[NDS32] Optimize nds32_register_priority.

gcc/
	* config/nds32/nds32.c (nds32_register_priority): Modify cost.

From-SVN: r258619

gcc/ChangeLog

+2018-03-17  Chung-Ju Wu  <jasonwucj@gmail.com>
+
+	* config/nds32/nds32.c (nds32_register_priority): Modify cost.
+
 2018-03-17  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/84902

gcc/config/nds32/nds32.c

@@ -1234,9 +1234,24 @@ static int
 nds32_register_priority (int hard_regno)
 {
   /* Encourage to use r0-r7 for LRA when optimize for size.  */
-  if (optimize_size && hard_regno < 8)
-    return 4;
-  return 3;
+  if (optimize_size)
+    {
+      if (hard_regno < 8)
+	return 4;
+      else if (hard_regno < 16)
+	return 3;
+      else if (hard_regno < 28)
+	return 2;
+      else
+	return 1;
+    }
+  else
+    {
+      if (hard_regno > 27)
+	return 1;
+      else
+	return 4;
+    }
 }