blob: c0a7ac9420149f7f79335841f00000a378a13bc1 [file] [log] [blame]
// Copyright 2011 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.
#ifndef V8_HANDLES_H_
#define V8_HANDLES_H_
#include <type_traits>
#include "include/v8.h"
#include "src/base/functional.h"
#include "src/base/macros.h"
#include "src/checks.h"
#include "src/globals.h"
#include "src/zone/zone.h"
namespace v8 {
namespace internal {
// Forward declarations.
class DeferredHandles;
class HandleScopeImplementer;
class Isolate;
template <typename T>
class MaybeHandle;
class Object;
// ----------------------------------------------------------------------------
// Base class for Handle instantiations. Don't use directly.
class HandleBase {
V8_INLINE explicit HandleBase(Object** location) : location_(location) {}
V8_INLINE explicit HandleBase(Object* object, Isolate* isolate);
// Check if this handle refers to the exact same object as the other handle.
V8_INLINE bool is_identical_to(const HandleBase that) const {
// Dereferencing deferred handles to check object equality is safe.
SLOW_DCHECK((this->location_ == nullptr ||
this->IsDereferenceAllowed(NO_DEFERRED_CHECK)) &&
(that.location_ == nullptr ||
if (this->location_ == that.location_) return true;
if (this->location_ == nullptr || that.location_ == nullptr) return false;
return *this->location_ == *that.location_;
V8_INLINE bool is_null() const { return location_ == nullptr; }
// Returns the raw address where this handle is stored. This should only be
// used for hashing handles; do not ever try to dereference it.
V8_INLINE Address address() const { return bit_cast<Address>(location_); }
// Provides the C++ dereference operator.
V8_INLINE Object* operator*() const {
return *location_;
// Returns the address to where the raw pointer is stored.
V8_INLINE Object** location() const {
SLOW_DCHECK(location_ == nullptr ||
return location_;
#ifdef DEBUG
bool V8_EXPORT_PRIVATE IsDereferenceAllowed(DereferenceCheckMode mode) const;
bool V8_EXPORT_PRIVATE IsDereferenceAllowed(DereferenceCheckMode mode) const {
return true;
#endif // DEBUG
Object** location_;
// ----------------------------------------------------------------------------
// A Handle provides a reference to an object that survives relocation by
// the garbage collector.
// Handles are only valid within a HandleScope. When a handle is created
// for an object a cell is allocated in the current HandleScope.
// Also note that Handles do not provide default equality comparison or hashing
// operators on purpose. Such operators would be misleading, because intended
// semantics is ambiguous between Handle location and object identity. Instead
// use either {is_identical_to} or {location} explicitly.
template <typename T>
class Handle final : public HandleBase {
V8_INLINE explicit Handle(T** location = nullptr)
: HandleBase(reinterpret_cast<Object**>(location)) {
// Type check:
static_assert(std::is_convertible<T*, Object*>::value,
"static type violation");
V8_INLINE Handle(T* object, Isolate* isolate);
// Allocate a new handle for the object, do not canonicalize.
V8_INLINE static Handle<T> New(T* object, Isolate* isolate);
// Constructor for handling automatic up casting.
// Ex. Handle<JSFunction> can be passed when Handle<Object> is expected.
template <typename S, typename = typename std::enable_if<
std::is_convertible<S*, T*>::value>::type>
V8_INLINE Handle(Handle<S> handle) : HandleBase(handle) {}
V8_INLINE T* operator->() const { return operator*(); }
// Provides the C++ dereference operator.
V8_INLINE T* operator*() const {
return reinterpret_cast<T*>(HandleBase::operator*());
// Returns the address to where the raw pointer is stored.
V8_INLINE T** location() const {
return reinterpret_cast<T**>(HandleBase::location());
template <typename S>
inline static const Handle<T> cast(Handle<S> that);
// TODO(yangguo): Values that contain empty handles should be declared as
// MaybeHandle to force validation before being used as handles.
static const Handle<T> null() { return Handle<T>(); }
// Location equality.
bool equals(Handle<T> other) const { return address() == other.address(); }
// Provide function object for location equality comparison.
struct equal_to : public std::binary_function<Handle<T>, Handle<T>, bool> {
V8_INLINE bool operator()(Handle<T> lhs, Handle<T> rhs) const {
return lhs.equals(rhs);
// Provide function object for location hashing.
struct hash : public std::unary_function<Handle<T>, size_t> {
V8_INLINE size_t operator()(Handle<T> const& handle) const {
return base::hash<Address>()(handle.address());
// Handles of different classes are allowed to access each other's location_.
template <typename>
friend class Handle;
// MaybeHandle is allowed to access location_.
template <typename>
friend class MaybeHandle;
template <typename T>
inline std::ostream& operator<<(std::ostream& os, Handle<T> handle);
// ----------------------------------------------------------------------------
// A stack-allocated class that governs a number of local handles.
// After a handle scope has been created, all local handles will be
// allocated within that handle scope until either the handle scope is
// deleted or another handle scope is created. If there is already a
// handle scope and a new one is created, all allocations will take
// place in the new handle scope until it is deleted. After that,
// new handles will again be allocated in the original handle scope.
// After the handle scope of a local handle has been deleted the
// garbage collector will no longer track the object stored in the
// handle and may deallocate it. The behavior of accessing a handle
// for which the handle scope has been deleted is undefined.
class HandleScope {
explicit inline HandleScope(Isolate* isolate);
inline ~HandleScope();
// Counts the number of allocated handles.
V8_EXPORT_PRIVATE static int NumberOfHandles(Isolate* isolate);
// Create a new handle or lookup a canonical handle.
V8_INLINE static Object** GetHandle(Isolate* isolate, Object* value);
// Creates a new handle with the given value.
V8_INLINE static Object** CreateHandle(Isolate* isolate, Object* value);
// Deallocates any extensions used by the current scope.
V8_EXPORT_PRIVATE static void DeleteExtensions(Isolate* isolate);
static Address current_next_address(Isolate* isolate);
static Address current_limit_address(Isolate* isolate);
static Address current_level_address(Isolate* isolate);
// Closes the HandleScope (invalidating all handles
// created in the scope of the HandleScope) and returns
// a Handle backed by the parent scope holding the
// value of the argument handle.
template <typename T>
Handle<T> CloseAndEscape(Handle<T> handle_value);
Isolate* isolate() { return isolate_; }
// Limit for number of handles with --check-handle-count. This is
// large enough to compile natives and pass unit tests with some
// slack for future changes to natives.
static const int kCheckHandleThreshold = 30 * 1024;
// Prevent heap allocation or illegal handle scopes.
void* operator new(size_t size);
void operator delete(void* size_t);
Isolate* isolate_;
Object** prev_next_;
Object** prev_limit_;
// Close the handle scope resetting limits to a previous state.
static inline void CloseScope(Isolate* isolate,
Object** prev_next,
Object** prev_limit);
// Extend the handle scope making room for more handles.
V8_EXPORT_PRIVATE static Object** Extend(Isolate* isolate);
// Zaps the handles in the half-open interval [start, end).
V8_EXPORT_PRIVATE static void ZapRange(Object** start, Object** end);
friend class v8::HandleScope;
friend class DeferredHandles;
friend class DeferredHandleScope;
friend class HandleScopeImplementer;
friend class Isolate;
// Forward declarations for CanonicalHandleScope.
template <typename V, class AllocationPolicy>
class IdentityMap;
class RootIndexMap;
// A CanonicalHandleScope does not open a new HandleScope. It changes the
// existing HandleScope so that Handles created within are canonicalized.
// This does not apply to nested inner HandleScopes unless a nested
// CanonicalHandleScope is introduced. Handles are only canonicalized within
// the same CanonicalHandleScope, but not across nested ones.
class V8_EXPORT_PRIVATE CanonicalHandleScope final {
explicit CanonicalHandleScope(Isolate* isolate);
Object** Lookup(Object* object);
Isolate* isolate_;
Zone zone_;
RootIndexMap* root_index_map_;
IdentityMap<Object**, ZoneAllocationPolicy>* identity_map_;
// Ordinary nested handle scopes within the current one are not canonical.
int canonical_level_;
// We may have nested canonical scopes. Handles are canonical within each one.
CanonicalHandleScope* prev_canonical_scope_;
friend class HandleScope;
// A DeferredHandleScope is a HandleScope in which handles are not destroyed
// when the DeferredHandleScope is left. Instead the DeferredHandleScope has to
// be detached with {Detach}, and the result of {Detach} has to be destroyed
// explicitly. A DeferredHandleScope should only be used with the following
// design pattern:
// 1) Open a HandleScope (not a DeferredHandleScope).
// HandleScope scope(isolate_);
// 2) Create handles.
// Handle<Object> h1 = handle(object1, isolate);
// Handle<Object> h2 = handle(object2, isolate);
// 3) Open a DeferredHandleScope.
// DeferredHandleScope deferred_scope(isolate);
// 4) Reopen handles which should be in the DeferredHandleScope, e.g only h1.
// h1 = handle(*h1, isolate);
// 5) Detach the DeferredHandleScope.
// DeferredHandles* deferred_handles = deferred_scope.Detach();
// 6) Destroy the deferred handles.
// delete deferred_handles;
// Note: A DeferredHandleScope must not be opened within a DeferredHandleScope.
class V8_EXPORT_PRIVATE DeferredHandleScope final {
explicit DeferredHandleScope(Isolate* isolate);
// The DeferredHandles object returned stores the Handles created
// since the creation of this DeferredHandleScope. The Handles are
// alive as long as the DeferredHandles object is alive.
DeferredHandles* Detach();
Object** prev_limit_;
Object** prev_next_;
HandleScopeImplementer* impl_;
#ifdef DEBUG
bool handles_detached_ = false;
int prev_level_;
friend class HandleScopeImplementer;
// Seal off the current HandleScope so that new handles can only be created
// if a new HandleScope is entered.
class SealHandleScope final {
#ifndef DEBUG
explicit SealHandleScope(Isolate* isolate) {}
~SealHandleScope() {}
explicit inline SealHandleScope(Isolate* isolate);
inline ~SealHandleScope();
Isolate* isolate_;
Object** prev_limit_;
int prev_sealed_level_;
struct HandleScopeData final {
Object** next;
Object** limit;
int level;
int sealed_level;
CanonicalHandleScope* canonical_scope;
void Initialize() {
next = limit = nullptr;
sealed_level = level = 0;
canonical_scope = nullptr;
} // namespace internal
} // namespace v8
#endif // V8_HANDLES_H_