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

[Richard Sandiford]

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.

Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
OK to install?

Richard


2018-01-22  Richard Sandiford  <richard.sandif...@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.

Index: gcc/tree-vect-patterns.c
===================================================================
--- gcc/tree-vect-patterns.c    2018-01-13 18:01:51.240735098 +0000
+++ gcc/tree-vect-patterns.c    2018-01-22 17:51:06.751462689 +0000
@@ -217,6 +217,16 @@ vect_recog_temp_ssa_var (tree type, gimp
   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
     {
       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 *> *s
     {
       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
   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;
 
Index: gcc/testsuite/gcc.dg/vect/pr83965.c
===================================================================
--- /dev/null   2018-01-22 13:31:44.608695204 +0000
+++ gcc/testsuite/gcc.dg/vect/pr83965.c 2018-01-22 17:51:06.751462689 +0000
@@ -0,0 +1,18 @@
+/* { 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;
+}