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From: "benjsith at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/109973] New: Wrong code for AVX2 since 13.1 by
 combining VPAND and VPTEST
Date: Fri, 26 May 2023 00:38:41 +0000
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X-Bugzilla-Component: rtl-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109973

            Bug ID: 109973
           Summary: Wrong code for AVX2 since 13.1 by combining VPAND and
                    VPTEST
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: benjsith at gmail dot com
  Target Milestone: ---

The following code is a minimal repro of the issue, when compiled with `gcc=
 -O1
-mavx2`:

#include <immintrin.h>

int do_stuff(__m256i Y0, __m256i Y1, __m128i X2) {
  __m256i And01 =3D _mm256_and_si256(Y0, Y1);
  int TestResult =3D _mm256_testc_si256(And01, And01);
  return TestResult;
}

I have also attached the preprocessed version of that minimal repro

12.3 produces the following assembly
        vpand   ymm0, ymm0, ymm1  ; <<<<< missing in 13.1
        mov     eax, 0
        vptest  ymm0, ymm0
        setb    al
        ret

While 13.1 generates:
        mov     eax, 0
        vptest  ymm0, ymm1
        setb    al
        ret

Note that as of 13.1, the VPAND is removed, and just the VPTEST remains.
However, this is incorrect because _mm256_testc_si256 returns the results of
the Carry Flag, which is based on the bitwise AND of the second operand and=
 the
bitwise NOT of the first operand, meaning these two can't be combined like =
that
for this intrinsic

Here is a Godbolt link showing the issue with a full execution case:
https://godbolt.org/z/x9or4WEWh

This issue is present in 12.3 but not 13.1. A bisect shows that it was most
likely introduced in a56c1641e9d25e46059168e811b4a2f185f07b6b

I have confirmed that this issue is still present on the latest trunk,
8d2fa90a41567670d2dbd4918d19d21d9bec4a8f

-O0 on trunk will also return the correct result

For triage/priority purposes: this bug was not found in manually written co=
de,
but instead from a fuzzer meant to test SIMD codegen

PS: This is my first bug on the GCC tracker, so if I've done anything wrong=
 let
me know. I marked it as in "rtl-optimization" though I'm not sure if that's
correct=