From: Arnd Bergmann <arnd@arndb.de>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Michael Ellerman <mpe@ellerman.id.au>,
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
Date: Tue, 30 Jul 2019 09:34:28 +0200 [thread overview]
Message-ID: <CAK8P3a1GQSyCj1L8fFG4Pah8dr5Lanw=1yuimX1o+53ARzOX+Q@mail.gmail.com> (raw)
In-Reply-To: <20190729215200.GN31406@gate.crashing.org>
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.
Arnd
WARNING: multiple messages have this Message-ID (diff)
From: Arnd Bergmann <arnd@arndb.de>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: kbuild test robot <lkp@intel.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Paul Mackerras <paulus@samba.org>,
Nathan Chancellor <natechancellor@gmail.com>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz
Date: Tue, 30 Jul 2019 09:34:28 +0200 [thread overview]
Message-ID: <CAK8P3a1GQSyCj1L8fFG4Pah8dr5Lanw=1yuimX1o+53ARzOX+Q@mail.gmail.com> (raw)
In-Reply-To: <20190729215200.GN31406@gate.crashing.org>
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.
Arnd
next prev parent reply other threads:[~2019-07-30 7:34 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-29 20:25 [PATCH] powerpc: workaround clang codegen bug in dcbz Nick Desaulniers
2019-07-29 20:25 ` Nick Desaulniers
2019-07-29 20:32 ` Nathan Chancellor
2019-07-29 20:32 ` Nathan Chancellor
2019-07-29 20:45 ` Nick Desaulniers
2019-07-29 20:45 ` Nick Desaulniers
2019-07-29 20:47 ` Nathan Chancellor
2019-07-29 20:47 ` Nathan Chancellor
2019-07-29 20:49 ` Nick Desaulniers
2019-07-29 20:49 ` Nick Desaulniers
2019-07-29 21:52 ` Segher Boessenkool
2019-07-29 21:52 ` Segher Boessenkool
2019-07-30 7:34 ` Arnd Bergmann [this message]
2019-07-30 7:34 ` Arnd Bergmann
2019-07-30 11:17 ` Michael Ellerman
2019-07-30 11:17 ` Michael Ellerman
2019-08-09 18:21 ` [PATCH] powerpc: fix inline asm constraints for dcbz Nick Desaulniers
2019-08-09 18:21 ` Nick Desaulniers
2019-08-09 18:28 ` Arnd Bergmann
2019-08-09 18:28 ` Arnd Bergmann
2019-08-09 20:03 ` Christophe Leroy
2019-08-09 20:03 ` Christophe Leroy
2019-08-09 20:12 ` Arnd Bergmann
2019-08-09 20:12 ` Arnd Bergmann
2019-08-09 22:03 ` Nick Desaulniers
2019-08-09 22:03 ` Nick Desaulniers
2019-08-09 22:10 ` Segher Boessenkool
2019-08-09 22:10 ` Segher Boessenkool
2019-08-09 22:00 ` Segher Boessenkool
2019-08-09 22:00 ` Segher Boessenkool
2019-08-09 22:03 ` [PATCH v3] Revert "powerpc: slightly improve cache helpers" Nick Desaulniers
2019-08-09 22:03 ` Nick Desaulniers
2019-08-10 9:09 ` Michael Ellerman
2019-08-10 9:09 ` Michael Ellerman
2019-08-09 21:55 ` [PATCH] powerpc: fix inline asm constraints for dcbz Segher Boessenkool
2019-08-09 21:55 ` Segher Boessenkool
2019-08-09 20:36 ` Nathan Chancellor
2019-08-09 20:36 ` Nathan Chancellor
2019-07-30 13:48 ` [PATCH] powerpc: workaround clang codegen bug in dcbz Segher Boessenkool
2019-07-30 13:48 ` Segher Boessenkool
2019-07-30 14:30 ` Arnd Bergmann
2019-07-30 14:30 ` Arnd Bergmann
2019-07-30 16:16 ` Segher Boessenkool
2019-07-30 16:16 ` Segher Boessenkool
2019-07-30 17:07 ` Segher Boessenkool
2019-07-30 18:24 ` Arnd Bergmann
2019-07-30 18:24 ` Arnd Bergmann
2019-07-30 18:24 ` Arnd Bergmann
2019-07-30 19:35 ` Segher Boessenkool
2019-07-30 19:35 ` Segher Boessenkool
2019-07-30 5:31 ` Christophe Leroy
2019-07-30 5:31 ` Christophe Leroy
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