Replace ad-hoc weakness in transition array with WeakCell.

src/transitions-inl.h

 // Copyright 2012 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.
 
 #ifndef V8_TRANSITIONS_INL_H_
 #define V8_TRANSITIONS_INL_H_
 
 #include "src/transitions.h"
 
 namespace v8 {
@@ skipping 29 matching lines @@
   return RawFieldOfElementAt(kPrototypeTransitionsIndex);
 }
 
 
 Object** TransitionArray::GetKeySlot(int transition_number) {
   DCHECK(transition_number < number_of_transitions());
   return RawFieldOfElementAt(ToKeyIndex(transition_number));
 }
 
 
+Object** TransitionArray::GetTargetSlot(int transition_number) {
+  DCHECK(transition_number < number_of_transitions());
+  return RawFieldOfElementAt(ToTargetIndex(transition_number));
+}
+
+
 Name* TransitionArray::GetKey(int transition_number) {
   DCHECK(transition_number < number_of_transitions());
   return Name::cast(get(ToKeyIndex(transition_number)));
 }
 
 
 Name* TransitionArray::GetKey(Object* raw_transitions, int transition_number) {
   if (IsSimpleTransition(raw_transitions)) {
     DCHECK(transition_number == 0);
     return GetSimpleTransitionKey(GetSimpleTransition(raw_transitions));
   }
   DCHECK(IsFullTransitionArray(raw_transitions));
   return TransitionArray::cast(raw_transitions)->GetKey(transition_number);
 }
 
 
 void TransitionArray::SetKey(int transition_number, Name* key) {
   DCHECK(transition_number < number_of_transitions());
   set(ToKeyIndex(transition_number), key);
 }
 
 
 Map* TransitionArray::GetTarget(int transition_number) {
   DCHECK(transition_number < number_of_transitions());
-  return Map::cast(get(ToTargetIndex(transition_number)));
+  WeakCell* cell = GetTargetCell(transition_number);
+  DCHECK(!cell->cleared());

[Jakob Kummerow, 2015/06/17 10:38:51]

This relies on ClearMapTransitions() making sure that all transition arrays are
always properly compacted and right-trimmed, so there are no cleared cells in
the array during normal execution. Is that intentional? Seems fine, but I'd like
a comment here stating this explicitly.

+  return Map::cast(cell->value());
 }
 
 
 Map* TransitionArray::GetTarget(Object* raw_transitions,
                                 int transition_number) {
   if (IsSimpleTransition(raw_transitions)) {
     DCHECK(transition_number == 0);
     return GetSimpleTransition(raw_transitions);
   }
   DCHECK(IsFullTransitionArray(raw_transitions));
   return TransitionArray::cast(raw_transitions)->GetTarget(transition_number);
 }
 
 
-void TransitionArray::SetTarget(int transition_number, Map* value) {
+WeakCell* TransitionArray::GetTargetCell(int transition_number) {
+  DCHECK(transition_number < number_of_transitions());
+  return WeakCell::cast(get(ToTargetIndex(transition_number)));
+}
+
+
+void TransitionArray::SetTargetCell(int transition_number, WeakCell* value) {
   DCHECK(transition_number < number_of_transitions());
   set(ToTargetIndex(transition_number), value);
 }
 
 
 int TransitionArray::SearchName(Name* name, int* out_insertion_index) {
   return internal::Search<ALL_ENTRIES>(this, name, 0, out_insertion_index);
 }
 
 
@@ skipping 51 matching lines @@
 PropertyDetails TransitionArray::GetTargetDetails(Name* name, Map* target) {
   DCHECK(!IsSpecialTransition(name));
   int descriptor = target->LastAdded();
   DescriptorArray* descriptors = target->instance_descriptors();
   // Transitions are allowed only for the last added property.
   DCHECK(descriptors->GetKey(descriptor)->Equals(name));
   return descriptors->GetDetails(descriptor);
 }
 
 
-void TransitionArray::NoIncrementalWriteBarrierSet(int transition_number,
-                                                   Name* key,
-                                                   Map* target) {
-  FixedArray::NoIncrementalWriteBarrierSet(
-      this, ToKeyIndex(transition_number), key);
-  FixedArray::NoIncrementalWriteBarrierSet(
-      this, ToTargetIndex(transition_number), target);
+void TransitionArray::Set(int transition_number, Name* key, WeakCell* target) {
+  set(ToKeyIndex(transition_number), key);
+  set(ToTargetIndex(transition_number), target);
 }
 
 
 void TransitionArray::SetNumberOfTransitions(int number_of_transitions) {
   DCHECK(number_of_transitions <= Capacity(this));
   set(kTransitionLengthIndex, Smi::FromInt(number_of_transitions));
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TRANSITIONS_INL_H_