We could vectorise:

     for (...)
       {
         a[0] = ...;
         a[1] = ...;
         a[2] = ...;
         a[3] = ...;
         a += stride;
       }

(including the case when stride == 8) but not:

     for (...)
       {
         a[0] = ...;
         a[1] = ...;
         a[2] = ...;
         a[3] = ...;
         a += 8;
       }

(where the stride is always 8).  The former was treated as a "grouped
and strided" store, while the latter was treated as a grouped store
with gaps, which we don't support.

This patch makes us treat groups of stores with gaps at the end as
strided groups too.  I tried to go through all uses of STMT_VINFO_STRIDED_P
and all vector uses of DR_STEP to see whether there were any hard-baked
assumptions, but couldn't see any.  I wondered whether we should relax:

  /* We do not have to consider dependences between accesses that belong
     to the same group, unless the stride could be smaller than the
     group size.  */
  if (DR_GROUP_FIRST_ELEMENT (stmtinfo_a)
      && (DR_GROUP_FIRST_ELEMENT (stmtinfo_a)
          == DR_GROUP_FIRST_ELEMENT (stmtinfo_b))
      && !STMT_VINFO_STRIDED_P (stmtinfo_a))
    return false;

for cases in which the step is constant and the absolute step is known
to be greater than the group size, but data dependence analysis should
already return chrec_known for those cases.

The new test is a version of vect-avg-15.c with the variable step
replaced by a constant one.

A natural follow-on would be to do the same for groups with gaps in
the middle:

          /* Check that the distance between two accesses is equal to the type
             size. Otherwise, we have gaps.  */
          diff = (TREE_INT_CST_LOW (DR_INIT (data_ref))
                  - TREE_INT_CST_LOW (prev_init)) / type_size;
          if (diff != 1)
            {
              [...]
              if (DR_IS_WRITE (data_ref))
                {
                  if (dump_enabled_p ())
                    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                                     "interleaved store with gaps\n");
                  return false;
                }

But I think we should do that separately and see what the fallout
from this change is first.

2018-08-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-data-refs.c (vect_analyze_group_access_1): Convert
	grouped stores with gaps to a strided group.

gcc/testsuite/
	* gcc.dg/vect/vect-avg-16.c: New test.
	* gcc.dg/vect/slp-37.c: Expect the loop to be vectorized.
	* gcc.dg/vect/vect-strided-u8-i8-gap4.c,
	* gcc.dg/vect/vect-strided-u8-i8-gap4-big-array.c: Likewise for
	the second loop in main1.

From-SVN: r263772