* rtlanal.c (rtx_cost): Handle TRUNCATE between tieable modes.

From-SVN: r247509

gcc/ChangeLog

+2017-05-02  Bin Cheng  <bin.cheng@arm.com>
+
+	* rtlanal.c (rtx_cost): Handle TRUNCATE between tieable modes.
+
 2017-05-02  Xi Ruoyao  <ryxi@stu.xidian.edu.cn>
 
 	* tree.h (EXPR_CILK_SPAWN): Use macro TREE_CHECK2 instead of
 	function tree_check2.
 
-
 2017-05-02  Martin Liska  <mliska@suse.cz>
 
 	* doc/gcov.texi: Add missing preposition.

gcc/rtlanal.c

@@ -4164,6 +4164,14 @@ rtx_cost (rtx x, machine_mode mode, enum rtx_code outer_code,
 	return COSTS_N_INSNS (2 + factor);
       break;
 
+    case TRUNCATE:
+      /* If we can tie these modes, make this cheap.  */
+      if (MODES_TIEABLE_P (mode, GET_MODE (SUBREG_REG (x))))
+	{
+	  total = 0;
+	  break;
+	}
+      /* FALLTHRU */
     default:
       if (targetm.rtx_costs (x, mode, outer_code, opno, &total, speed))
 	return total;