Source/JavaScriptCore/heap/SlotVisitor.cpp - WebKit - Git at Google

 /*
  * Copyright (C) 2012, 2015-2016 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. 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 with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
  * THE POSSIBILITY OF SUCH DAMAGE.
  */

 #include "config.h"
 #include "SlotVisitor.h"
 #include "SlotVisitorInlines.h"

 #include "ConservativeRoots.h"
 #include "HeapCellInlines.h"
 #include "HeapProfiler.h"
 #include "HeapSnapshotBuilder.h"
 #include "JSArray.h"
 #include "JSDestructibleObject.h"
 #include "JSObject.h"
 #include "JSString.h"
 #include "JSCInlines.h"
 #include "SuperSampler.h"
 #include "VM.h"
 #include <wtf/Lock.h>

 namespace JSC {

 #if ENABLE(GC_VALIDATION)
 static void validate(JSCell* cell)
 {
     RELEASE_ASSERT(cell);

     if (!cell->structure()) {
         dataLogF("cell at %p has a null structure\n" , cell);
         CRASH();
     }

     // Both the cell's structure, and the cell's structure's structure should be the Structure Structure.
     // I hate this sentence.
     if (cell->structure()->structure()->JSCell::classInfo() != cell->structure()->JSCell::classInfo()) {
         const char* parentClassName = 0;
         const char* ourClassName = 0;
         if (cell->structure()->structure() && cell->structure()->structure()->JSCell::classInfo())
             parentClassName = cell->structure()->structure()->JSCell::classInfo()->className;
         if (cell->structure()->JSCell::classInfo())
             ourClassName = cell->structure()->JSCell::classInfo()->className;
         dataLogF("parent structure (%p <%s>) of cell at %p doesn't match cell's structure (%p <%s>)\n",
             cell->structure()->structure(), parentClassName, cell, cell->structure(), ourClassName);
         CRASH();
     }

     // Make sure we can walk the ClassInfo chain
     const ClassInfo* info = cell->classInfo();
     do { } while ((info = info->parentClass));
 }
 #endif

 SlotVisitor::SlotVisitor(Heap& heap)
     : m_stack()
     , m_bytesVisited(0)
     , m_bytesCopied(0)
     , m_visitCount(0)
     , m_isInParallelMode(false)
     , m_version(MarkedSpace::initialVersion)
     , m_heap(heap)
 #if !ASSERT_DISABLED
     , m_isCheckingForDefaultMarkViolation(false)
     , m_isDraining(false)
 #endif
 {
 }

 SlotVisitor::~SlotVisitor()
 {
     clearMarkStack();
 }

 void SlotVisitor::didStartMarking()
 {
     if (heap()->operationInProgress() == FullCollection)
         ASSERT(m_opaqueRoots.isEmpty()); // Should have merged by now.
     else
         reset();

     if (HeapProfiler* heapProfiler = vm().heapProfiler())
         m_heapSnapshotBuilder = heapProfiler->activeSnapshotBuilder();

     m_version = heap()->objectSpace().version();
 }

 void SlotVisitor::reset()
 {
     m_bytesVisited = 0;
     m_bytesCopied = 0;
     m_visitCount = 0;
     m_heapSnapshotBuilder = nullptr;
     ASSERT(!m_currentCell);
 }

 void SlotVisitor::clearMarkStack()
 {
     m_stack.clear();
 }

 void SlotVisitor::append(ConservativeRoots& conservativeRoots)
 {
     HeapCell** roots = conservativeRoots.roots();
     size_t size = conservativeRoots.size();
     for (size_t i = 0; i < size; ++i)
         appendJSCellOrAuxiliary(roots[i]);
 }

 void SlotVisitor::appendJSCellOrAuxiliary(HeapCell* heapCell)
 {
     if (!heapCell)
         return;

     ASSERT(!m_isCheckingForDefaultMarkViolation);

     if (Heap::testAndSetMarked(m_version, heapCell))
         return;

     switch (heapCell->cellKind()) {
     case HeapCell::JSCell: {
         JSCell* jsCell = static_cast<JSCell*>(heapCell);

         if (!jsCell->structure()) {
             ASSERT_NOT_REACHED();
             return;
         }

         jsCell->setCellState(CellState::NewGrey);

         appendToMarkStack(jsCell);
         return;
     }

     case HeapCell::Auxiliary: {
         noteLiveAuxiliaryCell(heapCell);
         return;
     } }
 }

 void SlotVisitor::append(JSValue value)
 {
     if (!value || !value.isCell())
         return;

     if (UNLIKELY(m_heapSnapshotBuilder))
         m_heapSnapshotBuilder->appendEdge(m_currentCell, value.asCell());

     setMarkedAndAppendToMarkStack(value.asCell());
 }

 void SlotVisitor::appendHidden(JSValue value)
 {
     if (!value || !value.isCell())
         return;

     setMarkedAndAppendToMarkStack(value.asCell());
 }

 void SlotVisitor::setMarkedAndAppendToMarkStack(JSCell* cell)
 {
     SuperSamplerScope superSamplerScope(false);

     ASSERT(!m_isCheckingForDefaultMarkViolation);
     if (!cell)
         return;

 #if ENABLE(GC_VALIDATION)
     validate(cell);
 #endif

     if (cell->isLargeAllocation())
         setMarkedAndAppendToMarkStack(cell->largeAllocation(), cell);
     else
         setMarkedAndAppendToMarkStack(cell->markedBlock(), cell);
 }

 template<typename ContainerType>
 ALWAYS_INLINE void SlotVisitor::setMarkedAndAppendToMarkStack(ContainerType& container, JSCell* cell)
 {
     container.flipIfNecessaryConcurrently(m_version);

     if (container.testAndSetMarked(cell))
         return;

     ASSERT(cell->structure());

     // Indicate that the object is grey and that:
     // In case of concurrent GC: it's the first time it is grey in this GC cycle.
     // In case of eden collection: it's a new object that became grey rather than an old remembered object.
     cell->setCellState(CellState::NewGrey);

     appendToMarkStack(container, cell);
 }

 void SlotVisitor::appendToMarkStack(JSCell* cell)
 {
     if (cell->isLargeAllocation())
         appendToMarkStack(cell->largeAllocation(), cell);
     else
         appendToMarkStack(cell->markedBlock(), cell);
 }

 template<typename ContainerType>
 ALWAYS_INLINE void SlotVisitor::appendToMarkStack(ContainerType& container, JSCell* cell)
 {
     ASSERT(Heap::isMarkedConcurrently(cell));
     ASSERT(!cell->isZapped());

     container.noteMarked();

     // FIXME: These "just work" because the GC resets these fields before doing anything else. But
     // that won't be the case when we do concurrent GC.
     m_visitCount++;
     m_bytesVisited += container.cellSize();

     m_stack.append(cell);

     if (UNLIKELY(m_heapSnapshotBuilder))
         m_heapSnapshotBuilder->appendNode(cell);
 }

 void SlotVisitor::markAuxiliary(const void* base)
 {
     HeapCell* cell = bitwise_cast<HeapCell*>(base);

     ASSERT(cell->heap() == heap());

     if (Heap::testAndSetMarked(m_version, cell)) {
         RELEASE_ASSERT(Heap::isMarkedConcurrently(cell));
         return;
     }

     noteLiveAuxiliaryCell(cell);
 }

 void SlotVisitor::noteLiveAuxiliaryCell(HeapCell* cell)
 {
     // We get here once per GC under these circumstances:
     //
     // Eden collection: if the cell was allocated since the last collection and is live somehow.
     //
     // Full collection: if the cell is live somehow.

     CellContainer container = cell->cellContainer();

     container.noteMarked();

     m_visitCount++;
     m_bytesVisited += container.cellSize();
 }

 class SetCurrentCellScope {
 public:
     SetCurrentCellScope(SlotVisitor& visitor, const JSCell* cell)
         : m_visitor(visitor)
     {
         ASSERT(!m_visitor.m_currentCell);
         m_visitor.m_currentCell = const_cast<JSCell*>(cell);
     }

     ~SetCurrentCellScope()
     {
         ASSERT(m_visitor.m_currentCell);
         m_visitor.m_currentCell = nullptr;
     }

 private:
     SlotVisitor& m_visitor;
 };


 ALWAYS_INLINE void SlotVisitor::visitChildren(const JSCell* cell)
 {
     ASSERT(Heap::isMarkedConcurrently(cell));

     SetCurrentCellScope currentCellScope(*this, cell);

     m_currentObjectCellStateBeforeVisiting = cell->cellState();
     cell->setCellState(CellState::OldBlack);

     if (isJSString(cell)) {
         JSString::visitChildren(const_cast<JSCell*>(cell), *this);
         return;
     }

     if (isJSFinalObject(cell)) {
         JSFinalObject::visitChildren(const_cast<JSCell*>(cell), *this);
         return;
     }

     if (isJSArray(cell)) {
         JSArray::visitChildren(const_cast<JSCell*>(cell), *this);
         return;
     }

     cell->methodTable()->visitChildren(const_cast<JSCell*>(cell), *this);
 }

 void SlotVisitor::donateKnownParallel()
 {
     // NOTE: Because we re-try often, we can afford to be conservative, and
     // assume that donating is not profitable.

     // Avoid locking when a thread reaches a dead end in the object graph.
     if (m_stack.size() < 2)
         return;

     // If there's already some shared work queued up, be conservative and assume
     // that donating more is not profitable.
     if (m_heap.m_sharedMarkStack.size())
         return;

     // If we're contending on the lock, be conservative and assume that another
     // thread is already donating.
     std::unique_lock<Lock> lock(m_heap.m_markingMutex, std::try_to_lock);
     if (!lock.owns_lock())
         return;

     // Otherwise, assume that a thread will go idle soon, and donate.
     m_stack.donateSomeCellsTo(m_heap.m_sharedMarkStack);

     m_heap.m_markingConditionVariable.notifyAll();
 }

 void SlotVisitor::drain()
 {
     ASSERT(m_isInParallelMode);

     while (!m_stack.isEmpty()) {
         m_stack.refill();
         for (unsigned countdown = Options::minimumNumberOfScansBetweenRebalance(); m_stack.canRemoveLast() && countdown--;)
             visitChildren(m_stack.removeLast());
         donateKnownParallel();
     }

     mergeOpaqueRootsIfNecessary();
 }

 void SlotVisitor::drainFromShared(SharedDrainMode sharedDrainMode)
 {
     ASSERT(m_isInParallelMode);

     ASSERT(Options::numberOfGCMarkers());

     {
         std::lock_guard<Lock> lock(m_heap.m_markingMutex);
         m_heap.m_numberOfActiveParallelMarkers++;
     }
     while (true) {
         {
             std::unique_lock<Lock> lock(m_heap.m_markingMutex);
             m_heap.m_numberOfActiveParallelMarkers--;
             m_heap.m_numberOfWaitingParallelMarkers++;

             // How we wait differs depending on drain mode.
             if (sharedDrainMode == MasterDrain) {
                 // Wait until either termination is reached, or until there is some work
                 // for us to do.
                 while (true) {
                     // Did we reach termination?
                     if (!m_heap.m_numberOfActiveParallelMarkers
                         && m_heap.m_sharedMarkStack.isEmpty()) {
                         // Let any sleeping slaves know it's time for them to return;
                         m_heap.m_markingConditionVariable.notifyAll();
                         return;
                     }

                     // Is there work to be done?
                     if (!m_heap.m_sharedMarkStack.isEmpty())
                         break;

                     // Otherwise wait.
                     m_heap.m_markingConditionVariable.wait(lock);
                 }
             } else {
                 ASSERT(sharedDrainMode == SlaveDrain);

                 // Did we detect termination? If so, let the master know.
                 if (!m_heap.m_numberOfActiveParallelMarkers
                     && m_heap.m_sharedMarkStack.isEmpty())
                     m_heap.m_markingConditionVariable.notifyAll();

                 m_heap.m_markingConditionVariable.wait(
                     lock,
                     [this] {
                         return !m_heap.m_sharedMarkStack.isEmpty()
                             || m_heap.m_parallelMarkersShouldExit;
                     });

                 // Is the current phase done? If so, return from this function.
                 if (m_heap.m_parallelMarkersShouldExit)
                     return;
             }

             m_stack.stealSomeCellsFrom(
                 m_heap.m_sharedMarkStack, m_heap.m_numberOfWaitingParallelMarkers);
             m_heap.m_numberOfActiveParallelMarkers++;
             m_heap.m_numberOfWaitingParallelMarkers--;
         }

         drain();
     }
 }

 void SlotVisitor::addOpaqueRoot(void* root)
 {
     if (Options::numberOfGCMarkers() == 1) {
         // Put directly into the shared HashSet.
         m_heap.m_opaqueRoots.add(root);
         return;
     }
     // Put into the local set, but merge with the shared one every once in
     // a while to make sure that the local sets don't grow too large.
     mergeOpaqueRootsIfProfitable();
     m_opaqueRoots.add(root);
 }

 bool SlotVisitor::containsOpaqueRoot(void* root) const
 {
     ASSERT(!m_isInParallelMode);
     ASSERT(m_opaqueRoots.isEmpty());
     return m_heap.m_opaqueRoots.contains(root);
 }

 TriState SlotVisitor::containsOpaqueRootTriState(void* root) const
 {
     if (m_opaqueRoots.contains(root))
         return TrueTriState;
     std::lock_guard<Lock> lock(m_heap.m_opaqueRootsMutex);
     if (m_heap.m_opaqueRoots.contains(root))
         return TrueTriState;
     return MixedTriState;
 }

 int SlotVisitor::opaqueRootCount()
 {
     ASSERT(!m_isInParallelMode);
     ASSERT(m_opaqueRoots.isEmpty());
     return m_heap.m_opaqueRoots.size();
 }

 void SlotVisitor::mergeOpaqueRootsIfNecessary()
 {
     if (m_opaqueRoots.isEmpty())
         return;
     mergeOpaqueRoots();
 }

 void SlotVisitor::mergeOpaqueRootsIfProfitable()
 {
     if (static_cast<unsigned>(m_opaqueRoots.size()) < Options::opaqueRootMergeThreshold())
         return;
     mergeOpaqueRoots();
 }

 void SlotVisitor::donate()
 {
     ASSERT(m_isInParallelMode);
     if (Options::numberOfGCMarkers() == 1)
         return;

     donateKnownParallel();
 }

 void SlotVisitor::donateAndDrain()
 {
     donate();
     drain();
 }

 void SlotVisitor::mergeOpaqueRoots()
 {
     ASSERT(!m_opaqueRoots.isEmpty()); // Should only be called when opaque roots are non-empty.
     {
         std::lock_guard<Lock> lock(m_heap.m_opaqueRootsMutex);
         for (auto* root : m_opaqueRoots)
             m_heap.m_opaqueRoots.add(root);
     }
     m_opaqueRoots.clear();
 }

 void SlotVisitor::harvestWeakReferences()
 {
     for (WeakReferenceHarvester* current = m_heap.m_weakReferenceHarvesters.head(); current; current = current->next())
         current->visitWeakReferences(*this);
 }

 void SlotVisitor::finalizeUnconditionalFinalizers()
 {
     while (m_heap.m_unconditionalFinalizers.hasNext())
         m_heap.m_unconditionalFinalizers.removeNext()->finalizeUnconditionally();
 }

 void SlotVisitor::dump(PrintStream&) const
 {
     for (const JSCell* cell : markStack())
         dataLog(*cell, "\n");
 }

 } // namespace JSC
	/*
	* Copyright (C) 2012, 2015-2016 Apple Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. 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 with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
	* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
	* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
	* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
	* THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#include "config.h"
	#include "SlotVisitor.h"
	#include "SlotVisitorInlines.h"

	#include "ConservativeRoots.h"
	#include "HeapCellInlines.h"
	#include "HeapProfiler.h"
	#include "HeapSnapshotBuilder.h"
	#include "JSArray.h"
	#include "JSDestructibleObject.h"
	#include "JSObject.h"
	#include "JSString.h"
	#include "JSCInlines.h"
	#include "SuperSampler.h"
	#include "VM.h"
	#include <wtf/Lock.h>

	namespace JSC {

	#if ENABLE(GC_VALIDATION)
	static void validate(JSCell* cell)
	{
	RELEASE_ASSERT(cell);

	if (!cell->structure()) {
	dataLogF("cell at %p has a null structure\n" , cell);
	CRASH();
	}

	// Both the cell's structure, and the cell's structure's structure should be the Structure Structure.
	// I hate this sentence.
	if (cell->structure()->structure()->JSCell::classInfo() != cell->structure()->JSCell::classInfo()) {
	const char* parentClassName = 0;
	const char* ourClassName = 0;
	if (cell->structure()->structure() && cell->structure()->structure()->JSCell::classInfo())
	parentClassName = cell->structure()->structure()->JSCell::classInfo()->className;
	if (cell->structure()->JSCell::classInfo())
	ourClassName = cell->structure()->JSCell::classInfo()->className;
	dataLogF("parent structure (%p <%s>) of cell at %p doesn't match cell's structure (%p <%s>)\n",
	cell->structure()->structure(), parentClassName, cell, cell->structure(), ourClassName);
	CRASH();
	}

	// Make sure we can walk the ClassInfo chain
	const ClassInfo* info = cell->classInfo();
	do { } while ((info = info->parentClass));
	}
	#endif

	SlotVisitor::SlotVisitor(Heap& heap)
	: m_stack()
	, m_bytesVisited(0)
	, m_bytesCopied(0)
	, m_visitCount(0)
	, m_isInParallelMode(false)
	, m_version(MarkedSpace::initialVersion)
	, m_heap(heap)
	#if !ASSERT_DISABLED
	, m_isCheckingForDefaultMarkViolation(false)
	, m_isDraining(false)
	#endif
	{
	}

	SlotVisitor::~SlotVisitor()
	{
	clearMarkStack();
	}

	void SlotVisitor::didStartMarking()
	{
	if (heap()->operationInProgress() == FullCollection)
	ASSERT(m_opaqueRoots.isEmpty()); // Should have merged by now.
	else
	reset();

	if (HeapProfiler* heapProfiler = vm().heapProfiler())
	m_heapSnapshotBuilder = heapProfiler->activeSnapshotBuilder();

	m_version = heap()->objectSpace().version();
	}

	void SlotVisitor::reset()
	{
	m_bytesVisited = 0;
	m_bytesCopied = 0;
	m_visitCount = 0;
	m_heapSnapshotBuilder = nullptr;
	ASSERT(!m_currentCell);
	}

	void SlotVisitor::clearMarkStack()
	{
	m_stack.clear();
	}

	void SlotVisitor::append(ConservativeRoots& conservativeRoots)
	{
	HeapCell** roots = conservativeRoots.roots();
	size_t size = conservativeRoots.size();
	for (size_t i = 0; i < size; ++i)
	appendJSCellOrAuxiliary(roots[i]);
	}

	void SlotVisitor::appendJSCellOrAuxiliary(HeapCell* heapCell)
	{
	if (!heapCell)
	return;

	ASSERT(!m_isCheckingForDefaultMarkViolation);

	if (Heap::testAndSetMarked(m_version, heapCell))
	return;

	switch (heapCell->cellKind()) {
	case HeapCell::JSCell: {
	JSCell* jsCell = static_cast<JSCell*>(heapCell);

	if (!jsCell->structure()) {
	ASSERT_NOT_REACHED();
	return;
	}

	jsCell->setCellState(CellState::NewGrey);

	appendToMarkStack(jsCell);
	return;
	}

	case HeapCell::Auxiliary: {
	noteLiveAuxiliaryCell(heapCell);
	return;
	} }
	}

	void SlotVisitor::append(JSValue value)
	{
	if (!value \|\| !value.isCell())
	return;

	if (UNLIKELY(m_heapSnapshotBuilder))
	m_heapSnapshotBuilder->appendEdge(m_currentCell, value.asCell());

	setMarkedAndAppendToMarkStack(value.asCell());
	}

	void SlotVisitor::appendHidden(JSValue value)
	{
	if (!value \|\| !value.isCell())
	return;

	setMarkedAndAppendToMarkStack(value.asCell());
	}

	void SlotVisitor::setMarkedAndAppendToMarkStack(JSCell* cell)
	{
	SuperSamplerScope superSamplerScope(false);

	ASSERT(!m_isCheckingForDefaultMarkViolation);
	if (!cell)
	return;

	#if ENABLE(GC_VALIDATION)
	validate(cell);
	#endif

	if (cell->isLargeAllocation())
	setMarkedAndAppendToMarkStack(cell->largeAllocation(), cell);
	else
	setMarkedAndAppendToMarkStack(cell->markedBlock(), cell);
	}

	template<typename ContainerType>
	ALWAYS_INLINE void SlotVisitor::setMarkedAndAppendToMarkStack(ContainerType& container, JSCell* cell)
	{
	container.flipIfNecessaryConcurrently(m_version);

	if (container.testAndSetMarked(cell))
	return;

	ASSERT(cell->structure());

	// Indicate that the object is grey and that:
	// In case of concurrent GC: it's the first time it is grey in this GC cycle.
	// In case of eden collection: it's a new object that became grey rather than an old remembered object.
	cell->setCellState(CellState::NewGrey);

	appendToMarkStack(container, cell);
	}

	void SlotVisitor::appendToMarkStack(JSCell* cell)
	{
	if (cell->isLargeAllocation())
	appendToMarkStack(cell->largeAllocation(), cell);
	else
	appendToMarkStack(cell->markedBlock(), cell);
	}

	template<typename ContainerType>
	ALWAYS_INLINE void SlotVisitor::appendToMarkStack(ContainerType& container, JSCell* cell)
	{
	ASSERT(Heap::isMarkedConcurrently(cell));
	ASSERT(!cell->isZapped());

	container.noteMarked();

	// FIXME: These "just work" because the GC resets these fields before doing anything else. But
	// that won't be the case when we do concurrent GC.
	m_visitCount++;
	m_bytesVisited += container.cellSize();

	m_stack.append(cell);

	if (UNLIKELY(m_heapSnapshotBuilder))
	m_heapSnapshotBuilder->appendNode(cell);
	}

	void SlotVisitor::markAuxiliary(const void* base)
	{
	HeapCell* cell = bitwise_cast<HeapCell*>(base);

	ASSERT(cell->heap() == heap());

	if (Heap::testAndSetMarked(m_version, cell)) {
	RELEASE_ASSERT(Heap::isMarkedConcurrently(cell));
	return;
	}

	noteLiveAuxiliaryCell(cell);
	}

	void SlotVisitor::noteLiveAuxiliaryCell(HeapCell* cell)
	{
	// We get here once per GC under these circumstances:
	//
	// Eden collection: if the cell was allocated since the last collection and is live somehow.
	//
	// Full collection: if the cell is live somehow.

	CellContainer container = cell->cellContainer();

	container.noteMarked();

	m_visitCount++;
	m_bytesVisited += container.cellSize();
	}

	class SetCurrentCellScope {
	public:
	SetCurrentCellScope(SlotVisitor& visitor, const JSCell* cell)
	: m_visitor(visitor)
	{
	ASSERT(!m_visitor.m_currentCell);
	m_visitor.m_currentCell = const_cast<JSCell*>(cell);
	}

	~SetCurrentCellScope()
	{
	ASSERT(m_visitor.m_currentCell);
	m_visitor.m_currentCell = nullptr;
	}

	private:
	SlotVisitor& m_visitor;
	};


	ALWAYS_INLINE void SlotVisitor::visitChildren(const JSCell* cell)
	{
	ASSERT(Heap::isMarkedConcurrently(cell));

	SetCurrentCellScope currentCellScope(*this, cell);

	m_currentObjectCellStateBeforeVisiting = cell->cellState();
	cell->setCellState(CellState::OldBlack);

	if (isJSString(cell)) {
	JSString::visitChildren(const_cast<JSCell>(cell), this);
	return;
	}

	if (isJSFinalObject(cell)) {
	JSFinalObject::visitChildren(const_cast<JSCell>(cell), this);
	return;
	}

	if (isJSArray(cell)) {
	JSArray::visitChildren(const_cast<JSCell>(cell), this);
	return;
	}

	cell->methodTable()->visitChildren(const_cast<JSCell>(cell), this);
	}

	void SlotVisitor::donateKnownParallel()
	{
	// NOTE: Because we re-try often, we can afford to be conservative, and
	// assume that donating is not profitable.

	// Avoid locking when a thread reaches a dead end in the object graph.
	if (m_stack.size() < 2)
	return;

	// If there's already some shared work queued up, be conservative and assume
	// that donating more is not profitable.
	if (m_heap.m_sharedMarkStack.size())
	return;

	// If we're contending on the lock, be conservative and assume that another
	// thread is already donating.
	std::unique_lock<Lock> lock(m_heap.m_markingMutex, std::try_to_lock);
	if (!lock.owns_lock())
	return;

	// Otherwise, assume that a thread will go idle soon, and donate.
	m_stack.donateSomeCellsTo(m_heap.m_sharedMarkStack);

	m_heap.m_markingConditionVariable.notifyAll();
	}

	void SlotVisitor::drain()
	{
	ASSERT(m_isInParallelMode);

	while (!m_stack.isEmpty()) {
	m_stack.refill();
	for (unsigned countdown = Options::minimumNumberOfScansBetweenRebalance(); m_stack.canRemoveLast() && countdown--;)
	visitChildren(m_stack.removeLast());
	donateKnownParallel();
	}

	mergeOpaqueRootsIfNecessary();
	}

	void SlotVisitor::drainFromShared(SharedDrainMode sharedDrainMode)
	{
	ASSERT(m_isInParallelMode);

	ASSERT(Options::numberOfGCMarkers());

	{
	std::lock_guard<Lock> lock(m_heap.m_markingMutex);
	m_heap.m_numberOfActiveParallelMarkers++;
	}
	while (true) {
	{
	std::unique_lock<Lock> lock(m_heap.m_markingMutex);
	m_heap.m_numberOfActiveParallelMarkers--;
	m_heap.m_numberOfWaitingParallelMarkers++;

	// How we wait differs depending on drain mode.
	if (sharedDrainMode == MasterDrain) {
	// Wait until either termination is reached, or until there is some work
	// for us to do.
	while (true) {
	// Did we reach termination?
	if (!m_heap.m_numberOfActiveParallelMarkers
	&& m_heap.m_sharedMarkStack.isEmpty()) {
	// Let any sleeping slaves know it's time for them to return;
	m_heap.m_markingConditionVariable.notifyAll();
	return;
	}

	// Is there work to be done?
	if (!m_heap.m_sharedMarkStack.isEmpty())
	break;

	// Otherwise wait.
	m_heap.m_markingConditionVariable.wait(lock);
	}
	} else {
	ASSERT(sharedDrainMode == SlaveDrain);

	// Did we detect termination? If so, let the master know.
	if (!m_heap.m_numberOfActiveParallelMarkers
	&& m_heap.m_sharedMarkStack.isEmpty())
	m_heap.m_markingConditionVariable.notifyAll();

	m_heap.m_markingConditionVariable.wait(
	lock,
	[this] {
	return !m_heap.m_sharedMarkStack.isEmpty()
	\|\| m_heap.m_parallelMarkersShouldExit;
	});

	// Is the current phase done? If so, return from this function.
	if (m_heap.m_parallelMarkersShouldExit)
	return;
	}

	m_stack.stealSomeCellsFrom(
	m_heap.m_sharedMarkStack, m_heap.m_numberOfWaitingParallelMarkers);
	m_heap.m_numberOfActiveParallelMarkers++;
	m_heap.m_numberOfWaitingParallelMarkers--;
	}

	drain();
	}
	}

	void SlotVisitor::addOpaqueRoot(void* root)
	{
	if (Options::numberOfGCMarkers() == 1) {
	// Put directly into the shared HashSet.
	m_heap.m_opaqueRoots.add(root);
	return;
	}
	// Put into the local set, but merge with the shared one every once in
	// a while to make sure that the local sets don't grow too large.
	mergeOpaqueRootsIfProfitable();
	m_opaqueRoots.add(root);
	}

	bool SlotVisitor::containsOpaqueRoot(void* root) const
	{
	ASSERT(!m_isInParallelMode);
	ASSERT(m_opaqueRoots.isEmpty());
	return m_heap.m_opaqueRoots.contains(root);
	}

	TriState SlotVisitor::containsOpaqueRootTriState(void* root) const
	{
	if (m_opaqueRoots.contains(root))
	return TrueTriState;
	std::lock_guard<Lock> lock(m_heap.m_opaqueRootsMutex);
	if (m_heap.m_opaqueRoots.contains(root))
	return TrueTriState;
	return MixedTriState;
	}

	int SlotVisitor::opaqueRootCount()
	{
	ASSERT(!m_isInParallelMode);
	ASSERT(m_opaqueRoots.isEmpty());
	return m_heap.m_opaqueRoots.size();
	}

	void SlotVisitor::mergeOpaqueRootsIfNecessary()
	{
	if (m_opaqueRoots.isEmpty())
	return;
	mergeOpaqueRoots();
	}

	void SlotVisitor::mergeOpaqueRootsIfProfitable()
	{
	if (static_cast<unsigned>(m_opaqueRoots.size()) < Options::opaqueRootMergeThreshold())
	return;
	mergeOpaqueRoots();
	}

	void SlotVisitor::donate()
	{
	ASSERT(m_isInParallelMode);
	if (Options::numberOfGCMarkers() == 1)
	return;

	donateKnownParallel();
	}

	void SlotVisitor::donateAndDrain()
	{
	donate();
	drain();
	}

	void SlotVisitor::mergeOpaqueRoots()
	{
	ASSERT(!m_opaqueRoots.isEmpty()); // Should only be called when opaque roots are non-empty.
	{
	std::lock_guard<Lock> lock(m_heap.m_opaqueRootsMutex);
	for (auto* root : m_opaqueRoots)
	m_heap.m_opaqueRoots.add(root);
	}
	m_opaqueRoots.clear();
	}

	void SlotVisitor::harvestWeakReferences()
	{
	for (WeakReferenceHarvester* current = m_heap.m_weakReferenceHarvesters.head(); current; current = current->next())
	current->visitWeakReferences(*this);
	}

	void SlotVisitor::finalizeUnconditionalFinalizers()
	{
	while (m_heap.m_unconditionalFinalizers.hasNext())
	m_heap.m_unconditionalFinalizers.removeNext()->finalizeUnconditionally();
	}

	void SlotVisitor::dump(PrintStream&) const
	{
	for (const JSCell* cell : markStack())
	dataLog(*cell, "\n");
	}

	} // namespace JSC