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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/104610] memcmp () == 0 can be optimized better for
 avx512f
Date: Tue, 10 Oct 2023 07:37:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104610

--- Comment #22 from Hongtao.liu <crazylht at gmail dot com> ---
For 64-byte memory comparison

int compare (const char* s1, const char* s2)
{
  return __builtin_memcmp (s1, s2, 64) =3D=3D 0;
}

We're generating

            vmovdqu (%rsi), %ymm0
            vpxorq  (%rdi), %ymm0, %ymm0
            vptest  %ymm0, %ymm0
            jne     .L2
            vmovdqu 32(%rsi), %ymm0
            vpxorq  32(%rdi), %ymm0, %ymm0
            vptest  %ymm0, %ymm0
            je      .L5
    .L2:
            movl    $1, %eax
            xorl    $1, %eax
            vzeroupper
            ret

An alternative way is using vpcmpeq + kortest and check Carry bit

            vmovdqu64       (%rsi), %zmm0
            xorl    %eax, %eax
            vpcmpeqd        (%rdi), %zmm0, %k0
            kortestw        %k0, %k0
            setc    %al
            vzeroupper

Not sure if it's better or not.=