[PATCH v2] x86: Check AVX512 without mask instructions

["H.J. Lu" <hjl.tools@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 3151 bytes --]

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.
>
> 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?

Thanks.

-- 
H.J.

[-- Attachment #2: v2-0001-x86-Check-AVX512-without-mask-instructions.patch --]
[-- Type: text/x-patch, Size: 1317 bytes --]

From 7961c445f27de3e813d7332afc14e9844c0831f7 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 24 Jun 2021 04:43:41 -0700
Subject: [PATCH v2] x86: Check AVX512 without mask instructions

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.

	PR target/101185
	* gcc.target/i386/avx512-check.h: Disable AVX512F in AVX512 check
	with target pragma.
---
 gcc/testsuite/gcc.target/i386/avx512-check.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gcc/testsuite/gcc.target/i386/avx512-check.h b/gcc/testsuite/gcc.target/i386/avx512-check.h
index 0a377dba1d5..7ccf730c4f1 100644
--- a/gcc/testsuite/gcc.target/i386/avx512-check.h
+++ b/gcc/testsuite/gcc.target/i386/avx512-check.h
@@ -1,5 +1,8 @@
 #include <stdlib.h>
+#pragma GCC push_options
+#pragma GCC target ("no-mmx,no-sse")
 #include "cpuid.h"
+#pragma GCC pop_options
 #include "m512-check.h"
 #include "avx512f-os-support.h"
 
@@ -25,6 +28,9 @@ do_test (void)
 }
 #endif
 
+#pragma GCC push_options
+#pragma GCC target ("no-mmx,no-sse")
+
 static int
 check_osxsave (void)
 {
@@ -110,3 +116,4 @@ main ()
 #endif
   return 0;
 }
+#pragma GCC pop_options
-- 
2.31.1