Add comment to expand_mult max_cost change.

From-SVN: r104379

gcc/expmed.c

@@ -3022,7 +3022,7 @@ expand_mult (enum machine_mode mode, rtx op0, rtx op1, rtx target,
       && (unsignedp || !flag_trapv))
     {
       HOST_WIDE_INT coeff = 0;
-      rtx reg = gen_raw_REG (mode, LAST_VIRTUAL_REGISTER + 1);
+      rtx fake_reg = gen_raw_REG (mode, LAST_VIRTUAL_REGISTER + 1);
 
       /* synth_mult does an `unsigned int' multiply.  As long as the mode is
 	 less than or equal in size to `unsigned int' this doesn't matter.
@@ -3040,8 +3040,10 @@ expand_mult (enum machine_mode mode, rtx op0, rtx op1, rtx target,
 	      && GET_MODE_BITSIZE (mode) > HOST_BITS_PER_WIDE_INT)
 	    {
 	      /* Its safe to use -INTVAL (op1) even for INT_MIN, as the
-		 result is interpreted as an unsigned coefficient.  */
-	      max_cost = rtx_cost (gen_rtx_MULT (mode, reg, op1), SET)
+		 result is interpreted as an unsigned coefficient.
+		 Exclude cost of op0 from max_cost to match the cost
+		 calculation of the synth_mult.  */
+	      max_cost = rtx_cost (gen_rtx_MULT (mode, fake_reg, op1), SET)
 			 - neg_cost[mode];
 	      if (max_cost > 0
 		  && choose_mult_variant (mode, -INTVAL (op1), &algorithm,
@@ -3084,7 +3086,9 @@ expand_mult (enum machine_mode mode, rtx op0, rtx op1, rtx target,
 				 build_int_cst (NULL_TREE, floor_log2 (coeff)),
 				 target, unsignedp);
 
-	  max_cost = rtx_cost (gen_rtx_MULT (mode, reg, op1), SET);
+	  /* Exclude cost of op0 from max_cost to match the cost
+	     calculation of the synth_mult.  */
+	  max_cost = rtx_cost (gen_rtx_MULT (mode, fake_reg, op1), SET);
 	  if (choose_mult_variant (mode, coeff, &algorithm, &variant,
 				   max_cost))
 	    return expand_mult_const (mode, op0, coeff, target,