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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/checks.h"
#include "src/elements-kind.h"
#include "src/heap/heap.h"
#include "src/isolate.h"
#include "src/objects.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] Number of transitions
// [3] First transition
// [3 + number of transitions * kTransitionSize]: start of slack
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 Object* GetTargetValue(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;
if (length() <= kFirstIndex) return 0;
return Smi::cast(get(kTransitionLengthIndex))->value();
int number_of_transitions_storage() {
if (IsSimpleTransition()) return 1;
if (length() <= kFirstIndex) return 0;
return (length() - kFirstIndex) / kTransitionSize;
int NumberOfSlackTransitions() {
return number_of_transitions_storage() - number_of_transitions();
inline void SetNumberOfTransitions(int number_of_transitions);
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);
// Return a transition array, using the array from the owning map if it
// already has one (copying into a larger array if necessary), otherwise
// creating a new one according to flag.
// TODO(verwaest): This should not cause an existing transition to be
// overwritten.
static Handle<TransitionArray> Insert(Handle<Map> map, Handle<Name> name,
Handle<Map> target,
SimpleTransitionFlag flag);
// Search a transition for a given kind, property name and attributes.
int Search(PropertyKind kind, Name* name, PropertyAttributes attributes,
int* out_insertion_index = NULL);
// Search a non-property transition (like elements kind, observe or frozen
// transitions).
inline int SearchSpecial(Symbol* symbol, int* out_insertion_index = NULL) {
return SearchName(symbol, out_insertion_index);
static inline PropertyDetails GetTargetDetails(Name* name, Map* target);
// Allocates a TransitionArray.
static Handle<TransitionArray> Allocate(Isolate* isolate,
int number_of_transitions,
int slack = 0);
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 kTransitionLengthIndex = 2;
static const int kFirstIndex = 3;
// 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 +
static const int kTransitionLengthOffset =
kPrototypeTransitionsOffset + kPointerSize;
// Layout of map transition entries in full transition arrays.
static const int kTransitionKey = 0;
static const int kTransitionTarget = 1;
static const int kTransitionSize = 2;
#if defined(DEBUG) || defined(OBJECT_PRINT)
// For our gdb macros, we should perhaps change these in the future.
void Print();
// Print all the transitions.
void PrintTransitions(std::ostream& os, bool print_header = true); // NOLINT
#ifdef DEBUG
bool IsSortedNoDuplicates(int valid_entries = -1);
bool IsConsistentWithBackPointers(Map* current_map);
bool IsEqualTo(TransitionArray* other);
// Returns true for a non-property transitions like elements kind, observed
// or frozen transitions.
static inline bool IsSpecialTransition(Name* name);
// The maximum number of transitions we want in a transition array (should
// fit in a page).
static const int kMaxNumberOfTransitions = 1024 + 512;
// Returns the fixed array length required to hold number_of_transitions
// transitions.
static int LengthFor(int number_of_transitions) {
return ToKeyIndex(number_of_transitions);
// 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);
// Search a first transition for a given property name.
inline int SearchName(Name* name, int* out_insertion_index = NULL);
int SearchDetails(int transition, PropertyKind kind,
PropertyAttributes attributes, int* out_insertion_index);
// Compares two tuples <key, kind, attributes>, returns -1 if
// tuple1 is "less" than tuple2, 0 if tuple1 equal to tuple2 and 1 otherwise.
static inline int CompareKeys(Name* key1, uint32_t hash1, PropertyKind kind1,
PropertyAttributes attributes1, Name* key2,
uint32_t hash2, PropertyKind kind2,
PropertyAttributes attributes2);
// Compares keys, returns -1 if key1 is "less" than key2,
// 0 if key1 equal to key2 and 1 otherwise.
static inline int CompareNames(Name* key1, uint32_t hash1, Name* key2,
uint32_t hash2);
// Compares two details, returns -1 if details1 is "less" than details2,
// 0 if details1 equal to details2 and 1 otherwise.
static inline int CompareDetails(PropertyKind kind1,
PropertyAttributes attributes1,
PropertyKind kind2,
PropertyAttributes attributes2);
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_