(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

@@ -7122,6 +7122,11 @@ simplify_shift_const (x, code, result_mode, varop, count)
       else if (count < 0)
 	abort ();
 
+      /* 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
@@ -7141,6 +7146,7 @@ simplify_shift_const (x, code, result_mode, varop, count)
 	      && (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.