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[thirdparty/gcc.git] / gcc / doc / gcc / gcc-command-options / options-that-control-optimization.rst

..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: optimize options, options, optimization

.. _optimize-options:

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 completely disabled at :option:`-O0` or if an
:option:`-O` level is not set on the command line, even if individual
optimization flags are specified.  Similarly, :option:`-Og` suppresses
many optimization passes.

Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each :option:`-O` level than
those listed here.  You can invoke GCC with :option:`-Q --help=optimizers`
to find out the exact set of optimizations that are enabled at each level.
See :ref:`overall-options`, for examples.

.. option:: -O, -O1

  Optimize.  Optimizing compilation takes somewhat more time, and a lot
  more memory for a large function.

  With :option:`-O`, the compiler tries to reduce code size and execution
  time, without performing any optimizations that take a great deal of
  compilation time.

  .. Note that in addition to the default_options_table list in opts.cc,
     several optimization flags default to true but control optimization
     passes that are explicitly disabled at -O0.

  :option:`-O` turns on the following optimization flags:

  .. Please keep the following list alphabetized.

  :option:`-fauto-inc-dec` |gol|
  :option:`-fbranch-count-reg` |gol|
  :option:`-fcombine-stack-adjustments` |gol|
  :option:`-fcompare-elim` |gol|
  :option:`-fcprop-registers` |gol|
  :option:`-fdce` |gol|
  :option:`-fdefer-pop` |gol|
  :option:`-fdelayed-branch` |gol|
  :option:`-fdse` |gol|
  :option:`-fforward-propagate` |gol|
  :option:`-fguess-branch-probability` |gol|
  :option:`-fif-conversion` |gol|
  :option:`-fif-conversion2` |gol|
  :option:`-finline-functions-called-once` |gol|
  :option:`-fipa-modref` |gol|
  :option:`-fipa-profile` |gol|
  :option:`-fipa-pure-const` |gol|
  :option:`-fipa-reference` |gol|
  :option:`-fipa-reference-addressable` |gol|
  :option:`-fmerge-constants` |gol|
  :option:`-fmove-loop-invariants` |gol|
  :option:`-fmove-loop-stores` |gol|
  :option:`-fomit-frame-pointer` |gol|
  :option:`-freorder-blocks` |gol|
  :option:`-fshrink-wrap` |gol|
  :option:`-fshrink-wrap-separate` |gol|
  :option:`-fsplit-wide-types` |gol|
  :option:`-fssa-backprop` |gol|
  :option:`-fssa-phiopt` |gol|
  :option:`-ftree-bit-ccp` |gol|
  :option:`-ftree-ccp` |gol|
  :option:`-ftree-ch` |gol|
  :option:`-ftree-coalesce-vars` |gol|
  :option:`-ftree-copy-prop` |gol|
  :option:`-ftree-dce` |gol|
  :option:`-ftree-dominator-opts` |gol|
  :option:`-ftree-dse` |gol|
  :option:`-ftree-forwprop` |gol|
  :option:`-ftree-fre` |gol|
  :option:`-ftree-phiprop` |gol|
  :option:`-ftree-pta` |gol|
  :option:`-ftree-scev-cprop` |gol|
  :option:`-ftree-sink` |gol|
  :option:`-ftree-slsr` |gol|
  :option:`-ftree-sra` |gol|
  :option:`-ftree-ter` |gol|
  :option:`-funit-at-a-time`

.. option:: -O2

  Optimize even more.  GCC performs nearly all supported optimizations
  that do not involve a space-speed tradeoff.
  As compared to :option:`-O`, this option increases both compilation time
  and the performance of the generated code.

  :option:`-O2` turns on all optimization flags specified by :option:`-O1`.  It
  also turns on the following optimization flags:

  .. Please keep the following list alphabetized!

  :option:`-falign-functions`  :option:`-falign-jumps` |gol|
  :option:`-falign-labels`  :option:`-falign-loops` |gol|
  :option:`-fcaller-saves` |gol|
  :option:`-fcode-hoisting` |gol|
  :option:`-fcrossjumping` |gol|
  :option:`-fcse-follow-jumps`  :option:`-fcse-skip-blocks` |gol|
  :option:`-fdelete-null-pointer-checks` |gol|
  :option:`-fdevirtualize`  :option:`-fdevirtualize-speculatively` |gol|
  :option:`-fexpensive-optimizations` |gol|
  :option:`-ffinite-loops` |gol|
  :option:`-fgcse`  :option:`-fgcse-lm`  |gol|
  :option:`-fhoist-adjacent-loads` |gol|
  :option:`-finline-functions` |gol|
  :option:`-finline-small-functions` |gol|
  :option:`-findirect-inlining` |gol|
  :option:`-fipa-bit-cp`  :option:`-fipa-cp`  :option:`-fipa-icf` |gol|
  :option:`-fipa-ra`  :option:`-fipa-sra`  :option:`-fipa-vrp` |gol|
  :option:`-fisolate-erroneous-paths-dereference` |gol|
  :option:`-flra-remat` |gol|
  :option:`-foptimize-sibling-calls` |gol|
  :option:`-foptimize-strlen` |gol|
  :option:`-fpartial-inlining` |gol|
  :option:`-fpeephole2` |gol|
  :option:`-freorder-blocks-algorithm=stc` |gol|
  :option:`-freorder-blocks-and-partition`  :option:`-freorder-functions` |gol|
  :option:`-frerun-cse-after-loop`  |gol|
  :option:`-fschedule-insns`  :option:`-fschedule-insns2` |gol|
  :option:`-fsched-interblock`  :option:`-fsched-spec` |gol|
  :option:`-fstore-merging` |gol|
  :option:`-fstrict-aliasing` |gol|
  :option:`-fthread-jumps` |gol|
  :option:`-ftree-builtin-call-dce` |gol|
  :option:`-ftree-loop-vectorize` |gol|
  :option:`-ftree-pre` |gol|
  :option:`-ftree-slp-vectorize` |gol|
  :option:`-ftree-switch-conversion`  :option:`-ftree-tail-merge` |gol|
  :option:`-ftree-vrp` |gol|
  :option:`-fvect-cost-model=very-cheap`

  Please note the warning under :option:`-fgcse` about
  invoking :option:`-O2` on programs that use computed gotos.

.. option:: -O3

  Optimize yet more.  :option:`-O3` turns on all optimizations specified
  by :option:`-O2` and also turns on the following optimization flags:

  .. Please keep the following list alphabetized!

  :option:`-fgcse-after-reload` |gol|
  :option:`-fipa-cp-clone` |gol|
  :option:`-floop-interchange` |gol|
  :option:`-floop-unroll-and-jam` |gol|
  :option:`-fpeel-loops` |gol|
  :option:`-fpredictive-commoning` |gol|
  :option:`-fsplit-loops` |gol|
  :option:`-fsplit-paths` |gol|
  :option:`-ftree-loop-distribution` |gol|
  :option:`-ftree-partial-pre` |gol|
  :option:`-funswitch-loops` |gol|
  :option:`-fvect-cost-model=dynamic` |gol|
  :option:`-fversion-loops-for-strides`

.. option:: -O0

  Reduce compilation time and make debugging produce the expected
  results.  This is the default.

.. option:: -Os

  Optimize for size.  :option:`-Os` enables all :option:`-O2` optimizations
  except those that often increase code size:

  :option:`-falign-functions`  :option:`-falign-jumps` |gol|
  :option:`-falign-labels`  :option:`-falign-loops` |gol|
  :option:`-fprefetch-loop-arrays`  :option:`-freorder-blocks-algorithm=stc` |gol|
  It also enables :option:`-finline-functions`, causes the compiler to tune for
  code size rather than execution speed, and performs further optimizations
  designed to reduce code size.

.. option:: -Ofast

  Disregard strict standards compliance.  :option:`-Ofast` enables all
  :option:`-O3` optimizations.  It also enables optimizations that are not
  valid for all standard-compliant programs.
  It turns on :option:`-ffast-math`, :option:`-fallow-store-data-races`
  and the Fortran-specific :option:`-fstack-arrays`, unless
  :option:`-fmax-stack-var-size` is specified, and :option:`-fno-protect-parens`.
  It turns off :option:`-fsemantic-interposition`.

.. option:: -Og

  Optimize debugging experience.  :option:`-Og` 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.  It is a better choice than :option:`-O0`
  for producing debuggable code because some compiler passes
  that collect debug information are disabled at :option:`-O0`.

  Like :option:`-O0`, :option:`-Og` completely disables a number of
  optimization passes so that individual options controlling them have
  no effect.  Otherwise :option:`-Og` enables all :option:`-O1`
  optimization flags except for those that may interfere with debugging:

  :option:`-fbranch-count-reg`  :option:`-fdelayed-branch` |gol|
  :option:`-fdse`  :option:`-fif-conversion`  :option:`-fif-conversion2` |gol|
  :option:`-finline-functions-called-once` |gol|
  :option:`-fmove-loop-invariants`  :option:`-fmove-loop-stores`  :option:`-fssa-phiopt` |gol|
  :option:`-ftree-bit-ccp`  :option:`-ftree-dse`  :option:`-ftree-pta`  :option:`-ftree-sra`

.. option:: -Oz

  Optimize aggressively for size rather than speed.  This may increase
  the number of instructions executed if those instructions require
  fewer bytes to encode.  :option:`-Oz` behaves similarly to :option:`-Os`
  including enabling most :option:`-O2` optimizations.

If you use multiple :option:`-O` options, with or without level numbers,
the last such option is the one that is effective.

Options of the form :samp:`-fflag` specify machine-independent
flags.  Most flags have both positive and negative forms; the negative
form of :samp:`-ffoo` is :samp:`-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 :samp:`no-`
or adding it.

The following options control specific optimizations.  They are either
activated by :option:`-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.

.. option:: -fno-defer-pop

  For machines that must pop arguments after a function call, always pop
  the arguments as soon as each function returns.
  At levels :option:`-O1` and higher, :option:`-fdefer-pop` is the default;
  this allows the compiler to let arguments accumulate on the stack for several
  function calls and pop them all at once.

.. option:: -fdefer-pop

  Default setting; overrides :option:`-fno-defer-pop`.

.. option:: -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 :option:`-O1`,
  :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -ffp-contract={style}

  :option:`-ffp-contract=off` disables floating-point expression contraction.
  :option:`-ffp-contract=fast` enables floating-point expression contraction
  such as forming of fused multiply-add operations if the target has
  native support for them.
  :option:`-ffp-contract=on` enables floating-point expression contraction
  if allowed by the language standard.  This is currently not implemented
  and treated equal to :option:`-ffp-contract=off`.

  The default is :option:`-ffp-contract=fast`.

.. option:: -fomit-frame-pointer

  Omit the frame pointer in functions that don't need one.  This avoids the
  instructions to save, set up and restore the frame pointer; on many targets
  it also makes an extra register available.

  On some targets this flag has no effect because the standard calling sequence
  always uses a frame pointer, so it cannot be omitted.

  Note that :option:`-fno-omit-frame-pointer` doesn't guarantee the frame pointer
  is used in all functions.  Several targets always omit the frame pointer in
  leaf functions.

  Enabled by default at :option:`-O1` and higher.

.. option:: -foptimize-sibling-calls

  Optimize sibling and tail recursive calls.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O2`, :option:`-O3`.

.. option:: -fno-inline

  Do not expand any functions inline apart from those marked with
  the :fn-attr:`always_inline` attribute.  This is the default when not
  optimizing.

  Single functions can be exempted from inlining by marking them
  with the :fn-attr:`noinline` attribute.

.. option:: -finline

  Default setting; overrides :option:`-fno-inline`.

.. option:: -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 levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-finline-functions`
  or :option:`-finline-small-functions` options.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 levels :option:`-O2`, :option:`-O3`, :option:`-Os`.  Also enabled
  by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`-O1`, :option:`-O2`, :option:`-O3` and :option:`-Os`,
  but not :option:`-Og`.

.. option:: -fearly-inlining

  Inline functions marked by :fn-attr:`always_inline` and functions whose body seems
  smaller than the function call overhead early before doing
  :option:`-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.

.. option:: -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 :option:`-O2`, :option:`-O3` and :option:`-Os`.

.. option:: -finline-limit={n}

  By default, GCC limits the size of functions that can be inlined.  This flag
  allows coarse control of this limit.  :samp:`{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 :option:`--param name=value`.
  The :option:`-finline-limit=n` option sets some of these parameters
  as follows:

  ``max-inline-insns-single``
    is set to :samp:`{n}/2`.

  ``max-inline-insns-auto``
    is set to :samp:`{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 :option:`-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.

.. option:: -fno-keep-inline-dllexport

  This is a more fine-grained version of :option:`-fkeep-inline-functions`,
  which applies only to functions that are declared using the :microsoft-windows-fn-attr:`dllexport`
  attribute or declspec.  See :ref:`function-attributes`.

.. option:: -fkeep-inline-dllexport

  Default setting; overrides :option:`-fno-keep-inline-dllexport`.

.. option:: -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.

.. option:: -fkeep-static-functions

  Emit ``static`` functions into the object file, even if the function
  is never used.

.. option:: -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 :option:`-fno-keep-static-consts` option.

.. 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 :option:`-fno-merge-constants` to inhibit this
  behavior.

  Enabled at levels :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fmerge-all-constants

  Attempt to merge identical constants and identical variables.

  This option implies :option:`-fmerge-constants`.  In addition to
  :option:`-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.

.. option:: -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.

.. option:: -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
  :option:`-fmodulo-sched` enabled.

.. option:: -fno-branch-count-reg

  Disable the optimization pass that scans for opportunities to use
  'decrement and branch' instructions on a count register instead of
  instruction sequences that decrement a register, compare it against zero, and
  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.  Note that the :option:`-fno-branch-count-reg` option
  doesn't remove the decrement and branch instructions from the generated
  instruction stream introduced by other optimization passes.

  The default is :option:`-fbranch-count-reg` at :option:`-O1` and higher,
  except for :option:`-Og`.

.. option:: -fbranch-count-reg

  Default setting; overrides :option:`-fno-branch-count-reg`.

.. option:: -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 :option:`-ffunction-cse`

.. option:: -ffunction-cse

  Default setting; overrides :option:`-fno-function-cse`.

.. option:: -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 :option:`-fzero-initialized-in-bss`.

.. option:: -fzero-initialized-in-bss

  Default setting; overrides :option:`-fno-zero-initialized-in-bss`.

.. option:: -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 :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O1`, :option:`-O2`, :option:`-O3`,
  :option:`-Os`.

.. option:: -fsplit-wide-types-early

  Fully split wide types early, instead of very late.
  This option has no effect unless :option:`-fsplit-wide-types` is turned on.

  This is the default on some targets.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fcse-skip-blocks

  This is similar to :option:`-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,
  :option:`-fcse-skip-blocks` causes CSE to follow the jump around the
  body of the ``if``.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -frerun-cse-after-loop

  Re-run common subexpression elimination after loop optimizations are
  performed.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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
    :option:`-fno-gcse` to the command line.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fgcse-lm

  When :option:`-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 :option:`-fgcse` is enabled.

.. option:: -fgcse-sm

  When :option:`-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 :option:`-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.

.. option:: -fgcse-las

  When :option:`-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.

.. option:: -fgcse-after-reload

  When :option:`-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.

  Enabled by :option:`-O3`, :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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.

.. option:: -funconstrained-commons

  This option tells the compiler that variables declared in common blocks
  (e.g. Fortran) may later be overridden with longer trailing arrays. This
  prevents certain optimizations that depend on knowing the array bounds.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O1` and
  higher on architectures that support this.

.. option:: -fdce

  Perform dead code elimination (DCE) on RTL.
  Enabled by default at :option:`-O1` and higher.

.. option:: -fdse

  Perform dead store elimination (DSE) on RTL.
  Enabled by default at :option:`-O1` and higher.

.. option:: -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 :option:`-fif-conversion2`.

  Enabled at levels :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`, but
  not with :option:`-Og`.

.. option:: -fif-conversion2

  Use conditional execution (where available) to transform conditional jumps into
  branch-less equivalents.

  Enabled at levels :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`, but
  not with :option:`-Og`.

.. option:: -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 :option:`-Os`.

.. option:: -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 :option:`-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 MSP430, this option is completely disabled.

  Passes that use the dataflow information
  are enabled independently at different optimization levels.

.. option:: -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 (:option:`-findirect-inlining`) and interprocedural constant
  propagation (:option:`-fipa-cp`).
  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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.

.. option:: -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.

.. option:: -fexpensive-optimizations

  Perform a number of minor optimizations that are relatively expensive.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O2`,
  :option:`-O3`, :option:`-Os`.

.. option:: -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.  To preserve stores before the
  constructor starts (e.g. because your operator new clears the object
  storage) but still treat the object as dead after the destructor, you
  can use :option:`-flifetime-dse=1`.  The default behavior can be
  explicitly selected with :option:`-flifetime-dse=2`.
  :option:`-flifetime-dse=0` is equivalent to :option:`-fno-lifetime-dse`.

.. option:: -flifetime-dse

  Default setting; overrides :option:`-fno-lifetime-dse`.

.. option:: -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.

.. option:: -fira-algorithm={algorithm}

  Use the specified coloring algorithm for the integrated register
  allocator.  The :samp:`{algorithm}` argument can be :samp:`priority`, which
  specifies Chow's priority coloring, or :samp:`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.

.. option:: -fira-region={region}

  Use specified regions for the integrated register allocator.  The
  :samp:`{region}` argument should be one of the following:

  :samp:`all`
    Use all loops as register allocation regions.
    This can give the best results for machines with a small and/or
    irregular register set.

  :samp:`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
    (:option:`-O`, :option:`-O2`, ...).

  :samp:`one`
    Use all functions as a single region.
    This typically results in the smallest code size, and is enabled by default for
    :option:`-Os` or :option:`-O0`.

.. option:: -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 :option:`-Os` for all targets.

.. option:: -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 :option:`-O3` for some targets.

.. option:: -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.

.. option:: -fira-share-save-slots

  Default setting; overrides :option:`-fno-ira-share-save-slots`.

.. option:: -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.

.. option:: -fira-share-spill-slots

  Default setting; overrides :option:`-fno-ira-share-spill-slots`.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fdelayed-branch

  If supported for the target machine, attempt to reorder instructions
  to exploit instruction slots available after delayed branch
  instructions.

  Enabled at levels :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`,
  but not at :option:`-Og`.

.. option:: -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 :option:`-O2`, :option:`-O3`.

.. option:: -fschedule-insns2

  Similar to :option:`-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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fno-sched-interblock

  Disable instruction scheduling across basic blocks, which
  is normally enabled when scheduling before register allocation, i.e.
  with :option:`-fschedule-insns` or at :option:`-O2` or higher.

.. option:: -fsched-interblock

  Default setting; overrides :option:`-fno-sched-interblock`.

.. option:: -fno-sched-spec

  Disable speculative motion of non-load instructions, which
  is normally enabled when scheduling before register allocation, i.e.
  with :option:`-fschedule-insns` or at :option:`-O2` or higher.

.. option:: -fsched-spec

  Default setting; overrides :option:`-fno-sched-spec`.

.. option:: -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 :option:`-fschedule-insns` or at
  :option:`-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.

.. option:: -fsched-spec-load

  Allow speculative motion of some load instructions.  This only makes
  sense when scheduling before register allocation, i.e. with
  :option:`-fschedule-insns` or at :option:`-O2` or higher.

.. option:: -fsched-spec-load-dangerous

  Allow speculative motion of more load instructions.  This only makes
  sense when scheduling before register allocation, i.e. with
  :option:`-fschedule-insns` or at :option:`-O2` or higher.

.. option:: -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.
  :option:`-fno-sched-stalled-insns` means that no insns are moved
  prematurely, :option:`-fsched-stalled-insns=0` means there is no limit
  on how many queued insns can be moved prematurely.
  :option:`-fsched-stalled-insns` without a value is equivalent to
  :option:`-fsched-stalled-insns=1`.

.. option:: -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 :option:`-fsched-stalled-insns` is used.
  :option:`-fno-sched-stalled-insns-dep` is equivalent to
  :option:`-fsched-stalled-insns-dep=0`.
  :option:`-fsched-stalled-insns-dep` without a value is equivalent to
  :option:`-fsched-stalled-insns-dep=1`.

.. option:: -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
  :option:`-fschedule-insns2` or at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns`
  or :option:`-fschedule-insns2` or at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns`
  or :option:`-fschedule-insns2` or at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns` or :option:`-fschedule-insns2`
  or at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns` or :option:`-fschedule-insns2` or
  at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns` or :option:`-fschedule-insns2` or
  at :option:`-O2` or higher.

.. option:: -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 :option:`-fschedule-insns` or :option:`-fschedule-insns2` or
  at :option:`-O2` or higher.

.. option:: -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.

.. option:: -fselective-scheduling

  Schedule instructions using selective scheduling algorithm.  Selective
  scheduling runs instead of the first scheduler pass.

.. option:: -fselective-scheduling2

  Schedule instructions using selective scheduling algorithm.  Selective
  scheduling runs instead of the second scheduler pass.

.. option:: -fsel-sched-pipelining

  Enable software pipelining of innermost loops during selective scheduling.
  This option has no effect unless one of :option:`-fselective-scheduling` or
  :option:`-fselective-scheduling2` is turned on.

.. option:: -fsel-sched-pipelining-outer-loops

  When pipelining loops during selective scheduling, also pipeline outer loops.
  This option has no effect unless :option:`-fsel-sched-pipelining` is turned on.

.. option:: -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 :option:`-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.

.. option:: -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
  :option:`-O` and higher.

.. option:: -fshrink-wrap-separate

  Shrink-wrap separate parts of the prologue and epilogue separately, so that
  those parts are only executed when needed.
  This option is on by default, but has no effect unless :option:`-fshrink-wrap`
  is also turned on and the target supports this.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O1` and higher.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`, however the option
  is disabled if generated code will be instrumented for profiling
  (:option:`-p`, or :option:`-pg`) or if callee's register usage cannot be known
  exactly (this happens on targets that do not expose prologues
  and epilogues in RTL).

.. option:: -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.

.. option:: -ftree-reassoc

  Perform reassociation on trees.  This flag is enabled by default
  at :option:`-O1` and higher.

.. option:: -fcode-hoisting

  Perform code hoisting.  Code hoisting tries to move the
  evaluation of expressions executed on all paths to the function exit
  as early as possible.  This is especially useful as a code size
  optimization, but it often helps for code speed as well.
  This flag is enabled by default at :option:`-O2` and higher.

.. option:: -ftree-pre

  Perform partial redundancy elimination (PRE) on trees.  This flag is
  enabled by default at :option:`-O2` and :option:`-O3`.

.. option:: -ftree-partial-pre

  Make partial redundancy elimination (PRE) more aggressive.  This flag is
  enabled by default at :option:`-O3`.

.. option:: -ftree-forwprop

  Perform forward propagation on trees.  This flag is enabled by default
  at :option:`-O1` and higher.

.. option:: -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 :option:`-O1` and higher.

.. option:: -ftree-phiprop

  Perform hoisting of loads from conditional pointers on trees.  This
  pass is enabled by default at :option:`-O1` and higher.

.. option:: -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 :option:`-O2` and higher.

.. option:: -ftree-copy-prop

  Perform copy propagation on trees.  This pass eliminates unnecessary
  copy operations.  This flag is enabled by default at :option:`-O1` and
  higher.

.. option:: -fipa-pure-const

  Discover which functions are pure or constant.
  Enabled by default at :option:`-O1` and higher.

.. option:: -fipa-reference

  Discover which static variables do not escape the
  compilation unit.
  Enabled by default at :option:`-O1` and higher.

.. option:: -fipa-reference-addressable

  Discover read-only, write-only and non-addressable static variables.
  Enabled by default at :option:`-O1` and higher.

.. option:: -fipa-stack-alignment

  Reduce stack alignment on call sites if possible.
  Enabled by default.

.. option:: -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.

.. option:: -fipa-profile

  Perform interprocedural profile propagation.  The functions called only from
  cold functions are marked as cold. Also functions executed once (such as
  :fn-attr:`cold`, :fn-attr:`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 :option:`-O1` and higher.

.. option:: -fipa-modref

  Perform interprocedural mod/ref analysis.  This optimization analyzes the side
  effects of functions (memory locations that are modified or referenced) and
  enables better optimization across the function call boundary.  This flag is
  enabled by default at :option:`-O1` and higher.

.. option:: -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 :option:`-O2`, :option:`-Os` and :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`--param ipa-cp-unit-growth=value`).
  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -fipa-bit-cp

  When enabled, perform interprocedural bitwise constant
  propagation. This flag is enabled by default at :option:`-O2` and
  by :option:`-fprofile-use` and :option:`-fauto-profile`.
  It requires that :option:`-fipa-cp` is enabled.

.. option:: -fipa-vrp

  When enabled, perform interprocedural propagation of value
  ranges. This flag is enabled by default at :option:`-O2`. It requires
  that :option:`-fipa-cp` is enabled.

.. option:: -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.

  Although the behavior is similar to the Gold Linker's 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 :option:`-O2` and :option:`-Os`.

.. option:: -flive-patching={level}

  Control GCC's optimizations to produce output suitable for live-patching.

  If the compiler's optimization uses a function's body or information extracted
  from its body to optimize/change another function, the latter is called an
  impacted function of the former.  If a function is patched, its impacted
  functions should be patched too.

  The impacted functions are determined by the compiler's interprocedural
  optimizations.  For example, a caller is impacted when inlining a function
  into its caller,
  cloning a function and changing its caller to call this new clone,
  or extracting a function's pureness/constness information to optimize
  its direct or indirect callers, etc.

  Usually, the more IPA optimizations enabled, the larger the number of
  impacted functions for each function.  In order to control the number of
  impacted functions and more easily compute the list of impacted function,
  IPA optimizations can be partially enabled at two different levels.

  The :samp:`{level}` argument should be one of the following:

  :samp:`inline-clone`
    Only enable inlining and cloning optimizations, which includes inlining,
    cloning, interprocedural scalar replacement of aggregates and partial inlining.
    As a result, when patching a function, all its callers and its clones'
    callers are impacted, therefore need to be patched as well.

    :option:`-flive-patching=inline-clone` disables the following optimization flags:

    :option:`-fwhole-program`  :option:`-fipa-pta`  :option:`-fipa-reference`  :option:`-fipa-ra` |gol|
    :option:`-fipa-icf`  :option:`-fipa-icf-functions`  :option:`-fipa-icf-variables` |gol|
    :option:`-fipa-bit-cp`  :option:`-fipa-vrp`  :option:`-fipa-pure-const`  :option:`-fipa-reference-addressable` |gol|
    :option:`-fipa-stack-alignment` :option:`-fipa-modref`

  :samp:`inline-only-static`
    Only enable inlining of static functions.
    As a result, when patching a static function, all its callers are impacted
    and so need to be patched as well.

    In addition to all the flags that :option:`-flive-patching=inline-clone`
    disables,
    :option:`-flive-patching=inline-only-static` disables the following additional
    optimization flags:

    :option:`-fipa-cp-clone`  :option:`-fipa-sra`  :option:`-fpartial-inlining`  :option:`-fipa-cp`

    When :option:`-flive-patching` is specified without any value, the default value
    is :samp:`{inline-clone}`.

  This flag is disabled by default.

  Note that :option:`-flive-patching` is not supported with link-time optimization
  (:option:`-flto`).

.. option:: -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 :option:`-O2` and higher and depends on
  :option:`-fdelete-null-pointer-checks` also being enabled.

.. option:: -fisolate-erroneous-paths-attribute

  Detect paths that trigger erroneous or undefined behavior due to a null value
  being used in a way forbidden by a :fn-attr:`returns_nonnull` or :fn-attr:`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 :option:`-O2` in the future.

.. option:: -ftree-sink

  Perform forward store motion on trees.  This flag is
  enabled by default at :option:`-O1` and higher.

.. option:: -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 :option:`-O1` and higher, except for :option:`-Og`.
  It requires that :option:`-ftree-ccp` is enabled.

.. option:: -ftree-ccp

  Perform sparse conditional constant propagation (CCP) on trees.  This
  pass only operates on local scalar variables and is enabled by default
  at :option:`-O1` and higher.

.. option:: -fssa-backprop

  Propagate information about uses of a value up the definition chain
  in order to simplify the definitions.  For example, this pass strips
  sign operations if the sign of a value never matters.  The flag is
  enabled by default at :option:`-O1` and higher.

.. option:: -fssa-phiopt

  Perform pattern matching on SSA PHI nodes to optimize conditional
  code.  This pass is enabled by default at :option:`-O1` and higher,
  except for :option:`-Og`.

.. option:: -ftree-switch-conversion

  Perform conversion of simple initializations in a switch to
  initializations from a scalar array.  This flag is enabled by default
  at :option:`-O2` and higher.

.. option:: -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 :option:`-O2` and higher.  The compilation time
  in this pass can
  be limited using max-tail-merge-comparisons parameter and
  max-tail-merge-iterations parameter.

.. option:: -ftree-dce

  Perform dead code elimination (DCE) on trees.  This flag is enabled by
  default at :option:`-O1` and higher.

.. option:: -ftree-builtin-call-dce

  Perform conditional dead code elimination (DCE) for calls to built-in functions
  that may set ``errno`` but are otherwise free of side effects.  This flag is
  enabled by default at :option:`-O2` and higher if :option:`-Os` is not also
  specified.

.. option:: -ffinite-loops

  Assume that a loop with an exit will eventually take the exit and not loop
  indefinitely.  This allows the compiler to remove loops that otherwise have
  no side-effects, not considering eventual endless looping as such.

  This option is enabled by default at :option:`-O2` for C++ with -std=c++11
  or higher.

.. option:: -fno-finite-loops

  Default setting; overrides :option:`-ffinite-loops`.

.. option:: -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 :option:`-O1` and higher.

.. option:: -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 :option:`-O1` and higher.

.. option:: -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 :option:`-O1` and higher.  It is not enabled
  for :option:`-Os`, since it usually increases code size.

.. option:: -ftree-loop-optimize

  Perform loop optimizations on trees.  This flag is enabled by default
  at :option:`-O1` and higher.

.. option:: -ftree-loop-linear, -floop-strip-mine, -floop-block

  Perform loop nest optimizations.  Same as
  :option:`-floop-nest-optimize`.  To use this code transformation, GCC has
  to be configured with :option:`--with-isl` to enable the Graphite loop
  transformation infrastructure.

.. option:: -fgraphite-identity

  Enable the identity transformation for graphite.  For every SCoP we generate
  the polyhedral representation and transform it back to gimple.  Using
  :option:`-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.

.. option:: -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.

.. option:: -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.

.. option:: -ftree-coalesce-vars

  While transforming the program out of the SSA representation, attempt to
  reduce copying by coalescing versions of different user-defined
  variables, instead of just compiler temporaries.  This may severely
  limit the ability to debug an optimized program compiled with
  :option:`-fno-var-tracking-assignments`.  In the negated form, this flag
  prevents SSA coalescing of user variables.  This option is enabled by
  default if optimization is enabled, and it does very little otherwise.

.. option:: -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.

.. option:: -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

  .. code-block:: fortran

    DO I = 1, N
      A(I) = B(I) + C
      D(I) = E(I) * F
    ENDDO

  is transformed to

  .. code-block:: fortran

    DO I = 1, N
       A(I) = B(I) + C
    ENDDO
    DO I = 1, N
       D(I) = E(I) * F
    ENDDO

  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`-O2` and
  higher, and by :option:`-fprofile-use` and :option:`-fauto-profile`.

  This pass distributes the initialization loops and generates a call to
  memset zero.  For example, the loop

  .. code-block:: fortran

    DO I = 1, N
      A(I) = 0
      B(I) = A(I) + I
    ENDDO

  is transformed to

  .. code-block:: fortran

    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.
  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -floop-interchange

  Perform loop interchange outside of graphite.  This flag can improve cache
  performance on loop nest and allow further loop optimizations, like
  vectorization, to take place.  For example, the loop

  .. code-block:: c++

    for (int i = 0; i < N; i++)
      for (int j = 0; j < N; j++)
        for (int k = 0; k < N; k++)
          c[i][j] = c[i][j] + a[i][k]*b[k][j];

  is transformed to

  .. code-block:: c++

    for (int i = 0; i < N; i++)
      for (int k = 0; k < N; k++)
        for (int j = 0; j < N; j++)
          c[i][j] = c[i][j] + a[i][k]*b[k][j];

  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -floop-unroll-and-jam

  Apply unroll and jam transformations on feasible loops.  In a loop
  nest this unrolls the outer loop by some factor and fuses the resulting
  multiple inner loops.  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`-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.

.. option:: -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.

.. option:: -ftree-scev-cprop

  Perform final value replacement.  If a variable is modified in a loop
  in such a way that its value when exiting the loop can be determined using
  only its initial value and the number of loop iterations, replace uses of
  the final value by such a computation, provided it is sufficiently cheap.
  This reduces data dependencies and may allow further simplifications.
  Enabled by default at :option:`-O1` and higher.

.. option:: -fivopts

  Perform induction variable optimizations (strength reduction, induction
  variable merging and induction variable elimination) on trees.

.. option:: -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 :option:`-pthread`, and thus is only supported on targets
  that have support for :option:`-pthread`.

.. option:: -ftree-pta

  Perform function-local points-to analysis on trees.  This flag is
  enabled by default at :option:`-O1` and higher, except for :option:`-Og`.

.. option:: -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 :option:`-O1` and higher,
  except for :option:`-Og`.

.. option:: -fstore-merging

  Perform merging of narrow stores to consecutive memory addresses.  This pass
  merges contiguous stores of immediate values narrower than a word into fewer
  wider stores to reduce the number of instructions.  This is enabled by default
  at :option:`-O2` and higher as well as :option:`-Os`.

.. option:: -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 :option:`-O1` and higher.

.. option:: -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 :option:`-O1` and
  higher.

.. option:: -ftree-vectorize

  Perform vectorization on trees. This flag enables :option:`-ftree-loop-vectorize`
  and :option:`-ftree-slp-vectorize` if not explicitly specified.

.. option:: -ftree-loop-vectorize

  Perform loop vectorization on trees. This flag is enabled by default at
  :option:`-O2` and by :option:`-ftree-vectorize`, :option:`-fprofile-use`,
  and :option:`-fauto-profile`.

.. option:: -ftree-slp-vectorize

  Perform basic block vectorization on trees. This flag is enabled by default at
  :option:`-O2` and by :option:`-ftree-vectorize`, :option:`-fprofile-use`,
  and :option:`-fauto-profile`.

.. option:: -ftrivial-auto-var-init={choice}

  Initialize automatic variables with either a pattern or with zeroes to increase
  the security and predictability of a program by preventing uninitialized memory
  disclosure and use.
  GCC still considers an automatic variable that doesn't have an explicit
  initializer as uninitialized, :option:`-Wuninitialized` and
  :option:`-Wanalyzer-use-of-uninitialized-value` will still report
  warning messages on such automatic variables.
  With this option, GCC will also initialize any padding of automatic variables
  that have structure or union types to zeroes.
  However, the current implementation cannot initialize automatic variables that
  are declared between the controlling expression and the first case of a
  ``switch`` statement.  Using :option:`-Wtrivial-auto-var-init` to report all
  such cases.

  The three values of :samp:`{choice}` are:

  * :samp:`uninitialized` doesn't initialize any automatic variables.
    This is C and C++'s default.

  * :samp:`pattern` Initialize automatic variables with values which will likely
    transform logic bugs into crashes down the line, are easily recognized in a
    crash dump and without being values that programmers can rely on for useful
    program semantics.
    The current value is byte-repeatable pattern with byte "0xFE".
    The values used for pattern initialization might be changed in the future.

  * :samp:`zero` Initialize automatic variables with zeroes.

  The default is :samp:`uninitialized`.

  You can control this behavior for a specific variable by using the variable
  attribute :var-attr:`uninitialized` (see :ref:`variable-attributes`).

.. option:: -fvect-cost-model={model}

  Alter the cost model used for vectorization.  The :samp:`{model}` argument
  should be one of :samp:`unlimited`, :samp:`dynamic`, :samp:`cheap` or
  :samp:`very-cheap`.
  With the :samp:`unlimited` model the vectorized code-path is assumed
  to be profitable while with the :samp:`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 :samp:`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 :samp:`dynamic` model.  The :samp:`very-cheap` model only
  allows vectorization if the vector code would entirely replace the
  scalar code that is being vectorized.  For example, if each iteration
  of a vectorized loop would only be able to handle exactly four iterations
  of the scalar loop, the :samp:`very-cheap` model would only allow
  vectorization if the scalar iteration count is known to be a multiple
  of four.

  The default cost model depends on other optimization flags and is
  either :samp:`dynamic` or :samp:`cheap`.

.. option:: -fsimd-cost-model={model}

  Alter the cost model used for vectorization of loops marked with the OpenMP
  simd directive.  The :samp:`{model}` argument should be one of
  :samp:`unlimited`, :samp:`dynamic`, :samp:`cheap`.  All values of :samp:`{model}`
  have the same meaning as described in :option:`-fvect-cost-model` and by
  default a cost model defined with :option:`-fvect-cost-model` is used.

.. option:: -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 :option:`-O2` and higher.  Null pointer check
  elimination is only done if :option:`-fdelete-null-pointer-checks` is
  enabled.

.. option:: -fsplit-paths

  Split paths leading to loop backedges.  This can improve dead code
  elimination and common subexpression elimination.  This is enabled by
  default at :option:`-O3` and above.

.. option:: -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 :option:`-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.

.. option:: -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.

  This optimization is enabled by default for PowerPC targets, but disabled
  by default otherwise.

.. option:: -fpartial-inlining

  Inline parts of functions.  This option has any effect only
  when inlining itself is turned on by the :option:`-finline-functions`
  or :option:`-finline-small-functions` options.

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`-Os`.

.. option:: -fno-printf-return-value

  Do not substitute constants for known return value of formatted output
  functions such as ``sprintf``, ``snprintf``, ``vsprintf``, and
  ``vsnprintf`` (but not ``printf`` of ``fprintf``).  This
  transformation allows GCC to optimize or even eliminate branches based
  on the known return value of these functions called with arguments that
  are either constant, or whose values are known to be in a range that
  makes determining the exact return value possible.  For example, when
  :option:`-fprintf-return-value` is in effect, both the branch and the
  body of the ``if`` statement (but not the call to ``snprint``)
  can be optimized away when ``i`` is a 32-bit or smaller integer
  because the return value is guaranteed to be at most 8.

  .. code-block:: c++

    char buf[9];
    if (snprintf (buf, "%08x", i) >= sizeof buf)
      ...

  The :option:`-fprintf-return-value` option relies on other optimizations
  and yields best results with :option:`-O2` and above.  It works in tandem
  with the :option:`-Wformat-overflow` and :option:`-Wformat-truncation`
  options.  The :option:`-fprintf-return-value` option is enabled by default.

.. option:: -fprintf-return-value

  Default setting; overrides :option:`-fno-printf-return-value`.

.. option:: -fno-peephole, -fno-peephole2, -fpeephole, -fpeephole2

  Disable any machine-specific peephole optimizations.  The difference
  between :option:`-fno-peephole` and :option:`-fno-peephole2` is in how they
  are implemented in the compiler; some targets use one, some use the
  other, a few use both.

  :option:`-fpeephole` is enabled by default.
  :option:`-fpeephole2` enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 (:option:`-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.

  It is also possible to specify expected probability of the expression
  with ``__builtin_expect_with_probability`` built-in function.

  The default is :option:`-fguess-branch-probability` at levels
  :option:`-O`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fguess-branch-probability

  Default setting; overrides :option:`-fno-guess-branch-probability`.

.. option:: -freorder-blocks

  Reorder basic blocks in the compiled function in order to reduce number of
  taken branches and improve code locality.

  Enabled at levels :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -freorder-blocks-algorithm={algorithm}

  Use the specified algorithm for basic block reordering.  The
  :samp:`{algorithm}` argument can be :samp:`simple`, which does not increase
  code size (except sometimes due to secondary effects like alignment),
  or :samp:`stc`, the 'software trace cache' algorithm, which tries to
  put all often executed code together, minimizing the number of branches
  executed by making extra copies of code.

  The default is :samp:`simple` at levels :option:`-O1`, :option:`-Os`, and
  :samp:`stc` at levels :option:`-O2`, :option:`-O3`.

.. option:: -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 :samp:`.o` files, to improve
  paging and cache locality performance.

  This optimization is automatically turned off in the presence of
  exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined
  section attribute and on any architecture that does not support named
  sections.  When :option:`-fsplit-stack` is used this option is not
  enabled by default (to avoid linker errors), but may be enabled
  explicitly (if using a working linker).

  Enabled for x86 at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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.

  This option isn't effective unless you either provide profile feedback
  (see :option:`-fprofile-arcs` for details) or manually annotate functions with
  :fn-attr:`hot` or :fn-attr:`cold` attributes (see :ref:`common-function-attributes`).

  Enabled at levels :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. _type-punning:

.. option:: -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:

  .. code-block:: c++

    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
  :option:`-fstrict-aliasing`, type-punning is allowed, provided the memory
  is accessed through the union type.  So, the code above works as
  expected.  See :ref:`structures-unions-enumerations-and-bit-fields-implementation`.  However, this code might not:

  .. code-block:: c++

    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.:

  .. code-block:: c++

    int f() {
      double d = 3.0;
      return ((union a_union *) &d)->i;
    }

  The :option:`-fstrict-aliasing` option is enabled at levels
  :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -fipa-strict-aliasing

  Controls whether rules of :option:`-fstrict-aliasing` are applied across
  function boundaries.  Note that if multiple functions gets inlined into a
  single function the memory accesses are no longer considered to be crossing a
  function boundary.

  The :option:`-fipa-strict-aliasing` option is enabled by default and is
  effective only in combination with :option:`-fstrict-aliasing`.

.. option:: -falign-functions[={n}[:{m}[:{n2}[:{m2}]]]]

  Align the start of functions to the next power-of-two greater than or
  equal to :samp:`{n}`, skipping up to :samp:`{m}` -1 bytes.  This ensures that at
  least the first :samp:`{m}` bytes of the function can be fetched by the CPU
  without crossing an :samp:`{n}` -byte alignment boundary.

  If :samp:`{m}` is not specified, it defaults to :samp:`{n}`.

  Examples: :option:`-falign-functions=32` aligns functions to the next
  32-byte boundary, :option:`-falign-functions=24` aligns to the next
  32-byte boundary only if this can be done by skipping 23 bytes or less,
  :option:`-falign-functions=32:7` aligns to the next
  32-byte boundary only if this can be done by skipping 6 bytes or less.

  The second pair of :samp:`{n2}:{m2}` values allows you to specify
  a secondary alignment: :option:`-falign-functions=64:7:32:3` aligns to
  the next 64-byte boundary if this can be done by skipping 6 bytes or less,
  otherwise aligns to the next 32-byte boundary if this can be done
  by skipping 2 bytes or less.
  If :samp:`{m2}` is not specified, it defaults to :samp:`{n2}`.

  Some assemblers only support this flag when :samp:`{n}` is a power of two;
  in that case, it is rounded up.

  :option:`-fno-align-functions` and :option:`-falign-functions=1` are
  equivalent and mean that functions are not aligned.

  If :samp:`{n}` is not specified or is zero, use a machine-dependent default.
  The maximum allowed :samp:`{n}` option value is 65536.

  Enabled at levels :option:`-O2`, :option:`-O3`.

.. option:: -flimit-function-alignment

  If this option is enabled, the compiler tries to avoid unnecessarily
  overaligning functions. It attempts to instruct the assembler to align
  by the amount specified by :option:`-falign-functions`, but not to
  skip more bytes than the size of the function.

.. option:: -falign-labels[={n}[:{m}[:{n2}[:{m2}]]]]

  Align all branch targets to a power-of-two boundary.

  Parameters of this option are analogous to the :option:`-falign-functions` option.
  :option:`-fno-align-labels` and :option:`-falign-labels=1` are
  equivalent and mean that labels are not aligned.

  If :option:`-falign-loops` or :option:`-falign-jumps` are applicable and
  are greater than this value, then their values are used instead.

  If :samp:`{n}` is not specified or is zero, use a machine-dependent default
  which is very likely to be :samp:`1`, meaning no alignment.
  The maximum allowed :samp:`{n}` option value is 65536.

  Enabled at levels :option:`-O2`, :option:`-O3`.

.. option:: -falign-loops[={n}[:{m}[:{n2}[:{m2}]]]]

  Align loops to a power-of-two boundary.  If the loops are executed
  many times, this makes up for any execution of the dummy padding
  instructions.

  If :option:`-falign-labels` is greater than this value, then its value
  is used instead.

  Parameters of this option are analogous to the :option:`-falign-functions` option.
  :option:`-fno-align-loops` and :option:`-falign-loops=1` are
  equivalent and mean that loops are not aligned.
  The maximum allowed :samp:`{n}` option value is 65536.

  If :samp:`{n}` is not specified or is zero, use a machine-dependent default.

  Enabled at levels :option:`-O2`, :option:`-O3`.

.. option:: -falign-jumps[={n}[:{m}[:{n2}[:{m2}]]]]

  Align branch targets to a power-of-two boundary, for branch targets
  where the targets can only be reached by jumping.  In this case,
  no dummy operations need be executed.

  If :option:`-falign-labels` is greater than this value, then its value
  is used instead.

  Parameters of this option are analogous to the :option:`-falign-functions` option.
  :option:`-fno-align-jumps` and :option:`-falign-jumps=1` are
  equivalent and mean that loops are not aligned.

  If :samp:`{n}` is not specified or is zero, use a machine-dependent default.
  The maximum allowed :samp:`{n}` option value is 65536.

  Enabled at levels :option:`-O2`, :option:`-O3`.

.. option:: -fno-allocation-dce

  Do not remove unused C++ allocations in dead code elimination.

.. option:: -fallow-store-data-races

  Allow the compiler to perform optimizations that may introduce new data races
  on stores, without proving that the variable cannot be concurrently accessed
  by other threads.  Does not affect optimization of local data.  It is safe to
  use this option if it is known that global data will not be accessed by
  multiple threads.

  Examples of optimizations enabled by :option:`-fallow-store-data-races` include
  hoisting or if-conversions that may cause a value that was already in memory
  to be re-written with that same value.  Such re-writing is safe in a single
  threaded context but may be unsafe in a multi-threaded context.  Note that on
  some processors, if-conversions may be required in order to enable
  vectorization.

  Enabled at level :option:`-Ofast`.

.. option:: -funit-at-a-time

  This option is left for compatibility reasons. :option:`-funit-at-a-time`
  has no effect, while :option:`-fno-unit-at-a-time` implies
  :option:`-fno-toplevel-reorder` and :option:`-fno-section-anchors`.

  Enabled by default.

.. option:: -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.

  :option:`-ftoplevel-reorder` is the default at :option:`-O1` and higher, and
  also at :option:`-O0` if :option:`-fsection-anchors` is explicitly requested.
  Additionally :option:`-fno-toplevel-reorder` implies
  :option:`-fno-section-anchors`.

.. option:: -ftoplevel-reorder

  Default setting; overrides :option:`-fno-toplevel-reorder`.

.. option:: -funreachable-traps

  With this option, the compiler turns calls to
  ``__builtin_unreachable`` into traps, instead of using them for
  optimization.  This also affects any such calls implicitly generated
  by the compiler.

  This option has the same effect as :option:`-fsanitize=unreachable
  -fsanitize-trap=unreachable`, but does not affect the values of those
  options.  If :option:`-fsanitize=unreachable` is enabled, that option
  takes priority over this one.

  This option is enabled by default at :option:`-O0` and :option:`-Og`.

.. option:: -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 :option:`-funroll-loops`.

.. option:: -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 :fn-attr:`externally_visible` become static functions
  and in effect are optimized more aggressively by interprocedural optimizers.

  This option should not be used in combination with :option:`-flto`.
  Instead relying on a linker plugin should provide safer and more precise
  information.

.. option:: -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, :option:`-flto` and optimization
  options should be specified at compile time and during the final link.
  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.
  For example:

  .. code-block:: shell

    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 :samp:`foo.o` and
  :samp:`bar.o`.  The final invocation reads the GIMPLE bytecode from
  :samp:`foo.o` and :samp:`bar.o`, merges the two files into a single
  internal image, and compiles the result as usual.  Since both
  :samp:`foo.o` and :samp:`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
  :samp:`bar.o` into functions in :samp:`foo.o` and vice-versa.

  Another (simpler) way to enable link-time optimization is:

  .. code-block:: shell

    gcc -o myprog -flto -O2 foo.c bar.c

  The above generates bytecode for :samp:`foo.c` and :samp:`bar.c`,
  merges them together into a single GIMPLE representation and optimizes
  them as usual to produce :samp:`myprog`.

  The 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 automatically performs link-time optimization if any of the
  objects involved were compiled with the :option:`-flto` command-line option.
  You can always override
  the automatic decision to do link-time optimization
  by passing :option:`-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 :option:`-fuse-linker-plugin`) passes information
  to the compiler about used and externally visible symbols.  When
  the linker plugin is not available, :option:`-fwhole-program` should be
  used to allow the compiler to make these assumptions, which leads
  to more aggressive optimization decisions.

  When a file is compiled with :option:`-flto` without
  :option:`-fuse-linker-plugin`, the generated object file is larger than
  a regular object file because it contains GIMPLE bytecodes and the usual
  final code (see :option:`-ffat-lto-objects`).  This means that
  object files with LTO information can be linked as normal object
  files; if :option:`-fno-lto` is passed to the linker, no
  interprocedural optimizations are applied.  Note that when
  :option:`-fno-fat-lto-objects` is enabled the compile stage is faster
  but you cannot perform a regular, non-LTO link on them.

  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.

  Generally, options specified at link time override those
  specified at compile time, although in some cases GCC attempts to infer
  link-time options from the settings used to compile the input files.

  If you do not specify an optimization level option :option:`-O` at
  link time, then GCC uses the highest optimization level
  used when compiling the object files.  Note that it is generally
  ineffective to specify an optimization level option only at link time and
  not at compile time, for two reasons.  First, compiling without
  optimization suppresses compiler passes that gather information
  needed for effective optimization at link time.  Second, some early
  optimization passes can be performed only at compile time and
  not at link time.

  There are some code generation flags preserved by GCC when
  generating bytecodes, as they need to be used during the final link.
  Currently, the following options and their settings are taken from
  the first object file that explicitly specifies them:
  :option:`-fcommon`, :option:`-fexceptions`, :option:`-fnon-call-exceptions`,
  :option:`-fgnu-tm` and all the :option:`-m` target flags.

  The following options :option:`-fPIC`, :option:`-fpic`, :option:`-fpie` and
  :option:`-fPIE` are combined based on the following scheme:

  .. list-table::
      :header-rows: 1

      * - argument 1
        - argument 2
        - output

      * - :option:`-fPIC`
        - :option:`-fpic`
        - :option:`-fpic`
      * - :option:`-fPIC`
        - :option:`-fno-pic`
        - :option:`-fno-pic`
      * - :option:`-fpic`/:option:`-fPIC`
        - no option
        - no option
      * - :option:`-fPIC`
        - :option:`-fPIE`
        - :option:`-fPIE`
      * - :option:`-fpic`
        - :option:`-fPIE`
        - :option:`-fpie`
      * - :option:`-fPIC`/:option:`-fpic`
        - :option:`-fpie`
        - :option:`-fpie`

  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 :option:`-freg-struct-return`
  and :option:`-fpcc-struct-return`.

  Other options such as :option:`-ffp-contract`, :option:`-fno-strict-overflow`,
  :option:`-fwrapv`, :option:`-fno-trapv` or :option:`-fno-strict-aliasing`
  are passed through to the link stage and merged conservatively for
  conflicting translation units.  Specifically
  :option:`-fno-strict-overflow`, :option:`-fwrapv` and :option:`-fno-trapv` take
  precedence; and for example :option:`-ffp-contract=off` takes precedence
  over :option:`-ffp-contract=fast`.  You can override them at link time.

  Diagnostic options such as :option:`-Wstringop-overflow` are passed
  through to the link stage and their setting matches that of the
  compile-step at function granularity.  Note that this matters only
  for diagnostics emitted during optimization.  Note that code
  transforms such as inlining can lead to warnings being enabled
  or disabled for regions if code not consistent with the setting
  at compile time.

  When you need to pass options to the assembler via :option:`-Wa` or
  :option:`-Xassembler` make sure to either compile such translation
  units with :option:`-fno-lto` or consistently use the same assembler
  options on all translation units.  You can alternatively also
  specify assembler options at LTO link time.

  To enable debug info generation you need to supply :option:`-g` at
  compile time.  If any of the input files at link time were built
  with debug info generation enabled the link will enable debug info
  generation as well.  Any elaborate debug info settings
  like the dwarf level :option:`-gdwarf-5` need to be explicitly repeated
  at the linker command line and mixing different settings in different
  translation units is discouraged.

  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:

  .. code-block:: shell

    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 :command:`g++` to get the C++
  runtime libraries and :option:`-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
  :samp:`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 :command:`gcc-ar` and :command:`gcc-ranlib` instead of :command:`ar`
  and :command:`ranlib`;
  to show the symbols of object files with GIMPLE bytecode, use
  :command:`gcc-nm`.  Those commands require that :command:`ar`, :command:`ranlib`
  and :command:`nm` have been compiled with plugin support.  At link time, use the
  flag :option:`-fuse-linker-plugin` to ensure that the library participates in
  the LTO optimization process:

  .. code-block:: shell

    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 :samp:`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 :samp:`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
  :option:`-flto` :option:`-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 :option:`-flto` and :option:`-fwhole-program` to allow
  the interprocedural optimizers to use more aggressive assumptions which may
  lead to improved optimization opportunities.
  Use of :option:`-fwhole-program` is not needed when linker plugin is
  active (see :option:`-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 on systems other than those using a combination of ELF and
  DWARF.

  If you specify the optional :samp:`{n}`, the optimization and code
  generation done at link time is executed in parallel using :samp:`{n}`
  parallel jobs by utilizing an installed :command:`make` program.  The
  environment variable :envvar:`MAKE` may be used to override the program
  used.

  You can also specify :option:`-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 :samp:`+` to the command recipe in the parent Makefile
  for this to work.  This option likely only works if :envvar:`MAKE` is
  GNU make.  Even without the option value, GCC tries to automatically
  detect a running GNU make's job server.

  Use :option:`-flto=auto` to use GNU make's job server, if available,
  or otherwise fall back to autodetection of the number of CPU threads
  present in your system.

.. option:: -flto-partition={alg}

  Specify the partitioning algorithm used by the link-time optimizer.
  The value is either :samp:`1to1` to specify a partitioning mirroring
  the original source files or :samp:`balanced` to specify partitioning
  into equally sized chunks (whenever possible) or :samp:`max` to create
  new partition for every symbol where possible.  Specifying :samp:`none`
  as an algorithm disables partitioning and streaming completely.
  The default value is :samp:`balanced`. While :samp:`1to1` can be used
  as an workaround for various code ordering issues, the :samp:`max`
  partitioning is intended for internal testing only.
  The value :samp:`one` specifies that exactly one partition should be
  used while the value :samp:`none` bypasses partitioning and executes
  the link-time optimization step directly from the WPA phase.

.. option:: -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 (:option:`-flto`).  GCC currently supports two
  LTO compression algorithms. For zstd, valid values are 0 (no compression)
  to 19 (maximum compression), while zlib supports values from 0 to 9.
  Values outside this range are clamped to either minimum or maximum
  of the supported values.  If the option is not given,
  a default balanced compression setting is used.

.. option:: -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 :option:`-fwhole-program`.
  See :option:`-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).

.. option:: -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 :option:`-flto`
  and is ignored at link time.

  :option:`-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,
  :command:`nm`, :command:`ar` and :command:`ranlib`
  need to support linker plugins to allow a full-featured build environment
  (capable of building static libraries etc).  GCC provides the :command:`gcc-ar`,
  :command:`gcc-nm`, :command:`gcc-ranlib` wrappers to pass the right options
  to these tools. With non fat LTO makefiles need to be modified to use them.

  Note that modern binutils provide plugin auto-load mechanism.
  Installing the linker plugin into :samp:`$libdir/bfd-plugins` has the same
  effect as usage of the command wrappers (:command:`gcc-ar`, :command:`gcc-nm` and
  :command:`gcc-ranlib`).

  The default is :option:`-fno-fat-lto-objects` on targets with linker plugin
  support.

.. option:: -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 :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 :option:`-O1`, :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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.

  This option is enabled by :option:`-fauto-profile`.

.. option:: -fprofile-partial-training

  With ``-fprofile-use`` all portions of programs not executed during train
  run are optimized agressively for size rather than speed.  In some cases it is
  not practical to train all possible hot paths in the program. (For
  example, program may contain functions specific for a given hardware and
  trianing may not cover all hardware configurations program is run on.)  With
  ``-fprofile-partial-training`` profile feedback will be ignored for all
  functions not executed during the train run leading them to be optimized as if
  they were compiled without profile feedback. This leads to better performance
  when train run is not representative but also leads to significantly bigger
  code.

.. option:: -fprofile-use, -fprofile-use={path}

  Enable profile feedback-directed optimizations,
  and the following optimizations, many of which
  are generally profitable only with profile feedback available:

  :option:`-fbranch-probabilities`  :option:`-fprofile-values` |gol|
  :option:`-funroll-loops`  :option:`-fpeel-loops`  :option:`-ftracer`  :option:`-fvpt` |gol|
  :option:`-finline-functions`  :option:`-fipa-cp`  :option:`-fipa-cp-clone`  :option:`-fipa-bit-cp` |gol|
  :option:`-fpredictive-commoning`  :option:`-fsplit-loops`  :option:`-funswitch-loops` |gol|
  :option:`-fgcse-after-reload`  :option:`-ftree-loop-vectorize`  :option:`-ftree-slp-vectorize` |gol|
  :option:`-fvect-cost-model=dynamic`  :option:`-ftree-loop-distribute-patterns` |gol|
  :option:`-fprofile-reorder-functions`

  Before you can use this option, you must first generate profiling information.
  See :ref:`instrumentation-options`, for information about the
  :option:`-fprofile-generate` option.

  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
  :option:`-Wno-error=coverage-mismatch`.  Note this may result in poorly
  optimized code.  Additionally, by default, GCC also emits a warning message if
  the feedback profiles do not exist (see :option:`-Wmissing-profile`).

  If :samp:`{path}` is specified, GCC looks at the :samp:`{path}` to find
  the profile feedback data files. See :option:`-fprofile-dir`.

.. option:: -fauto-profile, -fauto-profile={path}

  Enable sampling-based feedback-directed optimizations,
  and the following optimizations,
  many of which are generally profitable only with profile feedback available:

  :option:`-fbranch-probabilities`  :option:`-fprofile-values` |gol|
  :option:`-funroll-loops`  :option:`-fpeel-loops`  :option:`-ftracer`  :option:`-fvpt` |gol|
  :option:`-finline-functions`  :option:`-fipa-cp`  :option:`-fipa-cp-clone`  :option:`-fipa-bit-cp` |gol|
  :option:`-fpredictive-commoning`  :option:`-fsplit-loops`  :option:`-funswitch-loops` |gol|
  :option:`-fgcse-after-reload`  :option:`-ftree-loop-vectorize`  :option:`-ftree-slp-vectorize` |gol|
  :option:`-fvect-cost-model=dynamic`  :option:`-ftree-loop-distribute-patterns` |gol|
  :option:`-fprofile-correction`

  :samp:`{path}` is the name of a file containing AutoFDO profile information.
  If omitted, it defaults to :samp:`fbdata.afdo` in the current directory.

  Producing an AutoFDO profile data file requires running your program
  with the :command:`perf` utility on a supported GNU/Linux target system.
  For more information, see https://perf.wiki.kernel.org/.

  E.g.

  .. code-block:: c++

    perf record -e br_inst_retired:near_taken -b -o perf.data \
        -- your_program

  Then use the :command:`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.

  .. code-block:: c++

    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.

.. option:: -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.

  .. index:: floating-point precision

  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 :option:`-ffloat-store` for such programs, after modifying
  them to store all pertinent intermediate computations into variables.

.. option:: -fexcess-precision={style}

  This option allows further control over excess precision on machines
  where floating-point operations occur in a format with more precision or
  range than the IEEE standard and interchange floating-point types.  By
  default, :option:`-fexcess-precision=fast` is in effect; this means that
  operations may be carried out in a wider precision than the types specified
  in the source if that would result in faster code, and it is unpredictable
  when rounding to the types specified in the source code takes place.
  When compiling C or C++, if :option:`-fexcess-precision=standard` is specified
  then excess precision follows the rules specified in ISO C99 or C++; in particular,
  both casts and assignments cause values to be rounded to their
  semantic types (whereas :option:`-ffloat-store` only affects
  assignments).  This option is enabled by default for C or C++ if a strict
  conformance option such as :option:`-std=c99` or :option:`-std=c++17` is used.
  :option:`-ffast-math` enables :option:`-fexcess-precision=fast` by default
  regardless of whether a strict conformance option is used.

  :option:`-fexcess-precision=standard` is not implemented for languages
  other than C or C++.  On the x86, it has no effect if :option:`-mfpmath=sse`
  or :option:`-mfpmath=sse+387` is specified; in the former case, IEEE
  semantics apply without excess precision, and in the latter, rounding
  is unpredictable.

.. option:: -ffast-math

  Sets the options :option:`-fno-math-errno`, :option:`-funsafe-math-optimizations`,
  :option:`-ffinite-math-only`, :option:`-fno-rounding-math`,
  :option:`-fno-signaling-nans`, :option:`-fcx-limited-range` and
  :option:`-fexcess-precision=fast`.

  This option causes the preprocessor macro ``__FAST_MATH__`` to be defined.

  This option is not turned on by any :option:`-O` option besides
  :option:`-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.

.. option:: -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 :option:`-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 :option:`-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 :option:`-fno-math-errno` is the default.

.. option:: -fmath-errno

  Default setting; overrides :option:`-fno-math-errno`.

.. option:: -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 :option:`-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 :option:`-fno-signed-zeros`, :option:`-fno-trapping-math`,
  :option:`-fassociative-math` and :option:`-freciprocal-math`.

  The default is :option:`-fno-unsafe-math-optimizations`.

.. option:: -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 :option:`-fno-signed-zeros` and
  :option:`-fno-trapping-math` be in effect.  Moreover, it doesn't make
  much sense with :option:`-frounding-math`. For Fortran the option
  is automatically enabled when both :option:`-fno-signed-zeros` and
  :option:`-fno-trapping-math` are in effect.

  The default is :option:`-fno-associative-math`.

.. option:: -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 :option:`-fno-reciprocal-math`.

.. option:: -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 :option:`-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 :option:`-fno-finite-math-only`.

.. option:: -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 :option:`-ffinite-math-only`).
  This option implies that the sign of a zero result isn't significant.

  The default is :option:`-fsigned-zeros`.

.. option:: -fsigned-zeros

  Default setting; overrides :option:`-fno-signed-zeros`.

.. option:: -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 :option:`-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 :option:`-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 :option:`-ftrapping-math`.

  Future versions of GCC may provide finer control of this setting
  using C99's ``FENV_ACCESS`` pragma.  This command-line option
  will be used along with :option:`-frounding-math` to specify the
  default state for ``FENV_ACCESS``.

.. option:: -ftrapping-math

  Default setting; overrides :option:`-fno-trapping-math`.

.. option:: -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 :option:`-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 along with :option:`-ftrapping-math` to specify the
  default state for ``FENV_ACCESS``.

.. option:: -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 :option:`-ftrapping-math`.

  This option causes the preprocessor macro ``__SUPPORT_SNAN__`` to
  be defined.

  The default is :option:`-fno-signaling-nans`.

  This option is experimental and does not currently guarantee to
  disable all GCC optimizations that affect signaling NaN behavior.

.. option:: -fno-fp-int-builtin-inexact

  Do not allow the built-in functions ``ceil``, ``floor``,
  ``round`` and ``trunc``, and their ``float`` and ``long
  double`` variants, to generate code that raises the 'inexact'
  floating-point exception for noninteger arguments.  ISO C99 and C11
  allow these functions to raise the 'inexact' exception, but ISO/IEC
  TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into
  ISO C2X, does not allow these functions to do so.

  The default is :option:`-ffp-int-builtin-inexact`, allowing the
  exception to be raised, unless C2X or a later C standard is selected.
  This option does nothing unless :option:`-ftrapping-math` is in effect.

  Even if :option:`-fno-fp-int-builtin-inexact` is used, if the functions
  generate a call to a library function then the 'inexact' exception
  may be raised if the library implementation does not follow TS 18661.

.. option:: -ffp-int-builtin-inexact

  Default setting; overrides :option:`-fno-fp-int-builtin-inexact`.

.. option:: -fsingle-precision-constant

  Treat floating-point constants as single precision instead of
  implicitly converting them to double-precision constants.

.. option:: -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 :option:`-fno-cx-limited-range`, but is enabled by
  :option:`-ffast-math`.

  This option controls the default setting of the ISO C99
  ``CX_LIMITED_RANGE`` pragma.  Nevertheless, the option applies to
  all languages.

.. option:: -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 :option:`-fno-cx-fortran-rules`.

The following options control optimizations that may improve
performance, but are not enabled by any :option:`-O` options.  This
section includes experimental options that may produce broken code.

.. option:: -fbranch-probabilities

  After running a program compiled with :option:`-fprofile-arcs`
  (see :ref:`instrumentation-options`),
  you can compile it a second time using
  :option:`-fbranch-probabilities`, to improve optimizations based on
  the number of times each branch was taken.  When a program
  compiled with :option:`-fprofile-arcs` exits, it saves arc execution
  counts to a file called :samp:`{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.
  See details about the file naming in :option:`-fprofile-arcs`.

  With :option:`-fbranch-probabilities`, GCC puts a
  :samp:`REG_BR_PROB` note on each :samp:`JUMP_INSN` and :samp:`CALL_INSN`.
  These can be used to improve optimization.  Currently, they are only
  used in one place: in :samp:`reorg.cc`, instead of guessing which path a
  branch is most likely to take, the :samp:`REG_BR_PROB` values are used to
  exactly determine which path is taken more often.

  Enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -fprofile-values

  If combined with :option:`-fprofile-arcs`, it adds code so that some
  data about values of expressions in the program is gathered.

  With :option:`-fbranch-probabilities`, it reads back the data gathered
  from profiling values of expressions for usage in optimizations.

  Enabled by :option:`-fprofile-generate`, :option:`-fprofile-use`, and
  :option:`-fauto-profile`.

.. option:: -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 :option:`-fprofile-use`.

.. option:: -fvpt

  If combined with :option:`-fprofile-arcs`, this option instructs the compiler
  to add code to gather information about values of expressions.

  With :option:`-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.

  Enabled with :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 :option:`-funroll-loops`.

.. option:: -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 :option:`-O2`, :option:`-O3`, :option:`-Os`.

.. option:: -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 by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -funroll-loops

  Unroll loops whose number of iterations can be determined at compile time or
  upon entry to the loop.  :option:`-funroll-loops` implies
  :option:`-frerun-cse-after-loop`, :option:`-fweb` and :option:`-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 by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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.
  :option:`-funroll-all-loops` implies the same options as
  :option:`-funroll-loops`.

.. option:: -fpeel-loops

  Peels loops for which there is enough information that they do not
  roll much (from profile feedback or static analysis).  It also turns on
  complete loop peeling (i.e. complete removal of loops with small constant
  number of iterations).

  Enabled by :option:`-O3`, :option:`-fprofile-use`, and :option:`-fauto-profile`.

.. option:: -fmove-loop-invariants

  Enables the loop invariant motion pass in the RTL loop optimizer.  Enabled
  at level :option:`-O1` and higher, except for :option:`-Og`.

.. option:: -fmove-loop-stores

  Enables the loop store motion pass in the GIMPLE loop optimizer.  This
  moves invariant stores to after the end of the loop in exchange for
  carrying the stored value in a register across the iteration.
  Note for this option to have an effect :option:`-ftree-loop-im` has to
  be enabled as well.  Enabled at level :option:`-O1` and higher, except
  for :option:`-Og`.

.. option:: -fsplit-loops

  Split a loop into two if it contains a condition that's always true
  for one side of the iteration space and false for the other.

  Enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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).

  Enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -fversion-loops-for-strides

  If a loop iterates over an array with a variable stride, create another
  version of the loop that assumes the stride is always one.  For example:

  .. code-block:: c++

    for (int i = 0; i < n; ++i)
      x[i * stride] = ...;

  becomes:

  .. code-block:: c++

    if (stride == 1)
      for (int i = 0; i < n; ++i)
        x[i] = ...;
    else
      for (int i = 0; i < n; ++i)
        x[i * stride] = ...;

  This is particularly useful for assumed-shape arrays in Fortran where
  (for example) it allows better vectorization assuming contiguous accesses.
  This flag is enabled by default at :option:`-O3`.
  It is also enabled by :option:`-fprofile-use` and :option:`-fauto-profile`.

.. option:: -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 have linkers with such optimizations.  On AIX, the linker
  rearranges sections (CSECTs) based on the call graph.  The performance impact
  varies.

  Together with a linker garbage collection (linker :option:`--gc-sections`
  option) these options may lead to smaller statically-linked executables (after
  stripping).

  On ELF/DWARF systems these options do not degenerate the quality of the debug
  information.  There could be issues with other object files/debug info formats.

  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.  These options affect code generation.
  They prevent optimizations by the compiler and assembler using relative
  locations inside a translation unit since the locations are unknown until
  link time.  An example of such an optimization is relaxing calls to short call
  instructions.

.. option:: -fstdarg-opt

  Optimize the prologue of variadic argument functions with respect to usage of
  those arguments.

.. option:: -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`` :

  .. code-block:: c++

    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 :option:`-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):

  .. code-block:: c++

    int foo (void)
    {
      register int *xr = &x;
      return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
    }

  Not all targets support this option.

.. option:: -fzero-call-used-regs={choice}

  Zero call-used registers at function return to increase program
  security by either mitigating Return-Oriented Programming (ROP)
  attacks or preventing information leakage through registers.

  The possible values of :samp:`{choice}` are the same as for the
  ``zero_call_used_regs`` attribute (see :ref:`function-attributes`).
  The default is :samp:`skip`.

  You can control this behavior for a specific function by using the function
  attribute ``zero_call_used_regs`` (see :ref:`function-attributes`).

.. 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
  :option:`--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 order to get minimal, maximal and default value of a parameter,
  one can use :option:`--help=param -Q` options.

  In each case, the :samp:`{value}` is an integer.  The following choices
  of :samp:`{name}` are recognized for all targets:

  .. gcc-param:: predictable-branch-outcome

    When branch is predicted to be taken with probability lower than this threshold
    (in percent), then it is considered well predictable.

  .. gcc-param:: max-rtl-if-conversion-insns

    RTL if-conversion tries to remove conditional branches around a block and
    replace them with conditionally executed instructions.  This parameter
    gives the maximum number of instructions in a block which should be
    considered for if-conversion.  The compiler will
    also use other heuristics to decide whether if-conversion is likely to be
    profitable.

  .. gcc-param:: max-rtl-if-conversion-predictable-cost

    RTL if-conversion will try to remove conditional branches around a block
    and replace them with conditionally executed instructions.  These parameters
    give the maximum permissible cost for the sequence that would be generated
    by if-conversion depending on whether the branch is statically determined
    to be predictable or not.  The units for this parameter are the same as
    those for the GCC internal seq_cost metric.  The compiler will try to
    provide a reasonable default for this parameter using the BRANCH_COST
    target macro.

  .. gcc-param:: max-crossjump-edges

    The maximum number of incoming edges to consider for cross-jumping.
    The algorithm used by :option:`-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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: max-gcse-memory

    The approximate maximum amount of memory in ``kB`` 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: max-inline-functions-called-once-loop-depth

    Maximal loop depth of a call considered by inline heuristics that tries to
    inline all functions called once.

  .. gcc-param:: max-inline-functions-called-once-insns

    Maximal estimated size of functions produced while inlining functions called
    once.

  .. gcc-param:: 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++).

  .. gcc-param:: max-inline-insns-auto

    When you use :option:`-finline-functions` (included in :option:`-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 (:option:`--param` :gcc-param:`max-inline-insns-auto`).

  .. gcc-param:: max-inline-insns-small

    This is bound applied to calls which are considered relevant with
    :option:`-finline-small-functions`.

  .. gcc-param:: max-inline-insns-size

    This is bound applied to calls which are optimized for size. Small growth
    may be desirable to anticipate optimization oppurtunities exposed by inlining.

  .. gcc-param:: uninlined-function-insns

    Number of instructions accounted by inliner for function overhead such as
    function prologue and epilogue.

  .. gcc-param:: uninlined-function-time

    Extra time accounted by inliner for function overhead such as time needed to
    execute function prologue and epilogue.

  .. gcc-param:: inline-heuristics-hint-percent

    The scale (in percents) applied to inline-insns-single,
    inline-insns-single-O2, inline-insns-auto
    when inline heuristics hints that inlining is
    very profitable (will enable later optimizations).

  .. gcc-param:: uninlined-thunk-insns
                 uninlined-thunk-time

    Same as :option:`--param` :gcc-param:`uninlined-function-insns` and
    :option:`--param` :gcc-param:`uninlined-function-time` but applied to function thunks.

  .. gcc-param:: inline-min-speedup

    When estimated performance improvement of caller + callee runtime exceeds this
    threshold (in percent), the function can be inlined regardless of the limit on
    :option:`--param` :gcc-param:`max-inline-insns-single` and :option:`--param`
    :gcc-param:`max-inline-insns-auto`.

  .. gcc-param:: large-function-insns

    The limit specifying really large functions.  For functions larger than this
    limit after inlining, inlining is constrained by
    :option:`--param` :gcc-param:`large-function-growth`.  This parameter is useful primarily
    to avoid extreme compilation time caused by non-linear algorithms used by the
    back end.

  .. gcc-param:: large-function-growth

    Specifies maximal growth of large function caused by inlining in percents.
    For example, parameter value 100 limits large function growth to 2.0 times
    the original size.

  .. gcc-param:: large-unit-insns

    The limit specifying large translation unit.  Growth caused by inlining of
    units larger than this limit is limited by :option:`--param` :gcc-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 :option:`--param` :gcc-param:`large-unit-insns`
    before applying :option:`--param` :gcc-param:`inline-unit-growth`.

  .. gcc-param:: lazy-modules

    Maximum number of concurrently open C++ module files when lazy loading.

  .. gcc-param:: inline-unit-growth

    Specifies maximal overall growth of the compilation unit caused by inlining.
    For example, parameter value 20 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.

  .. gcc-param:: ipa-cp-unit-growth

    Specifies maximal overall growth of the compilation unit caused by
    interprocedural constant propagation.  For example, parameter value 10 limits
    unit growth to 1.1 times the original size.

  .. gcc-param:: ipa-cp-large-unit-insns

    The size of translation unit that IPA-CP pass considers large.

  .. gcc-param:: large-stack-frame

    The limit specifying large stack frames.  While inlining the algorithm is trying
    to not grow past this limit too much.

  .. gcc-param:: large-stack-frame-growth

    Specifies maximal growth of large stack frames caused by inlining in percents.
    For example, parameter value 1000 limits large stack frame growth to 11 times
    the original size.

  .. gcc-param:: 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.

    :option:`--param` :gcc-param:`max-inline-insns-recursive` applies to functions
    declared inline.
    For functions not declared inline, recursive inlining
    happens only when :option:`-finline-functions` (included in :option:`-O3`) is
    enabled; :option:`--param` :gcc-param:`max-inline-insns-recursive-auto` applies instead.

  .. gcc-param:: max-inline-recursive-depth
                 max-inline-recursive-depth-auto

    Specifies the maximum recursion depth used for recursive inlining.

    :option:`--param` :gcc-param:`max-inline-recursive-depth` applies to functions
    declared inline.  For functions not declared inline, recursive inlining
    happens only when :option:`-finline-functions` (included in :option:`-O3`) is
    enabled; :option:`--param` :gcc-param:`max-inline-recursive-depth-auto` applies instead.

  .. gcc-param:: 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 :option:`-fprofile-generate`) the actual
    recursion depth can be guessed from the 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).

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: comdat-sharing-probability

    Probability (in percent) that C++ inline function with comdat visibility
    are shared across multiple compilation units.

  .. gcc-param:: modref-max-bases
                 modref-max-refs
                 modref-max-accesses

    Specifies the maximal number of base pointers, references and accesses stored
    for a single function by mod/ref analysis.

  .. gcc-param:: modref-max-tests

    Specifies the maxmal number of tests alias oracle can perform to disambiguate
    memory locations using the mod/ref information.  This parameter ought to be
    bigger than :option:`--param` :gcc-param:`modref-max-bases` and :option:`--param
    :gcc-param:`modref-max-refs`.

  .. gcc-param:: modref-max-depth

    Specifies the maximum depth of DFS walk used by modref escape analysis.
    Setting to 0 disables the analysis completely.

  .. gcc-param:: modref-max-escape-points

    Specifies the maximum number of escape points tracked by modref per SSA-name.

  .. gcc-param:: modref-max-adjustments

    Specifies the maximum number the access range is enlarged during modref dataflow
    analysis.

  .. gcc-param:: 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.

  .. gcc-param:: min-vect-loop-bound

    The minimum number of iterations under which loops are not vectorized
    when :option:`-ftree-vectorize` is used.  The number of iterations after
    vectorization needs to be greater than the value specified by this option
    to allow vectorization.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: store-merging-allow-unaligned

    Allow the store merging pass to introduce unaligned stores if it is legal to
    do so.

  .. gcc-param:: max-stores-to-merge

    The maximum number of stores to attempt to merge into wider stores in the store
    merging pass.

  .. gcc-param:: max-store-chains-to-track

    The maximum number of store chains to track at the same time in the attempt
    to merge them into wider stores in the store merging pass.

  .. gcc-param:: max-stores-to-track

    The maximum number of stores to track at the same time in the attemt to
    to merge them into wider stores in the store merging pass.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: max-unroll-times

    The maximum number of unrollings of a single loop.

  .. gcc-param:: 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.

  .. gcc-param:: max-peel-times

    The maximum number of peelings of a single loop.

  .. gcc-param:: max-peel-branches

    The maximum number of branches on the hot path through the peeled sequence.

  .. gcc-param:: max-completely-peeled-insns

    The maximum number of insns of a completely peeled loop.

  .. gcc-param:: max-completely-peel-times

    The maximum number of iterations of a loop to be suitable for complete peeling.

  .. gcc-param:: max-completely-peel-loop-nest-depth

    The maximum depth of a loop nest suitable for complete peeling.

  .. gcc-param:: max-unswitch-insns

    The maximum number of insns of an unswitched loop.

  .. gcc-param:: lim-expensive

    The minimum cost of an expensive expression in the loop invariant motion.

  .. gcc-param:: min-loop-cond-split-prob

    When FDO profile information is available, min-loop-cond-split-prob
    specifies minimum threshold for probability of semi-invariant condition
    statement to trigger loop split.

  .. gcc-param:: 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.

  .. gcc-param:: iv-max-considered-uses

    The induction variable optimizations give up on loops that contain more
    induction variable uses.

  .. gcc-param:: 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.

  .. gcc-param:: avg-loop-niter

    Average number of iterations of a loop.

  .. gcc-param:: dse-max-object-size

    Maximum size (in bytes) of objects tracked bytewise by dead store elimination.
    Larger values may result in larger compilation times.

  .. gcc-param:: dse-max-alias-queries-per-store

    Maximum number of queries into the alias oracle per store.
    Larger values result in larger compilation times and may result in more
    removed dead stores.

  .. gcc-param:: scev-max-expr-size

    Bound on size of expressions used in the scalar evolutions analyzer.
    Large expressions slow the analyzer.

  .. gcc-param:: scev-max-expr-complexity

    Bound on the complexity of the expressions in the scalar evolutions analyzer.
    Complex expressions slow the analyzer.

  .. gcc-param:: max-tree-if-conversion-phi-args

    Maximum number of arguments in a PHI supported by TREE if conversion
    unless the loop is marked with simd pragma.

  .. gcc-param:: vect-max-layout-candidates

    The maximum number of possible vector layouts (such as permutations)
    to consider when optimizing to-be-vectorized code.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: vect-max-peeling-for-alignment

    The maximum number of loop peels to enhance access alignment
    for vectorizer. Value -1 means no limit.

  .. gcc-param:: 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.

  .. gcc-param:: hot-bb-count-fraction

    The denominator n of fraction 1/n of the maximal execution count of a
    basic block in the entire program that a basic block needs to at least
    have in order to be considered hot.  The default is 10000, which means
    that a basic block is considered hot if its execution count is greater
    than 1/10000 of the maximal execution count.  0 means that it is never
    considered hot.  Used in non-LTO mode.

  .. gcc-param:: hot-bb-count-ws-permille

    The number of most executed permilles, ranging from 0 to 1000, of the
    profiled execution of the entire program to which the execution count
    of a basic block must be part of in order to be considered hot.  The
    default is 990, which means that a basic block is considered hot if
    its execution count contributes to the upper 990 permilles, or 99.0%,
    of the profiled execution of the entire program.  0 means that it is
    never considered hot.  Used in LTO mode.

  .. gcc-param:: hot-bb-frequency-fraction

    The denominator n of fraction 1/n of the execution frequency of the
    entry block of a function that a basic block of this function needs
    to at least have in order to be considered hot.  The default is 1000,
    which means that a basic block is considered hot in a function if it
    is executed more frequently than 1/1000 of the frequency of the entry
    block of the function.  0 means that it is never considered hot.

  .. gcc-param:: unlikely-bb-count-fraction

    The denominator n of fraction 1/n of the number of profiled runs of
    the entire program below which the execution count of a basic block
    must be in order for the basic block to be considered unlikely executed.
    The default is 20, which means that a basic block is considered unlikely
    executed if it is executed in fewer than 1/20, or 5%, of the runs of
    the program.  0 means that it is always considered unlikely executed.

  .. gcc-param:: 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.

  .. gcc-param:: builtin-expect-probability

    Control the probability of the expression having the specified value. This
    parameter takes a percentage (i.e. 0 ... 100) as input.

  .. gcc-param:: builtin-string-cmp-inline-length

    The maximum length of a constant string for a builtin string cmp call
    eligible for inlining.

  .. gcc-param:: align-threshold

    Select fraction of the maximal frequency of executions of a basic block in
    a function to align the basic block.

  .. gcc-param:: align-loop-iterations

    A loop expected to iterate at least the selected number of iterations is
    aligned.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: tracer-min-branch-ratio

    Stop reverse growth when the reverse probability of best edge is less than this
    threshold (in percent).

  .. gcc-param:: tracer-min-branch-probability
                 tracer-min-branch-probability-feedback

    Stop forward growth if the best edge has probability lower than this
    threshold.

    Similarly to tracer-dynamic-coverage two parameters are
    provided.  tracer-min-branch-probability-feedback is used for
    compilation with profile feedback and tracer-min-branch-probability
    compilation without.  The value for compilation with profile feedback
    needs to be more conservative (higher) in order to make tracer
    effective.

  .. gcc-param:: stack-clash-protection-guard-size

    Specify the size of the operating system provided stack guard as
    2 raised to :samp:`{num}` bytes.  Higher values may reduce the
    number of explicit probes, but a value larger than the operating system
    provided guard will leave code vulnerable to stack clash style attacks.

  .. gcc-param:: stack-clash-protection-probe-interval

    Stack clash protection involves probing stack space as it is allocated.  This
    param controls the maximum distance between probes into the stack as 2 raised
    to :samp:`{num}` bytes.  Higher values may reduce the number of explicit probes, but a value
    larger than the operating system provided guard will leave code vulnerable to
    stack clash style attacks.

  .. gcc-param:: max-cse-path-length

    The maximum number of basic blocks on path that CSE considers.

  .. gcc-param:: max-cse-insns

    The maximum number of instructions CSE processes before flushing.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: max-sched-region-blocks

    The maximum number of blocks in a region to be considered for
    interblock scheduling.

  .. gcc-param:: max-pipeline-region-blocks

    The maximum number of blocks in a region to be considered for
    pipelining in the selective scheduler.

  .. gcc-param:: max-sched-region-insns

    The maximum number of insns in a region to be considered for
    interblock scheduling.

  .. gcc-param:: max-pipeline-region-insns

    The maximum number of insns in a region to be considered for
    pipelining in the selective scheduler.

  .. gcc-param:: min-spec-prob

    The minimum probability (in percents) of reaching a source block
    for interblock speculative scheduling.

  .. gcc-param:: max-sched-extend-regions-iters

    The maximum number of iterations through CFG to extend regions.
    A value of 0 disables region extensions.

  .. gcc-param:: max-sched-insn-conflict-delay

    The maximum conflict delay for an insn to be considered for speculative motion.

  .. gcc-param:: sched-spec-prob-cutoff

    The minimal probability of speculation success (in percents), so that
    speculative insns are scheduled.

  .. gcc-param:: sched-state-edge-prob-cutoff

    The minimum probability an edge must have for the scheduler to save its
    state across it.

  .. gcc-param:: sched-mem-true-dep-cost

    Minimal distance (in CPU cycles) between store and load targeting same
    memory locations.

  .. gcc-param:: selsched-max-lookahead

    The maximum size of the lookahead window of selective scheduling.  It is a
    depth of search for available instructions.

  .. gcc-param:: 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.

  .. gcc-param:: selsched-insns-to-rename

    The maximum number of best instructions in the ready list that are considered
    for renaming in the selective scheduler.

  .. gcc-param:: sms-min-sc

    The minimum value of stage count that swing modulo scheduler
    generates.

  .. gcc-param:: 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.

  .. gcc-param:: max-combine-insns

    The maximum number of instructions the RTL combiner tries to combine.

  .. gcc-param:: 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.

  .. gcc-param:: ssp-buffer-size

    The minimum size of buffers (i.e. arrays) that receive stack smashing
    protection when :option:`-fstack-protector` is used.

  .. gcc-param:: min-size-for-stack-sharing

    The minimum size of variables taking part in stack slot sharing when not
    optimizing.

  .. gcc-param:: max-jump-thread-duplication-stmts

    Maximum number of statements allowed in a block that needs to be
    duplicated when threading jumps.

  .. gcc-param:: max-jump-thread-paths

    The maximum number of paths to consider when searching for jump threading
    opportunities.  When arriving at a block, incoming edges are only considered
    if the number of paths to be searched so far multiplied by the number of
    incoming edges does not exhaust the specified maximum number of paths to
    consider.

  .. gcc-param:: max-fields-for-field-sensitive

    Maximum number of fields in a structure treated in
    a field sensitive manner during pointer analysis.

  .. gcc-param:: 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 :gcc-param:`simultaneous-prefetches`).

  .. gcc-param:: simultaneous-prefetches

    Maximum number of prefetches that can run at the same time.

  .. gcc-param:: l1-cache-line-size

    The size of cache line in L1 data cache, in bytes.

  .. gcc-param:: l1-cache-size

    The size of L1 data cache, in kilobytes.

  .. gcc-param:: l2-cache-size

    The size of L2 data cache, in kilobytes.

  .. gcc-param:: prefetch-dynamic-strides

    Whether the loop array prefetch pass should issue software prefetch hints
    for strides that are non-constant.  In some cases this may be
    beneficial, though the fact the stride is non-constant may make it
    hard to predict when there is clear benefit to issuing these hints.

    Set to 1 if the prefetch hints should be issued for non-constant
    strides.  Set to 0 if prefetch hints should be issued only for strides that
    are known to be constant and below prefetch-minimum-stride.

  .. gcc-param:: prefetch-minimum-stride

    Minimum constant stride, in bytes, to start using prefetch hints for.  If
    the stride is less than this threshold, prefetch hints will not be issued.

    This setting is useful for processors that have hardware prefetchers, in
    which case there may be conflicts between the hardware prefetchers and
    the software prefetchers.  If the hardware prefetchers have a maximum
    stride they can handle, it should be used here to improve the use of
    software prefetchers.

    A value of -1 means we don't have a threshold and therefore
    prefetch hints can be issued for any constant stride.

    This setting is only useful for strides that are known and constant.

  .. gcc-param:: destructive-interference-size
                 constructive-interference-size

    The values for the C++17 variables
    ``std::hardware_destructive_interference_size`` and
    ``std::hardware_constructive_interference_size``.  The destructive
    interference size is the minimum recommended offset between two
    independent concurrently-accessed objects; the constructive
    interference size is the maximum recommended size of contiguous memory
    accessed together.  Typically both will be the size of an L1 cache
    line for the target, in bytes.  For a generic target covering a range of L1
    cache line sizes, typically the constructive interference size will be
    the small end of the range and the destructive size will be the large
    end.

    The destructive interference size is intended to be used for layout,
    and thus has ABI impact.  The default value is not expected to be
    stable, and on some targets varies with :option:`-mtune`, so use of
    this variable in a context where ABI stability is important, such as
    the public interface of a library, is strongly discouraged; if it is
    used in that context, users can stabilize the value using this
    option.

    The constructive interference size is less sensitive, as it is
    typically only used in a :samp:`static_assert` to make sure that a type
    fits within a cache line.

    See also :option:`-Winterference-size`.

  .. gcc-param:: loop-interchange-max-num-stmts

    The maximum number of stmts in a loop to be interchanged.

  .. gcc-param:: loop-interchange-stride-ratio

    The minimum ratio between stride of two loops for interchange to be profitable.

  .. gcc-param:: min-insn-to-prefetch-ratio

    The minimum ratio between the number of instructions and the
    number of prefetches to enable prefetching in a loop.

  .. gcc-param:: 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.

  .. gcc-param:: use-canonical-types

    Whether the compiler should use the 'canonical' type system.
    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.

  .. gcc-param:: 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.

  .. gcc-param:: max-partial-antic-length

    Maximum length of the partial antic set computed during the tree
    partial redundancy elimination optimization (:option:`-ftree-pre`) when
    optimizing at :option:`-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.

  .. gcc-param:: rpo-vn-max-loop-depth

    Maximum loop depth that is value-numbered optimistically.
    When the limit hits the innermost
    :samp:`{rpo-vn-max-loop-depth}` loops and the outermost loop in the
    loop nest are value-numbered optimistically and the remaining ones not.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: ira-loop-reserved-regs

    IRA can be used to evaluate more accurate register pressure in loops
    for decisions to move loop invariants (see :option:`-O3`).  The number
    of available registers reserved for some other purposes is given
    by this parameter.  Default of the parameter
    is the best found from numerous experiments.

  .. gcc-param:: ira-consider-dup-in-all-alts

    Make IRA to consider matching constraint (duplicated operand number)
    heavily in all available alternatives for preferred register class.
    If it is set as zero, it means IRA only respects the matching
    constraint when it's in the only available alternative with an
    appropriate register class.  Otherwise, it means IRA will check all
    available alternatives for preferred register class even if it has
    found some choice with an appropriate register class and respect the
    found qualified matching constraint.

  .. gcc-param:: 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 was chosen
    from numerous runs of SPEC2000 on x86-64.

  .. gcc-param:: 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.

  .. gcc-param:: loop-max-datarefs-for-datadeps

    Building data dependencies 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.

  .. gcc-param:: 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.

  .. gcc-param:: 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.

  .. gcc-param:: max-debug-marker-count

    Sets a threshold on the number of debug markers (e.g. begin stmt
    markers) to avoid complexity explosion at inlining or expanding to RTL.
    If a function has more such gimple stmts than the set limit, such stmts
    will be dropped from the inlined copy of a function, and from its RTL
    expansion.

  .. gcc-param:: 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
    :option:`-fvar-tracking-assignments`, but debug insns may get
    (non-overlapping) uids above it if the reserved range is exhausted.

  .. gcc-param:: 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.

  .. gcc-param:: ipa-sra-max-replacements

    Maximum pieces of an aggregate that IPA-SRA tracks.  As a
    consequence, it is also the maximum number of replacements of a formal
    parameter.

  .. gcc-param:: 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
    (:gcc-param:`sra-max-scalarization-size-Ospeed``) or size
    (:gcc-param:`sra-max-scalarization-size-Osize``) respectively.

  .. gcc-param:: sra-max-propagations

    The maximum number of artificial accesses that Scalar Replacement of
    Aggregates (SRA) will track, per one local variable, in order to
    facilitate copy propagation.

  .. gcc-param:: 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
    :option:`-fgnu-tm`.

  .. gcc-param:: 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.
    A value of zero can be used to lift
    the bound.  A variable whose value is unknown at compilation time and
    defined outside a SCoP is a parameter of the SCoP.

  .. gcc-param:: loop-block-tile-size

    Loop blocking or strip mining transforms, enabled with
    :option:`-floop-block` or :option:`-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.

  .. gcc-param:: ipa-jump-function-lookups

    Specifies number of statements visited during jump function offset discovery.

  .. gcc-param:: 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.
    :gcc-param:`ipa-cp-value-list-size` is the maximum number of values and types it
    stores per one formal parameter of a function.

  .. gcc-param:: ipa-cp-eval-threshold

    IPA-CP calculates its own score of cloning profitability heuristics
    and performs those cloning opportunities with scores that exceed
    :gcc-param:`ipa-cp-eval-threshold`.

  .. gcc-param:: ipa-cp-max-recursive-depth

    Maximum depth of recursive cloning for self-recursive function.

  .. gcc-param:: ipa-cp-min-recursive-probability

    Recursive cloning only when the probability of call being executed exceeds
    the parameter.

  .. gcc-param:: ipa-cp-profile-count-base

    When using :option:`-fprofile-use` option, IPA-CP will consider the measured
    execution count of a call graph edge at this percentage position in their
    histogram as the basis for its heuristics calculation.

  .. gcc-param:: ipa-cp-recursive-freq-factor

    The number of times interprocedural copy propagation expects recursive
    functions to call themselves.

  .. gcc-param:: ipa-cp-recursion-penalty

    Percentage penalty the recursive functions will receive when they
    are evaluated for cloning.

  .. gcc-param:: ipa-cp-single-call-penalty

    Percentage penalty functions containing a single call to another
    function will receive when they are evaluated for cloning.

  .. gcc-param:: ipa-max-agg-items

    IPA-CP is also capable to propagate a number of scalar values passed
    in an aggregate. :gcc-param:`ipa-max-agg-items`` controls the maximum
    number of such values per one parameter.

  .. gcc-param:: 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.

  .. gcc-param:: ipa-max-loop-predicates

    The maximum number of different predicates IPA will use to describe when
    loops in a function have known properties.

  .. gcc-param:: 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
    :gcc-param:`ipa-max-aa-steps` statements modifying memory.

  .. gcc-param:: ipa-max-switch-predicate-bounds

    Maximal number of boundary endpoints of case ranges of switch statement.
    For switch exceeding this limit, IPA-CP will not construct cloning cost
    predicate, which is used to estimate cloning benefit, for default case
    of the switch statement.

  .. gcc-param:: ipa-max-param-expr-ops

    IPA-CP will analyze conditional statement that references some function
    parameter to estimate benefit for cloning upon certain constant value.
    But if number of operations in a parameter expression exceeds
    :gcc-param:`ipa-max-param-expr-ops`, the expression is treated as complicated
    one, and is not handled by IPA analysis.

  .. gcc-param:: lto-partitions

    Specify desired number of partitions produced during WHOPR compilation.
    The number of partitions should exceed the number of CPUs used for compilation.

  .. gcc-param:: lto-min-partition

    Size of minimal partition for WHOPR (in estimated instructions).
    This prevents expenses of splitting very small programs into too many
    partitions.

  .. gcc-param:: lto-max-partition

    Size of max partition for WHOPR (in estimated instructions).
    to provide an upper bound for individual size of partition.
    Meant to be used only with balanced partitioning.

  .. gcc-param:: lto-max-streaming-parallelism

    Maximal number of parallel processes used for LTO streaming.

  .. gcc-param:: cxx-max-namespaces-for-diagnostic-help

    The maximum number of namespaces to consult for suggestions when C++
    name lookup fails for an identifier.

  .. gcc-param:: 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.
    A small positive adjustment is applied for
    statements with memory operands as those are even more profitable so sink.

  .. gcc-param:: max-stores-to-sink

    The maximum number of conditional store pairs that can be sunk.  Set to 0
    if either vectorization (:option:`-ftree-vectorize`) or if-conversion
    (:option:`-ftree-loop-if-convert`) is disabled.

  .. gcc-param:: 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.

  .. gcc-param:: jump-table-max-growth-ratio-for-size

    The maximum code size growth ratio when expanding
    into a jump table (in percent).  The parameter is used when
    optimizing for size.

  .. gcc-param:: jump-table-max-growth-ratio-for-speed

    The maximum code size growth ratio when expanding
    into a jump table (in percent).  The parameter is used when
    optimizing for speed.

  .. gcc-param:: 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.

  .. gcc-param:: sched-pressure-algorithm

    Choose between the two available implementations of
    :option:`-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 :samp:`haifa-sched.cc` in the GCC sources for more details.

    The default choice depends on the target.

  .. gcc-param:: 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.

  .. gcc-param:: asan-globals

    Enable buffer overflow detection for global objects.  This kind
    of protection is enabled by default if you are using
    :option:`-fsanitize=address` option.
    To disable global objects protection use :option:`--param` :gcc-param:`asan-globals`:samp:`=0`.

  .. gcc-param:: asan-stack

    Enable buffer overflow detection for stack objects.  This kind of
    protection is enabled by default when using :option:`-fsanitize=address`.
    To disable stack protection use :option:`--param` :gcc-param:`asan-stack`:samp:`=0` option.

  .. gcc-param:: asan-instrument-reads

    Enable buffer overflow detection for memory reads.  This kind of
    protection is enabled by default when using :option:`-fsanitize=address`.
    To disable memory reads protection use
    :option:`--param` :gcc-param:`asan-instrument-reads`:samp:`=0`.

  .. gcc-param:: asan-instrument-writes

    Enable buffer overflow detection for memory writes.  This kind of
    protection is enabled by default when using :option:`-fsanitize=address`.
    To disable memory writes protection use
    :option:`--param` :gcc-param:`asan-instrument-writes`:samp:`=0` option.

  .. gcc-param:: asan-memintrin

    Enable detection for built-in functions.  This kind of protection
    is enabled by default when using :option:`-fsanitize=address`.
    To disable built-in functions protection use
    :option:`--param` :gcc-param:`asan-memintrin`:samp:`=0`.

  .. gcc-param:: asan-use-after-return

    Enable detection of use-after-return.  This kind of protection
    is enabled by default when using the :option:`-fsanitize=address` option.
    To disable it use :option:`--param` :gcc-param:`asan-use-after-return`:samp:`=0`.

    .. note::

      By default the check is disabled at run time.  To enable it,
      add ``detect_stack_use_after_return=1`` to the environment variable
      :envvar:`ASAN_OPTIONS`.

  .. gcc-param:: 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
    :option:`--param` :gcc-param:`asan-instrumentation-with-call-threshold`:samp:`=0`.

  .. gcc-param:: hwasan-instrument-stack

    Enable hwasan instrumentation of statically sized stack-allocated variables.
    This kind of instrumentation is enabled by default when using
    :option:`-fsanitize=hwaddress` and disabled by default when using
    :option:`-fsanitize=kernel-hwaddress`.
    To disable stack instrumentation use
    :option:`--param` :gcc-param:`hwasan-instrument-stack`:samp:`=0`, and to enable it use
    :option:`--param` :gcc-param:`hwasan-instrument-stack`:samp:`=1`.

  .. gcc-param:: hwasan-random-frame-tag

    When using stack instrumentation, decide tags for stack variables using a
    deterministic sequence beginning at a random tag for each frame.  With this
    parameter unset tags are chosen using the same sequence but beginning from 1.
    This is enabled by default for :option:`-fsanitize=hwaddress` and unavailable
    for :option:`-fsanitize=kernel-hwaddress`.
    To disable it use :option:`--param` :gcc-param:`hwasan-random-frame-tag`:samp:`=0`.

  .. gcc-param:: hwasan-instrument-allocas

    Enable hwasan instrumentation of dynamically sized stack-allocated variables.
    This kind of instrumentation is enabled by default when using
    :option:`-fsanitize=hwaddress` and disabled by default when using
    :option:`-fsanitize=kernel-hwaddress`.
    To disable instrumentation of such variables use
    :option:`--param` :gcc-param:`hwasan-instrument-allocas`:samp:`=0`, and to enable it use
    :option:`--param` :gcc-param:`hwasan-instrument-allocas`:samp:`=1`.

  .. gcc-param:: hwasan-instrument-reads

    Enable hwasan checks on memory reads.  Instrumentation of reads is enabled by
    default for both :option:`-fsanitize=hwaddress` and
    :option:`-fsanitize=kernel-hwaddress`.
    To disable checking memory reads use
    :option:`--param` :gcc-param:`hwasan-instrument-reads`:samp:`=0`.

  .. gcc-param:: hwasan-instrument-writes

    Enable hwasan checks on memory writes.  Instrumentation of writes is enabled by
    default for both :option:`-fsanitize=hwaddress` and
    :option:`-fsanitize=kernel-hwaddress`.
    To disable checking memory writes use
    :option:`--param` :gcc-param:`hwasan-instrument-writes`:samp:`=0`.

  .. gcc-param:: hwasan-instrument-mem-intrinsics

    Enable hwasan instrumentation of builtin functions.  Instrumentation of these
    builtin functions is enabled by default for both :option:`-fsanitize=hwaddress`
    and :option:`-fsanitize=kernel-hwaddress`.
    To disable instrumentation of builtin functions use
    :option:`--param` :gcc-param:`hwasan-instrument-mem-intrinsics`:samp:`=0`.

  .. gcc-param:: use-after-scope-direct-emission-threshold

    If the size of a local variable in bytes is smaller or equal to this
    number, directly poison (or unpoison) shadow memory instead of using
    run-time callbacks.

  .. gcc-param:: tsan-distinguish-volatile

    Emit special instrumentation for accesses to volatiles.

  .. gcc-param:: tsan-instrument-func-entry-exit

    Emit instrumentation calls to :samp:`__tsan_func_entry()` and :samp:`__tsan_func_exit()`.

  .. gcc-param:: max-fsm-thread-path-insns

    Maximum number of instructions to copy when duplicating blocks on a
    finite state automaton jump thread path.

  .. gcc-param:: threader-debug

    threader-debug=[none|all]
    Enables verbose dumping of the threader solver.

  .. gcc-param:: parloops-chunk-size

    Chunk size of omp schedule for loops parallelized by parloops.

  .. gcc-param:: parloops-schedule

    Schedule type of omp schedule for loops parallelized by parloops (static,
    dynamic, guided, auto, runtime).

  .. gcc-param:: parloops-min-per-thread

    The minimum number of iterations per thread of an innermost parallelized
    loop for which the parallelized variant is preferred over the single threaded
    one.  Note that for a parallelized loop nest the
    minimum number of iterations of the outermost loop per thread is two.

  .. gcc-param:: max-ssa-name-query-depth

    Maximum depth of recursion when querying properties of SSA names in things
    like fold routines.  One level of recursion corresponds to following a
    use-def chain.

  .. gcc-param:: max-speculative-devirt-maydefs

    The maximum number of may-defs we analyze when looking for a must-def
    specifying the dynamic type of an object that invokes a virtual call
    we may be able to devirtualize speculatively.

  .. gcc-param:: max-vrp-switch-assertions

    The maximum number of assertions to add along the default edge of a switch
    statement during VRP.

  .. gcc-param:: evrp-sparse-threshold

    Maximum number of basic blocks before EVRP uses a sparse cache.

  .. gcc-param:: vrp1-mode

    Specifies the mode VRP pass 1 should operate in.

  .. gcc-param:: vrp2-mode

    Specifies the mode VRP pass 2 should operate in.

  .. gcc-param:: ranger-debug

    Specifies the type of debug output to be issued for ranges.

  .. gcc-param:: evrp-switch-limit

    Specifies the maximum number of switch cases before EVRP ignores a switch.

  .. gcc-param:: unroll-jam-min-percent

    The minimum percentage of memory references that must be optimized
    away for the unroll-and-jam transformation to be considered profitable.

  .. gcc-param:: unroll-jam-max-unroll

    The maximum number of times the outer loop should be unrolled by
    the unroll-and-jam transformation.

  .. gcc-param:: max-rtl-if-conversion-unpredictable-cost

    Maximum permissible cost for the sequence that would be generated
    by the RTL if-conversion pass for a branch that is considered unpredictable.

  .. gcc-param:: max-variable-expansions-in-unroller

    If :option:`-fvariable-expansion-in-unroller` is used, the maximum number
    of times that an individual variable will be expanded during loop unrolling.

  .. gcc-param:: partial-inlining-entry-probability

    Maximum probability of the entry BB of split region
    (in percent relative to entry BB of the function)
    to make partial inlining happen.

  .. gcc-param:: max-tracked-strlens

    Maximum number of strings for which strlen optimization pass will
    track string lengths.

  .. gcc-param:: gcse-after-reload-partial-fraction

    The threshold ratio for performing partial redundancy
    elimination after reload.

  .. gcc-param:: gcse-after-reload-critical-fraction

    The threshold ratio of critical edges execution count that
    permit performing redundancy elimination after reload.

  .. gcc-param:: max-loop-header-insns

    The maximum number of insns in loop header duplicated
    by the copy loop headers pass.

  .. gcc-param:: vect-epilogues-nomask

    Enable loop epilogue vectorization using smaller vector size.

  .. gcc-param:: vect-partial-vector-usage

    Controls when the loop vectorizer considers using partial vector loads
    and stores as an alternative to falling back to scalar code.  0 stops
    the vectorizer from ever using partial vector loads and stores.  1 allows
    partial vector loads and stores if vectorization removes the need for the
    code to iterate.  2 allows partial vector loads and stores in all loops.
    The parameter only has an effect on targets that support partial
    vector loads and stores.

  .. gcc-param:: vect-inner-loop-cost-factor

    The maximum factor which the loop vectorizer applies to the cost of statements
    in an inner loop relative to the loop being vectorized.  The factor applied
    is the maximum of the estimated number of iterations of the inner loop and
    this parameter.  The default value of this parameter is 50.

  .. gcc-param:: vect-induction-float

    Enable loop vectorization of floating point inductions.

  .. gcc-param:: avoid-fma-max-bits

    Maximum number of bits for which we avoid creating FMAs.

  .. gcc-param:: sms-loop-average-count-threshold

    A threshold on the average loop count considered by the swing modulo scheduler.

  .. gcc-param:: sms-dfa-history

    The number of cycles the swing modulo scheduler considers when checking
    conflicts using DFA.

  .. gcc-param:: graphite-allow-codegen-errors

    Whether codegen errors should be ICEs when :option:`-fchecking`.

  .. gcc-param:: sms-max-ii-factor

    A factor for tuning the upper bound that swing modulo scheduler
    uses for scheduling a loop.

  .. gcc-param:: lra-max-considered-reload-pseudos

    The max number of reload pseudos which are considered during
    spilling a non-reload pseudo.

  .. gcc-param:: max-pow-sqrt-depth

    Maximum depth of sqrt chains to use when synthesizing exponentiation
    by a real constant.

  .. gcc-param:: max-dse-active-local-stores

    Maximum number of active local stores in RTL dead store elimination.

  .. gcc-param:: asan-instrument-allocas

    Enable asan allocas/VLAs protection.

  .. gcc-param:: max-iterations-computation-cost

    Bound on the cost of an expression to compute the number of iterations.

  .. gcc-param:: max-isl-operations

    Maximum number of isl operations, 0 means unlimited.

  .. gcc-param:: graphite-max-arrays-per-scop

    Maximum number of arrays per scop.

  .. gcc-param:: max-vartrack-reverse-op-size

    Max. size of loc list for which reverse ops should be added.

  .. gcc-param:: fsm-scale-path-stmts

    Scale factor to apply to the number of statements in a threading path
    when comparing to the number of (scaled) blocks.

  .. gcc-param:: uninit-control-dep-attempts

    Maximum number of nested calls to search for control dependencies
    during uninitialized variable analysis.

  .. gcc-param:: fsm-scale-path-blocks

    Scale factor to apply to the number of blocks in a threading path
    when comparing to the number of (scaled) statements.

  .. gcc-param:: sched-autopref-queue-depth

    Hardware autoprefetcher scheduler model control flag.
    Number of lookahead cycles the model looks into; at '
    ' only enable instruction sorting heuristic.

  .. gcc-param:: loop-versioning-max-inner-insns

    The maximum number of instructions that an inner loop can have
    before the loop versioning pass considers it too big to copy.

  .. gcc-param:: loop-versioning-max-outer-insns

    The maximum number of instructions that an outer loop can have
    before the loop versioning pass considers it too big to copy,
    discounting any instructions in inner loops that directly benefit
    from versioning.

  .. gcc-param:: ssa-name-def-chain-limit

    The maximum number of SSA_NAME assignments to follow in determining
    a property of a variable such as its value.  This limits the number
    of iterations or recursive calls GCC performs when optimizing certain
    statements or when determining their validity prior to issuing
    diagnostics.

  .. gcc-param:: store-merging-max-size

    Maximum size of a single store merging region in bytes.

  .. gcc-param:: hash-table-verification-limit

    The number of elements for which hash table verification is done
    for each searched element.

  .. gcc-param:: max-find-base-term-values

    Maximum number of VALUEs handled during a single find_base_term call.

  .. gcc-param:: analyzer-max-enodes-per-program-point

    The maximum number of exploded nodes per program point within
    the analyzer, before terminating analysis of that point.

  .. gcc-param:: analyzer-max-constraints

    The maximum number of constraints per state.

  .. gcc-param:: analyzer-min-snodes-for-call-summary

    The minimum number of supernodes within a function for the
    analyzer to consider summarizing its effects at call sites.

  .. gcc-param:: analyzer-max-enodes-for-full-dump

    The maximum depth of exploded nodes that should appear in a dot dump
    before switching to a less verbose format.

  .. gcc-param:: analyzer-max-recursion-depth

    The maximum number of times a callsite can appear in a call stack
    within the analyzer, before terminating analysis of a call that would
    recurse deeper.

  .. gcc-param:: analyzer-max-svalue-depth

    The maximum depth of a symbolic value, before approximating
    the value as unknown.

  .. gcc-param:: analyzer-max-infeasible-edges

    The maximum number of infeasible edges to reject before declaring
    a diagnostic as infeasible.

  .. gcc-param:: gimple-fe-computed-hot-bb-threshold

    The number of executions of a basic block which is considered hot.
    The parameter is used only in GIMPLE FE.

  .. gcc-param:: analyzer-bb-explosion-factor

    The maximum number of 'after supernode' exploded nodes within the analyzer
    per supernode, before terminating analysis.

  .. gcc-param:: ranger-logical-depth

    Maximum depth of logical expression evaluation ranger will look through
    when evaluating outgoing edge ranges.

  .. gcc-param:: relation-block-limit

    Maximum number of relations the oracle will register in a basic block.

  .. gcc-param:: min-pagesize

    Minimum page size for warning purposes.

  .. gcc-param:: openacc-kernels

    Specify mode of OpenACC 'kernels' constructs handling.
    With :option:`--param` :gcc-param:`openacc-kernels`:samp:`=decompose`, OpenACC 'kernels'
    constructs are decomposed into parts, a sequence of compute
    constructs, each then handled individually.
    This is work in progress.
    With :option:`--param` :gcc-param:`openacc-kernels`:samp:`=parloops`, OpenACC 'kernels'
    constructs are handled by the :samp:`parloops` pass, en bloc.
    This is the current default.

  .. gcc-param:: openacc-privatization

    Specify mode of OpenACC privatization diagnostics for
    :option:`-fopt-info-omp-note` and applicable
    :option:`-fdump-tree-*-details`.
    With :option:`--param` :gcc-param:`openacc-privatization`:samp:`=quiet`, don't diagnose.
    This is the current default.
    With :option:`--param` :gcc-param:`openacc-privatization`:samp:`=noisy`, do diagnose.

    The following choices of :samp:`{name}` are available on AArch64 targets:

  .. gcc-param:: aarch64-sve-compare-costs

    When vectorizing for SVE, consider using 'unpacked' vectors for
    smaller elements and use the cost model to pick the cheapest approach.
    Also use the cost model to choose between SVE and Advanced SIMD vectorization.

    Using unpacked vectors includes storing smaller elements in larger
    containers and accessing elements with extending loads and truncating
    stores.

  .. gcc-param:: aarch64-float-recp-precision

    The number of Newton iterations for calculating the reciprocal for float type.
    The precision of division is proportional to this param when division
    approximation is enabled.  The default value is 1.

  .. gcc-param:: aarch64-double-recp-precision

    The number of Newton iterations for calculating the reciprocal for double type.
    The precision of division is propotional to this param when division
    approximation is enabled.  The default value is 2.

  .. gcc-param:: aarch64-autovec-preference

    Force an ISA selection strategy for auto-vectorization.  Accepts values from
    0 to 4, inclusive.

    :samp:`0`
      Use the default heuristics.

    :samp:`1`
      Use only Advanced SIMD for auto-vectorization.

    :samp:`2`
      Use only SVE for auto-vectorization.

    :samp:`3`
      Use both Advanced SIMD and SVE.  Prefer Advanced SIMD when the costs are
      deemed equal.

    :samp:`4`
      Use both Advanced SIMD and SVE.  Prefer SVE when the costs are deemed equal.

    The default value is 0.

  .. gcc-param:: aarch64-loop-vect-issue-rate-niters

    The tuning for some AArch64 CPUs tries to take both latencies and issue
    rates into account when deciding whether a loop should be vectorized
    using SVE, vectorized using Advanced SIMD, or not vectorized at all.
    If this parameter is set to :samp:`{n}`, GCC will not use this heuristic
    for loops that are known to execute in fewer than :samp:`{n}` Advanced
    SIMD iterations.

  .. gcc-param:: aarch64-vect-unroll-limit

    The vectorizer will use available tuning information to determine whether it
    would be beneficial to unroll the main vectorized loop and by how much.  This
    parameter set's the upper bound of how much the vectorizer will unroll the main
    loop.  The default value is four.

    The following choices of :samp:`{name}` are available on i386 and x86_64 targets:

  .. gcc-param:: x86-stlf-window-ninsns

    Instructions number above which STFL stall penalty can be compensated.