[Bug target/102591] New: Failure to optimize search for value in vector-sized area to use SIMD

["gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102591

            Bug ID: 102591
           Summary: Failure to optimize search for value in vector-sized
                    area to use SIMD
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool match8(char *tpl) 
{
    int found = 0;
    for (int at = 0; at < 16; at++)
        if (tpl[at] == 0)
            found = 1;
    return found;
}

This function can be greatly optimized by using SIMD. It can be optimized to
something like this:

typedef char v16i8 __attribute__((vector_size(16)));

bool match8v2(char *tpl)
{
    v16i8 values;
    __builtin_memcpy(&values, tpl, 16);
    v16i8 compared = (values == 0);
    return _mm_movemask_epi8((__m128i)compared) != 0;
}

This optimization is done by LLVM, but not by GCC.

PS: I've marked this as an x86 bug, but only because I could not find a
portable way of expressing `_mm_movemask_epi8((__m128i)compared)`, I would
assume other architectures have similar ways of expressing the same thing
cheaply.

(For example, Altivec should be able to implement that operation with a
`vec_extract(vec_vbpermq((__vector unsigned char)compared, perm), 1)` with
`perm` looking like this: `{120, 112, 104, 96, 88, 80, 72, 64, 56, 48, 40, 32,
24, 16, 8, 0}` and the 1 replaced with 14 on big-endian)