[Bug target/104611] New: memcmp/strcmp/strncmp can be optimized when the result is tested for [in]equality with 0 on aarch64

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104611

            Bug ID: 104611
           Summary: memcmp/strcmp/strncmp can be optimized when the result
                    is tested for [in]equality with 0 on aarch64
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:

bool f(char *a)
{
    char t[] = "0123456789012345678901234567890";
    return __builtin_memcmp(a, &t[0], sizeof(t)) == 0;
}

Right now GCC uses branches to optimize this but this could be done via a few
loads followed by xor (eor) of the two sides and then oring the results of xor
and then umavx and then comparing that to 0. This can be done for the
middle-end code too if there is a max reduction opcode.