[turbofan] Improve handling of TransitionElementsKind in load elimination.

src/compiler/load-elimination.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/load-elimination.h"
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-properties.h"
 #include "src/compiler/simplified-operator.h"
@@ skipping 848 matching lines @@
             state = state->KillField(
                 object, FieldIndexOf(JSObject::kElementsOffset), zone());
             if (flags & GrowFastElementsFlag::kArrayObject) {
               state = state->KillField(
                   object, FieldIndexOf(JSArray::kLengthOffset), zone());
             }
             break;
           }
           case IrOpcode::kTransitionElementsKind: {
             Node* const object = NodeProperties::GetValueInput(current, 0);
+            Node* const target_map = NodeProperties::GetValueInput(current, 2);
+            Node* const object_map = state->LookupField(
+                object, FieldIndexOf(HeapObject::kMapOffset));
+            if (target_map == object_map) {

[Benedikt Meurer, 2016/11/16 20:22:19]

Nit: Please do 

if (target_map != object_map) {
  ... kill
}
break;

instead

+              break;
+            }
             state = state->KillField(
                 object, FieldIndexOf(HeapObject::kMapOffset), zone());
             state = state->KillField(
                 object, FieldIndexOf(JSObject::kElementsOffset), zone());
             break;
           }
           case IrOpcode::kStoreField: {
             FieldAccess const& access = FieldAccessOf(current->op());
             Node* const object = NodeProperties::GetValueInput(current, 0);
             int field_index = FieldIndexOf(access);
@@ skipping 74 matching lines @@
 
 CommonOperatorBuilder* LoadElimination::common() const {
   return jsgraph()->common();
 }
 
 Graph* LoadElimination::graph() const { return jsgraph()->graph(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8