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

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Date:   Wed, 31 Oct 2018 12:54:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     joel@....id.au
Cc:     mpe@...erman.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 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?

> +# clang ppc port does not yet support -maltifec when -msoft-float is
> +# enabled. A future release of clang will resolve this
> +# https://bugs.llvm.org/show_bug.cgi?id=31177
> +CFLAGS_REMOVE_altivec1.o  += -msoft-float
> +CFLAGS_REMOVE_altivec2.o  += -msoft-float
> +CFLAGS_REMOVE_altivec4.o  += -msoft-float
> +CFLAGS_REMOVE_altivec8.o  += -msoft-float
> +CFLAGS_REMOVE_altivec8.o  += -msoft-float
> +CFLAGS_REMOVE_vpermxor1.o += -msoft-float
> +CFLAGS_REMOVE_vpermxor2.o += -msoft-float
> +CFLAGS_REMOVE_vpermxor4.o += -msoft-float
> +CFLAGS_REMOVE_vpermxor8.o += -msoft-float
> +endif
>  endif
>
>  # The GCC option -ffreestanding is required in order to compile code containing
> --
> 2.19.1
>


-- 
Thanks,
~Nick Desaulniers