[Bug middle-end/65962] New: Missed vectorization of strided stores

["alalaw01 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65962

            Bug ID: 65962
           Summary: Missed vectorization of strided stores
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alalaw01 at gcc dot gnu.org
  Target Milestone: ---

This does not vectorize at -O3 on x86_64/-mavx or aarch64:
int
loop (int *data)
{
  int tot = 0;
  for (int i = 0; i < 256; i++)
    data[i * 2] += 7;
  return tot;
}

-fdump-tree-vect-details reveals:

loadstore.c:6:3: note: === vect_analyze_data_ref_accesses ===
loadstore.c:6:3: note: Detected single element interleaving *_8 step 8
loadstore.c:6:3: note: Data access with gaps requires scalar epilogue loop
loadstore.c:6:3: note: not consecutive access *_8 = _10;

loadstore.c:6:3: note: not vectorized: complicated access pattern.
loadstore.c:6:3: note: bad data access.

However, a similar testcase that only reads from those locations, vectorizes
ok:
int
loop_12 (int *data)
{
  int tot = 0;
  for (int i = 0; i < 256; i++)
    tot += data[i * 2];
  return tot;
}
blocksort.c:6:3: note: === vect_analyze_data_ref_accesses ===
blocksort.c:6:3: note: Detected single element interleaving *_7 step 8
blocksort.c:6:3: note: Data access with gaps requires scalar epilogue loop