Promoting elements transitions to their own field.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 687b525f64026179fe36006abc6a40d6258b4dd1..84971213eb72fcd2715928f28bd27ad9ff82d09a 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1870,10 +1870,17 @@ Object** FixedArray::data_start() {
 
 
 bool DescriptorArray::IsEmpty() {
-  ASSERT(this->IsSmi() ||
-         this->length() > kFirstIndex ||
-         this == HEAP->empty_descriptor_array());
-  return this->IsSmi() || length() <= kFirstIndex;
+  // TODO(verwaest) Reenable and change >= above when descriptor array is
+  // immutable again.

[danno, 2012/06/01 13:49:55]

Re-enable this assert appropriately.

[Toon Verwaest, 2012/06/04 09:17:48]

Done.

+  // ASSERT(this->IsSmi() ||
+  //        this->length() > kFirstIndex ||
+  //        this == HEAP->empty_descriptor_array());
+  return this->IsSmi() || length() < kFirstIndex;
+}
+
+
+bool DescriptorArray::MayContainTransitions() {
+  return length() >= kTransitionsIndex;
 }
 
 
@@ -1883,7 +1890,7 @@ int DescriptorArray::bit_field3_storage() {
 }
 
 void DescriptorArray::set_bit_field3_storage(int value) {
-  ASSERT(!IsEmpty());
+  ASSERT(this->MayContainTransitions());
   WRITE_FIELD(this, kBitField3StorageOffset, Smi::FromInt(value));
 }
 
@@ -1925,6 +1932,30 @@ int DescriptorArray::SearchWithCache(String* name) {
 }
 
 
+Map* DescriptorArray::elements_transition() {
+  if (!this->MayContainTransitions()) {
+    return NULL;
+  }
+  Object* transition_map = get(kTransitionsIndex);
+  if (transition_map == Smi::FromInt(0)) {
+    return NULL;
+  } else {
+    return Map::cast(transition_map);
+  }
+}
+
+
+void DescriptorArray::set_elements_transition(
+    Map* transition_map, WriteBarrierMode mode) {
+  ASSERT(this->length() >= kFirstIndex);
+  Heap* heap = GetHeap();
+  WRITE_FIELD(this, kTransitionsOffset, transition_map);
+  CONDITIONAL_WRITE_BARRIER(
+      heap, this, kTransitionsOffset, transition_map, mode);
+  ASSERT(DescriptorArray::cast(this));
+}
+
+
 Object** DescriptorArray::GetKeySlot(int descriptor_number) {
   ASSERT(descriptor_number < number_of_descriptors());
   return HeapObject::RawField(
@@ -2011,7 +2042,6 @@ bool DescriptorArray::IsTransitionOnly(int descriptor_number) {
   switch (GetType(descriptor_number)) {
     case MAP_TRANSITION:
     case CONSTANT_TRANSITION:
-    case ELEMENTS_TRANSITION:
       return true;
     case CALLBACKS: {
       Object* value = GetValue(descriptor_number);
@@ -3453,6 +3483,16 @@ Object* Map::GetBackPointer() {
 }
 
 
+Map* Map::elements_transition() {
+  return instance_descriptors()->elements_transition();
+}
+
+
+void Map::set_elements_transition(Map* transitioned_map) {
+  return instance_descriptors()->set_elements_transition(transitioned_map);
+}
+
+
 void Map::SetBackPointer(Object* value, WriteBarrierMode mode) {
   Heap* heap = GetHeap();
   ASSERT(instance_type() >= FIRST_JS_RECEIVER_TYPE);
@@ -4050,15 +4090,12 @@ MaybeObject* JSFunction::set_initial_map_and_cache_transitions(
     maps->set(kind, current_map);
     for (int i = GetSequenceIndexFromFastElementsKind(kind) + 1;
          i < kFastElementsKindCount; ++i) {
-      ElementsKind transitioned_kind = GetFastElementsKindFromSequenceIndex(i);
-      MaybeObject* maybe_new_map = current_map->CopyDropTransitions();
-      Map* new_map = NULL;
-      if (!maybe_new_map->To<Map>(&new_map)) return maybe_new_map;
-      new_map->set_elements_kind(transitioned_kind);
-      maybe_new_map = current_map->AddElementsTransition(transitioned_kind,
-                                                         new_map);
-      if (maybe_new_map->IsFailure()) return maybe_new_map;
-      maps->set(transitioned_kind, new_map);
+      Map* new_map;
+      ElementsKind next_kind = GetFastElementsKindFromSequenceIndex(i);
+      MaybeObject* maybe_new_map =
+          current_map->CreateNextElementsTransition(next_kind);
+      if (!maybe_new_map->To(&new_map)) return maybe_new_map;
+      maps->set(next_kind, new_map);
       current_map = new_map;
     }
     global_context->set_js_array_maps(maps);