[Bug tree-optimization/66051] New: can't vectorize reductions inside an SLP group

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66051

            Bug ID: 66051
           Summary: can't vectorize reductions inside an SLP group
           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: ---

void foo (int *p, short *q, int n)
{
  int i;
  for (i = 0; i < n; ++i)
    {
      p[i*4+0] = q[i*8+0] + q[i*8+4];
      p[i*4+1] = q[i*8+1] + q[i*8+5];
      p[i*4+2] = q[i*8+2] + q[i*8+6];
      p[i*4+3] = q[i*8+3] + q[i*8+7];
    }
}

is vectorized by unrolling the loop 4 times instead of using SLP because

t3.c:4:3: note: Build SLP for _15 = *_14;

t3.c:4:3: note: Build SLP failed: grouped loads have gaps _15 = *_14;

which isn't the whole story (I don't think we support this kind of
"reductions").  The SLP build runs into two load children, one loading
[0, 3] and one loading [4, 7] of a single group (and thus producing gaps).
Ideally we vectorize this with a single load of [0, 7], shuffle, add
and only unpack the low part.


Referenced Bugs:

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