[backport from gcc-4.8/trunk r188359 <mikpe@it.uu.se>]

Date: Sat, 09 Jun 2012 08:47:03 -0400
From: Kenneth Zadeck <zadeck at naturalbridge dot com>
Subject: obvious fix to simplify-rtx.c
List-Archive: <http://gcc.gnu.org/ml/gcc-patches/>

The shift count was being truncated by the wrong amount.    It is now 
zero extended from log2 of the mode size rather than the mode size.    
This is a (mostly) obvious fix - iant needed a hint to see what was 
wrong.   The location of this code meant that this would be a very rare 
bug:   only ti mode shifts on an llp64 platform.

bootstrapped and regression tested on x86-64.

committed as revision 188359.

Kenny

gcc/

2012-06-09  Kenneth Zadeck <zadeck@naturalbridge.com>

	* simplify-rtx.c (simplify_const_binary_operation): Fixed shift
	count trucation.

--- gcc-4.6.3/gcc/simplify-rtx.c.~1~	2011-12-23 10:16:08.000000000 +0100
+++ gcc-4.6.3/gcc/simplify-rtx.c	2012-06-09 20:38:47.000000000 +0200
@@ -3459,7 +3459,10 @@ simplify_const_binary_operation (enum rt
 	    unsigned HOST_WIDE_INT cnt;
 
 	    if (SHIFT_COUNT_TRUNCATED)
-	      o1 = double_int_zext (o1, GET_MODE_BITSIZE (mode));
+	      {
+		o1.high = 0; 
+		o1.low &= GET_MODE_BITSIZE (mode) - 1;
+	      }
 
 	    if (!double_int_fits_in_uhwi_p (o1)
 	        || double_int_to_uhwi (o1) >= GET_MODE_BITSIZE (mode))