[RFC PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

[Vincent Mailhol]

Aside of the __builtin_foo() ones, x86 does not directly rely on any
builtin functions.

However, such builtin functions are not explicitly deactivated,
creating some collisions, concrete example being ffs() from bitops.h,
c.f.:

| ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]
|   283 | static __always_inline int ffs(int x)

This patch adds -fno-builtin to KBUILD_CFLAGS for the x86
architectures in order to prevent shadowing of builtin functions.

Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
FYI, I tested this patch on a "make allyesconfig" for both x86_32 and
x86_64.

arch/x86/Makefile.um already adds the -fno-builtin but
does not get included in arch/x86/Makefile (the only consumer of
Makefile.um is arch/um/Makefile). I do not understand what is the role
of Makefile.um here.

Because of my lack of confidence on this Makefile.um, and because it
is the first time for me to send a patch for x86/build I am sending
this as an RFC.

Regarless, this patch is my best shot on this issue. I hope I did not
miss anything obvious.
---
 arch/x86/Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index e84cdd409b64..5ff7b6571dd2 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -53,6 +53,8 @@ export REALMODE_CFLAGS
 # e.g.: obj-y += foo_$(BITS).o
 export BITS
 
+KBUILD_CFLAGS += -fno-builtin
+
 #
 # Prevent GCC from generating any FP code by mistake.
 #
-- 
2.34.1