gimple-ssa-strength-reduction.c (valid_mem_ref_cand_p): New function.

	* gimple-ssa-strength-reduction.c (valid_mem_ref_cand_p): New function.
	(replace_ref): Do not replace a chain of only two candidates which are
	valid memory references.

From-SVN: r275297

gcc/ChangeLog

+2019-09-02  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gimple-ssa-strength-reduction.c (valid_mem_ref_cand_p): New function.
+	(replace_ref): Do not replace a chain of only two candidates which are
+	valid memory references.
+
 2019-09-02  Martin Liska  <mliska@suse.cz>
 
 	* tree-switch-conversion.c (jump_table_cluster::find_jump_tables):

gcc/gimple-ssa-strength-reduction.c

@@ -1999,6 +1999,23 @@ replace_ref (tree *expr, slsr_cand_t c)
   update_stmt (c->cand_stmt);
 }
 
+/* Return true if CAND_REF candidate C is a valid memory reference.  */
+
+static bool
+valid_mem_ref_cand_p (slsr_cand_t c)
+{
+  if (TREE_CODE (TREE_OPERAND (c->stride, 1)) != INTEGER_CST)
+    return false;
+
+  struct mem_address addr
+    = { NULL_TREE, c->base_expr, TREE_OPERAND (c->stride, 0),
+	TREE_OPERAND (c->stride, 1), wide_int_to_tree (sizetype, c->index) };
+
+  return
+    valid_mem_ref_p (TYPE_MODE (c->cand_type), TYPE_ADDR_SPACE (c->cand_type),
+		     &addr);
+}
+
 /* Replace CAND_REF candidate C, each sibling of candidate C, and each
    dependent of candidate C with an equivalent strength-reduced data
    reference.  */
@@ -2006,6 +2023,16 @@ replace_ref (tree *expr, slsr_cand_t c)
 static void
 replace_refs (slsr_cand_t c)
 {
+  /* Replacing a chain of only 2 candidates which are valid memory references
+     is generally counter-productive because you cannot recoup the additional
+     calculation added in front of them.  */
+  if (c->basis == 0
+      && c->dependent
+      && !lookup_cand (c->dependent)->dependent
+      && valid_mem_ref_cand_p (c)
+      && valid_mem_ref_cand_p (lookup_cand (c->dependent)))
+    return;
+
   if (dump_file && (dump_flags & TDF_DETAILS))
     {
       fputs ("Replacing reference: ", dump_file);

gcc/testsuite/ChangeLog

+2019-09-02  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gcc.dg/tree-ssa/slsr-42.c: New test.
+
 2019-09-02  Martin Liska  <mliska@suse.cz>
 
 	PR c++/91155

gcc/testsuite/gcc.dg/tree-ssa/slsr-42.c

+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-slsr-details" } */
+
+struct x
+{
+  int a[16];
+  int b[16];
+};
+
+void
+set (struct x *p, unsigned int n, int i)
+{
+  p->a[n] = i;
+  p->b[n] = i;
+}
+
+/* { dg-final { scan-tree-dump-not "Replacing reference: " "slsr"  { target i?86-*-* x86_64-*-* } } } */