[backport gcc-4.8/trunk r194899 <mikpe@it.uu.se>]

gcc/

2013-01-04  Richard Biener  <rguenther@suse.de>

	PR middle-end/55863
	* fold-const.c (split_tree): Undo -X - 1 to ~X folding for
	reassociation.

gcc/testsuite/

2013-01-04  Richard Biener  <rguenther@suse.de>

	PR middle-end/55863
	* gcc.dg/fold-reassoc-2.c: New testcase.

--- gcc-4.7.2/gcc/fold-const.c.~1~	2012-06-01 19:03:19.000000000 +0200
+++ gcc-4.7.2/gcc/fold-const.c	2013-01-05 13:33:54.000000000 +0100
@@ -836,6 +836,13 @@ split_tree (tree in, enum tree_code code
       if (neg_var_p)
 	var = negate_expr (var);
     }
+  else if (TREE_CODE (in) == BIT_NOT_EXPR
+	   && code == PLUS_EXPR)
+    {
+      /* -X - 1 is folded to ~X, undo that here.  */
+      *minus_litp = build_one_cst (TREE_TYPE (in));
+      var = negate_expr (TREE_OPERAND (in, 0));
+    }
   else if (TREE_CONSTANT (in))
     *conp = in;
   else
--- gcc-4.7.2/gcc/testsuite/gcc.dg/fold-reassoc-2.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.7.2/gcc/testsuite/gcc.dg/fold-reassoc-2.c	2013-01-05 13:33:54.000000000 +0100
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-original" } */
+
+int foo (int i)
+{
+  return (i + 2) - (i + 1);
+}
+int bar (int i)
+{
+  return (i + 2) + ~i;
+}
+
+/* { dg-final { scan-tree-dump "return 1;" "original" } } */
+/* { dg-final { cleanup-tree-dump "original" } } */