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// 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.
#include "src/heap/objects-visiting.h"
#include "src/heap/heap-inl.h"
#include "src/heap/mark-compact-inl.h"
#include "src/heap/objects-visiting-inl.h"
namespace v8 {
namespace internal {
// We don't record weak slots during marking or scavenges. Instead we do it
// once when we complete mark-compact cycle. Note that write barrier has no
// effect if we are already in the middle of compacting mark-sweep cycle and we
// have to record slots manually.
static bool MustRecordSlots(Heap* heap) {
return heap->gc_state() == Heap::MARK_COMPACT &&
template <class T>
struct WeakListVisitor;
template <class T>
Object* VisitWeakList(Heap* heap, Object* list, WeakObjectRetainer* retainer) {
Object* undefined = heap->undefined_value();
Object* head = undefined;
T* tail = nullptr;
bool record_slots = MustRecordSlots(heap);
while (list != undefined) {
// Check whether to keep the candidate in the list.
T* candidate = reinterpret_cast<T*>(list);
Object* retained = retainer->RetainAs(list);
// Move to the next element before the WeakNext is cleared.
list = WeakListVisitor<T>::WeakNext(candidate);
if (retained != nullptr) {
if (head == undefined) {
// First element in the list.
head = retained;
} else {
// Subsequent elements in the list.
WeakListVisitor<T>::SetWeakNext(tail, retained);
if (record_slots) {
HeapObject* slot_holder = WeakListVisitor<T>::WeakNextHolder(tail);
int slot_offset = WeakListVisitor<T>::WeakNextOffset();
Object** slot = HeapObject::RawField(slot_holder, slot_offset);
MarkCompactCollector::RecordSlot(slot_holder, slot, retained);
// Retained object is new tail.
candidate = reinterpret_cast<T*>(retained);
tail = candidate;
// tail is a live object, visit it.
WeakListVisitor<T>::VisitLiveObject(heap, tail, retainer);
} else {
WeakListVisitor<T>::VisitPhantomObject(heap, candidate);
// Terminate the list if there is one or more elements.
if (tail != nullptr) WeakListVisitor<T>::SetWeakNext(tail, undefined);
return head;
template <class T>
static void ClearWeakList(Heap* heap, Object* list) {
Object* undefined = heap->undefined_value();
while (list != undefined) {
T* candidate = reinterpret_cast<T*>(list);
list = WeakListVisitor<T>::WeakNext(candidate);
WeakListVisitor<T>::SetWeakNext(candidate, undefined);
template <>
struct WeakListVisitor<Code> {
static void SetWeakNext(Code* code, Object* next) {
static Object* WeakNext(Code* code) {
return code->code_data_container()->next_code_link();
static HeapObject* WeakNextHolder(Code* code) {
return code->code_data_container();
static int WeakNextOffset() { return CodeDataContainer::kNextCodeLinkOffset; }
static void VisitLiveObject(Heap*, Code*, WeakObjectRetainer*) {}
static void VisitPhantomObject(Heap* heap, Code* code) {
// Even though the code is dying, its code_data_container can still be
// alive. Clear the next_code_link slot to avoid a dangling pointer.
SetWeakNext(code, heap->undefined_value());
template <>
struct WeakListVisitor<Context> {
static void SetWeakNext(Context* context, Object* next) {
static Object* WeakNext(Context* context) {
return context->next_context_link();
static HeapObject* WeakNextHolder(Context* context) { return context; }
static int WeakNextOffset() {
return FixedArray::SizeFor(Context::NEXT_CONTEXT_LINK);
static void VisitLiveObject(Heap* heap, Context* context,
WeakObjectRetainer* retainer) {
if (heap->gc_state() == Heap::MARK_COMPACT) {
// Record the slots of the weak entries in the native context.
for (int idx = Context::FIRST_WEAK_SLOT;
idx < Context::NATIVE_CONTEXT_SLOTS; ++idx) {
Object** slot = Context::cast(context)->RawFieldOfElementAt(idx);
MarkCompactCollector::RecordSlot(context, slot, *slot);
// Code objects are always allocated in Code space, we do not have to
// visit them during scavenges.
DoWeakList<Code>(heap, context, retainer, Context::OPTIMIZED_CODE_LIST);
DoWeakList<Code>(heap, context, retainer, Context::DEOPTIMIZED_CODE_LIST);
template <class T>
static void DoWeakList(Heap* heap, Context* context,
WeakObjectRetainer* retainer, int index) {
// Visit the weak list, removing dead intermediate elements.
Object* list_head = VisitWeakList<T>(heap, context->get(index), retainer);
// Update the list head.
context->set(index, list_head, UPDATE_WRITE_BARRIER);
if (MustRecordSlots(heap)) {
// Record the updated slot if necessary.
Object** head_slot =
HeapObject::RawField(context, FixedArray::SizeFor(index));
heap->mark_compact_collector()->RecordSlot(context, head_slot, list_head);
static void VisitPhantomObject(Heap* heap, Context* context) {
ClearWeakList<Code>(heap, context->get(Context::OPTIMIZED_CODE_LIST));
ClearWeakList<Code>(heap, context->get(Context::DEOPTIMIZED_CODE_LIST));
template <>
struct WeakListVisitor<AllocationSite> {
static void SetWeakNext(AllocationSite* obj, Object* next) {
obj->set_weak_next(next, UPDATE_WEAK_WRITE_BARRIER);
static Object* WeakNext(AllocationSite* obj) { return obj->weak_next(); }
static HeapObject* WeakNextHolder(AllocationSite* obj) { return obj; }
static int WeakNextOffset() { return AllocationSite::kWeakNextOffset; }
static void VisitLiveObject(Heap*, AllocationSite*, WeakObjectRetainer*) {}
static void VisitPhantomObject(Heap*, AllocationSite*) {}
template Object* VisitWeakList<Context>(Heap* heap, Object* list,
WeakObjectRetainer* retainer);
template Object* VisitWeakList<AllocationSite>(Heap* heap, Object* list,
WeakObjectRetainer* retainer);
} // namespace internal
} // namespace v8