Fix loop reversal bug reported by Joern Rennecke.

	* loop.c (check_dbra_loop): When normalize comparison_val, add check
	to verify it is non-negative.

From-SVN: r19030

gcc/ChangeLog

+Tue Apr  7 11:58:04 1998  Jim Wilson  <wilson@cygnus.com>
+
+	* loop.c (check_dbra_loop): When normalize comparison_val, add check
+	to verify it is non-negative.
+
 Tue Apr  7 02:01:47 1998  Richard Henderson  <rth@cygnus.com>
 
 	* alpha.c (alpha_expand_block_move): Correctly collect block offsets.

gcc/loop.c

@@ -6281,7 +6281,10 @@ check_dbra_loop (loop_end, insn_count, loop_start)
 		  && GET_CODE (initial_value) == CONST_INT)
 		{
 		  comparison_val = comparison_val - INTVAL (bl->initial_value);
-		  initial_value = const0_rtx;
+		  /* Check for overflow.  If comparison_val ends up as a
+		     negative value, then we can't reverse the loop.  */
+		  if (comparison_val >= 0)
+		    initial_value = const0_rtx;
 		}
 
 	      /* If the initial value is not zero, or if the comparison