[Bug c/54803] New: Manual constant unfolding breaks vectorization

[jasongross9+bugzilla at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54803

             Bug #: 54803
           Summary: Manual constant unfolding breaks vectorization
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jasongross9+bugzilla@gmail.com


Created attachment 28348
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28348
code files

Manually unfolding constants sometimes prevents vectorization.

For example, these loops vectorize:

void multi_left_shift0(uint64_t *const array, size_t len, size_t num_bits) {
  for (size_t i = 0; i < len; i++) {
    array[i] = (array[i] >> 31) | (array[i] << 31);
  }
}

void multi_left_shift2(uint64_t *const array, size_t len, size_t num_bits) {
  for (size_t i = 0; i < len; i++) {
    const uint64_t tempa = array[i] >> 32;
    const uint64_t tempb = array[i] << 32;
    array[i] = tempa | tempb;
  }
}


but this loops does not:

void multi_left_shiftb0(uint64_t *const array, size_t len, size_t num_bits) {
  for (size_t i = 0; i < len; i++) {
    array[i] = (array[i] >> 32) | (array[i] << 32);
  }
}


See attached file for the code, preprocessed code, gcc command line log, and
assembly.