[backport from gcc-4.7.1 r188119, sans the restrict-2.c change <mikpe@it.uu.se>]

gcc/

2012-06-01  Eric Botcazou  <ebotcazou@adacore.com>

	PR middle-end/53501
	* fold-const.c (fold_binary_loc): Refine previous change.

--- gcc-4.6.3/gcc/fold-const.c.~1~	2012-06-09 22:14:21.000000000 +0200
+++ gcc-4.6.3/gcc/fold-const.c	2012-06-09 22:18:29.000000000 +0200
@@ -9644,15 +9644,18 @@ fold_binary_loc (location_t loc,
 	    }
 	}
 
-      /* Handle (A1 * C1) + (A2 * C2) with A1, A2 or C1, C2 being the
-	 same or one.  Make sure type is not saturating.
-	 fold_plusminus_mult_expr will re-associate.  */
-      if ((TREE_CODE (op0) == MULT_EXPR
-	   || TREE_CODE (op1) == MULT_EXPR)
+      /* Handle (A1 * C1) + (A2 * C2) with A1, A2 or C1, C2 being the same or
+	 one.  Make sure the type is not saturating and has the signedness of
+	 the stripped operands, as fold_plusminus_mult_expr will re-associate.
+	 ??? The latter condition should use TYPE_OVERFLOW_* flags instead.  */
+      if ((TREE_CODE (arg0) == MULT_EXPR
+	   || TREE_CODE (arg1) == MULT_EXPR)
 	  && !TYPE_SATURATING (type)
+	  && TYPE_UNSIGNED (type) == TYPE_UNSIGNED (TREE_TYPE (arg0))
+	  && TYPE_UNSIGNED (type) == TYPE_UNSIGNED (TREE_TYPE (arg1))
 	  && (!FLOAT_TYPE_P (type) || flag_associative_math))
         {
-	  tree tem = fold_plusminus_mult_expr (loc, code, type, op0, op1);
+	  tree tem = fold_plusminus_mult_expr (loc, code, type, arg0, arg1);
 	  if (tem)
 	    return tem;
 	}
@@ -10263,15 +10266,18 @@ fold_binary_loc (location_t loc,
 	  && (tem = distribute_real_division (loc, code, type, arg0, arg1)))
 	return tem;
 
-      /* Handle (A1 * C1) - (A2 * C2) with A1, A2 or C1, C2 being the
-	 same or one.  Make sure type is not saturating.
-	 fold_plusminus_mult_expr will re-associate.  */
-      if ((TREE_CODE (op0) == MULT_EXPR
-	   || TREE_CODE (op1) == MULT_EXPR)
+      /* Handle (A1 * C1) - (A2 * C2) with A1, A2 or C1, C2 being the same or
+	 one.  Make sure the type is not saturating and has the signedness of
+	 the stripped operands, as fold_plusminus_mult_expr will re-associate.
+	 ??? The latter condition should use TYPE_OVERFLOW_* flags instead.  */
+      if ((TREE_CODE (arg0) == MULT_EXPR
+	   || TREE_CODE (arg1) == MULT_EXPR)
 	  && !TYPE_SATURATING (type)
+	  && TYPE_UNSIGNED (type) == TYPE_UNSIGNED (TREE_TYPE (arg0))
+	  && TYPE_UNSIGNED (type) == TYPE_UNSIGNED (TREE_TYPE (arg1))
 	  && (!FLOAT_TYPE_P (type) || flag_associative_math))
         {
-	  tree tem = fold_plusminus_mult_expr (loc, code, type, op0, op1);
+	  tree tem = fold_plusminus_mult_expr (loc, code, type, arg0, arg1);
 	  if (tem)
 	    return tem;
 	}