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From: "kretz at kde dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94343] New: [10 Regression] invalid AVX512VL vpternlogd
 instruction emitted for -march=knl
Date: Thu, 26 Mar 2020 15:24:35 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization, wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: kretz at kde dot org
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94343

            Bug ID: 94343
           Summary: [10 Regression] invalid AVX512VL vpternlogd
                    instruction emitted for -march=3Dknl
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kretz at kde dot org
  Target Milestone: ---
            Target: i386,x86-64

Test case (`-O1 -march=3Dknl`, cf. https://godbolt.org/z/qQc3Sf):

using W [[gnu::vector_size(16)]] =3D long long;
using V [[gnu::vector_size(16)]] =3D int;

auto f(V a) {
    return __builtin_ia32_pandn128(reinterpret_cast<W>(~V() ^ a), ~W());
}


This emits a XMM variant of `vpternlogd` which requires AVX512VL. But it was
supposed to compile for KNL.

Besides the bug, there's also a missed optimization here: `~V() ^ a` flips =
all
bits and pandn flips all bits again. Thus it should compile to a single `re=
t`
instruction. Note that the variation:

auto f(V a) {
    return ~reinterpret_cast<W>(~V() ^ a) & ~W();
}

compiles to

  vpternlogd xmm0, xmm0, xmm0, 0x55
  vpternlogq xmm0, xmm0, xmm0, 0x55

for KNL.=