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

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Message-ID: <CAKwvOdndyeRZApOqbp-6evEpYgzHyGo4qqaMKd_DQ4U9x8_5wA@mail.gmail.com>
Date:   Mon, 3 Dec 2018 10:45:01 -0800
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 v2] raid6/ppc: Fix build for clang

On Mon, Dec 3, 2018 at 2:24 AM Joel Stanley <joel@....id.au> wrote:
>
> 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.

As in clang itself crashes, and cc-option/kbuild can't handle that gracefully?

>
> 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



-- 
Thanks,
~Nick Desaulniers