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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 3 Dec 2018 14:55:37 -0800
Message-ID: <CAKwvOdm953JFNaR6wMtDJDiwkaqXM83s2Qr=RFJ5OZAzeNQwEA@mail.gmail.com>
Subject: Re: [PATCH v2] raid6/ppc: Fix build for clang
To:     joel@jms.id.au
Cc:     mpe@ellerman.id.au, LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev@lists.ozlabs.org, Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>, jji@us.ibm.com
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On Mon, Dec 3, 2018 at 2:14 PM Joel Stanley <joel@jms.id.au> wrote:
>
> On Tue, 4 Dec 2018 at 05:15, Nick Desaulniers <ndesaulniers@google.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".
> > >

Eek, that means cc-option is not hardened against flags that crash the
compiler (misbehaving compiler).  We'll have to get some version check
in place should we ever want to use those flags for these object
files, but for now:

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
-- 
Thanks,
~Nick Desaulniers