re PR tree-optimization/28364 (poor optimization choices when iterating over a std::string (probably not c++-specific))

	PR tree-optimization/28364
	* tree-ssa-loop-ivopts.c (aff_combination_to_tree): Handle zero
	correctly.
	(fold_affine_expr): New function.
	(may_eliminate_iv): Use fold_affine_expr.

From-SVN: r116189

1 files changed, 26 insertions, 4 deletions

diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 8a69664..0e8fa94 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -2917,10 +2917,17 @@ aff_combination_to_tree (struct affine_tree_combination *comb)
   unsigned i;
   unsigned HOST_WIDE_INT off, sgn;
 
-  /* Handle the special case produced by get_computation_aff when
-     the type does not fit in HOST_WIDE_INT.  */
   if (comb->n == 0 && comb->offset == 0)
-    return fold_convert (type, expr);
+    {
+      if (expr)
+	{
+	  /* Handle the special case produced by get_computation_aff when
+	     the type does not fit in HOST_WIDE_INT.  */
+	  return fold_convert (type, expr);
+	}
+      else
+	return build_int_cst (type, 0);
+    }
 
   gcc_assert (comb->n == MAX_AFF_ELTS || comb->rest == NULL_TREE);
 
@@ -2943,6 +2950,21 @@ aff_combination_to_tree (struct affine_tree_combination *comb)
 			  comb->mask);
 }
 
+/* Folds EXPR using the affine expressions framework.  */
+
+static tree
+fold_affine_expr (tree expr)
+{
+  tree type = TREE_TYPE (expr);
+  struct affine_tree_combination comb;
+
+  if (TYPE_PRECISION (type) > HOST_BITS_PER_WIDE_INT)
+    return expr;
+
+  tree_to_aff_combination (expr, type, &comb);
+  return aff_combination_to_tree (&comb);
+}
+
 /* Determines the expression by that USE is expressed from induction variable
    CAND at statement AT in LOOP.  The expression is stored in a decomposed
    form into AFF.  Returns false if USE cannot be expressed using CAND.  */
@@ -4029,7 +4051,7 @@ may_eliminate_iv (struct ivopts_data *data,
 				      fold_convert (wider_type, nit))))
     return false;
 
-  *bound = cand_value_at (loop, cand, use->stmt, nit);
+  *bound = fold_affine_expr (cand_value_at (loop, cand, use->stmt, nit));
   return true;
 }