Allow patterns in SLP reductions

The following removes the over-broad rejection of patterns for SLP
reductions which is done by removing them from LOOP_VINFO_REDUCTIONS
during pattern detection.  That's also insufficient in case the
pattern only appears on the reduction path.  Instead this implements
the proper correctness check in vectorizable_reduction and guides
SLP discovery to heuristically avoid forming later invalid groups.

I also couldn't find any testcase that FAILs when allowing the SLP
reductions to form so I've added one.

I came across this for single-lane SLP reductions with the all-SLP
work where we rely on patterns to properly vectorize COND_EXPR
reductions.

	* tree-vect-patterns.cc (vect_pattern_recog_1): Do not
	remove reductions involving patterns.
	* tree-vect-loop.cc (vectorizable_reduction): Reject SLP
	reduction groups with multiple lane-reducing reductions.
	* tree-vect-slp.cc (vect_analyze_slp_instance): When discovering
	SLP reduction groups avoid including lane-reducing ones.

	* gcc.dg/vect/vect-reduc-sad-9.c: New testcase.

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 704df7b..361aec0 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -7667,6 +7667,21 @@ vectorizable_reduction (loop_vec_info loop_vinfo,
       return false;
     }
 
+  /* Lane-reducing ops also never can be used in a SLP reduction group
+     since we'll mix lanes belonging to different reductions.  But it's
+     OK to use them in a reduction chain or when the reduction group
+     has just one element.  */
+  if (lane_reduc_code_p
+      && slp_node
+      && !REDUC_GROUP_FIRST_ELEMENT (stmt_info)
+      && SLP_TREE_LANES (slp_node) > 1)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			 "lane-reducing reduction in reduction group.\n");
+      return false;
+    }
+
   /* All uses but the last are expected to be defined in the loop.
      The last use is the reduction variable.  In case of nested cycle this
      assumption is not true: we use reduc_index to record the index of the