re PR middle-end/37135 (code size increase for struct assignment)

	PR middle-end/37135
	* dse.c (find_shift_sequence): Optimize extraction from a constant.

From-SVN: r142157

gcc/ChangeLog

+2008-11-24  Jakub Jelinek  <jakub@redhat.com>
+
+	PR middle-end/37135
+	* dse.c (find_shift_sequence): Optimize extraction from a constant.
+
 2008-11-23  John David Anglin  <dave.anglin@nrc-cnrc.gc.ca>
 
 	* pa.c (function_arg): Revert 2008-10-26 change.

gcc/dse.c

@@ -1447,6 +1447,29 @@ find_shift_sequence (int access_size,
       rtx target, new_reg, shift_seq, insn, new_lhs;
       int cost;
 
+      /* If a constant was stored into memory, try to simplify it here,
+	 otherwise the cost of the shift might preclude this optimization
+	 e.g. at -Os, even when no actual shift will be needed.  */
+      if (CONSTANT_P (store_info->rhs))
+	{
+	  unsigned int byte = subreg_lowpart_offset (new_mode, store_mode);
+	  rtx ret = simplify_subreg (new_mode, store_info->rhs, store_mode,
+				     byte);
+	  if (ret && CONSTANT_P (ret))
+	    {
+	      ret = simplify_const_binary_operation (LSHIFTRT, new_mode,
+						     ret, GEN_INT (shift));
+	      if (ret && CONSTANT_P (ret))
+		{
+		  byte = subreg_lowpart_offset (read_mode, new_mode);
+		  ret = simplify_subreg (read_mode, ret, new_mode, byte);
+		  if (ret && CONSTANT_P (ret)
+		      && rtx_cost (ret, SET, speed) <= COSTS_N_INSNS (1))
+		    return ret;
+		}
+	    }
+	}
+
       /* Try a wider mode if truncating the store mode to NEW_MODE
 	 requires a real instruction.  */
       if (GET_MODE_BITSIZE (new_mode) < GET_MODE_BITSIZE (store_mode)