tree-ssa-propagate.c (substitute_and_fold): Use do_dce flag to deside...

2011-07-21  Kai Tietz  <ktietz@redhat.com>

	* tree-ssa-propagate.c (substitute_and_fold): Use
	do_dce flag to deside, if BB's statements are scanned
	in last to first, or first to last order.

From-SVN: r176556

gcc/ChangeLog

+2011-07-21  Kai Tietz  <ktietz@redhat.com>
+
+	* tree-ssa-propagate.c (substitute_and_fold): Use
+	do_dce flag to deside, if BB's statements are scanned
+	in last to first, or first to last order.
+
 2011-07-21  Georg-Johann Lay  <avr@gjlay.de>
 	
 	* config/avr/avr.c (avr_rtx_costs): Set cost of CONST, LABEL_REF to 0.

gcc/tree-ssa-propagate.c

@@ -966,6 +966,9 @@ replace_phi_args_in (gimple phi, ssa_prop_get_value_fn get_value)
 
    DO_DCE is true if trivially dead stmts can be removed.
 
+   If DO_DCE is true, the statements within a BB are walked from
+   last to first element.  Otherwise we scan from first to last element.
+
    Return TRUE when something changed.  */
 
 bool
@@ -1046,9 +1049,10 @@ substitute_and_fold (ssa_prop_get_value_fn get_value_fn,
 	for (i = gsi_start_phis (bb); !gsi_end_p (i); gsi_next (&i))
 	  replace_phi_args_in (gsi_stmt (i), get_value_fn);
 
-      /* Propagate known values into stmts.  Do a backward walk to expose
-	 more trivially deletable stmts.  */
-      for (i = gsi_last_bb (bb); !gsi_end_p (i);)
+      /* Propagate known values into stmts.  Do a backward walk if
+         do_dce is true. In some case it exposes
+	 more trivially deletable stmts to walk backward.  */
+      for (i = (do_dce ? gsi_last_bb (bb) : gsi_start_bb (bb)); !gsi_end_p (i);)
 	{
           bool did_replace;
 	  gimple stmt = gsi_stmt (i);
@@ -1057,7 +1061,10 @@ substitute_and_fold (ssa_prop_get_value_fn get_value_fn,
 	  gimple_stmt_iterator oldi;
 
 	  oldi = i;
-	  gsi_prev (&i);
+	  if (do_dce)
+	    gsi_prev (&i);
+	  else
+	    gsi_next (&i);
 
 	  /* Ignore ASSERT_EXPRs.  They are used by VRP to generate
 	     range information for names and they are discarded