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

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Message-ID: <74e770fb614c41208203732b095bd20f@AcuMS.aculab.com>
Date: Sun, 10 Mar 2024 18:37:27 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Nathan Chancellor' <nathan@...nel.org>, Masahiro Yamada
	<masahiroy@...nel.org>
CC: "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
	"linux-kernel@...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" <llvm@...ts.linux.dev>
Subject: RE: [PATCH] kbuild: remove GCC's default -Wpacked-bitfield-compat
 flag

From: Nathan Chancellor
> Sent: 06 March 2024 16:50
> 
> 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>

gcc generates pretty crap code for the above.
clang is ok unless you extend the structure to 3 bytes.
See:
https://godbolt.org/z/9c1csonY9


	David

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