expmed.c (synth_mult): When trying out a shift, pass the result of a signed shift.

	* expmed.c (synth_mult): When trying out a shift, pass the result
	of a signed shift.

From-SVN: r147087

gcc/ChangeLog

 2009-05-04  Kazu Hirata  <kazu@codesourcery.com>
 
+	* expmed.c (synth_mult): When trying out a shift, pass the result
+	of a signed shift.
+
+2009-05-04  Kazu Hirata  <kazu@codesourcery.com>
+
 	* expmed.c (shiftsub_cost): Rename to shiftsub0_cost.
 	(shiftsub1_cost): New.
 	(init_expmed): Compute shiftsub1_cost.

gcc/expmed.c

@@ -2551,6 +2551,38 @@ synth_mult (struct algorithm *alg_out, unsigned HOST_WIDE_INT t,
 	      best_alg->log[best_alg->ops] = m;
 	      best_alg->op[best_alg->ops] = alg_shift;
 	    }
+
+	  /* See if treating ORIG_T as a signed number yields a better
+	     sequence.  Try this sequence only for a negative ORIG_T
+	     as it would be useless for a non-negative ORIG_T.  */
+	  if ((HOST_WIDE_INT) orig_t < 0)
+	    {
+	      /* Shift ORIG_T as follows because a right shift of a
+		 negative-valued signed type is implementation
+		 defined.  */
+	      q = ~(~orig_t >> m);
+	      /* The function expand_shift will choose between a shift
+		 and a sequence of additions, so the observed cost is
+		 given as MIN (m * add_cost[speed][mode],
+		 shift_cost[speed][mode][m]).  */
+	      op_cost = m * add_cost[speed][mode];
+	      if (shift_cost[speed][mode][m] < op_cost)
+		op_cost = shift_cost[speed][mode][m];
+	      new_limit.cost = best_cost.cost - op_cost;
+	      new_limit.latency = best_cost.latency - op_cost;
+	      synth_mult (alg_in, q, &new_limit, mode);
+
+	      alg_in->cost.cost += op_cost;
+	      alg_in->cost.latency += op_cost;
+	      if (CHEAPER_MULT_COST (&alg_in->cost, &best_cost))
+		{
+		  struct algorithm *x;
+		  best_cost = alg_in->cost;
+		  x = alg_in, alg_in = best_alg, best_alg = x;
+		  best_alg->log[best_alg->ops] = m;
+		  best_alg->op[best_alg->ops] = alg_shift;
+		}
+	    }
 	}
       if (cache_hit)
 	goto done;