linux-kernel - Re: [PATCH] compiler-gcc: require gcc 4.8 for powerpc __builtin

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Date:	Mon, 9 May 2016 10:59:27 +1000
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] compiler-gcc: require gcc 4.8 for powerpc
 __builtin_bswap16()

Hi Josh,

On Fri, 6 May 2016 09:22:25 -0500 Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> I've also seen no problems on powerpc with 4.4 and 4.8.  I suspect it's
> specific to gcc 4.6.  Stephen, can you confirm this patch fixes it?

That will obviously fix the problem for us (since it will effectively
restore the code to what it was before the other commit for our gcc
4.6.3 builds and we have not seen it in other builds).  I will add this
patch to linux-next today.

And since "byteswap: try to avoid __builtin_constant_p gcc bug" is not
in Linus' tree, hopefully we can have this fix applied soon.

> From: Josh Poimboeuf <jpoimboe@...hat.com>
> Subject: [PATCH] compiler-gcc: require gcc 4.8 for powerpc __builtin_bswap16()
> 
> gcc support for __builtin_bswap16() was supposedly added for powerpc in
> gcc 4.6, and was then later added for other architectures in gcc 4.8.
> 
> However, Stephen Rothwell reported that attempting to use it on powerpc
> in gcc 4.6 fails with:
> 
>   lib/vsprintf.c:160:2: error: initializer element is not constant
>   lib/vsprintf.c:160:2: error: (near initialization for 'decpair[0]')
>   lib/vsprintf.c:160:2: error: initializer element is not constant
>   lib/vsprintf.c:160:2: error: (near initialization for 'decpair[1]')
>   lib/vsprintf.c:160:2: error: initializer element is not constant
>   lib/vsprintf.c:160:2: error: (near initialization for 'decpair[2]')
>   lib/vsprintf.c:160:2: error: initializer element is not constant
>   lib/vsprintf.c:160:2: error: (near initialization for 'decpair[3]')
>   lib/vsprintf.c:160:2: error: initializer element is not constant
> 
> I'm not entirely sure what those errors mean, but I don't see them on
> gcc 4.8.  So let's consider gcc 4.8 to be the official starting point
> for __builtin_bswap16().
> 
> Fixes: 7322dd755e7d ("byteswap: try to avoid __builtin_constant_p gcc bug")
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  include/linux/compiler-gcc.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index eeae401..3d5202e 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -246,7 +246,7 @@
>  #define __HAVE_BUILTIN_BSWAP32__
>  #define __HAVE_BUILTIN_BSWAP64__
>  #endif
> -#if GCC_VERSION >= 40800 || (defined(__powerpc__) && GCC_VERSION >= 40600)
> +#if GCC_VERSION >= 40800
>  #define __HAVE_BUILTIN_BSWAP16__
>  #endif
>  #endif /* CONFIG_ARCH_USE_BUILTIN_BSWAP */
> -- 
> 2.4.11

-- 
Cheers,
Stephen Rothwell