* simplify-rtx.c (simplify_const_binary_operation): Use the
	shift_truncation_mask hook instead of performing modulo by
	width.  Compare against mode precision, not bitsize.
	* combine.c (combine_simplify_rtx, simplify_shift_const_1):
	Use shift_truncation_mask instead of constructing the value
	manually.

Index: gcc/simplify-rtx.c
===================================================================
--- gcc/simplify-rtx.c.orig
+++ gcc/simplify-rtx.c
@@ -3704,8 +3704,8 @@ simplify_const_binary_operation (enum rt
 	     shift_truncation_mask, since the shift might not be part of an
 	     ashlM3, lshrM3 or ashrM3 instruction.  */
 	  if (SHIFT_COUNT_TRUNCATED)
-	    arg1 = (unsigned HOST_WIDE_INT) arg1 % width;
-	  else if (arg1 < 0 || arg1 >= GET_MODE_BITSIZE (mode))
+	    arg1 &= targetm.shift_truncation_mask (mode);
+	  else if (arg1 < 0 || arg1 >= GET_MODE_PRECISION (mode))
 	    return 0;
 
 	  val = (code == ASHIFT
Index: gcc/combine.c
===================================================================
--- gcc/combine.c.orig
+++ gcc/combine.c
@@ -5941,9 +5941,7 @@ combine_simplify_rtx (rtx x, enum machin
       else if (SHIFT_COUNT_TRUNCATED && !REG_P (XEXP (x, 1)))
 	SUBST (XEXP (x, 1),
 	       force_to_mode (XEXP (x, 1), GET_MODE (XEXP (x, 1)),
-			      ((unsigned HOST_WIDE_INT) 1
-			       << exact_log2 (GET_MODE_BITSIZE (GET_MODE (x))))
-			      - 1,
+			      targetm.shift_truncation_mask (GET_MODE (x)),
 			      0));
       break;
 
@@ -9896,7 +9894,7 @@ simplify_shift_const_1 (enum rtx_code co
      want to do this inside the loop as it makes it more difficult to
      combine shifts.  */
   if (SHIFT_COUNT_TRUNCATED)
-    orig_count &= GET_MODE_BITSIZE (mode) - 1;
+    orig_count &= targetm.shift_truncation_mask (mode);
 
   /* If we were given an invalid count, don't do anything except exactly
      what was requested.  */