[Bug tree-optimization/78205] New: BB vectorization confused by too large load groups

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78205

            Bug ID: 78205
           Summary: BB vectorization confused by too large load groups
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
            Blocks: 53947
  Target Milestone: ---

double x, a[4], b[4], c[5];

void foo ()
{
  a[0] = c[0];
  a[1] = c[1];
  a[2] = c[0];
  a[3] = c[1];
  b[0] = c[2];
  b[1] = c[3];
  b[2] = c[2];
  b[3] = c[3];
  x = c[4];
}

if you comment the load from c[4] the testcase will be vectorized.  If not the
we run into

      /* ???  The following is overly pessimistic (as well as the loop
         case above) in the case we can statically determine the excess
         elements loaded are within the bounds of a decl that is accessed.
         Likewise for BB vectorizations using masked loads is a possibility. 
*/
      if (bb_vinfo && slp_perm && group_size % nunits != 0)
        {
          dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                           "BB vectorization with gaps at the end of a load "
                           "is not supported\n");
          return false;
        }


I do have a hackish patch.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations