linux-kernel - Re: [PATCH v2] raid6/ppc: Fix build for clang

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Date:   Mon, 3 Dec 2018 20:54:40 +1030
From:   Joel Stanley <joel@....id.au>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org, Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>, Jinsong Ji <jji@...ibm.com>
Subject: Re: [PATCH v2] raid6/ppc: Fix build for clang

On Sat, 3 Nov 2018 at 04:04, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Thu, Nov 1, 2018 at 5:45 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>
> > ---
> > v2: fix typo in comment, thanks Jinsong
> >
> >  lib/raid6/Makefile | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
> > index 2f8b61dfd9b0..7ed43eaa02ef 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
> > +# clang ppc port does not yet support -maltivec 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
>
> Hi Joel, thanks for this patch!  My same thoughts about
> CONFIG_CC_IS_CLANG vs cc-option from
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2018-November/180939.html
> apply here as well.  I don't feel strongly about either though.  What
> are your thoughts?

I'm not sure that we can test for this one with cc-option. The result
of having -maltivec with -msoft-float is a error about the internals
of clang, which isn't something that kbuild is set up to test for.

When clang is fixed to allow this combination we will still build this
code in the same way, so in that sense it fails "open".

Cheers,

Joel