Reduce space usage of simple transitions and descriptors holders.

src/transitions-inl.h

Index: src/transitions-inl.h
diff --git a/src/transitions-inl.h b/src/transitions-inl.h
index 5030616e4ea4ecc21bcf1092f80c753e0f849eb8..17c42edce2e50daf51a7a9c212cf33e4cde30807 100644
--- a/src/transitions-inl.h
+++ b/src/transitions-inl.h
@@ -69,12 +69,14 @@ void TransitionArray::ClearElementsTransition() {
 
 
 bool TransitionArray::HasElementsTransition() {
-  return get(kElementsTransitionIndex) != Smi::FromInt(0);
+  return IsFullTransitionArray() &&
+      get(kElementsTransitionIndex) != Smi::FromInt(0);
 }
 
 
 void TransitionArray::set_elements_transition(Map* transition_map,
                                               WriteBarrierMode mode) {
+  ASSERT(IsFullTransitionArray());
   Heap* heap = GetHeap();
   WRITE_FIELD(this, kElementsTransitionOffset, transition_map);
   CONDITIONAL_WRITE_BARRIER(
@@ -121,12 +123,13 @@ void TransitionArray::set_back_pointer_storage(Object* back_pointer,
 
 
 bool TransitionArray::HasPrototypeTransitions() {
-  Object* prototype_transitions = get(kPrototypeTransitionsIndex);
-  return prototype_transitions != Smi::FromInt(0);
+  return IsFullTransitionArray() &&
+      get(kPrototypeTransitionsIndex) != Smi::FromInt(0);
 }
 
 
 FixedArray* TransitionArray::GetPrototypeTransitions() {
+  ASSERT(IsFullTransitionArray());
   Object* prototype_transitions = get(kPrototypeTransitionsIndex);
   return FixedArray::cast(prototype_transitions);
 }
@@ -140,7 +143,7 @@ HeapObject* TransitionArray::UncheckedPrototypeTransitions() {
 
 void TransitionArray::SetPrototypeTransitions(FixedArray* transitions,
                                               WriteBarrierMode mode) {
-  ASSERT(this != NULL);
+  ASSERT(IsFullTransitionArray());
   ASSERT(transitions->IsFixedArray());
   Heap* heap = GetHeap();
   WRITE_FIELD(this, kPrototypeTransitionsOffset, transitions);
@@ -156,6 +159,7 @@ Object** TransitionArray::GetPrototypeTransitionsSlot() {
 
 
 Object** TransitionArray::GetKeySlot(int transition_number) {
+  ASSERT(!IsSimpleTransition());
   ASSERT(transition_number < number_of_transitions());
   return HeapObject::RawField(
       reinterpret_cast<HeapObject*>(this),
@@ -164,24 +168,39 @@ Object** TransitionArray::GetKeySlot(int transition_number) {
 
 
 String* TransitionArray::GetKey(int transition_number) {
+  if (IsSimpleTransition()) {
+    Map* target = GetTarget(kSimpleTransitionIndex);
+    int descriptor = target->LastAdded();
+    String* key = target->instance_descriptors()->GetKey(descriptor);
+    return key;
+  }
   ASSERT(transition_number < number_of_transitions());
   return String::cast(get(ToKeyIndex(transition_number)));
 }
 
 
 void TransitionArray::SetKey(int transition_number, String* key) {
+  ASSERT(!IsSimpleTransition());
   ASSERT(transition_number < number_of_transitions());
   set(ToKeyIndex(transition_number), key);
 }
 
 
 Map* TransitionArray::GetTarget(int transition_number) {
+  if (IsSimpleTransition()) {
+    ASSERT(transition_number == kSimpleTransitionIndex);
+    return Map::cast(get(kSimpleTransitionTarget));
+  }
   ASSERT(transition_number < number_of_transitions());
   return Map::cast(get(ToTargetIndex(transition_number)));
 }
 
 
 void TransitionArray::SetTarget(int transition_number, Map* value) {
+  if (IsSimpleTransition()) {
+    ASSERT(transition_number == kSimpleTransitionIndex);
+    return set(kSimpleTransitionTarget, value);
+  }
   ASSERT(transition_number < number_of_transitions());
   set(ToTargetIndex(transition_number), value);
 }