Fix bug in load elimination with multiple phis.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
===================================================================
--- runtime/vm/flow_graph_optimizer.cc	(revision 33112)
+++ runtime/vm/flow_graph_optimizer.cc	(working copy)
@@ -5624,7 +5624,6 @@
                       result->id());
           }
         }
-
         phi_moves->CreateOutgoingMove(block->PredecessorAt(j),
                                       result->id(),
                                       place->id());
@@ -5770,8 +5769,6 @@
   // Loads that are locally redundant will be replaced as we go through
   // instructions.
   void ComputeInitialSets() {
-    BitVector* forwarded_loads = new BitVector(aliased_set_->max_place_id());
-
     for (BlockIterator block_it = graph_->reverse_postorder_iterator();
          !block_it.Done();
          block_it.Advance()) {
@@ -5930,12 +5927,6 @@
         (*out_values)[place_id] = defn;
       }
 
-      PhiPlaceMoves::MovesList phi_moves =
-          aliased_set_->phi_moves()->GetOutgoingMoves(block);
-      if (phi_moves != NULL) {
-        PerformPhiMoves(phi_moves, gen, forwarded_loads);
-      }
-
       exposed_values_[preorder_number] = exposed_values;
       out_values_[preorder_number] = out_values;
     }
@@ -5972,6 +5963,7 @@
   void ComputeOutSets() {
     BitVector* temp = new BitVector(aliased_set_->max_place_id());
     BitVector* forwarded_loads = new BitVector(aliased_set_->max_place_id());
+    BitVector* temp_out = new BitVector(aliased_set_->max_place_id());
 
     bool changed = true;
     while (changed) {
@@ -6000,7 +5992,17 @@
             BlockEntryInstr* pred = block->PredecessorAt(i);
             BitVector* pred_out = out_[pred->preorder_number()];
             if (pred_out != NULL) {

[Vyacheslav Egorov (Chromium), 2014/02/27 16:25:09]

I would write it slightly differently:

if (pred_out == NULL) continue;

PhiPlaceMoves::MovesList phi_moves =
aliased_set_->phi_moves()->GetOutgoingMoves(pred);
if (phi_moves != NULL) {
  // If there are phi moves, perform intersection with
  // a copy of pred_out where the phi moves are applied.
  temp_out->CopyFrom(pred_out);
  PerformPhiMoves(phi_moves, temp_out, forwarded_loads);
  pred_out = temp_out;
}

temp->Intersect(pred_out);

[Florian Schneider, 2014/02/28 13:38:57]

Done.

-              temp->Intersect(pred_out);
+              PhiPlaceMoves::MovesList phi_moves =
+                  aliased_set_->phi_moves()->GetOutgoingMoves(pred);
+              if (phi_moves != NULL) {
+                // If there are phi moves, perform intersection with
+                // a copy of pred_out where the phi moves are applied.
+                temp_out->CopyFrom(pred_out);
+                PerformPhiMoves(phi_moves, temp_out, forwarded_loads);
+                temp->Intersect(temp_out);
+              } else {
+                temp->Intersect(pred_out);
+              }
             }
           }
         }
@@ -6012,12 +6014,6 @@
           temp->RemoveAll(block_kill);
           temp->AddAll(block_gen);
 
-          PhiPlaceMoves::MovesList phi_moves =
-              aliased_set_->phi_moves()->GetOutgoingMoves(block);
-          if (phi_moves != NULL) {
-            PerformPhiMoves(phi_moves, temp, forwarded_loads);
-          }
-
           if ((block_out == NULL) || !block_out->Equals(*temp)) {
             if (block_out == NULL) {
               block_out = out_[preorder_number] =