combine.c (force_to_mode): Avoid undefined behaviour due to negative shift amount.

	* combine.c (force_to_mode) [LSHIFTRT]: Avoid undefined behaviour
	due to negative shift amount.

From-SVN: r189347

gcc/ChangeLog

+2012-07-07  Ulrich Weigand  <ulrich.weigand@linaro.org>
+
+	* combine.c (force_to_mode) [LSHIFTRT]: Avoid undefined behaviour
+	due to negative shift amount.
+
 2012-07-07  Hans-Peter Nilsson  <hp@axis.com>
 
 	Fix configure test for "stack protector support in target C library".

gcc/combine.c

@@ -8433,6 +8433,7 @@ force_to_mode (rtx x, enum machine_mode mode, unsigned HOST_WIDE_INT mask,
 	 in OP_MODE.  */
 
       if (CONST_INT_P (XEXP (x, 1))
+	  && INTVAL (XEXP (x, 1)) >= 0
 	  && INTVAL (XEXP (x, 1)) < HOST_BITS_PER_WIDE_INT
 	  && HWI_COMPUTABLE_MODE_P (op_mode))
 	{