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

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Date:   Tue, 4 Dec 2018 08:43:47 +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 Tue, 4 Dec 2018 at 05:15, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > > +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.
>
> As in clang itself crashes, and cc-option/kbuild can't handle that gracefully?

The developer gets something like this:

SplitVectorResult #0: t196: v16i8 = llvm.ppc.altivec.vcmpgtsb
TargetConstant:i64<4823>, t146, t195
fatal error: error in backend: Do not know how to split the result of
this operator!
clang-8: error: clang frontend command failed with exit code 70 (use
-v to see invocation)

>
> >
> > 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".
> >