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

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

 - commit e74fc973b6e5 ("Turn off -Wmaybe-uninitialized when building
   with -Os") turned off this option for -Os.

 - commit 815eb71e7149 ("Kbuild: disable 'maybe-uninitialized' warning
   for CONFIG_PROFILE_ALL_BRANCHES") turned off this option for
   CONFIG_PROFILE_ALL_BRANCHES

 - commit a76bcf557ef4 ("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: https://github.com/ClangBuiltLinux/linux/issues/350
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 513fa544a134..ce43083b681d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -26,6 +26,22 @@ config CLANG_VERSION
 config CC_HAS_ASM_GOTO
         def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
 
+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
@@ -1102,6 +1118,7 @@ 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.