kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig

commit b303c6df80c9f8f13785aa83a0471fca7e38b24d upstream.

Since -Wmaybe-uninitialized was introduced by GCC 4.7, we have patched
various false positives:

 - commit e74fc97 ("Turn off -Wmaybe-uninitialized when building
   with -Os") turned off this option for -Os.

 - commit 815eb71 ("Kbuild: disable 'maybe-uninitialized' warning
   for CONFIG_PROFILE_ALL_BRANCHES") turned off this option for
   CONFIG_PROFILE_ALL_BRANCHES

 - commit a76bcf5 ("Kbuild: enable -Wmaybe-uninitialized warning
   for "make W=1"") turned off this option for GCC < 4.9
   Arnd provided more explanation in https://lkml.org/lkml/2017/3/14/903

I think this looks better by shifting the logic from Makefile to Kconfig.

Link: ClangBuiltLinux/linux#350
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Makefile

@@ -656,17 +656,14 @@ KBUILD_CFLAGS	+= $(call cc-disable-warning, address-of-packed-member)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, attribute-alias)
 
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
-KBUILD_CFLAGS	+= -Os $(call cc-disable-warning,maybe-uninitialized,)
-else
-ifdef CONFIG_PROFILE_ALL_BRANCHES
-KBUILD_CFLAGS	+= -O2 $(call cc-disable-warning,maybe-uninitialized,)
+KBUILD_CFLAGS   += -Os
 else
 KBUILD_CFLAGS   += -O2
 endif
-endif
 
-KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
-			$(call cc-disable-warning,maybe-uninitialized,))
+ifdef CONFIG_CC_DISABLE_WARN_MAYBE_UNINITIALIZED
+KBUILD_CFLAGS   += -Wno-maybe-uninitialized
+endif
 
 # Tell gcc to never replace conditional load with a non-conditional one
 KBUILD_CFLAGS	+= $(call cc-option,--param=allow-store-data-races=0)

init/Kconfig

@@ -16,6 +16,22 @@ config DEFCONFIG_LIST
 	default "$ARCH_DEFCONFIG"
 	default "arch/$ARCH/defconfig"
 
+config CC_HAS_WARN_MAYBE_UNINITIALIZED
+	def_bool $(cc-option,-Wmaybe-uninitialized)
+	help
+	  GCC >= 4.7 supports this option.
+
+config CC_DISABLE_WARN_MAYBE_UNINITIALIZED
+	bool
+	depends on CC_HAS_WARN_MAYBE_UNINITIALIZED
+	default CC_IS_GCC && GCC_VERSION < 40900  # unreliable for GCC < 4.9
+	help
+	  GCC's -Wmaybe-uninitialized is not reliable by definition.
+	  Lots of false positive warnings are produced in some cases.
+
+	  If this option is enabled, -Wno-maybe-uninitialzed is passed
+	  to the compiler to suppress maybe-uninitialized warnings.
+
 config CONSTRUCTORS
 	bool
 	depends on !UML
@@ -1044,6 +1060,7 @@ config CC_OPTIMIZE_FOR_PERFORMANCE
 
 config CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size"
+	imply CC_DISABLE_WARN_MAYBE_UNINITIALIZED  # avoid false positives
 	help
 	  Enabling this option will pass "-Os" instead of "-O2" to
 	  your compiler resulting in a smaller kernel.

kernel/trace/Kconfig

@@ -345,6 +345,7 @@ config PROFILE_ANNOTATED_BRANCHES
 config PROFILE_ALL_BRANCHES
 	bool "Profile all if conditionals" if !FORTIFY_SOURCE
 	select TRACE_BRANCH_PROFILING
+	imply CC_DISABLE_WARN_MAYBE_UNINITIALIZED  # avoid false positives
 	help
 	  This tracer profiles all branch conditions. Every if ()
 	  taken in the kernel is recorded whether it hit or miss.