Relax TransitionElementsKind DependsOn/Changes dependencies.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1174 matching lines @@
           // Keep it.
           lists_[current].next = kept;
           kept = current;
           present_flags_.Add(value->gvn_flags());
         }
       }
       array_[i].next = kept;
 
       // Now possibly drop directly indexed element.
       value = array_[i].value;
-      if (value->gvn_flags().ContainsAnyOf(depends_flags)) {  // Drop it.
+      if (value->gvn_flags().ContainsAnyOf(depends_flags) &&
+          !value->IsTransitionElementsKind()) {  // Drop it.

[fschneider, 2012/02/15 15:36:48]

I don't understand why skipping transition instructions here is this necessary
(and correct). 
The dependency flags should be enough to control GVN for the
HTransitionElementsKind.

[danno, 2012/02/16 13:08:52]

Done.

         count_--;
         int head = array_[i].next;
         if (head == kNil) {
           array_[i].value = NULL;
         } else {
           array_[i].value = lists_[head].value;
           array_[i].next = lists_[head].next;
           lists_[head].next = free_list_head_;
           free_list_head_ = head;
         }
@@ skipping 3241 matching lines @@
         map->FindTransitionedMap(&possible_transitioned_maps);
     transition_target.Add(transitioned_map);
   }
 
   int num_untransitionable_maps = 0;
   Handle<Map> untransitionable_map;
   for (int i = 0; i < maps->length(); ++i) {
     Handle<Map> map = maps->at(i);
     ASSERT(map->IsMap());
     if (!transition_target.at(i).is_null()) {
-      object = AddInstruction(new(zone()) HTransitionElementsKind(
+      AddInstruction(new(zone()) HTransitionElementsKind(
           object, map, transition_target.at(i)));
     } else {
       type_todo[map->elements_kind()] = true;
       if (map->elements_kind() >= FIRST_EXTERNAL_ARRAY_ELEMENTS_KIND) {
         todo_external_array = true;
       }
       num_untransitionable_maps++;
       untransitionable_map = map;
     }
   }
@@ skipping 3239 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal