linux-kernel - Re: [PATCH] raid6/ppc: Fix build for clang

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Date:   Thu, 1 Nov 2018 09:05:29 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     mpe@...erman.id.au
Cc:     joel@....id.au, LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org, Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>, jji@...ibm.com
Subject: Re: [PATCH] raid6/ppc: Fix build for clang

On Wed, Oct 31, 2018 at 9:38 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Nick Desaulniers <ndesaulniers@...gle.com> writes:
> > On Tue, Oct 30, 2018 at 8:28 PM Joel Stanley <joel@....id.au> wrote:
> >>
> >> We cannot build these files with clang as it does not allow altivec
> >> instructions in assembly when -msoft-float is passed.
> >>
> >> Jinsong Ji <jji@...ibm.com> wrote:
> >> > We currently disable Altivec/VSX support when enabling soft-float.  So
> >> > any usage of vector builtins will break.
> >> >
> >> > Enable Altivec/VSX with soft-float may need quite some clean up work, so
> >> > I guess this is currently a limitation.
> >> >
> >> > Removing -msoft-float will make it work (and we are lucky that no
> >> > floating point instructions will be generated as well).
> >>
> >> This is a workaround until the issue is resolved in clang.
> >>
> >> Link: https://bugs.llvm.org/show_bug.cgi?id=31177
> >> Link: https://github.com/ClangBuiltLinux/linux/issues/239
> >> Signed-off-by: Joel Stanley <joel@....id.au>
> >
> > Hopefully we can have this fixed up soon.  I'll try to see who works
> > on Clang+PPC here and see if they can help move that bug along. Thanks
> > for the patch!
> >
> >> ---
> >>  lib/raid6/Makefile | 15 +++++++++++++++
> >>  1 file changed, 15 insertions(+)
> >>
> >> diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
> >> index 2f8b61dfd9b0..3a844e6fd01c 100644
> >> --- a/lib/raid6/Makefile
> >> +++ b/lib/raid6/Makefile
> >> @@ -18,6 +18,21 @@ quiet_cmd_unroll = UNROLL  $@
> >>
> >>  ifeq ($(CONFIG_ALTIVEC),y)
> >>  altivec_flags := -maltivec $(call cc-option,-mabi=altivec)
> >> +
> >> +ifdef CONFIG_CC_IS_CLANG
> >
> > Note that the top level makefile detects clang via:
> >
> > ifeq ($(cc-name),clang)
> >
> > Does that not work here? I'd prefer to keep compiler detection in
> > Makefiles consistent, if it works?
>
> cc-name is being removed in favor of CONFIG_CC_IS_CLANG:
>
> https://lore.kernel.org/lkml/1540905994-6073-1-git-send-email-yamada.masahiro@socionext.com/
>
> cheers

Cool, thanks for the link!

-- 
Thanks,
~Nick Desaulniers