Disable some patterns for fold-left reductions (PR 83965)

In this PR we recognised a PLUS_EXPR as a fold-left reduction,
then applied pattern matching to convert it to a WIDEN_SUM_EXPR.
We need to keep the original code in this case since we implement
the reduction using scalar rather than vector operations.

2018-01-23  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83965
	* tree-vect-patterns.c (vect_reassociating_reduction_p): New function.
	(vect_recog_dot_prod_pattern, vect_recog_sad_pattern): Use it
	instead of checking only for a reduction.
	(vect_recog_widen_sum_pattern): Likewise.

gcc/testsuite/
	PR tree-optimization/83965
	* gcc.dg/vect/pr83965.c: New test.

From-SVN: r256976

gcc/ChangeLog

+2018-01-23  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/83965
+	* tree-vect-patterns.c (vect_reassociating_reduction_p): New function.
+	(vect_recog_dot_prod_pattern, vect_recog_sad_pattern): Use it
+	instead of checking only for a reduction.
+	(vect_recog_widen_sum_pattern): Likewise.
+
 2018-01-23  Jan Hubicka  <hubicka@ucw.cz>
 
 	* predict.c (probably_never_executed): Only use precise profile info.

gcc/testsuite/ChangeLog

+2018-01-23  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/83965
+	* gcc.dg/vect/pr83965.c: New test.
+
 2018-01-23  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/83963

gcc/testsuite/gcc.dg/vect/pr83965.c

+/* { dg-do compile } */
+/* { dg-additional-options "-ftrapv" } */
+
+int
+mac (const short *a, const short *b, int sqr, int *sum)
+{
+  int i;
+  int dotp = *sum;
+
+  for (i = 0; i < 150; i++)
+    {
+      dotp += b[i] * a[i];
+      sqr += b[i] * b[i];
+    }
+
+  *sum = dotp;
+  return sqr;
+}

gcc/tree-vect-patterns.c

@@ -217,6 +217,16 @@ vect_recog_temp_ssa_var (tree type, gimple *stmt)
   return make_temp_ssa_name (type, stmt, "patt");
 }
 
+/* Return true if STMT_VINFO describes a reduction for which reassociation
+   is allowed.  */
+
+static bool
+vect_reassociating_reduction_p (stmt_vec_info stmt_vinfo)
+{
+  return (STMT_VINFO_DEF_TYPE (stmt_vinfo) == vect_reduction_def
+	  && STMT_VINFO_REDUC_TYPE (stmt_vinfo) != FOLD_LEFT_REDUCTION);
+}
+
 /* Function vect_recog_dot_prod_pattern
 
    Try to find the following pattern:
@@ -336,7 +346,7 @@ vect_recog_dot_prod_pattern (vec<gimple *> *stmts, tree *type_in,
     {
       gimple *def_stmt;
 
-      if (STMT_VINFO_DEF_TYPE (stmt_vinfo) != vect_reduction_def
+      if (!vect_reassociating_reduction_p (stmt_vinfo)
 	  && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
 	return NULL;
       oprnd0 = gimple_assign_rhs1 (last_stmt);
@@ -557,7 +567,7 @@ vect_recog_sad_pattern (vec<gimple *> *stmts, tree *type_in,
     {
       gimple *def_stmt;
 
-      if (STMT_VINFO_DEF_TYPE (stmt_vinfo) != vect_reduction_def
+      if (!vect_reassociating_reduction_p (stmt_vinfo)
 	  && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
 	return NULL;
       plus_oprnd0 = gimple_assign_rhs1 (last_stmt);
@@ -1181,7 +1191,7 @@ vect_recog_widen_sum_pattern (vec<gimple *> *stmts, tree *type_in,
   if (gimple_assign_rhs_code (last_stmt) != PLUS_EXPR)
     return NULL;
 
-  if (STMT_VINFO_DEF_TYPE (stmt_vinfo) != vect_reduction_def
+  if (!vect_reassociating_reduction_p (stmt_vinfo)
       && ! STMT_VINFO_GROUP_FIRST_ELEMENT (stmt_vinfo))
     return NULL;