[Bug tree-optimization/68559] New: Excessive peeling for gaps

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68559

            Bug ID: 68559
           Summary: Excessive peeling for gaps
           Product: gcc
           Version: 6.0
            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: ---

With N == 20 gcc.dg/vect/pr45752.c only contains the peeling-for-gap copy
because of the following premature check which doesn't consider the
group being vectorized using SLP (or three-element interleaving).
SLP will never require gap peeling for exact_log2 (groupsize) == -1
and three-element interleaving neither.

The LOOP_VINFO_PEELING_FOR_GAPS analysis should be postponed (apart from
the single-element interleaving case).


static bool
vect_analyze_group_access_1 (struct data_reference *dr)
{
...
      /* If there is a gap in the end of the group or the group size cannot
         be made a multiple of the vector element count then we access excess
         elements in the last iteration and thus need to peel that off.  */
      if (loop_vinfo
          && (groupsize - last_accessed_element > 0
              || exact_log2 (groupsize) == -1))

        {
          if (dump_enabled_p ())
            dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                             "Data access with gaps requires scalar "
                             "epilogue loop\n");
          if (loop->inner)
            {
              if (dump_enabled_p ())
                dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                                 "Peeling for outer loop is not supported\n");
              return false;
            }

          LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) = true;


Referenced Bugs:

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