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// 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.
#include "src/elements-kind.h"
#include "src/heap.h"
#include "src/isolate.h"
#include "src/objects.h"
#include "src/v8checks.h"
namespace v8 {
namespace internal {
// TransitionArrays are fixed arrays used to hold map transitions for property,
// constant, and element changes. They can either be simple transition arrays
// that store a single property transition, or a full transition array that has
// prototype transitions and multiple property transitons. The details related
// to property transitions are accessed in the descriptor array of the target
// map. In the case of a simple transition, the key is also read from the
// descriptor array of the target map.
// The simple format of the these objects is:
// [0] Undefined or back pointer map
// [1] Single transition
// The full format is:
// [0] Undefined or back pointer map
// [1] Smi(0) or fixed array of prototype transitions
// [2] First transition
// [length() - kTransitionSize] Last transition
class TransitionArray: public FixedArray {
// Accessors for fetching instance transition at transition number.
inline Name* GetKey(int transition_number);
inline void SetKey(int transition_number, Name* value);
inline Object** GetKeySlot(int transition_number);
int GetSortedKeyIndex(int transition_number) { return transition_number; }
Name* GetSortedKey(int transition_number) {
return GetKey(transition_number);
inline Map* GetTarget(int transition_number);
inline void SetTarget(int transition_number, Map* target);
inline PropertyDetails GetTargetDetails(int transition_number);
inline bool HasElementsTransition();
inline Object* back_pointer_storage();
inline void set_back_pointer_storage(
Object* back_pointer,
WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
inline FixedArray* GetPrototypeTransitions();
inline void SetPrototypeTransitions(
FixedArray* prototype_transitions,
WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
inline Object** GetPrototypeTransitionsSlot();
inline bool HasPrototypeTransitions();
// Returns the number of transitions in the array.
int number_of_transitions() {
if (IsSimpleTransition()) return 1;
int len = length();
return len <= kFirstIndex ? 0 : (len - kFirstIndex) / kTransitionSize;
inline int number_of_entries() { return number_of_transitions(); }
// Creates a FullTransitionArray from a SimpleTransitionArray in
// containing_map.
static Handle<TransitionArray> ExtendToFullTransitionArray(
Handle<Map> containing_map);
// Create a transition array, copying from the owning map if it already has
// one, otherwise creating a new one according to flag.
// TODO(verwaest): This should not cause an existing transition to be
// overwritten.
static Handle<TransitionArray> CopyInsert(Handle<Map> map,
Handle<Name> name,
Handle<Map> target,
SimpleTransitionFlag flag);
// Search a transition for a given property name.
inline int Search(Name* name);
// Allocates a TransitionArray.
static Handle<TransitionArray> Allocate(
Isolate* isolate, int number_of_transitions);
bool IsSimpleTransition() {
return length() == kSimpleTransitionSize &&
get(kSimpleTransitionTarget)->IsHeapObject() &&
// The IntrusivePrototypeTransitionIterator may have set the map of the
// prototype transitions array to a smi. In that case, there are
// prototype transitions, hence this transition array is a full
// transition array.
HeapObject::cast(get(kSimpleTransitionTarget))->map()->IsMap() &&
bool IsFullTransitionArray() {
return length() > kFirstIndex ||
(length() == kFirstIndex && !IsSimpleTransition());
// Casting.
static inline TransitionArray* cast(Object* obj);
// Constant for denoting key was not found.
static const int kNotFound = -1;
static const int kBackPointerStorageIndex = 0;
// Layout for full transition arrays.
static const int kPrototypeTransitionsIndex = 1;
static const int kFirstIndex = 2;
// Layout for simple transition arrays.
static const int kSimpleTransitionTarget = 1;
static const int kSimpleTransitionSize = 2;
static const int kSimpleTransitionIndex = 0;
STATIC_ASSERT(kSimpleTransitionIndex != kNotFound);
static const int kBackPointerStorageOffset = FixedArray::kHeaderSize;
// Layout for the full transition array header.
static const int kPrototypeTransitionsOffset = kBackPointerStorageOffset +
// Layout of map transition entries in full transition arrays.
static const int kTransitionKey = 0;
static const int kTransitionTarget = 1;
static const int kTransitionSize = 2;
// Print all the transitions.
inline void PrintTransitions() {
void PrintTransitions(FILE* out);
#ifdef DEBUG
bool IsSortedNoDuplicates(int valid_entries = -1);
bool IsConsistentWithBackPointers(Map* current_map);
bool IsEqualTo(TransitionArray* other);
// The maximum number of transitions we want in a transition array (should
// fit in a page).
static const int kMaxNumberOfTransitions = 1024 + 512;
// Conversion from transition number to array indices.
static int ToKeyIndex(int transition_number) {
return kFirstIndex +
(transition_number * kTransitionSize) +
static int ToTargetIndex(int transition_number) {
return kFirstIndex +
(transition_number * kTransitionSize) +
static Handle<TransitionArray> AllocateSimple(
Isolate* isolate, Handle<Map> target);
// Allocate a new transition array with a single entry.
static Handle<TransitionArray> NewWith(Handle<Map> map,
Handle<Name> name,
Handle<Map> target,
SimpleTransitionFlag flag);
inline void NoIncrementalWriteBarrierSet(int transition_number,
Name* key,
Map* target);
// Copy a single transition from the origin array.
inline void NoIncrementalWriteBarrierCopyFrom(TransitionArray* origin,
int origin_transition,
int target_transition);
} } // namespace v8::internal
#endif // V8_TRANSITIONS_H_