Re: PING^1 [PATCH v2] x86: Check AVX512 without mask instructions

[Hongtao Liu <crazylht@gmail.com>]

On Wed, Jul 14, 2021 at 8:27 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Fri, Jun 25, 2021 at 5:39 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Fri, Jun 25, 2021 at 12:50 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > On Fri, Jun 25, 2021 at 4:51 AM Hongtao Liu <crazylht@gmail.com> wrote:
> > > >
> > > > On Fri, Jun 25, 2021 at 12:13 AM Uros Bizjak via Gcc-patches
> > > > <gcc-patches@gcc.gnu.org> wrote:
> > > > >
> > > > > On Thu, Jun 24, 2021 at 2:12 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > > >
> > > > > > CPUID functions are used to detect CPU features.  If vector ISAs
> > > > > > are enabled, compiler is free to use them in these functions.  Add
> > > > > > __attribute__ ((target("general-regs-only"))) to CPUID functions
> > > > > > to avoid vector instructions.
> > > > >
> > > > > These functions are intended to be inlined, so how does target
> > > > > attribute affect inlining?
> > > > I guess w/ -O0. they may not be inlined, that's why H.J adds those
> > > > attributes to those functions.
> > >
> > > The problem is not with these functions, but with surrounding checks
> > > for cpuid features. These checks are implemented with logic
> > > instructions, and nothing prevents RA from allocating mask registers,
> > > and consequently mask insn is emitted. Regarding mentioned functions,
> > > cpuid insn pattern has four GPR single-reg constraints, so mask
> > > registers can't be allocated here.
> > >
> > > > pr96814.dump:
> > > > 0804aa40 <main>:
> > > >  804aa40: 8d 4c 24 04          lea    0x4(%esp),%ecx
> > > > ...
> > > >  804aa63: 6a 07                push   $0x7
> > > >  804aa65: e8 e0 e7 ff ff        call   804924a <__get_cpuid_count>
> > > >
> > > > Also we need to add a target attribute to avx512f_os_support (), and
> > > > that would be enough to fix the AVX512 part.
> > > >
> > > > Moreover, all check functions in below files may also need to deal with:
> > > > adx-check.h
> > > > aes-avx-check.h
> > > > aes-check.h
> > > > amx-check.h
> > > > attr-nocf-check-1a.c
> > > > attr-nocf-check-3a.c
> > > > avx2-check.h
> > > > avx2-vpop-check.h
> > > > avx512bw-check.h
> > > > avx512-check.h
> > > > avx512dq-check.h
> > > > avx512er-check.h
> > > > avx512f-check.h
> > > > avx512vl-check.h
> > > > avx-check.h
> > > > bmi2-check.h
> > > > bmi-check.h
> > > > cf_check-1.c
> > > > cf_check-2.c
> > > > cf_check-3.c
> > > > cf_check-4.c
> > > > cf_check-5.c
> > > > f16c-check.h
> > > > fma4-check.h
> > > > fma-check.h
> > > > isa-check.h
> > > > lzcnt-check.h
> > > > m128-check.h
> > > > m256-check.h
> > > > m512-check.h
> > > > mmx-3dnow-check.h
> > > > mmx-check.h
> > > > pclmul-avx-check.h
> > > > pclmul-check.h
> > > > pr39315-check.c
> > > > rtm-check.h
> > > > sha-check.h
> > > > spellcheck-options-1.c
> > > > spellcheck-options-2.c
> > > > spellcheck-options-3.c
> > > > spellcheck-options-4.c
> > > > spellcheck-options-5.c
> > > > sse2-check.h
> > > > sse3-check.h
> > > > sse4_1-check.h
> > > > sse4_2-check.h
> > > > sse4a-check.h
> > > > sse-check.h
> > > > ssse3-check.h
> > > > stack-check-11.c
> > > > stack-check-12.c
> > > > stack-check-17.c
> > > > stack-check-18.c
> > > > stack-check-19.c
> > > > xop-check.h
> > >
> > > True, but this would just paper over the real problem. Now, it is
> > > expected that the user decorates the function that checks CPUID
> > > features with the target attribute. I'm not sure if this is OK.
vmovw is enabled by AVX512FP16, and compile cpuid check function w/
avx512fp16 may result in SIGILL on non-avx512fp16 target(though, we
didn't get a testcase yet).
Would that be a sufficient reason to disable avx512 for cpuid check?
> > >
> > > Uros.
> >
> > CPUID functions are used to detect CPU features.  If mask instructions
> > are enabled, compiler is free to use them in these functions.  Disable
> > AVX512F in AVX512 check with target pragma to avoid mask instructions.
> >
> > OK for master?
> >
>
> PING:
>
> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573717.html
>
>
> --
> H.J.



-- 
BR,
Hongtao