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

[Vincent MAILHOL]

Hi Nick,

On Tue. 10 May 2022 at 04:50, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> On Mon, May 9, 2022 at 8:01 AM Vincent MAILHOL
> <mailhol.vincent@xxxxxxxxxx> wrote:
> >
> > Instead, I am thinking of just using -fno-builtin-ffs to remove
> > the annoying -Wshadow warning. Would that make more sense?
>
> Perhaps a pragma would be the best tool to silence this instance of
> -Wshadow?  I understand what GCC is trying to express, but the kernel
> does straddle a weird place between -ffreestanding and a "hosted" env.

I was a bit reluctant to propose the use of pragma because I received
negative feedback in another patch for using the __diag_ignore()
c.f.:
https://lore.kernel.org/all/YmhZSZWg9YZZLRHA@yury-laptop/

But the context here is a bit different, I guess. If I receive your support, I
am fully OK to silence this with some #pragma.

The patch would look as below (I just need to test with clang
before submitting).

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index a288ecd230ab..e44911253bdf 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -269,6 +269,9 @@ static __always_inline unsigned long
__fls(unsigned long word)
 #undef ADDR

 #ifdef __KERNEL__
+__diag_push();
+__diag_ignore_all("-Wshadow",
+                  "-fno-builtin-foo would remove optimization, just
silence it instead");
 /**
  * ffs - find first set bit in word
  * @x: the word to search
@@ -309,6 +312,7 @@ static __always_inline int ffs(int x)
 #endif
        return r + 1;
 }
+__diag_pop(); /* ignore -Wshadow */

 /**
  * fls - find last set bit in word