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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Arnd Bergmann <arnd@arndb.de>,
        Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        christophe leroy <christophe.leroy@c-s.fr>,
        kbuild test robot <lkp@intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz
In-Reply-To: <CAK8P3a1GQSyCj1L8fFG4Pah8dr5Lanw=1yuimX1o+53ARzOX+Q@mail.gmail.com>
References: <20190729202542.205309-1-ndesaulniers@google.com> <20190729203246.GA117371@archlinux-threadripper> <20190729215200.GN31406@gate.crashing.org> <CAK8P3a1GQSyCj1L8fFG4Pah8dr5Lanw=1yuimX1o+53ARzOX+Q@mail.gmail.com>
Date:   Tue, 30 Jul 2019 21:17:43 +1000
Message-ID: <87h873zs88.fsf@concordia.ellerman.id.au>
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Arnd Bergmann <arnd@arndb.de> writes:
> On Mon, Jul 29, 2019 at 11:52 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
>> On Mon, Jul 29, 2019 at 01:32:46PM -0700, Nathan Chancellor wrote:
>> > 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.
>>
>> Try this:
>>
>>   https://godbolt.org/z/6_ZfVi
>>
>> This matters in non-trivial loops, for example.  But all current cases
>> where such non-trivial loops are done with cache block instructions are
>> actually written in real assembler already, using two registers.
>> Because performance matters.  Not that I recommend writing code as
>> critical as memset in C with inline asm :-)
>
> Upon a second look, I think the issue is that the "Z" is an input argument
> when it should be an output. clang decides that it can make a copy of the
> input and pass that into the inline asm. This is not the most efficient
> way, but it seems entirely correct according to the constraints.
>
> Changing it to an output "=Z" constraint seems to make it work:
>
> https://godbolt.org/z/FwEqHf
>
> Clang still doesn't use the optimum form, but it passes the correct pointer.

Thanks Arnd. This seems like a better solution.

I'll drop the revert I have staged.

Segher does this look OK to you?

Nathan/Nick, are one of you able to test this with your clang CI?

cheers