Moving prototype transitions into the header of the transition array.

src/objects-inl.h

 // 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 3471 matching lines @@
     WRITE_FIELD(this,
                 kInstanceDescriptorsOrBitField3Offset,
                 Smi::FromInt(value));
   } else {
     DescriptorArray::cast(object)->set_bit_field3_storage(value);
   }
 }
 
 
 Object* Map::GetBackPointer() {
-  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
-  if (object->IsFixedArray()) {
-    return FixedArray::cast(object)->get(kProtoTransitionBackPointerOffset);
-  } else {
-    return object;
-  }
+  return READ_FIELD(this, kBackPointerOffset);
 }
 
 
 bool Map::HasElementsTransition() {
   return HasTransitionArray() && transitions()->HasElementsTransition();
 }
 
 
 bool Map::HasTransitionArray() {
   return instance_descriptors()->HasTransitionArray();
@@ skipping 17 matching lines @@
   if (map->instance_descriptors()->MayContainTransitions()) return map;
   DescriptorArray* descriptors;
   MaybeObject* maybe_descriptors =
       DescriptorArray::Allocate(0, DescriptorArray::CANNOT_BE_SHARED);
   if (!maybe_descriptors->To(&descriptors)) return maybe_descriptors;
   map->set_instance_descriptors(descriptors);
   return descriptors;
 }
 
 
-// If the descriptor does not have a transition array, install a new
-// transition array that has room for an element transition.
-static MaybeObject* AllowElementsTransition(Map* map) {
+// If the descriptor is using the empty transition array, install a new empty
+// transition array that will have place for an element transition.
+static MaybeObject* EnsureHasTransitionArray(Map* map) {
   if (map->HasTransitionArray()) return map;
 
   AllowTransitions(map);
 
   TransitionArray* transitions;
   MaybeObject* maybe_transitions = TransitionArray::Allocate(0);
   if (!maybe_transitions->To(&transitions)) return maybe_transitions;
   MaybeObject* added_transitions = map->set_transitions(transitions);
   if (added_transitions->IsFailure()) return added_transitions;
   return transitions;
 }
 
 
 MaybeObject* Map::set_elements_transition_map(Map* transitioned_map) {
-  MaybeObject* allow_elements = AllowElementsTransition(this);
+  MaybeObject* allow_elements = EnsureHasTransitionArray(this);
   if (allow_elements->IsFailure()) return allow_elements;
   transitions()->set_elements_transition(transitioned_map);
   return this;
 }
 
 
+FixedArray* Map::prototype_transitions() {

[Michael Starzinger, 2012/07/06 13:53:18]

Since these methods are far more than generic accessors we should use camel-case
naming for them.

+  if (!HasTransitionArray()) return GetHeap()->empty_fixed_array();
+  if (!transitions()->HasPrototypeTransitions()) {
+    return GetHeap()->empty_fixed_array();
+  }
+  return transitions()->prototype_transitions();
+}
+
+
+MaybeObject* Map::set_prototype_transitions(FixedArray* proto_transitions) {

[Michael Starzinger, 2012/07/06 13:53:18]

Likewise.

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

+  MaybeObject* allow_prototype = EnsureHasTransitionArray(this);
+  if (allow_prototype->IsFailure()) return allow_prototype;
+#ifdef DEBUG
+  if (HasPrototypeTransitions() &&
+      prototype_transitions() == proto_transitions) {

[Michael Starzinger, 2012/07/06 13:53:18]

I think this should actually be "!=" here. Maybe we should turn that into an
assertion if it can actually never be "==" here.

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

+    ZapPrototypeTransitions();
+  }
+#endif
+  transitions()->set_prototype_transitions(proto_transitions);
+  return this;
+}
+
+
+bool Map::HasPrototypeTransitions() {
+  return HasTransitionArray() && transitions()->HasPrototypeTransitions();
+}
+
+
 TransitionArray* Map::transitions() {
   return instance_descriptors()->transitions();
 }
 
 
 void Map::ClearTransitions() {
 #ifdef DEBUG
   ZapTransitions();
 #endif
   DescriptorArray* descriptors = instance_descriptors();
@@ skipping 12 matching lines @@
   if (HasTransitionArray()) {
     ASSERT(transitions() != transitions_array);
     ZapTransitions();
   }
 #endif
   instance_descriptors()->set_transitions(transitions_array);
   return this;
 }
 
 
+void Map::init_back_pointer(Object* undefined) {
+  ASSERT(undefined->IsUndefined());
+  WRITE_FIELD(this, kBackPointerOffset, undefined);
+}
+
+
 void Map::SetBackPointer(Object* value, WriteBarrierMode mode) {
   Heap* heap = GetHeap();
   ASSERT(instance_type() >= FIRST_JS_RECEIVER_TYPE);
   ASSERT((value->IsUndefined() && GetBackPointer()->IsMap()) ||
          (value->IsMap() && GetBackPointer()->IsUndefined()));
-  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
-  if (object->IsFixedArray()) {
-    FixedArray::cast(object)->set(
-        kProtoTransitionBackPointerOffset, value, mode);
-  } else {
-    WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, value);
-    CONDITIONAL_WRITE_BARRIER(
-        heap, this, kPrototypeTransitionsOrBackPointerOffset, value, mode);
-  }
+  WRITE_FIELD(this, kBackPointerOffset, value);
+  CONDITIONAL_WRITE_BARRIER(

[Michael Starzinger, 2012/07/06 13:53:18]

This should fit into one line now.

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

+      heap, this, kBackPointerOffset, value, mode);
 }
 
 
-FixedArray* Map::prototype_transitions() {
-  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
-  if (object->IsFixedArray()) {
-    return FixedArray::cast(object);
-  } else {
-    return GetHeap()->empty_fixed_array();
-  }
-}
-
-
-void Map::set_prototype_transitions(FixedArray* value, WriteBarrierMode mode) {
-  Heap* heap = GetHeap();
-  ASSERT(value != heap->empty_fixed_array());
-  value->set(kProtoTransitionBackPointerOffset, GetBackPointer());
-#ifdef DEBUG
-  if (value != prototype_transitions()) {
-    ZapPrototypeTransitions();
-  }
-#endif
-  WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, value);
-  CONDITIONAL_WRITE_BARRIER(
-      heap, this, kPrototypeTransitionsOrBackPointerOffset, value, mode);
-}
-
-
-void Map::init_prototype_transitions(Object* undefined) {
-  ASSERT(undefined->IsUndefined());
-  WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, undefined);
+// Can either be Smi (no transitions), normal transition array, or a transition
+// array with the header overwritten as a Smi (thus iterating).
+TransitionArray* Map::unchecked_transition_array() {
+  ASSERT(HasTransitionArray());
+  Object* object = *HeapObject::RawField(instance_descriptors(),
+                                         DescriptorArray::kTransitionsOffset);
+  ASSERT(!object->IsSmi());
+  TransitionArray* transition_array = static_cast<TransitionArray*>(object);
+  return transition_array;
 }
 
 
 HeapObject* Map::unchecked_prototype_transitions() {
-  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
-  return reinterpret_cast<HeapObject*>(object);
+  ASSERT(HasTransitionArray());
+  ASSERT(unchecked_transition_array()->HasPrototypeTransitions());
+  return unchecked_transition_array()->unchecked_prototype_transitions();
 }
 
 
 ACCESSORS(Map, code_cache, Object, kCodeCacheOffset)
 ACCESSORS(Map, constructor, Object, kConstructorOffset)
 
 ACCESSORS(JSFunction, shared, SharedFunctionInfo, kSharedFunctionInfoOffset)
 ACCESSORS(JSFunction, literals_or_bindings, FixedArray, kLiteralsOffset)
 ACCESSORS(JSFunction, next_function_link, Object, kNextFunctionLinkOffset)
 
@@ skipping 1628 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_