BUILD: makefile: avoid testing all -Wno-* options when not needed

We already have 9 different warning shutup options and this list grows
with each new version. Testing for their support takes some time at the
makefile's initialisation which is visible on all options (make clean
etc). Some compilers like clang are extremely slow to validate them all
and spend roughly half a second on modern machines to validate all
options. And some compilers are happier than others when passed a -Wno-*
option they do not know:
  - gcc < 4 complains loudly
  - gcc 4 and above do not say anything, unless there is already another
    warning, in which case they will report about the unknown option as
    well, but without affecting the return code
  - clang by default rejects unknown options but supports a special option
    -Wno-unknown-warning-option to silently ignore them

This patch improves the situation a bit by detecting if the compiler
already supports random options, only supports them when called with
-Wno-unknown-warning-option, or not at all. Based on this, a variable
is set to indicate if we can avoid testing for all unknown options and
assume they are supported, and another one is set to hold the optionally
required option to shut the warning. This results in almost halving the
makefile's startup time, which is particularly appreciable with latest
compilers which become really fat (the other half is caused by the same
tests on various cc-opt).

diff --git a/Makefile b/Makefile
index 57c7510a129c5cd5ff1c495ce193c561a35f0df4..cae5b6f4685b6bc665b7ac3292c3bafc55f31460 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -138,11 +138,27 @@ 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;)
 
+
+# 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
+# will allow to get rid of testing when the compiler doesn't care. The result
+# is made of two variables:
+#  - cc-anywno that's non-empty if the compiler supports disabling anything
+#  - cc-wnouwo that may contain an option needed to enable this behavior
+# Gcc 4.x and above do not need any option but will still complain about unknown
+# options if another warning or error happens, and as such they're not testable.
+# Clang needs a special option -Wno-unknown-warning-option. Compilers not
+# supporting this option will check all warnings individually.
+cc-anywno := $(call cc-opt,-Wno-haproxy-warning)
+cc-wnouwo := $(if $(cc-anywno),,$(call cc-opt,-Wno-unknown-warning-option))
+cc-anywno := $(if $(cc-anywno)$(cc-wnouwo),1)
+
 # Disable a warning when supported by the compiler. Don't put spaces around the
 # warning! And don't use cc-opt which doesn't always report an error until
-# another one is also returned.
+# another one is also returned. If "cc-anywno" is set, the compiler supports
+# -Wno- followed by anything so we don't even need to start the compiler.
 # Usage: CFLAGS += $(call cc-nowarn,warning). Eg: $(call cc-opt,format-truncation)
-cc-nowarn = $(shell set -e; if $(CC) -Werror -W$(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "-Wno-$(1)"; fi;)
+cc-nowarn = $(if $(cc-anywno),-Wno-$(1),$(shell set -e; if $(CC) -Werror -W$(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "-Wno-$(1)"; fi;))
 
 #### Installation options.
 DESTDIR =
@@ -195,6 +211,7 @@ REG_TEST_SCRIPT=./scripts/run-regtests.sh
 # to be sure we get the intended behavior.
 SPEC_CFLAGS := -Wall -Wextra -Wundef -Wdeclaration-after-statement
 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)
 SPEC_CFLAGS += $(call cc-nowarn,unused-label)
 SPEC_CFLAGS += $(call cc-nowarn,sign-compare)