to a new group.

-mcall-sysv
     On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement.  This is the default unless you configured GCC using powerpc-*-eabiaix.

-mcall-sysv-eabi
-mcall-eabi
     Specify both -mcall-sysv and -meabi options.

-mcall-sysv-noeabi
     Specify both -mcall-sysv and -mno-eabi options.

-mcall-aixdesc
     On System V.4 and embedded PowerPC systems compile code for the AIX operating system.

-mcall-linux
     On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mcall-freebsd
     On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.

-mcall-netbsd
     On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.

-mcall-openbsd
     On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.

-maix-struct-return
     Return all structures in memory (as specified by the AIX ABI).

-msvr4-struct-return
     Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).

-mabi=ABI-TYPE
     Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec, no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble, elfv1, elfv2.

-mabi=spe
     Extend the current ABI with SPE ABI extensions.  This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI.

-mabi=no-spe
     Disable Book-E SPE ABI extensions for the current ABI.

-mabi=ibmlongdouble
     Change the current ABI to use IBM extended-precision long double. This is a PowerPC 32-bit SYSV ABI option.

-mabi=ieeelongdouble
     Change the current ABI to use IEEE extended-precision long double. This is a PowerPC 32-bit Linux ABI option.

-mabi=elfv1
     Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways.

-mabi=elfv2
     Change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways.

-mprototype
-mno-prototype
     On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped.  Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments.  With -mprototype, only calls to prototyped variable argument functions set or clear the bit.

-msim
     On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a.  This is the default for powerpc-*-eabisim configurations.

-mmvme
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.

-mads
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.

-myellowknife
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a.

-mvxworks
     On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.

-memb
     On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used.

-meabi
-mno-eabi
     On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications.  Selecting -meabi means that the stack is aligned to an 8-byte boundary, a function __eabi is called from main to set up the EABI environment, and the -msdata option can use both r2 and r13 to point to two separate small data areas.  Selecting -mno-eabi means that the stack is aligned to a 16-byte boundary, no EABI initialization function is called from main, and the -msdata option only uses r13 to point to a single small data area.  The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.

-msdata=eabi
     On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the .sdata2 section, which is pointed to by register r2.  Put small initialized non-const global and static data in the .sdata section, which is pointed to by register r13.  Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section.  The -msdata=eabi option is incompatible with the -mrelocatable option.  The -msdata=eabi option also sets the -memb option.

-msdata=sysv
     On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register r13.  Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable option.

-msdata=default
-msdata
     On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.

-msdata=data
     On System V.4 and embedded PowerPC systems, put small global data in the .sdata section.  Put small uninitialized global data in the .sbss section.  Do not use register r13 to address small data however.  This is the default behavior unless other -msdata options are used.

-msdata=none
-mno-sdata
     On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.

-mblock-move-inline-limit=NUM
     Inline all block moves (such as calls to memcpy or structure copies) less than or equal to NUM bytes.  The minimum value for NUM is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets.  The default value is target-specific.

-G NUM
     On embedded PowerPC systems, put global and static items less than or equal to NUM bytes into the small data or BSS sections instead of the normal data or BSS section.  By default, NUM is 8.  The -G NUM switch is also passed to the linker. All modules should be compiled with the same -G NUM value.

-mregnames
-mno-regnames
     On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.

-mlongcall
-mno-longcall
     By default assume that all calls are far away so that a longer and more expensive calling sequence is required.  This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call cannot be that far away.  This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

     Some linkers are capable of detecting out-of-range calls and generating glue code on the fly.  On these systems, long calls are unnecessary and generate slower code.  As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64.  It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

     On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code).  The two target addresses represent the callee and the branch island.  The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island.  The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

     On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

     In the future, GCC may ignore all longcall specifications when the linker is known to generate glue.

-mtls-markers
-mno-tls-markers
     Mark (do not mark) calls to __tls_get_addr with a relocation specifying the function argument.  The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence.

-pthread
     Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

-mrecip
-mno-recip
     This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments.  You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -finite-math-only, -freciprocal-math and -fno-trapping-math).  Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.

-mrecip=OPT
     This option controls which reciprocal estimate instructions may be used.  OPT is a comma-separated list of options, which may be preceded by a ! to invert the option:

     all
          Enable all estimate instructions.

     default
          Enable the default instructions, equivalent to -mrecip.

     none
          Disable all estimate instructions, equivalent to -mno-recip.

     div
          Enable the reciprocal approximation instructions for both single and double precision.

     divf
          Enable the single-precision reciprocal approximation instructions.

     divd
          Enable the double-precision reciprocal approximation instructions.

     rsqrt
          Enable the reciprocal square root approximation instructions for both single and double precision.

     rsqrtf
          Enable the single-precision reciprocal square root approximation instructions.

     rsqrtd
          Enable the double-precision reciprocal square root approximation instructions.

     So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations.

-mrecip-precision
-mno-recip-precision
     Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI.  Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps.

-mveclibabi=TYPE
     Specifies the ABI type to use for vectorizing intrinsics using an external library.  The only type supported at present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7.  Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled.  The MASS libraries must be specified at link time.

-mfriz
-mno-friz
     Generate (do not generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point.  The friz instruction does not return the same value if the floating-point number is too large to fit in an integer.

-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
     Generate (do not generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, TOC value to be loaded in register r2, and static chain value to be loaded in register r11.  The -mpointers-to-nested-functions is on by default.  You cannot call through pointers to nested functions or pointers to functions compiled in other languages that use the static chain if you use -mno-pointers-to-nested-functions.

-msave-toc-indirect
-mno-save-toc-indirect
     Generate (do not generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems.  If the TOC value is not saved in the prologue, it is saved just before the call through the pointer.  The -mno-save-toc-indirect option is the default.

-mcompat-align-parm
-mno-compat-align-parm
     Generate (do not generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

     Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment.  This is corrected in more recent versions of GCC.  This option may be used to generate code that is compatible with functions compiled with older versions of GCC.

     The -mno-compat-align-parm option is the default.

RX Options
----------

These command-line options are defined for RX targets:

-m64bit-doubles
-m32bit-doubles
     Make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size.  The default is -m32bit-doubles.  Note RX floating-point hardware only works on 32-bit values, which is why the default is -m32bit-doubles.

-fpu
-nofpu
     Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware.  The default is enabled for the RX600 series and disabled for the RX200 series.

     Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

     Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe.

-mcpu=NAME
     Selects the type of RX CPU to be targeted.  Currently three types are supported, the generic RX600 and RX200 series hardware and the specific RX610 CPU.  The default is RX600.

     The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction.

     The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default when this type is selected.

-mbig-endian-data
-mlittle-endian-data
     Store data (but not code) in the big-endian format.  The default is -mlittle-endian-data, i.e. to store data in the little-endian format.

-msmall-data-limit=N
     Specifies the maximum size in bytes of global and static variables which can be placed into the small data area.  Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow.  Also when the small data area is used one of the RX's registers (usually r13) is reserved for use pointing to this area, so it is no longer available for use by the compiler.  This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.

     Note, common variables (variables that have not been initialized) and constants are not placed into the small data area as they are assigned to other sections in the output executable.

     The default value is zero, which disables this feature.  Note, this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register.  It is up to the programmer to experiment and discover whether this feature is of benefit to their program.  See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen.

-msim
-mno-sim
     Use the simulator runtime.  The default is to use the libgloss board-specific runtime.

-mas100-syntax
-mno-as100-syntax
     When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler.  This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.

-mmax-constant-size=N
     Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction.  Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction.  Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions.  Constants that are too big are instead placed into a constant pool and referenced via register indirection.

     The value N can be between 0 and 4.  A value of 0 (the default) or 4 means that constants of any size are allowed.

-mrelax
     Enable linker relaxation.  Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions.  Disabled by default.

-mint-register=N
     Specify the number of registers to reserve for fast interrupt handler functions.  The value N can be between 0 and 4.  A value of 1 means that register r13 is reserved for the exclusive use of fast interrupt handlers.  A value of 2 reserves r13 and r12.  A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, the default, does not reserve any registers.

-msave-acc-in-interrupts
     Specifies that interrupt handler functions should preserve the accumulator register.  This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications.  The default is to ignore the accumulator as this makes the interrupt handlers faster.

-mpid
-mno-pid
     Enables the generation of position independent data.  When enabled any access to constant data is done via an offset from a base address held in a register.  This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints.  Data that can be modified is not affected by this option.

     Note, using this feature reserves a register, usually r13, for the constant data base address.  This can result in slower and/or larger code, especially in complicated functions.

     The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled.  Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit.  Thus it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line.

     By default this feature is not enabled.  The default can be restored via the -mno-pid command-line option.

-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
     Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file.  The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt.

-mallow-string-insns
-mno-allow-string-insns
     Enables or disables the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL SWHILE and also the RMPA instruction.  These instructions may prefetch data, which is not safe to do if accessing an I/O register.  (See section 12.2.7 of the RX62N Group User's Manual for more information).

     The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically.  Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

     When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__.

Note: The generic GCC command-line option -ffixed-REG has special significance to the RX port when used with the interrupt function attribute.  This attribute indicates a function intended to process fast interrupts.  GCC ensures that it only uses the registers r10, r11, r12 and/or r13 and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-REG or -mint-register command-line options.

S/390 and zSeries Options
-------------------------

These are the -m options defined for the S/390 and zSeries architecture.

-mhard-float
-msoft-float
     Use (do not use) the hardware floating-point instructions and registers for floating-point operations.  When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations.  When -mhard-float is specified, the compiler generates IEEE floating-point instructions.  This is the default.

-mhard-dfp
-mno-hard-dfp
     Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations.  When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations.  When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions.  This is the default for -march=z9-ec or higher.

-mlong-double-64
-mlong-double-128
     These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default.

-mbackchain
-mno-backchain
     Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF 2 call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

     In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain.  Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.  In order to build a linux kernel use -msoft-float.

     The default is to not maintain the backchain.

-mpacked-stack
-mno-packed-stack
     Use (do not use) the packed stack layout.  When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

     As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack.  Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes.  Such code is not call-compatible with code compiled with -mpacked-stack.  Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.  In order to build a linux kernel use -msoft-float.

     The default is to not use the packed stack layout.

-msmall-exec
-mno-small-exec
     Generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k.  The default is to use the basr instruction instead, which does not have this limitation.

-m64
-m31
     When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI.  When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI.  This allows GCC in particular to generate 64-bit instructions.  For the s390 targets, the default is -m31, while the s390x targets default to -m64.

-mzarch
-mesa
     When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390.  Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa.  When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.

-mmvcle
-mno-mvcle
     Generate (or do not generate) code using the mvcle instruction to perform block moves.  When -mno-mvcle is specified, use a mvc loop instead.  This is the default unless optimizing for size.

-mdebug
-mno-debug
     Print (or do not print) additional debug information when compiling. The default is to not print debug information.

-march=CPU-TYPE
     Generate code that runs on CPU-TYPE, which is the name of a system representing a certain processor type.  Possible values for CPU-TYPE are g5, g6, z900, z990, z9-109, z9-ec, z10,  z196, zEC12, and z13. When generating code using the instructions available on z/Architecture, the default is -march=z900.  Otherwise, the default is -march=g5.

-mtune=CPU-TYPE
     Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions. The list of CPU-TYPE values is the same as for -march. The default is the value used for -march.

-mtpf-trace
-mno-tpf-trace
     Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system.  This option is off by default, even when compiling for the TPF OS.

-mfused-madd
-mno-fused-madd
     Generate code that uses (does not use) the floating-point multiply and accumulate instructions.  These instructions are generated by default if hardware floating point is used.

-mwarn-framesize=FRAMESIZE
     Emit a warning if the current function exceeds the given frame size.  Because this is a compile-time check it doesn't need to be a real problem when the program runs.  It is intended to identify functions that most probably cause a stack overflow.  It is useful to be used in an environment with limited stack size e.g. the linux kernel.

-mwarn-dynamicstack
     Emit a warning if the function calls alloca or uses dynamically-sized arrays.  This is generally a bad idea with a limited stack size.

-mstack-guard=STACK-GUARD
-mstack-size=STACK-SIZE
     If these options are provided the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is STACK-GUARD bytes above the STACK-SIZE (remember that the stack on S/390 grows downward). If the STACK-GUARD option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation.  The given values have to be exact powers of 2 and STACK-SIZE has to be greater than STACK-GUARD without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by STACK-SIZE. The STACK-GUARD option can only be used in conjunction with STACK-SIZE.

-mhotpatch=PRE-HALFWORDS,POST-HALFWORDS
     If the hotpatch option is enabled, a ``hot-patching'' function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (PRE-HALFWORDS, maximum 1000000).  After the label, 2 * POST-HALFWORDS bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).

     If both arguments are zero, hotpatching is disabled.

     This option can be overridden for individual functions with the hotpatch attribute.

Score Options
-------------

These options are defined for Score implementations:

-meb
     Compile code for big-endian mode.  This is the default.

-mel
     Compile code for little-endian mode.

-mnhwloop
     Disable generation of bcnz instructions.

-muls
     Enable generation of unaligned load and store instructions.

-mmac
     Enable the use of multiply-accumulate instructions. Disabled by default.

-mscore5
     Specify the SCORE5 as the target architecture.

-mscore5u
     Specify the SCORE5U of the target architecture.

-mscore7
     Specify the SCORE7 as the target architecture. This is the default.

-mscore7d
     Specify the SCORE7D as the target architecture.

SH Options
----------

These -m options are defined for the SH implementations:

-m1
     Generate code for the SH1.

-m2
     Generate code for the SH2.

-m2e
     Generate code for the SH2e.

-m2a-nofpu
     Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.

-m2a-single-only
     Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.

-m2a-single
     Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.

-m2a
     Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.

-m3
     Generate code for the SH3.

-m3e
     Generate code for the SH3e.

-m4-nofpu
     Generate code for the SH4 without a floating-point unit.

-m4-single-only
     Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.

-m4-single
     Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.

-m4
     Generate code for the SH4.

-m4-100
     Generate code for SH4-100.

-m4-100-nofpu
     Generate code for SH4-100 in such a way that the floating-point unit is not used.

-m4-100-single
     Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.

-m4-100-single-only
     Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.

-m4-200
     Generate code for SH4-200.

-m4-200-nofpu
     Generate code for SH4-200 without in such a way that the floating-point unit is not used.

-m4-200-single
     Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.

-m4-200-single-only
     Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.

-m4-300
     Generate code for SH4-300.

-m4-300-nofpu
     Generate code for SH4-300 without in such a way that the floating-point unit is not used.

-m4-300-single
     Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.

-m4-300-single-only
     Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.

-m4-340
     Generate code for SH4-340 (no MMU, no FPU).

-m4-500
     Generate code for SH4-500 (no FPU).  Passes -isa=sh4-nofpu to the assembler.

-m4a-nofpu
     Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.

-m4a-single-only
     Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.

-m4a-single
     Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.

-m4a
     Generate code for the SH4a.

-m4al
     Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler.  GCC doesn't generate any DSP instructions at the moment.

-m5-32media
     Generate 32-bit code for SHmedia.

-m5-32media-nofpu
     Generate 32-bit code for SHmedia in such a way that the floating-point unit is not used.

-m5-64media
     Generate 64-bit code for SHmedia.

-m5-64media-nofpu
     Generate 64-bit code for SHmedia in such a way that the floating-point unit is not used.

-m5-compact
     Generate code for SHcompact.

-m5-compact-nofpu
     Generate code for SHcompact in such a way that the floating-point unit is not used.

-mb
     Compile code for the processor in big-endian mode.

-ml
     Compile code for the processor in little-endian mode.

-mdalign
     Align doubles at 64-bit boundaries.  Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign.

-mrelax
     Shorten some address references at link time, when possible; uses the linker option -relax.

-mbigtable
     Use 32-bit offsets in switch tables.  The default is to use 16-bit offsets.

-mbitops
     Enable the use of bit manipulation instructions on SH2A.

-mfmovd
     Enable the use of the instruction fmovd.  Check -mdalign for alignment constraints.

-mrenesas
     Comply with the calling conventions defined by Renesas.

-mno-renesas
     Comply with the calling conventions defined for GCC before the Renesas conventions were available.  This option is the default for all targets of the SH toolchain.

-mnomacsave
     Mark the MAC register as call-clobbered, even if -mrenesas is given.

-mieee
-mno-ieee
     Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered.  By default -mieee is implicitly enabled.  If -ffinite-math-only is enabled -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons.  The implcit settings can be overridden by specifying either -mieee or -mno-ieee.

-minline-ic_invalidate
     Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write.  This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped.

-misize
     Dump instruction size and location in the assembly code.

-mpadstruct
     This option is deprecated.  It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.

-matomic-model=MODEL
     Sets the model of atomic operations and additional parameters as a comma separated list.  For details on the atomic built-in functions see __atomic Builtins.  The following models and parameters are supported:

     none
          Disable compiler generated atomic sequences and emit library calls for atomic operations.  This is the default if the target is not sh*-*-linux*.

     soft-gusa
          Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions.  The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems.  This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*.  When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified.

     soft-tcb
          Generate software atomic sequences that use a variable in the thread control block.  This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets.  The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems.  When using this model, the gbr-offset= parameter has to be specified as well.

     soft-imask
          Generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111.  This model works only when the program runs in privileged mode and is only suitable for single-core systems.  Additional support from the interrupt/exception handling code of the system is not required.  This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.

     hard-llcs
          Generate hardware atomic sequences using the movli.l and movco.l instructions only.  This is only available on SH4A and is suitable for multi-core systems.  Since the hardware instructions support only 32 bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system.  Additional support from the interrupt/exception handling code of the system is not required for this model.

     gbr-offset=
          This parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected.  For other models this parameter is ignored.  The specified value must be an integer multiple of four and in the range 0-1020.

     strict
          This parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only.

-mtas
     Generate the tas.b opcode for __atomic_test_and_set. Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction.  On multi-core SH4A processors the tas.b instruction must be used with caution since it can result in data corruption for certain cache configurations.

-mprefergot
     When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.

-musermode
-mno-usermode
     Don't allow (allow) the compiler generating privileged mode code.  Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode.  -musermode is the default when the target is sh*-*-linux*.  If the target is SH1* or SH2* -musermode has no effect, since there is no user mode.

-multcost=NUMBER
     Set the cost to assume for a multiply insn.

-mdiv=STRATEGY
     Set the division strategy to be used for integer division operations. For SHmedia STRATEGY can be one of:

     fp
          Performs the operation in floating point.  This has a very high latency, but needs only a few instructions, so it might be a good choice if your code has enough easily-exploitable ILP to allow the compiler to schedule the floating-point instructions together with other instructions. Division by zero causes a floating-point exception.

     inv
          Uses integer operations to calculate the inverse of the divisor, and then multiplies the dividend with the inverse.  This strategy allows CSE and hoisting of the inverse calculation.  Division by zero calculates an unspecified result, but does not trap.

     inv:minlat
          A variant of inv where, if no CSE or hoisting opportunities have been found, or if the entire operation has been hoisted to the same place, the last stages of the inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense of using a few more instructions, and thus offering fewer scheduling opportunities with other code.

     call
          Calls a library function that usually implements the inv:minlat strategy. This gives high code density for m5-*media-nofpu compilations.

     call2
          Uses a different entry point of the same library function, where it assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to CSE and code hoisting optimizations.

     inv:call
     inv:call2
     inv:fp
          Use the inv algorithm for initial code generation, but if the code stays unoptimized, revert to the call, call2, or fp strategies, respectively.  Note that the potentially-trapping side effect of division by zero is carried by a separate instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the side effect stays where it is. A recombination to floating-point operations or a call is not possible in that case.

     inv20u
     inv20l
          Variants of the inv:minlat strategy.  In the case that the inverse calculation is not separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable) by inserting a test to skip a number of operations in this case; this test slows down the case of larger dividends.  inv20u assumes the case of a such a small dividend to be unlikely, and inv20l assumes it to be likely.

     For targets other than SHmedia STRATEGY can be one of:

     call-div1
          Calls a library function that uses the single-step division instruction div1 to perform the operation.  Division by zero calculates an unspecified result and does not trap.  This is the default except for SH4, SH2A and SHcompact.

     call-fp
          Calls a library function that performs the operation in double precision floating point.  Division by zero causes a floating-point exception.  This is the default for SHcompact with FPU.  Specifying this for targets that do not have a double precision FPU defaults to call-div1.

     call-table
          Calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors.  Division by zero calculates an unspecified result and does not trap.  This is the default for SH4.  Specifying this for targets that do not have dynamic shift instructions defaults to call-div1.

     When a division strategy has not been specified the default strategy is selected based on the current target.  For SH2A the default strategy is to use the divs and divu instructions instead of library function calls.

-maccumulate-outgoing-args
     Reserve space once for outgoing arguments in the function prologue rather than around each call.  Generally beneficial for performance and size.  Also needed for unwinding to avoid changing the stack frame around conditional code.

-mdivsi3_libfunc=NAME
     Set the name of the library function used for 32-bit signed division to NAME. This only affects the name used in the call and inv:call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mindexed-addressing
     Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware and/or OS implement 32-bit wrap-around semantics for the indexed addressing mode.  The architecture allows the implementation of processors with 64-bit MMU, which the OS could use to get 32-bit addressing, but since no current hardware implementation supports this or any other way to make the indexed addressing mode safe to use in the 32-bit ABI, the default is -mno-indexed-addressing.

-mgettrcost=NUMBER
     Set the cost assumed for the gettr instruction to NUMBER. The default is 2 if -mpt-fixed is in effect, 100 otherwise.

-mpt-fixed
     Assume pt* instructions won't trap.  This generally generates better-scheduled code, but is unsafe on current hardware. The current architecture definition says that ptabs and ptrel trap when the target anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule these instructions before a branch, or hoist them out of a loop.  For example, __do_global_ctors, a part of libgcc that runs constructors at program startup, calls functions in a list which is delimited by -1.  With the -mpt-fixed option, the ptabs is done before testing against -1. That means that all the constructors run a bit more quickly, but when the loop comes to the end of the list, the program crashes because ptabs loads -1 into a target register.

     Since this option is unsafe for any hardware implementing the current architecture specification, the default is -mno-pt-fixed.  Unless specified explicitly with -mgettrcost, -mno-pt-fixed also implies -mgettrcost=100; this deters register allocation from using target registers for storing ordinary integers.

-minvalid-symbols
     Assume symbols might be invalid.  Ordinary function symbols generated by the compiler are always valid to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it is possible to generate symbols that cause ptabs or ptrel to trap. This option is only meaningful when -mno-pt-fixed is in effect. It prevents cross-basic-block CSE, hoisting and most scheduling of symbol loads.  The default is -mno-invalid-symbols.

-mbranch-cost=NUM
     Assume NUM to be the cost for a branch instruction.  Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.

-mzdcbranch
-mno-zdcbranch
     Assume (do not assume) that zero displacement conditional branch instructions bt and bf are fast.  If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences.  This is enabled by default when generating code for SH4 and SH4A.  It can be explicitly disabled by specifying -mno-zdcbranch.

-mcbranch-force-delay-slot
     Force the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction can't be found.  By default this option is disabled.  It can be enabled to work around hardware bugs as found in the original SH7055.

-mfused-madd
-mno-fused-madd
     Generate code that uses (does not use) the floating-point multiply and accumulate instructions.  These instructions are generated by default if hardware floating point is used.  The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.

-mfsca
-mno-fsca
     Allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations.  The option -mfsca must be used in combination with -funsafe-math-optimizations.  It is enabled by default when generating code for SH4A.  Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.

-mfsrra
-mno-fsrra
     Allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations.  The option -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only.  It is enabled by default when generating code for SH4A.  Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.

-mpretend-cmove
     Prefer zero-displacement conditional branches for conditional move instruction patterns.  This can result in faster code on the SH4 processor.

Solaris 2 Options
-----------------

These -m options are supported on Solaris 2:

-mclear-hwcap
     -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler.  This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.

-mimpure-text
     -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object.  Using this option, you can link position-dependent code into a shared object.

     -mimpure-text suppresses the ``relocations remain against allocatable but non-writable sections'' linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes.  Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

These switches are supported in addition to the above on Solaris 2:

-pthreads
     Add support for multithreading using the POSIX threads library.  This option sets flags for both the preprocessor and linker.  This option does not affect the thread safety of object code produced  by the compiler or that of libraries supplied with it.

-pthread
     This is a synonym for -pthreads.

SPARC Options
-------------

These -m options are supported on the SPARC:

-mno-app-regs
-mapp-regs
     Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications.  Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls.  This is the default.

     To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs.  You should compile libraries and system software with this option.

-mflat
-mno-flat
     With -mflat, the compiler does not generate save/restore instructions and uses a ``flat'' or single register window model.  This model is compatible with the regular register window model.  The local registers and the input registers (0--5) are still treated as ``call-saved'' registers and are saved on the stack as needed.

     With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions).  This is the normal operating mode.

-mfpu
-mhard-float
     Generate output containing floating-point instructions.  This is the default.

-mno-fpu
-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets.  Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.  The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mhard-quad-float
     Generate output containing quad-word (long double) floating-point instructions.

-msoft-quad-float
     Generate output containing library calls for quad-word (long double) floating-point instructions.  The functions called are those specified in the SPARC ABI.  This is the default.

     As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions.  They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction.  Because of the trap handler overhead, this is much slower than calling the ABI library routines.  Thus the -msoft-quad-float option is the default.

-mno-unaligned-doubles
-munaligned-doubles
     Assume that doubles have 8-byte alignment.  This is the default.

     With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address.  Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers.  It is not the default because it results in a performance loss, especially for floating-point code.

-muser-mode
-mno-user-mode
     Do not generate code that can only run in supervisor mode.  This is relevant only for the casa instruction emitted for the LEON3 processor.  The default is -mno-user-mode.

-mno-faster-structs
-mfaster-structs
     With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment.  This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI.  Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.

-mcpu=CPU_TYPE
     Set the instruction set, register set, and instruction scheduling parameters for machine type CPU_TYPE.  Supported values for CPU_TYPE are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3 and niagara4.

     Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

     Default instruction scheduling parameters are used for values that select an architecture and not an implementation.  These are v7, v8, sparclite, sparclet, v9.

     Here is a list of each supported architecture and their supported implementations.

     v7
          cypress, leon3v7

     v8
          supersparc, hypersparc, leon, leon3

     sparclite
          f930, f934, sparclite86x

     sparclet
          tsc701

     v9
          ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4

     By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture.  With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series.  This is also appropriate for the older SPARCStation 1, 2, IPX etc.

     With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture.  The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7.  With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

     With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture.  This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU.  With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

     With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture.  This adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7.  With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

     With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture.  This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions.  With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips.  With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips.  With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips.  With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips.  With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the instruction set or register set that the option -mcpu=CPU_TYPE does.

     The same values for -mcpu=CPU_TYPE can be used for -mtune=CPU_TYPE, but the only useful values are those that select a particular CPU implementation.  Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3 and niagara4.  With native Solaris and GNU/Linux toolchains, native can also be used.

-mv8plus
-mno-v8plus
     With -mv8plus, GCC generates code for the SPARC-V8+ ABI.  The difference from the V8 ABI is that the global and out registers are considered 64 bits wide.  This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.

-mvis
-mno-vis
     With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions.  The default is -mno-vis.

-mvis2
-mno-vis2
     With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions.  The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later.  Setting -mvis2 also sets -mvis.

-mvis3
-mno-vis3
     With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions.  The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later.  Setting -mvis3 also sets -mvis2 and -mvis.

-mcbcond
-mno-cbcond
     With -mcbcond, GCC generates code that takes advantage of compare-and-branch instructions, as defined in the Sparc Architecture 2011. The default is -mcbcond when targeting a cpu that supports such instructions, such as niagara-4 and later.

-mpopc
-mno-popc
     With -mpopc, GCC generates code that takes advantage of the UltraSPARC population count instruction.  The default is -mpopc when targeting a cpu that supports such instructions, such as Niagara-2 and later.

-mfmaf
-mno-fmaf
     With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point extensions.  The default is -mfmaf when targeting a cpu that supports such instructions, such as Niagara-3 and later.

-mfix-at697f
     Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).

-mfix-ut699
     Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:

-m32
-m64
     Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.

-mcmodel=WHICH
     Set the code model to one of

     medlow
          The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory.  Programs can be statically or dynamically linked.

     medmid
          The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.

     medany
          The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.

     embmedany
          The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time).  The global register %g4 points to the base of the data segment.  Programs are statically linked and PIC is not supported.

-mmemory-model=MEM-MODEL
     Set the memory model in force on the processor to one of

     default
          The default memory model for the processor and operating system.

     rmo
          Relaxed Memory Order

     pso
          Partial Store Order

     tso
          Total Store Order

     sc
          Sequential Consistency

     These memory models are formally defined in Appendix D of the Sparc V9 architecture manual, as set in the processor's PSTATE.MM field.

-mstack-bias
-mno-stack-bias
     With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references.  This is the default in 64-bit mode. Otherwise, assume no such offset is present.

SPU Options
-----------

These -m options are supported on the SPU:

-mwarn-reloc
-merror-reloc

     The loader for SPU does not handle dynamic relocations.  By default, GCC gives an error when it generates code that requires a dynamic relocation.  -mno-error-reloc disables the error, -mwarn-reloc generates a warning instead.

-msafe-dma
-munsafe-dma

     Instructions that initiate or test completion of DMA must not be reordered with respect to loads and stores of the memory that is being accessed. With -munsafe-dma you must use the volatile keyword to protect memory accesses, but that can lead to inefficient code in places where the memory is known to not change.  Rather than mark the memory as volatile, you can use -msafe-dma to tell the compiler to treat the DMA instructions as potentially affecting all memory.

-mbranch-hints

     By default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or probably-taken branches.  A hint is not generated closer than 8 instructions away from its branch. There is little reason to disable them, except for debugging purposes, or to make an object a little bit smaller.

-msmall-mem
-mlarge-mem

     By default, GCC generates code assuming that addresses are never larger than 18 bits.  With -mlarge-mem code is generated that assumes a full 32-bit address.

-mstdmain

     By default, GCC links against startup code that assumes the SPU-style main function interface (which has an unconventional parameter list). With -mstdmain, GCC links your program against startup code that assumes a C99-style interface to main, including a local copy of argv strings.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mea32
-mea64
     Compile code assuming that pointers to the PPU address space accessed via the __ea named address space qualifier are either 32 or 64 bits wide.  The default is 32 bits.  As this is an ABI-changing option, all object code in an executable must be compiled with the same setting.

-maddress-space-conversion
-mno-address-space-conversion
     Allow/disallow treating the __ea address space as superset of the generic address space.  This enables explicit type casts between __ea and generic pointer as well as implicit conversions of generic pointers to __ea pointers.  The default is to allow address space pointer conversions.

-mcache-size=CACHE-SIZE
     This option controls the version of libgcc that the compiler links to an executable and selects a software-managed cache for accessing variables in the __ea address space with a particular cache size.  Possible options for CACHE-SIZE are 8, 16, 32, 64 and 128.  The default cache size is 64KB.

-matomic-updates
-mno-atomic-updates
     This option controls the version of libgcc that the compiler links to an executable and selects whether atomic updates to the software-managed cache of PPU-side variables are used.  If you use atomic updates, changes to a PPU variable from SPU code using the __ea named address space qualifier do not interfere with changes to other PPU variables residing in the same cache line from PPU code.  If you do not use atomic updates, such interference may occur; however, writing back cache lines is more efficient.  The default behavior is to use atomic updates.

-mdual-nops
-mdual-nops=N
     By default, GCC inserts nops to increase dual issue when it expects it to increase performance.  N can be a value from 0 to 10.  A smaller N inserts fewer nops.  10 is the default, 0 is the same as -mno-dual-nops.  Disabled with -Os.

-mhint-max-nops=N
     Maximum number of nops to insert for a branch hint.  A branch hint must be at least 8 instructions away from the branch it is affecting.  GCC inserts up to N nops to enforce this, otherwise it does not generate the branch hint.

-mhint-max-distance=N
     The encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it is affecting.  By default, GCC makes sure it is within 125.

-msafe-hints
     Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts the hbrp instruction to make sure this stall won't happen.

Options for System V
--------------------

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

-G
     Create a shared object. It is recommended that -symbolic or -shared be used instead.

-Qy
     Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.

-Qn
     Refrain from adding .ident directives to the output file (this is the default).

-YP,DIRS
     Search the directories DIRS, and no others, for libraries specified with -l.

-Ym,DIR
     Look in the directory DIR to find the M4 preprocessor. The assembler uses this option.

TILE-Gx Options
---------------

These -m options are supported on the TILE-Gx:

-mcmodel=small
     Generate code for the small model.  The distance for direct calls is limited to 500M in either direction.  PC-relative addresses are 32 bits.  Absolute addresses support the full address range.

-mcmodel=large
     Generate code for the large model.  There is no limitation on call distance, pc-relative addresses, or absolute addresses.

-mcpu=NAME
     Selects the type of CPU to be targeted.  Currently the only supported type is tilegx.

-m32
-m64
     Generate code for a 32-bit or 64-bit environment.  The 32-bit environment sets int, long, and pointer to 32 bits.  The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.

-mbig-endian
-mlittle-endian
     Generate code in big/little endian mode, respectively.

TILEPro Options
---------------

These -m options are supported on the TILEPro:

-mcpu=NAME
     Selects the type of CPU to be targeted.  Currently the only supported type is tilepro.

-m32
     Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits.  This is the only supported behavior so the flag is essentially ignored.

V850 Options
------------

These -m options are defined for V850 implementations:

-mlong-calls
-mno-long-calls
     Treat all calls as being far away (near).  If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer.

-mno-ep
-mep
     Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions.  The -mep option is on by default if you optimize.

-mno-prolog-function
-mprolog-function
     Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function.  The external functions are slower, but use less code space if more than one function saves the same number of registers.  The -mprolog-function option is on by default if you optimize.

-mspace
     Try to make the code as small as possible.  At present, this just turns on the -mep and -mprolog-function options.

-mtda=N
     Put static or global variables whose size is N bytes or less into the tiny data area that register ep points to.  The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).

-msda=N
     Put static or global variables whose size is N bytes or less into the small data area that register gp points to.  The small data area can hold up to 64 kilobytes.

-mzda=N
     Put static or global variables whose size is N bytes or less into the first 32 kilobytes of memory.

-mv850
     Specify that the target processor is the V850.

-mv850e3v5
     Specify that the target processor is the V850E3V5.  The preprocessor constant __v850e3v5__ is defined if this option is used.

-mv850e2v4
     Specify that the target processor is the V850E3V5.  This is an alias for the -mv850e3v5 option.

-mv850e2v3
     Specify that the target processor is the V850E2V3.  The preprocessor constant __v850e2v3__ is defined if this option is used.

-mv850e2
     Specify that the target processor is the V850E2.  The preprocessor constant __v850e2__ is defined if this option is used.

-mv850e1
     Specify that the target processor is the V850E1.  The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.

-mv850es
     Specify that the target processor is the V850ES.  This is an alias for the -mv850e1 option.

-mv850e
     Specify that the target processor is the V850E.  The preprocessor constant __v850e__ is defined if this option is used.

     If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

     The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.

-mdisable-callt
-mno-disable-callt
     This option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

     This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use.  If CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined.

-mrelax
-mno-relax
     Pass on (or do not pass on) the -mrelax command-line option to the assembler.

-mlong-jumps
-mno-long-jumps
     Disable (or re-enable) the generation of PC-relative jump instructions.

-msoft-float
-mhard-float
     Disable (or re-enable) the generation of hardware floating point instructions.  This option is only significant when the target architecture is V850E2V3 or higher.  If hardware floating point instructions are being generated then the C preprocessor symbol __FPU_OK__ is defined, otherwise the symbol __NO_FPU__ is defined.

-mloop
     Enables the use of the e3v5 LOOP instruction.  The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental.

-mrh850-abi
-mghs
     Enables support for the RH850 version of the V850 ABI.  This is the default.  With this version of the ABI the following rules apply:

       * Integer sized structures and unions are returned via a memory pointer rather than a register.

       * Large structures and unions (more than 8 bytes in size) are passed by value.

       * Functions are aligned to 16-bit boundaries.

       * The -m8byte-align command-line option is supported.

       * The -mdisable-callt command-line option is enabled by default.  The -mno-disable-callt command-line option is not supported.

     When this version of the ABI is enabled the C preprocessor symbol __V850_RH850_ABI__ is defined.

-mgcc-abi
     Enables support for the old GCC version of the V850 ABI.  With this version of the ABI the following rules apply:

       * Integer sized structures and unions are returned in register r10.

       * Large structures and unions (more than 8 bytes in size) are passed by reference.

       * Functions are aligned to 32-bit boundaries, unless optimizing for size.

       * The -m8byte-align command-line option is not supported.

       * The -mdisable-callt command-line option is supported but not enabled by default.

     When this version of the ABI is enabled the C preprocessor symbol __V850_GCC_ABI__ is defined.

-m8byte-align
-mno-8byte-align
     Enables support for double and long long types to be aligned on 8-byte boundaries.  The default is to restrict the alignment of all objects to at most 4-bytes.  When -m8byte-align is in effect the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined.

-mbig-switch
     Generate code suitable for big switch tables.  Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mapp-regs
     This option causes r2 and r5 to be used in the code generated by the compiler.  This setting is the default.

-mno-app-regs
     This option causes r2 and r5 to be treated as fixed registers.

VAX Options
-----------

These -m options are defined for the VAX:

-munix
     Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.

-mgnu
     Do output those jump instructions, on the assumption that the GNU assembler is being used.

-mg
     Output code for G-format floating-point numbers instead of D-format.

Visium Options
--------------

-mdebug
     A program which performs file I/O and is destined to run on an MCM target should be linked with this option.  It causes the libraries libc.a and libdebug.a to be linked.  The program should be run on the target under the control of the GDB remote debugging stub.

-msim
     A program which performs file I/O and is destined to run on the simulator should be linked with option.  This causes libraries libc.a and libsim.a to be linked.

-mfpu
-mhard-float
     Generate code containing floating-point instructions.  This is the default.

-mno-fpu
-msoft-float
     Generate code containing library calls for floating-point.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mcpu=CPU_TYPE
     Set the instruction set, register set, and instruction scheduling parameters for machine type CPU_TYPE.  Supported values for CPU_TYPE are mcm, gr5 and gr6.

     mcm is a synonym of gr5 present for backward compatibility.

     By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

     With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture.  The only difference from GR5 code is that the compiler will generate block move instructions.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the instruction set or register set that the option -mcpu=CPU_TYPE would.

-msv-mode
     Generate code for the supervisor mode, where there are no restrictions on the access to general registers.  This is the default.

-muser-mode
     Generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected.

VMS Options
-----------

These -m options are defined for the VMS implementations:

-mvms-return-codes
     Return VMS condition codes from main. The default is to return POSIX-style condition (e.g. error) codes.

-mdebug-main=PREFIX
     Flag the first routine whose name starts with PREFIX as the main routine for the debugger.

-mmalloc64
     Default to 64-bit memory allocation routines.

-mpointer-size=SIZE
     Set the default size of pointers. Possible options for SIZE are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables pragma pointer_size.

VxWorks Options
---------------

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.

-mrtp
     GCC can generate code for both VxWorks kernels and real time processes (RTPs).  This option switches from the former to the latter.  It also defines the preprocessor macro __RTP__.

-non-static
     Link an RTP executable against shared libraries rather than static libraries.  The options -static and -shared can also be used for RTPs (see Link Options); -static is the default.

-Bstatic
-Bdynamic
     These options are passed down to the linker.  They are defined for compatibility with Diab.

-Xbind-lazy
     Enable lazy binding of function calls.  This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.

-Xbind-now
     Disable lazy binding of function calls.  This option is the default and is defined for compatibility with Diab.

x86 Options
-----------

These -m options are defined for the x86 family of computers.

-march=CPU-TYPE
     Generate instructions for the machine type CPU-TYPE.  In contrast to -mtune=CPU-TYPE, which merely tunes the generated code for the specified CPU-TYPE, -march=CPU-TYPE allows GCC to generate code that may not run at all on processors other than the one indicated.  Specifying -march=CPU-TYPE implies -mtune=CPU-TYPE.

     The choices for CPU-TYPE are:

     native
          This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine.  Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines).  Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.

     i386
          Original Intel i386 CPU.

     i486
          Intel i486 CPU.  (No scheduling is implemented for this chip.)

     i586
     pentium
          Intel Pentium CPU with no MMX support.

     iamcu
          Intel MCU, based on Intel Pentium CPU.

     pentium-mmx
          Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.

     pentiumpro
          Intel Pentium Pro CPU.

     i686
          When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.

     pentium2
          Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.

     pentium3
     pentium3m
          Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.

     pentium-m
          Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support.  Used by Centrino notebooks.

     pentium4
     pentium4m
          Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.

     prescott
          Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.

     nocona
          Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.

     core2
          Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

     nehalem
          Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.

     westmere
          Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.

     sandybridge
          Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.

     ivybridge
          Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.

     haswell
          Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.

     broadwell
          Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support.

     bonnell
          Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

     silvermont
          Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND instruction set support.

     knl
          Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.

     k6
          AMD K6 CPU with MMX instruction set support.

     k6-2
     k6-3
          Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.

     athlon
     athlon-tbird
          AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.

     athlon-4
     athlon-xp
     athlon-mp
          Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.

     k8
     opteron
     athlon64
     athlon-fx
          Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)

     k8-sse3
     opteron-sse3
     athlon64-sse3
          Improved versions of AMD K8 cores with SSE3 instruction set support.

     amdfam10
     barcelona
          CPUs based on AMD Family 10h cores with x86-64 instruction set support.  (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)

     bdver1
          CPUs based on AMD Family 15h cores with x86-64 instruction set support.  (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

     bdver2
          AMD Family 15h core based CPUs with x86-64 instruction set support.  (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

     bdver3
          AMD Family 15h core based CPUs with x86-64 instruction set support.  (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.

     bdver4
          AMD Family 15h core based CPUs with x86-64 instruction set support.  (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.

     btver1
          CPUs based on AMD Family 14h cores with x86-64 instruction set support.  (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)

     btver2
          CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.

     winchip-c6
          IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.

     winchip2
          IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.

     c3
          VIA C3 CPU with MMX and 3DNow! instruction set support.  (No scheduling is implemented for this chip.)

     c3-2
          VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

     geode
          AMD Geode embedded processor with MMX and 3DNow! instruction set support.

-mtune=CPU-TYPE
     Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific CPU-TYPE schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=CPU-TYPE option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.

     The choices for CPU-TYPE are the same as for -march. In addition, -mtune supports 2 extra choices for CPU-TYPE:

     generic
          Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic.  But, if you do not know exactly what CPU users of your application will have, then you should use this option.

          As new processors are deployed in the marketplace, the behavior of this option will change.  Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

          There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors.  In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.

     intel
          Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC.  If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.

          As new Intel processors are deployed in the marketplace, the behavior of this option will change.  Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

          There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors.  In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.

-mcpu=CPU-TYPE
     A deprecated synonym for -mtune.

-mfpmath=UNIT
     Generate floating-point arithmetic for selected unit UNIT.  The choices for UNIT are:

     387
          Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise.  Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips.  See -ffloat-store for more detailed description.

          This is the default choice for x86-32 targets.

     sse
          Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips.  The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387.  A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.

          For the x86-32 compiler, you must use -march=CPU-TYPE, -msse or -msse2 switches to enable SSE extensions and make this option effective.  For the x86-64 compiler, these extensions are enabled by default.

          The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits.

          This is the default choice for the x86-64 compiler.

     sse,387
     sse+387
     both
          Attempt to utilize both instruction sets at once.  This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too.  Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance.

-masm=DIALECT
     Output assembly instructions using selected DIALECT.  Also affects which dialect is used for basic asm (see Basic Asm) and extended asm (see Extended Asm). Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel.

-mieee-fp
-mno-ieee-fp
     Control whether or not the compiler uses IEEE floating-point comparisons.  These correctly handle the case where the result of a comparison is unordered.

-msoft-float
     Generate output containing library calls for floating point.

     Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.

     On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

-mno-fp-ret-in-387
     Do not use the FPU registers for return values of functions.

     The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU.  The idea is that the operating system should emulate an FPU.

     The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

-mno-fancy-math-387
     Some 387 emulators do not support the sin, cos and sqrt instructions for the 387.  Specify this option to avoid generating those instructions.  This option is the default on OpenBSD and NetBSD.  This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation.  These instructions are not generated unless you also use the -funsafe-math-optimizations switch.

-malign-double
-mno-align-double
     Control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary.  Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.

     On x86-64, -malign-double is enabled by default.

     Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.

-m96bit-long-double
-m128bit-long-double
     These switches control the size of long double type.  The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

     Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary.  In arrays or structures conforming to the ABI, this is not possible.  So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero.

     In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary.

     Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

     Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double.  Hence they are not binary-compatible with code compiled without that switch.

-mlong-double-64
-mlong-double-80
-mlong-double-128
     These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library.  A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library.

     Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double.  Hence they are not binary-compatible with code compiled without that switch.

-malign-data=TYPE
     Control how GCC aligns variables.  Supported values for TYPE are compat uses increased alignment value compatible uses GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased alignment value to match the cache line size.  compat is the default.

-mlarge-data-threshold=THRESHOLD
     When -mcmodel=medium is specified, data objects larger than THRESHOLD are placed in the large data section.  This value must be the same across all objects linked into the binary, and defaults to 65535.

-mrtd
     Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret NUM instruction, which pops their arguments while returning.  This saves one instruction in the caller since there is no need to pop the arguments there.

     You can specify that an individual function is called with this calling sequence with the function attribute stdcall.  You can also override the -mrtd option by using the function attribute cdecl.  See Function Attributes.

     Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions.

     In addition, seriously incorrect code results if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

-mregparm=NUM
     Control how many registers are used to pass integer arguments.  By default, no registers are used to pass arguments, and at most 3 registers can be used.  You can control this behavior for a specific function by using the function attribute regparm. See Function Attributes.

     Warning: if you use this switch, and NUM is nonzero, then you must build all modules with the same value, including any libraries.  This includes the system libraries and startup modules.

-msseregparm
     Use SSE register passing conventions for float and double arguments and return values.  You can control this behavior for a specific function by using the function attribute sseregparm. See Function Attributes.

     Warning: if you use this switch then you must build all modules with the same value, including any libraries.  This includes the system libraries and startup modules.

-mvect8-ret-in-mem
     Return 8-byte vectors in memory instead of MMX registers.  This is the default on Solaris 8 and 9 and VxWorks to match the ABI of the Sun Studio compilers until version 12.  Later compiler versions (starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on Solaris 10 and later.  Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.

-mpc32
-mpc64
-mpc80

     Set 80387 floating-point precision to 32, 64 or 80 bits.  When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default.  When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.

     Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more.  Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called ``catastrophic cancellation'', when this option is used to set the precision to less than extended precision.

-mstackrealign
     Realign the stack at entry.  On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary.  This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility.  See also the attribute force_align_arg_pointer, applicable to individual functions.

-mpreferred-stack-boundary=NUM
     Attempt to keep the stack boundary aligned to a 2 raised to NUM byte boundary.  If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

     Warning: When generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary.  Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation.  This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack.  In this case, SSE instructions may lead to misaligned memory access traps.  In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results.  You must build all modules with -mpreferred-stack-boundary=3, including any libraries.  This includes the system libraries and startup modules.

-mincoming-stack-boundary=NUM
     Assume the incoming stack is aligned to a 2 raised to NUM byte boundary.  If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

     On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties.  On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.

     To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned.  Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack.  It is recommended that libraries that use callbacks always use the default setting.

     This extra alignment does consume extra stack space, and generally increases code size.  Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-msha
-maes
-mpclmul
-mclfushopt
-mfsgsbase
-mrdrnd
-mf16c
-mfma
-mfma4
-mno-fma4
-mprefetchwt1
-mxop
-mlwp
-m3dnow
-mpopcnt
-mabm
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mtbm
-mmpx
-mmwaitx
     These switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, SHA, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT, RTM, MPX, MWAITX or 3DNow! extended instruction sets.  Each has a corresponding -mno- option to disable use of these instructions.

     These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.

     To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.

     GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.

     These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse.  Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags.  In particular, the file containing the CPU detection code should be compiled without these options.

-mdump-tune-features
     This option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=FEATURE-LIST.

-mtune-ctrl=FEATURE-LIST
     This option is used to do fine grain control of x86 code generation features. FEATURE-LIST is a comma separated list of FEATURE names. See also -mdump-tune-features. When specified, the FEATURE is turned on if it is not preceded with ^, otherwise, it is turned off. -mtune-ctrl=FEATURE-LIST is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors.

-mno-default
     This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=FEATURE-LIST and -mdump-tune-features.

-mcld
     This option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions.  String instructions depend on the DF flag to select between autoincrement or autodecrement mode.  While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers.  The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option.  Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case.

-mvzeroupper
     This option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics.

-mprefer-avx128
     This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.

-mcx16
     This option enables GCC to generate CMPXCHG16B instructions. CMPXCHG16B allows for atomic operations on 128-bit double quadword (or oword) data types. This is useful for high-resolution counters that can be updated by multiple processors (or cores).  This instruction is generated as part of atomic built-in functions: see __sync Builtins or __atomic Builtins for details.

-msahf
     This option enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details.

-mmovbe
     This option enables use of the movbe instruction to implement __builtin_bswap32 and __builtin_bswap64.

-mcrc32
     This option enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction.

-mrecip
     This option enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments.  These instructions are generated only when -funsafe-math-optimizations is enabled together with -finite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).

     Note that GCC implements 1.0f/sqrtf(X) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

     Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(X) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

-mrecip=OPT
     This option controls which reciprocal estimate instructions may be used.  OPT is a comma-separated list of options, which may be preceded by a ! to invert the option:

     all
          Enable all estimate instructions.

     default
          Enable the default instructions, equivalent to -mrecip.

     none
          Disable all estimate instructions, equivalent to -mno-recip.

     div
          Enable the approximation for scalar division.

     vec-div
          Enable the approximation for vectorized division.

     sqrt
          Enable the approximation for scalar square root.

     vec-sqrt
          Enable the approximation for vectorized square root.

     So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root.

-mveclibabi=TYPE
     Specifies the ABI type to use for vectorizing intrinsics using an external library.  Supported values for TYPE are svml for the Intel short vector math library and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.

     GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used.

-mabi=NAME
     Generate code for the specified calling convention.  Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI.  The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. See Function Attributes.

-mtls-dialect=TYPE
     Generate code to access thread-local storage using the gnu or gnu2 conventions.  gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems.

-mpush-args
-mno-push-args
     Use PUSH operations to store outgoing parameters.  This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default.  In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.

-maccumulate-outgoing-args
     If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue.  This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2.  The drawback is a notable increase in code size.  This switch implies -mno-push-args.

-mthreads
     Support thread-safe exception handling on MinGW.  Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option.  When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data.

-mno-align-stringops
     Do not align the destination of inlined string operations.  This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it.

-minline-all-stringops
     By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy, strlen, and memset for short lengths.

-minline-stringops-dynamically
     For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks.

-mstringop-strategy=ALG
     Override the internal decision heuristic for the particular algorithm to use for inlining string operations.  The allowed values for ALG are:

     rep_byte
     rep_4byte
     rep_8byte
          Expand using i386 rep prefix of the specified size.

     byte_loop
     loop
     unrolled_loop
          Expand into an inline loop.

     libcall
          Always use a library call.

-mmemcpy-strategy=STRATEGY
     Override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. STRATEGY is a comma-separated list of ALG:MAX_SIZE:DEST_ALIGN triplets. ALG is specified in -mstringop-strategy, MAX_SIZE specifies the max byte size with which inline algorithm ALG is allowed.  For the last triplet, the MAX_SIZE must be -1. The MAX_SIZE of the triplets in the list must be specified in increasing order.  The minimal byte size for ALG is 0 for the first triplet and MAX_SIZE + 1 of the preceding range.

-mmemset-strategy=STRATEGY
     The option is similar to -mmemcpy-strategy= except that it is to control __builtin_memset expansion.

-momit-leaf-frame-pointer
     Don't keep the frame pointer in a register for leaf functions.  This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions.  The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder.

-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
     Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added.  Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

     For systems that use the GNU C Library, the default is on.

-msse2avx
-mno-sse2avx
     Specify that the assembler should encode SSE instructions with VEX prefix.  The option -mavx turns this on by default.

-mfentry
-mno-fentry
     If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg.

-mrecord-mcount
-mno-record-mcount
     If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls.

-mnop-mcount
-mno-nop-mcount
     If profiling is active (-pg), generate the calls to the profiling functions as nops. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount.

-mskip-rax-setup
-mno-skip-rax-setup
     When generating code for the x86-64 architecture with SSE extensions disabled, -skip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers.

     Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location.  GCC 4.4 or newer don't have those issues, regardless the RAX register value.

-m8bit-idiv
-mno-8bit-idiv
     On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide.  This option generates a run-time check.  If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.

-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
     Split 32-byte AVX unaligned load and store.

-mstack-protector-guard=GUARD
     Generate stack protection code using canary at GUARD.  Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default).  This option has effect only when -fstack-protector or -fstack-protector-all is specified.

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.

-m32
-m64
-mx32
-m16
-miamcu
     Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs on any i386 system.

     The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.

     The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.

     The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.

     The -miamcu option generates code which conforms to Intel MCU psABI.  It requires the -m32 option to be turned on.

-mno-red-zone
     Do not use a so-called ``red zone'' for x86-64 code.  The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer.  The flag -mno-red-zone disables this red zone.

-mcmodel=small
     Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space.  Pointers are 64 bits. Programs can be statically or dynamically linked.  This is the default code model.

-mcmodel=kernel
     Generate code for the kernel code model.  The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.

-mcmodel=medium
     Generate code for the medium model: the program is linked in the lower 2 GB of the address space.  Small symbols are also placed there.  Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB.  Programs can be statically or dynamically linked.

-mcmodel=large
     Generate code for the large model.  This model makes no assumptions about addresses and sizes of sections.

-maddress-mode=long
     Generate code for long address mode.  This is only supported for 64-bit and x32 environments.  It is the default address mode for 64-bit environments.

-maddress-mode=short
     Generate code for short address mode.  This is only supported for 32-bit and x32 environments.  It is the default address mode for 32-bit and x32 environments.

x86 Windows Options
-------------------

These additional options are available for Microsoft Windows targets:

-mconsole
     This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets.

-mdll
     This option is available for Cygwin and MinGW targets.  It specifies that a DLL---a dynamic link library---is to be generated, enabling the selection of the required runtime startup object and entry point.

-mnop-fun-dllimport
     This option is available for Cygwin and MinGW targets.  It specifies that the dllimport attribute should be ignored.

-mthread
     This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.

-municode
     This option is available for MinGW-w64 targets.  It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.

-mwin32
     This option is available for Cygwin and MinGW targets.  It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.

-mwindows
     This option is available for Cygwin and MinGW targets.  It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.

-fno-set-stack-executable
     This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available.

-fwritable-relocated-rdata
     This option is available for MinGW and Cygwin targets.  It specifies that relocated-data in read-only section is put into .data section.  This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation.

-mpe-aligned-commons
     This option is available for Cygwin and MinGW targets.  It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code.  It is enabled by default if GCC detects that the target assembler found during configuration supports the feature.

See also under x86 Options for standard options.

Xstormy16 Options
-----------------

These options are defined for Xstormy16:

-msim
     Choose startup files and linker script suitable for the simulator.

Xtensa Options
--------------

These options are supported for Xtensa targets:

-mconst16
-mno-const16
     Enable or disable use of CONST16 instructions for loading constant values.  The CONST16 instruction is currently not a standard option from Tensilica.  When enabled, CONST16 instructions are always used in place of the standard L32R instructions.  The use of CONST16 is enabled by default only if the L32R instruction is not available.

-mfused-madd
-mno-fused-madd
     Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option.  This has no effect if the floating-point option is not also enabled.  Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations.  This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard.  Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations.

-mserialize-volatile
-mno-serialize-volatile
     When this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile.  Use -mno-serialize-volatile to omit the MEMW instructions.

-mforce-no-pic
     For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code.

-mtext-section-literals
-mno-text-section-literals
     These options control the treatment of literal pools.  The default is -mno-text-section-literals, which places literals in a separate section in the output file.  This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size.  With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references.  This may be necessary for large assembly files.

-mtarget-align
-mno-target-align
     When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density.  The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions.  If there are not enough preceding safe density instructions to align a target, no widening is performed.  The default is -mtarget-align.  These options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions.

-mlongcalls
-mno-longcalls
     When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction.  This translation typically occurs for calls to functions in other source files.  Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls.  This option should be used in programs where the call target can potentially be out of range.  This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions.  Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range.

zSeries Options
---------------

These are listed under see S/390 and zSeries Options.

Options for Code Generation Conventions
=======================================

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo.  In the table below, only one of the forms is listed---the one that is not the default.  You can figure out the other form by either removing no- or adding it.

-fbounds-check
     For front ends that support it, generate additional code to check that indices used to access arrays are within the declared range.  This is currently only supported by the Java and Fortran front ends, where this option defaults to true and false respectively.

-fstack-reuse=REUSE-LEVEL
     This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.  REUSE_LEVEL can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language.  When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization.

     For example,

             int *p;
             {
               int local1;

               p = &local1;
               local1 = 10;
               ....
             }
             {
                int local2;
                local2 = 20;
                ...
             }

             if (*p == 10)  // out of scope use of local1
               {

               }

     Another example:

             struct A
             {
                 A(int k) : i(k), j(k) { }
                 int i;
                 int j;
             };

             A *ap;

             void foo(const A& ar)
             {
                ap = &ar;
             }

             void bar()
             {
                foo(A(10)); // temp object's lifetime ends when foo returns

                {
                  A a(20);
                  ....
                }
                ap->i+= 10;  // ap references out of scope temp whose space
                             // is reused with a. What is the value of ap->i?
             }

     The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization.

-ftrapv
     This option generates traps for signed overflow on addition, subtraction, multiplication operations.

-fwrapv
     This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation.  This flag enables some optimizations and disables others.  This option is enabled by default for the Java front end, as required by the Java language specification.

-fexceptions
     Enable exception handling.  Generates extra code needed to propagate exceptions.  For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution.  If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it.  However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++.  You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.

-fnon-call-exceptions
     Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere.  Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions.  It does not allow exceptions to be thrown from arbitrary signal handlers such as SIGALRM.

-fdelete-dead-exceptions
     Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.

-funwind-tables
     Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.

-fasynchronous-unwind-tables
     Generate unwind table in DWARF 2 format, if supported by target machine.  The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).

-fno-gnu-unique
     On systems with recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of RTLD_LOCAL; this is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol.  But this causes dlclose to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via dlclose and dlopen, you can use -fno-gnu-unique.

-fpcc-struct-return
     Return ``short'' struct and union values in memory like longer ones, rather than in registers.  This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

     The precise convention for returning structures in memory depends on the target configuration macros.

     Short structures and unions are those whose size and alignment match that of some integer type.

     Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.

-freg-struct-return
     Return struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.

     If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target.  If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler.  In those cases, we can choose the standard, and we chose the more efficient register return alternative.

     Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

-fshort-enums
     Allocate to an enum type only as many bytes as it needs for the declared range of possible values.  Specifically, the enum type is equivalent to the smallest integer type that has enough room.

     Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-double
     Use the same size for double as for float.

     Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-wchar
     Override the underlying type for wchar_t to be short unsigned int instead of the default for the target.  This option is useful for building programs to run under WINE.

     Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fno-common
     In C code, controls the placement of uninitialized global variables. Unix C compilers have traditionally permitted multiple definitions of such variables in different compilation units by placing the variables in a common block. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. The -fno-common option specifies that the compiler should place uninitialized global variables in the data section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you get a multiple-definition error when you link them. In this case, you must compile with -fcommon instead. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable declarations this way.

-fno-ident
     Ignore the #ident directive.

-finhibit-size-directive
     Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory.  This option is used when compiling crtstuff.c; you should not need to use it for anything else.

-fverbose-asm
     Put extra commentary information in the generated assembly code to make it more readable.  This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

     -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

-frecord-gcc-switches
     This switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text.  This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file.

-fpic
     Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine.  Such code accesses all constant addresses through a global offset table (GOT).  The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system).  If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead.  (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000.  The x86 has no such limit.)

     Position-independent code requires special support, and therefore works only on certain machines.  For the x86, GCC supports PIC for System V but not for the Sun 386i.  Code generated for the IBM RS/6000 is always position-independent.

     When this flag is set, the macros __pic__ and __PIC__ are defined to 1.

-fPIC
     If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table.  This option makes a difference on the m68k, PowerPC and SPARC.

     Position-independent code requires special support, and therefore works only on certain machines.

     When this flag is set, the macros __pic__ and __PIC__ are defined to 2.

-fpie
-fPIE
     These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into executables. Usually these options are used when -pie GCC option is used during linking.

     -fpie and -fPIE both define the macros __pie__ and __PIE__.  The macros have the value 1 for -fpie and 2 for -fPIE.

-fno-plt
     Do not use PLT for external function calls in position-independent code. Instead, load callee address at call site from GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT load to optimizations.  On architectures such as 32-bit x86 where PLT stubs expect GOT pointer in a specific register, this gives more register allocation freedom to the compiler.  Lazy binding requires PLT: with -fno-plt all external symbols are resolved at load time.

     Alternatively, function attribute noplt can be used to avoid PLT for calls to specific external functions by marking those functions with this attribute.

     Additionally, a few targets also convert calls to those functions that are marked to not use the PLT to use the GOT instead for non-position independent code.

-fno-jump-tables
     Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies.  This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table.  On some targets, jump tables do not require a GOT and this option is not needed.

-ffixed-REG
     Treat the register named REG as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

     REG must be the name of a register.  The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

     This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-REG
     Treat the register named REG as an allocable register that is clobbered by function calls.  It may be allocated for temporaries or variables that do not live across a call.  Functions compiled this way do not save and restore the register REG.

     It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

     This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-REG
     Treat the register named REG as an allocable register saved by functions.  It may be allocated even for temporaries or variables that live across a call.  Functions compiled this way save and restore the register REG if they use it.

     It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

     A different sort of disaster results from the use of this flag for a register in which function values may be returned.

     This flag does not have a negative form, because it specifies a three-way choice.

-fpack-struct[=N]
     Without a value specified, pack all structure members together without holes.  When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.

     Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

-finstrument-functions
     Generate instrumentation calls for entry and exit to functions.  Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site.  (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

          void __cyg_profile_func_enter (void *this_fn,
                                         void *call_site);
          void __cyg_profile_func_exit  (void *this_fn,
                                         void *call_site);

     The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

     This instrumentation is also done for functions expanded inline in other functions.  The profiling calls indicate where, conceptually, the inline function is entered and exited.  This means that addressable versions of such functions must be available.  If all your uses of a function are expanded inline, this may mean an additional expansion of code size.  If you use extern inline in your C code, an addressable version of such functions must be provided.  (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

     A function may be given the attribute no_instrument_function, in which case this instrumentation is not done.  This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

-finstrument-functions-exclude-file-list=FILE,FILE,...

     Set the list of functions that are excluded from instrumentation (see the description of -finstrument-functions).  If the file that contains a function definition matches with one of FILE, then that function is not instrumented.  The match is done on substrings: if the FILE parameter is a substring of the file name, it is considered to be a match.

     For example:

          -finstrument-functions-exclude-file-list=/bits/stl,include/sys

     excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

     If, for some reason, you want to include letter , in one of SYM, write \,. For example, -finstrument-functions-exclude-file-list='\,\,tmp' (note the single quote surrounding the option).

-finstrument-functions-exclude-function-list=SYM,SYM,...

     This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation.  The function name to be matched is its user-visible name, such as vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE).  The match is done on substrings: if the SYM parameter is a substring of the function name, it is considered to be a match.  For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.

-fstack-check
     Generate code to verify that you do not go beyond the boundary of the stack.  You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

     Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that.  The switch causes generation of code to ensure that they see the stack being extended.

     You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

     Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

      1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold.

      2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.

      3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

     Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-limit-register=REG
-fstack-limit-symbol=SYM
-fno-stack-limit
     Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol.  If a larger stack is required, a signal is raised at run time.  For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

     For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB.  Note that this may only work with the GNU linker.

-fsplit-stack
     Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory.  This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread.  This is currently only implemented for the x86 targets running GNU/Linux.

     When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run.  If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack.  Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.

-fleading-underscore
     This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file.  One use is to help link with legacy assembly code.

     Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

-ftls-model=MODEL
     Alter the thread-local storage model to be used (see Thread-Local). The MODEL argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line.

     The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

-fvisibility=[default|internal|hidden|protected]
     Set the default ELF image symbol visibility to the specified option---all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

     Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.

     A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by ``How To Write Shared Libraries'' by Ulrich Drepper (which can be found at http://www.akkadia.org/drepper/)---however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public.  This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax.  This is a great boon to those working with cross-platform projects.

     For those adding visibility support to existing code, you may find #pragma GCC visibility of use.  This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

     Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default.  You may need to explicitly say #pragma GCC visibility push(default) before including any such headers.

     extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications.  However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

     Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs.

     An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/wiki/Visibility.

-fstrict-volatile-bitfields
     This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible.  For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.

     If this option is disabled, the compiler uses the most efficient instruction.  In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.

     In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine.  In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.

     Note:  Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members.  It is therefore recommended to define all bits of the field's type as bit-field members.

     The default value of this option is determined by the application binary interface for the target processor.

-fsync-libcalls
     This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions.

     The default value of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls.  This option is used in the implementation of the libatomic runtime library.

Environment Variables Affecting GCC
===================================

This section describes several environment variables that affect how GCC operates.  Some of them work by specifying directories or prefixes to use when searching for various kinds of files.  Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as -B, -I and -L (see Directory Options).  These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Controlling the Compilation Driver gcc (GNU Compiler Collection (GCC) Internals).

LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
     These environment variables control the way that GCC uses localization information which allows GCC to work with different national conventions.  GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so.  These locale categories can be set to any value supported by your installation.  A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

     The LC_CTYPE environment variable specifies character classification.  GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape.

     The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.

     If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable.  If none of these variables are set, GCC defaults to traditional C English behavior.

TMPDIR
     If TMPDIR is set, it specifies the directory to use for temporary files.  GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

GCC_COMPARE_DEBUG
     Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver.  See the documentation of this option for more details.

GCC_EXEC_PREFIX
     If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler.  No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

     If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

     If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

     The default value of GCC_EXEC_PREFIX is PREFIX/lib/gcc/ where PREFIX is the prefix to the installed compiler. In many cases PREFIX is the value of prefix when you ran the configure script.

     Other prefixes specified with -B take precedence over this prefix.

     This prefix is also used for finding files such as crt0.o that are used for linking.

     In addition, the prefix is used in an unusual way in finding the directories to search for header files.  For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name.  Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured PREFIX then the value of PREFIX is replaced by GCC_EXEC_PREFIX when looking for header files.

COMPILER_PATH
     The value of COMPILER_PATH is a colon-separated list of directories, much like PATH.  GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH
     The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH.  When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX.  Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).

LANG
     This variable is used to pass locale information to the compiler.  One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

     C-JIS
          Recognize JIS characters.

     C-SJIS
          Recognize SJIS characters.

     C-EUCJP
          Recognize EUCJP characters.

     If LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environment variables affect the behavior of the preprocessor.

     [file cppenv.texi not included]

Using Precompiled Headers
=========================

Often large projects have many header files that are included in every source file.  The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project.  To make builds faster, GCC allows you to precompile a header file.

To create a precompiled header file, simply compile it as you would any other file, if necessary using the -x option to make the driver treat it as a C or C++ header file.  You may want to use a tool like make to keep the precompiled header up-to-date when the headers it contains change.

A precompiled header file is searched for when #include is seen in the compilation.  As it searches for the included file (see Search Path (The C Preprocessor)) the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory.  The name searched for is the name specified in the #include with .gch appended.  If the precompiled header file can't be used, it is ignored.

For instance, if you have #include "all.h", and you have all.h.gch in the same directory as all.h, then the precompiled header file is used if possible, and the original header is used otherwise.

Alternatively, you might decide to put the precompiled header file in a directory and use -I to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an #error command.

This also works with -include.  So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and -include the precompiled header.  If the header files have guards against multiple inclusion, they are skipped because they've already been included (in the precompiled header).

If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like all.h.gch, and put each precompiled header in the directory, perhaps using -o.  It doesn't matter what you call the files in the directory; every precompiled header in the directory is considered.  The first precompiled header encountered in the directory that is valid for this compilation is used; they're searched in no particular order.

There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.

A precompiled header file can be used only when these conditions apply:

  * Only one precompiled header can be used in a particular compilation.

  * A precompiled header can't be used once the first C token is seen.  You can have preprocessor directives before a precompiled header; you cannot include a precompiled header from inside another header.

  * The precompiled header file must be produced for the same language as the current compilation.  You can't use a C precompiled header for a C++ compilation.

  * The precompiled header file must have been produced by the same compiler binary as the current compilation is using.

  * Any macros defined before the precompiled header is included must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that they don't appear in the precompiled header at all.

     The -D option is one way to define a macro before a precompiled header is included; using a #define can also do it. There are also some options that define macros implicitly, like -O and -Wdeprecated; the same rule applies to macros defined this way.

  * If debugging information is output when using the precompiled header, using -g or similar, the same kind of debugging information must have been output when building the precompiled header.  However, a precompiled header built using -g can be used in a compilation when no debugging information is being output.

  * The same -m options must generally be used when building and using the precompiled header.  See Submodel Options, for any cases where this rule is relaxed.

  * Each of the following options must be the same when building and using the precompiled header:

     -fexceptions

  * Some other command-line options starting with -f, -p, or -O must be defined in the same way as when the precompiled header was generated.  At present, it's not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header.  The following are known to be safe:

     -fmessage-length=  -fpreprocessed  -fsched-interblock -fsched-spec  -fsched-spec-load  -fsched-spec-load-dangerous -fsched-verbose=NUMBER  -fschedule-insns  -fvisibility= -pedantic-errors

For all of these except the last, the compiler automatically ignores the precompiled header if the conditions aren't met.  If you find an option combination that doesn't work and doesn't cause the precompiled header to be ignored, please consider filing a bug report, see Bugs.

If you do use differing options when generating and using the precompiled header, the actual behavior is a mixture of the behavior for the options.  For instance, if you use -g to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.