(simplify_shift_const): Inhibit hacks based on

num_sign_bit_copies if shift_mode differs from the mode of varop.

From-SVN: r4994

gcc/combine.c

@@ -7122,25 +7122,31 @@ simplify_shift_const (x, code, result_mode, varop, count)
       else if (count < 0)
 	abort ();
 
-      /* An arithmetic right shift of a quantity known to be -1 or 0
-	 is a no-op.  */
-      if (code == ASHIFTRT
-	  && (num_sign_bit_copies (varop, shift_mode)
-	      == GET_MODE_BITSIZE (shift_mode)))
-	{
-	  count = 0;
-	  break;
-	}
+      /* If we have replaced VAROP with something wider
+	 (such as, the SUBREG_REG), then this won't work;
+	 num_sign_bit_copies will give the wrong answer in that case.  */
+      if (shift_mode == GET_MODE (varop))
+	{
+	  /* An arithmetic right shift of a quantity known to be -1 or 0
+	     is a no-op.  */
+	  if (code == ASHIFTRT
+	      && (num_sign_bit_copies (varop, shift_mode)
+		  == GET_MODE_BITSIZE (shift_mode)))
+	    {
+	      count = 0;
+	      break;
+	    }
 
-      /* If we are doing an arithmetic right shift and discarding all but
-	 the sign bit copies, this is equivalent to doing a shift by the
-	 bitsize minus one.  Convert it into that shift because it will often
-	 allow other simplifications.  */
+	  /* If we are doing an arithmetic right shift and discarding all but
+	     the sign bit copies, this is equivalent to doing a shift by the
+	     bitsize minus one.  Convert it into that shift because it will often
+	     allow other simplifications.  */
 
-      if (code == ASHIFTRT
-	  && (count + num_sign_bit_copies (varop, shift_mode)
-	      >= GET_MODE_BITSIZE (shift_mode)))
-	count = GET_MODE_BITSIZE (shift_mode) - 1;
+	  if (code == ASHIFTRT
+	      && (count + num_sign_bit_copies (varop, shift_mode)
+		  >= GET_MODE_BITSIZE (shift_mode)))
+	    count = GET_MODE_BITSIZE (shift_mode) - 1;
+	}
 
       /* We simplify the tests below and elsewhere by converting
 	 ASHIFTRT to LSHIFTRT if we know the sign bit is clear.