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

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Message-ID: <20160509163302.59f3f6a2@canb.auug.org.au>
Date:	Mon, 9 May 2016 16:33:02 +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 all,

On Mon, 9 May 2016 10:59:27 +1000 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>
> 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  

Tested-by: Stephen Rothwell <sfr@...b.auug.org.au>

Michael, do you want to pass the fix patch on, or will I submit it
directly to Linus?

-- 
Cheers,
Stephen Rothwell