linux-kernel - Re: [PATCH] powerpc: workaround clang codegen bug in dcbz

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Message-ID: <20190729204755.GA118622@archlinux-threadripper>
Date:   Mon, 29 Jul 2019 13:47:55 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        kbuild test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz

On Mon, Jul 29, 2019 at 01:45:35PM -0700, Nick Desaulniers wrote:
> On Mon, Jul 29, 2019 at 1:32 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > On Mon, Jul 29, 2019 at 01:25:41PM -0700, Nick Desaulniers wrote:
> > > But I'm not sure how the inlined code generated would be affected.
> >
> > For the record:
> >
> > https://godbolt.org/z/z57VU7
> >
> > This seems consistent with what Michael found so I don't think a revert
> > is entirely unreasonable.
> 
> Thanks for debugging/reporting/testing and the Godbolt link which
> clearly shows that the codegen for out of line versions is no
> different.  The case I can't comment on is what happens when those
> `static inline` functions get inlined (maybe the original patch
> improves those cases?).
> -- 
> Thanks,
> ~Nick Desaulniers

I'll try to build with various versions of GCC and compare the
disassembly of the one problematic location that I found and see
what it looks like.

Cheers,
Nathan