linux-kernel - Re: [PATCH] kbuild: remove GCC's default -Wpacked-bitfield-compat flag

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Date: Wed, 6 Mar 2024 09:49:50 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Nicolas Schier <nicolas@...sle.eu>, llvm@...ts.linux.dev
Subject: Re: [PATCH] kbuild: remove GCC's default -Wpacked-bitfield-compat
 flag

On Wed, Mar 06, 2024 at 09:47:09PM +0900, Masahiro Yamada wrote:
> Commit 4a5838ad9d2d ("kbuild: Add extra gcc checks") added the
> -Wpacked-bitfield-compat flag.
> 
> GCC manual says:
>   "This warning is enabled by default. Use -Wno-packed-bitfield-compat
>    to disable this warning."
> 
> The test code in the manual:
> 
>   struct foo
>   {
>     char a:4;
>     char b:8;
>   } __attribute__ ((packed));
> 
> ... emits "note: offset of packed bit-field ‘b’ has changed in GCC 4.4"
> without W=3.
> 
> Let's remove it, as it is a default with GCC.
> 
> Clang does not support this flag, so its removal will not affect Clang
> builds.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> 
>  scripts/Makefile.extrawarn | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index a9e552a1e910..8192b497aae8 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -185,7 +185,6 @@ KBUILD_CFLAGS += -Wpointer-arith
>  KBUILD_CFLAGS += -Wredundant-decls
>  KBUILD_CFLAGS += -Wsign-compare
>  KBUILD_CFLAGS += -Wswitch-default
> -KBUILD_CFLAGS += $(call cc-option, -Wpacked-bitfield-compat)
>  
>  KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN3
>  
> -- 
> 2.40.1
> 
>