BUILD: makefile: validate support for extra warnings by batches

The makefile takes quite some time to check supported warning options
and that's getting quite annoying. Most of the time all the tested ones
are quite legacy and well supported, so let's first try to validate
them all at once, and only if they fail, test them individually.

Doing so reduces the number of calls to the compiler to ~4 during the
startup, which is much better.

diff --git a/Makefile b/Makefile
index cae5b6f4685b6bc665b7ac3292c3bafc55f31460..1facdcebf15237f1e682d9cf7775ea25e9a30d81 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -138,6 +138,13 @@ cc-opt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null
 # same but emits $2 if $1 is not supported
 cc-opt-alt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "$(1)"; else echo "$(2)"; fi;)
 
+# validate a list of options one at a time
+cc-all-opts  = $(foreach a,$(1),$(call cc-opt,$(a)))
+
+# try to pass plenty of options at once, take them on success or try them
+# one at a time on failure and keep successful ones. This is handy to quickly
+# validate most common options.
+cc-all-fast = $(if $(call cc-opt,$(1)),$(1),$(call cc-all-opts,$(1)))
 
 # Below we verify that the compiler supports any -Wno-something option to
 # disable any warning, or if a special option is needed to achieve that. This
@@ -209,7 +216,11 @@ REG_TEST_SCRIPT=./scripts/run-regtests.sh
 # We rely on signed integer wraparound on overflow, however clang think it
 # can do whatever it wants since it's an undefined behavior, so use -fwrapv
 # to be sure we get the intended behavior.
+WARN_CFLAGS := -Wtype-limits -Wshift-negative-value -Wshift-overflow=2 \
+               -Wduplicated-cond -Wnull-dereference
 SPEC_CFLAGS := -Wall -Wextra -Wundef -Wdeclaration-after-statement
+SPEC_CFLAGS += $(call cc-all-fast,$(WARN_CFLAGS))
+
 SPEC_CFLAGS += $(call cc-opt-alt,-fwrapv,$(call cc-opt,-fno-strict-overflow))
 SPEC_CFLAGS += $(cc-wnouwo)
 SPEC_CFLAGS += $(call cc-nowarn,address-of-packed-member)
@@ -221,11 +232,6 @@ SPEC_CFLAGS += $(call cc-nowarn,missing-field-initializers)
 SPEC_CFLAGS += $(call cc-nowarn,cast-function-type)
 SPEC_CFLAGS += $(call cc-nowarn,string-plus-int)
 SPEC_CFLAGS += $(call cc-nowarn,atomic-alignment)
-SPEC_CFLAGS += $(call cc-opt,-Wtype-limits)
-SPEC_CFLAGS += $(call cc-opt,-Wshift-negative-value)
-SPEC_CFLAGS += $(call cc-opt,-Wshift-overflow=2)
-SPEC_CFLAGS += $(call cc-opt,-Wduplicated-cond)
-SPEC_CFLAGS += $(call cc-opt,-Wnull-dereference)
 
 ifneq ($(ERR),)
 SPEC_CFLAGS += -Werror