(non_lvalue): Result must not be null ptr constant.

(omit_one_operand): Use non_lvalue.
(fold, case COMPOUND_EXPR): Use non_lvalue if value is 0.

From-SVN: r3276

gcc/fold-const.c

@@ -1580,7 +1580,8 @@ fold_convert (t, arg1)
   return t;
 }
 
-/* Return an expr equal to X but certainly not valid as an lvalue.  */
+/* Return an expr equal to X but certainly not valid as an lvalue.
+   Also make sure it is not valid as an null pointer constant.  */
 
 tree
 non_lvalue (x)
@@ -1594,7 +1595,15 @@ non_lvalue (x)
       || TREE_CODE (x) == REAL_CST
       || TREE_CODE (x) == STRING_CST
       || TREE_CODE (x) == ADDR_EXPR)
-    return x;
+    {
+      if (TREE_CODE (x) == INTEGER_CST && integer_zerop (x))
+	{
+	  result = build1 (NOP_EXPR, TREE_TYPE (x), x);
+	  TREE_CONSTANT (result) = TREE_CONSTANT (x);
+	  return result;
+	}
+      return x;
+    }
 
   result = build1 (NON_LVALUE_EXPR, TREE_TYPE (x), x);
   TREE_CONSTANT (result) = TREE_CONSTANT (x);
@@ -1990,7 +1999,7 @@ omit_one_operand (type, result, omitted)
   if (TREE_SIDE_EFFECTS (omitted))
     return build (COMPOUND_EXPR, type, omitted, t);
 
-  return t;
+  return non_lvalue (t);
 }
 
 /* Return a simplified tree node for the truth-negation of ARG.  This
@@ -4339,9 +4348,12 @@ fold (expr)
       return t;
 
     case COMPOUND_EXPR:
-      if (!TREE_SIDE_EFFECTS (arg0))
-	return arg1;
-      return t;
+      if (TREE_SIDE_EFFECTS (arg0))
+	return t;
+      /* Don't let (0, 0) be null pointer constant.  */
+      if (integer_zerop (arg1))
+	return non_lvalue (arg1);
+      return arg1;
 
     default:
       return t;