(merge_ranges): In (+,-) case, treat subset specially

if lower bounds are the same.

From-SVN: r13127

gcc/fold-const.c

@@ -2963,16 +2963,20 @@ merge_ranges (pin_p, plow, phigh, in0_p, low0, high0, in1_p, low1, high1)
     {
       /* If they don't overlap, the result is the first range.  If the
 	 second range is a subset of the first, we can't describe this as
-	 a single range unless both ranges end at the same place, in which
-	 case we can ignore the fact that it's a subset.  Otherwise, we go
-	 from the start of the first range to just before the start of the
-	 second.  */
+	 a single range unless both ranges end at the same place.  If both
+	 ranges also start in the same place, then the result is false.
+	 Otherwise, we go from the start of the first range to just before
+	 the start of the second.  */
       if (no_overlap)
 	in_p = 1, low = low0, high = high0;
       else if (subset
 	       && integer_zerop (range_binop (EQ_EXPR, integer_type_node,
 					      high0, 1, high1, 0)))
 	return 0;
+      else if (subset
+	       && integer_onep (range_binop (EQ_EXPR, integer_type_node,
+					     low0, 0, low1, 0)))
+	in_p = 0, low = high = 0;
       else
 	{
 	  in_p = 1, low = low0;