[Bug target/104610] memcmp () == 0 can be optimized better for avx512f

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104610

--- 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) == 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.