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

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Date:	Mon, 09 May 2016 12:16:11 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] compiler-gcc: require gcc 4.8 for powerpc __builtin_bswap16()

On Monday 09 May 2016 19:34:53 Michael Ellerman wrote:
> On Mon, 2016-05-09 at 16:33 +1000, Stephen Rothwell wrote:
> > 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:
> > > > 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 */
> > 
> > Tested-by: Stephen Rothwell <sfr@...b.auug.org.au>

Acked-by: Arnd Bergmann <arnd@...db.de>

> > Michael, do you want to pass the fix patch on, or will I submit it
> > directly to Linus?
> 
> I'm happy for you to send it, I haven't actually hit the bug myself.

I found the commit in gcc-4.8 that replaced the powerpc-specific implementation
of __builtin_bswap16 with an architecture-independent one. Apparently the
powerpc version (gcc-4.6 and 4.7) just mapped to the lhbrx/sthbrx instructions,
so it ended up not being a constant, though the intent of the patch was
mainly to add support for the builtin to x86.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624 has the patch that went
into gcc-4.8 and more information.

	Arnd