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

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

[Masahiro Yamada]

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@kernel.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

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

[Nathan Chancellor]

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@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.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
> 
> 

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

[David Laight]

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