Add pointer to PR92772

2019-12-17  Andrew Stubbs  <ams@codesourcery.com>

	* tree-vect-loop.c (vect_create_epilog_for_reduction): Mention pr92772
	in the comments.

From-SVN: r279460

gcc/ChangeLog

 2019-12-17  Andrew Stubbs  <ams@codesourcery.com>
 
+	* tree-vect-loop.c (vect_create_epilog_for_reduction): Mention pr92772
+	in the comments.
+
+2019-12-17  Andrew Stubbs  <ams@codesourcery.com>
+
 	* config/gcn/gcn-valu.md (extract_last_<mode>): New expander.
 	(fold_extract_last_<mode>): New expander.
 

gcc/tree-vect-loop.c

@@ -4534,7 +4534,10 @@ vect_create_epilog_for_reduction (stmt_vec_info stmt_info,
      containing the last time the condition passed for that vector lane.
      The first match will be a 1 to allow 0 to be used for non-matching
      indexes.  If there are no matches at all then the vector will be all
-     zeroes.  */
+     zeroes.
+   
+     PR92772: This algorithm is broken for architectures that support
+     masked vectors, but do not provide fold_extract_last.  */
   if (STMT_VINFO_REDUC_TYPE (reduc_info) == COND_REDUCTION)
     {
       auto_vec<std::pair<tree, bool>, 2> ccompares;