Source/JavaScriptCore/runtime/JSString.cpp - WebKit - Git at Google

 /*
  *  Copyright (C) 1999-2002 Harri Porten (porten@kde.org)
  *  Copyright (C) 2001 Peter Kelly (pmk@post.com)
  *  Copyright (C) 2004-2019 Apple Inc. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Library General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  *  Library General Public License for more details.
  *
  *  You should have received a copy of the GNU Library General Public License
  *  along with this library; see the file COPYING.LIB.  If not, write to
  *  the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  *  Boston, MA 02110-1301, USA.
  *
  */

 #include "config.h"
 #include "JSString.h"

 #include "JSGlobalObject.h"
 #include "JSGlobalObjectFunctions.h"
 #include "JSObject.h"
 #include "JSCInlines.h"
 #include "StringObject.h"
 #include "StringPrototype.h"
 #include "StrongInlines.h"

 namespace JSC {

 const ClassInfo JSString::s_info = { "string", nullptr, nullptr, nullptr, CREATE_METHOD_TABLE(JSString) };

 Structure* JSString::createStructure(VM& vm, JSGlobalObject* globalObject, JSValue proto)
 {
     return Structure::create(vm, globalObject, proto, TypeInfo(StringType, StructureFlags), info());
 }

 JSString* JSString::createEmptyString(VM& vm)
 {
     JSString* newString = new (NotNull, allocateCell<JSString>(vm.heap)) JSString(vm, *StringImpl::empty());
     newString->finishCreation(vm);
     return newString;
 }

 template<>
 void JSRopeString::RopeBuilder<RecordOverflow>::expand()
 {
     RELEASE_ASSERT(!this->hasOverflowed());
     ASSERT(m_strings.size() == JSRopeString::s_maxInternalRopeLength);
     static_assert(3 == JSRopeString::s_maxInternalRopeLength, "");
     ASSERT(m_length);
     ASSERT(asString(m_strings.at(0))->length());
     ASSERT(asString(m_strings.at(1))->length());
     ASSERT(asString(m_strings.at(2))->length());

     JSString* string = JSRopeString::create(m_vm, asString(m_strings.at(0)), asString(m_strings.at(1)), asString(m_strings.at(2)));
     ASSERT(string->length() == m_length);
     m_strings.clear();
     m_strings.append(string);
 }

 void JSString::dumpToStream(const JSCell* cell, PrintStream& out)
 {
     VM& vm = cell->vm();
     const JSString* thisObject = jsCast<const JSString*>(cell);
     out.printf("<%p, %s, [%u], ", thisObject, thisObject->className(vm), thisObject->length());
     uintptr_t pointer = thisObject->m_fiber;
     if (pointer & isRopeInPointer) {
         if (pointer & JSRopeString::isSubstringInPointer)
             out.printf("[substring]");
         else
             out.printf("[rope]");
     } else {
         if (WTF::StringImpl* ourImpl = bitwise_cast<StringImpl*>(pointer)) {
             if (ourImpl->is8Bit())
                 out.printf("[8 %p]", ourImpl->characters8());
             else
                 out.printf("[16 %p]", ourImpl->characters16());
         }
     }
     out.printf(">");
 }

 bool JSString::equalSlowCase(JSGlobalObject* globalObject, JSString* other) const
 {
     VM& vm = globalObject->vm();
     auto scope = DECLARE_THROW_SCOPE(vm);
     String str1 = value(globalObject);
     RETURN_IF_EXCEPTION(scope, false);
     String str2 = other->value(globalObject);
     RETURN_IF_EXCEPTION(scope, false);
     return WTF::equal(*str1.impl(), *str2.impl());
 }

 size_t JSString::estimatedSize(JSCell* cell, VM& vm)
 {
     JSString* thisObject = asString(cell);
     uintptr_t pointer = thisObject->m_fiber;
     if (pointer & isRopeInPointer)
         return Base::estimatedSize(cell, vm);
     return Base::estimatedSize(cell, vm) + bitwise_cast<StringImpl*>(pointer)->costDuringGC();
 }

 void JSString::visitChildren(JSCell* cell, SlotVisitor& visitor)
 {
     JSString* thisObject = asString(cell);
     ASSERT_GC_OBJECT_INHERITS(thisObject, info());
     Base::visitChildren(thisObject, visitor);

     uintptr_t pointer = thisObject->m_fiber;
     if (pointer & isRopeInPointer) {
         if (pointer & JSRopeString::isSubstringInPointer) {
             visitor.appendUnbarriered(static_cast<JSRopeString*>(thisObject)->fiber1());
             return;
         }
         for (unsigned index = 0; index < JSRopeString::s_maxInternalRopeLength; ++index) {
             JSString* fiber = nullptr;
             switch (index) {
             case 0:
                 fiber = bitwise_cast<JSString*>(pointer & JSRopeString::stringMask);
                 break;
             case 1:
                 fiber = static_cast<JSRopeString*>(thisObject)->fiber1();
                 break;
             case 2:
                 fiber = static_cast<JSRopeString*>(thisObject)->fiber2();
                 break;
             default:
                 ASSERT_NOT_REACHED();
                 return;
             }
             if (!fiber)
                 break;
             visitor.appendUnbarriered(fiber);
         }
         return;
     }
     if (StringImpl* impl = bitwise_cast<StringImpl*>(pointer))
         visitor.reportExtraMemoryVisited(impl->costDuringGC());
 }

 static constexpr unsigned maxLengthForOnStackResolve = 2048;

 void JSRopeString::resolveRopeInternal8(LChar* buffer) const
 {
     if (isSubstring()) {
         StringImpl::copyCharacters(buffer, substringBase()->valueInternal().characters8() + substringOffset(), length());
         return;
     }

     resolveRopeInternalNoSubstring(buffer);
 }

 void JSRopeString::resolveRopeInternal16(UChar* buffer) const
 {
     if (isSubstring()) {
         StringImpl::copyCharacters(
             buffer, substringBase()->valueInternal().characters16() + substringOffset(), length());
         return;
     }

     resolveRopeInternalNoSubstring(buffer);
 }

 template<typename CharacterType>
 void JSRopeString::resolveRopeInternalNoSubstring(CharacterType* buffer) const
 {
     for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i) {
         if (fiber(i)->isRope()) {
             resolveRopeSlowCase(buffer);
             return;
         }
     }

     CharacterType* position = buffer;
     for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i) {
         const StringImpl& fiberString = *fiber(i)->valueInternal().impl();
         unsigned length = fiberString.length();
         if (fiberString.is8Bit())
             StringImpl::copyCharacters(position, fiberString.characters8(), length);
         else
             StringImpl::copyCharacters(position, fiberString.characters16(), length);
         position += length;
     }
     ASSERT((buffer + length()) == position);
 }

 AtomString JSRopeString::resolveRopeToAtomString(JSGlobalObject* globalObject) const
 {
     VM& vm = globalObject->vm();
     auto scope = DECLARE_THROW_SCOPE(vm);

     if (length() > maxLengthForOnStackResolve) {
         scope.release();
         return resolveRopeWithFunction(globalObject, [&] (Ref<StringImpl>&& newImpl) {
             return AtomStringImpl::add(newImpl.ptr());
         });
     }

     if (is8Bit()) {
         LChar buffer[maxLengthForOnStackResolve];
         resolveRopeInternal8(buffer);
         convertToNonRope(AtomStringImpl::add(buffer, length()));
     } else {
         UChar buffer[maxLengthForOnStackResolve];
         resolveRopeInternal16(buffer);
         convertToNonRope(AtomStringImpl::add(buffer, length()));
     }

     // If we resolved a string that didn't previously exist, notify the heap that we've grown.
     if (valueInternal().impl()->hasOneRef())
         vm.heap.reportExtraMemoryAllocated(valueInternal().impl()->cost());
     return valueInternal();
 }

 inline void JSRopeString::convertToNonRope(String&& string) const
 {
     // Concurrent compiler threads can access String held by JSString. So we always emit
     // store-store barrier here to ensure concurrent compiler threads see initialized String.
     ASSERT(JSString::isRope());
     WTF::storeStoreFence();
     new (&uninitializedValueInternal()) String(WTFMove(string));
     static_assert(sizeof(String) == sizeof(RefPtr<StringImpl>), "JSString's String initialization must be done in one pointer move.");
     // We do not clear the trailing fibers and length information (fiber1 and fiber2) because we could be reading the length concurrently.
     ASSERT(!JSString::isRope());
 }

 RefPtr<AtomStringImpl> JSRopeString::resolveRopeToExistingAtomString(JSGlobalObject* globalObject) const
 {
     VM& vm = globalObject->vm();
     auto scope = DECLARE_THROW_SCOPE(vm);

     if (length() > maxLengthForOnStackResolve) {
         RefPtr<AtomStringImpl> existingAtomString;
         resolveRopeWithFunction(globalObject, [&] (Ref<StringImpl>&& newImpl) -> Ref<StringImpl> {
             existingAtomString = AtomStringImpl::lookUp(newImpl.ptr());
             if (existingAtomString)
                 return makeRef(*existingAtomString);
             return WTFMove(newImpl);
         });
         RETURN_IF_EXCEPTION(scope, nullptr);
         return existingAtomString;
     }

     if (is8Bit()) {
         LChar buffer[maxLengthForOnStackResolve];
         resolveRopeInternal8(buffer);
         if (RefPtr<AtomStringImpl> existingAtomString = AtomStringImpl::lookUp(buffer, length())) {
             convertToNonRope(*existingAtomString);
             return existingAtomString;
         }
     } else {
         UChar buffer[maxLengthForOnStackResolve];
         resolveRopeInternal16(buffer);
         if (RefPtr<AtomStringImpl> existingAtomString = AtomStringImpl::lookUp(buffer, length())) {
             convertToNonRope(*existingAtomString);
             return existingAtomString;
         }
     }

     return nullptr;
 }

 template<typename Function>
 const String& JSRopeString::resolveRopeWithFunction(JSGlobalObject* nullOrGlobalObjectForOOM, Function&& function) const
 {
     ASSERT(isRope());

     VM& vm = this->vm();
     if (isSubstring()) {
         ASSERT(!substringBase()->isRope());
         auto newImpl = substringBase()->valueInternal().substringSharingImpl(substringOffset(), length());
         convertToNonRope(function(newImpl.releaseImpl().releaseNonNull()));
         return valueInternal();
     }

     if (is8Bit()) {
         LChar* buffer;
         auto newImpl = StringImpl::tryCreateUninitialized(length(), buffer);
         if (!newImpl) {
             outOfMemory(nullOrGlobalObjectForOOM);
             return nullString();
         }
         vm.heap.reportExtraMemoryAllocated(newImpl->cost());

         resolveRopeInternalNoSubstring(buffer);
         convertToNonRope(function(newImpl.releaseNonNull()));
         return valueInternal();
     }

     UChar* buffer;
     auto newImpl = StringImpl::tryCreateUninitialized(length(), buffer);
     if (!newImpl) {
         outOfMemory(nullOrGlobalObjectForOOM);
         return nullString();
     }
     vm.heap.reportExtraMemoryAllocated(newImpl->cost());

     resolveRopeInternalNoSubstring(buffer);
     convertToNonRope(function(newImpl.releaseNonNull()));
     return valueInternal();
 }

 const String& JSRopeString::resolveRope(JSGlobalObject* nullOrGlobalObjectForOOM) const
 {
     return resolveRopeWithFunction(nullOrGlobalObjectForOOM, [] (Ref<StringImpl>&& newImpl) {
         return WTFMove(newImpl);
     });
 }

 // Overview: These functions convert a JSString from holding a string in rope form
 // down to a simple String representation. It does so by building up the string
 // backwards, since we want to avoid recursion, we expect that the tree structure
 // representing the rope is likely imbalanced with more nodes down the left side
 // (since appending to the string is likely more common) - and as such resolving
 // in this fashion should minimize work queue size.  (If we built the queue forwards
 // we would likely have to place all of the constituent StringImpls into the
 // Vector before performing any concatenation, but by working backwards we likely
 // only fill the queue with the number of substrings at any given level in a
 // rope-of-ropes.)
 template<typename CharacterType>
 void JSRopeString::resolveRopeSlowCase(CharacterType* buffer) const
 {
     CharacterType* position = buffer + length(); // We will be working backwards over the rope.
     Vector<JSString*, 32, UnsafeVectorOverflow> workQueue; // These strings are kept alive by the parent rope, so using a Vector is OK.

     for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i)
         workQueue.append(fiber(i));

     while (!workQueue.isEmpty()) {
         JSString* currentFiber = workQueue.last();
         workQueue.removeLast();

         if (currentFiber->isRope()) {
             JSRopeString* currentFiberAsRope = static_cast<JSRopeString*>(currentFiber);
             if (currentFiberAsRope->isSubstring()) {
                 ASSERT(!currentFiberAsRope->substringBase()->isRope());
                 StringImpl* string = static_cast<StringImpl*>(
                     currentFiberAsRope->substringBase()->valueInternal().impl());
                 unsigned offset = currentFiberAsRope->substringOffset();
                 unsigned length = currentFiberAsRope->length();
                 position -= length;
                 if (string->is8Bit())
                     StringImpl::copyCharacters(position, string->characters8() + offset, length);
                 else
                     StringImpl::copyCharacters(position, string->characters16() + offset, length);
                 continue;
             }
             for (size_t i = 0; i < s_maxInternalRopeLength && currentFiberAsRope->fiber(i); ++i)
                 workQueue.append(currentFiberAsRope->fiber(i));
             continue;
         }

         StringImpl* string = static_cast<StringImpl*>(currentFiber->valueInternal().impl());
         unsigned length = string->length();
         position -= length;
         if (string->is8Bit())
             StringImpl::copyCharacters(position, string->characters8(), length);
         else
             StringImpl::copyCharacters(position, string->characters16(), length);
     }

     ASSERT(buffer == position);
 }

 void JSRopeString::outOfMemory(JSGlobalObject* nullOrGlobalObjectForOOM) const
 {
     ASSERT(isRope());
     if (nullOrGlobalObjectForOOM) {
         VM& vm = nullOrGlobalObjectForOOM->vm();
         auto scope = DECLARE_THROW_SCOPE(vm);
         throwOutOfMemoryError(nullOrGlobalObjectForOOM, scope);
     }
 }

 JSValue JSString::toPrimitive(JSGlobalObject*, PreferredPrimitiveType) const
 {
     return const_cast<JSString*>(this);
 }

 bool JSString::getPrimitiveNumber(JSGlobalObject* globalObject, double& number, JSValue& result) const
 {
     VM& vm = globalObject->vm();
     auto scope = DECLARE_THROW_SCOPE(vm);
     StringView view = unsafeView(globalObject);
     RETURN_IF_EXCEPTION(scope, false);
     result = this;
     number = jsToNumber(view);
     return false;
 }

 double JSString::toNumber(JSGlobalObject* globalObject) const
 {
     VM& vm = globalObject->vm();
     auto scope = DECLARE_THROW_SCOPE(vm);
     StringView view = unsafeView(globalObject);
     RETURN_IF_EXCEPTION(scope, 0);
     return jsToNumber(view);
 }

 inline StringObject* StringObject::create(VM& vm, JSGlobalObject* globalObject, JSString* string)
 {
     StringObject* object = new (NotNull, allocateCell<StringObject>(vm.heap)) StringObject(vm, globalObject->stringObjectStructure());
     object->finishCreation(vm, string);
     return object;
 }

 JSObject* JSString::toObject(JSGlobalObject* globalObject) const
 {
     return StringObject::create(globalObject->vm(), globalObject, const_cast<JSString*>(this));
 }

 JSValue JSString::toThis(JSCell* cell, JSGlobalObject* globalObject, ECMAMode ecmaMode)
 {
     if (ecmaMode == StrictMode)
         return cell;
     return StringObject::create(globalObject->vm(), globalObject, asString(cell));
 }

 bool JSString::getStringPropertyDescriptor(JSGlobalObject* globalObject, PropertyName propertyName, PropertyDescriptor& descriptor)
 {
     VM& vm = globalObject->vm();
     if (propertyName == vm.propertyNames->length) {
         descriptor.setDescriptor(jsNumber(length()), PropertyAttribute::DontEnum | PropertyAttribute::DontDelete | PropertyAttribute::ReadOnly);
         return true;
     }

     Optional<uint32_t> index = parseIndex(propertyName);
     if (index && index.value() < length()) {
         descriptor.setDescriptor(getIndex(globalObject, index.value()), PropertyAttribute::DontDelete | PropertyAttribute::ReadOnly);
         return true;
     }

     return false;
 }

 JSString* jsStringWithCacheSlowCase(VM& vm, StringImpl& stringImpl)
 {
     if (JSString* string = vm.stringCache.get(&stringImpl))
         return string;

     JSString* string = jsString(vm, String(stringImpl));
     vm.lastCachedString.set(vm, string);
     return string;
 }

 } // namespace JSC
	/*
	* Copyright (C) 1999-2002 Harri Porten (porten@kde.org)
	* Copyright (C) 2001 Peter Kelly (pmk@post.com)
	* Copyright (C) 2004-2019 Apple Inc. All rights reserved.
	*
	* This library is free software; you can redistribute it and/or
	* modify it under the terms of the GNU Library General Public
	* License as published by the Free Software Foundation; either
	* version 2 of the License, or (at your option) any later version.
	*
	* This library is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
	* Library General Public License for more details.
	*
	* You should have received a copy of the GNU Library General Public License
	* along with this library; see the file COPYING.LIB. If not, write to
	* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
	* Boston, MA 02110-1301, USA.
	*
	*/

	#include "config.h"
	#include "JSString.h"

	#include "JSGlobalObject.h"
	#include "JSGlobalObjectFunctions.h"
	#include "JSObject.h"
	#include "JSCInlines.h"
	#include "StringObject.h"
	#include "StringPrototype.h"
	#include "StrongInlines.h"

	namespace JSC {

	const ClassInfo JSString::s_info = { "string", nullptr, nullptr, nullptr, CREATE_METHOD_TABLE(JSString) };

	Structure* JSString::createStructure(VM& vm, JSGlobalObject* globalObject, JSValue proto)
	{
	return Structure::create(vm, globalObject, proto, TypeInfo(StringType, StructureFlags), info());
	}

	JSString* JSString::createEmptyString(VM& vm)
	{
	JSString* newString = new (NotNull, allocateCell<JSString>(vm.heap)) JSString(vm, *StringImpl::empty());
	newString->finishCreation(vm);
	return newString;
	}

	template<>
	void JSRopeString::RopeBuilder<RecordOverflow>::expand()
	{
	RELEASE_ASSERT(!this->hasOverflowed());
	ASSERT(m_strings.size() == JSRopeString::s_maxInternalRopeLength);
	static_assert(3 == JSRopeString::s_maxInternalRopeLength, "");
	ASSERT(m_length);
	ASSERT(asString(m_strings.at(0))->length());
	ASSERT(asString(m_strings.at(1))->length());
	ASSERT(asString(m_strings.at(2))->length());

	JSString* string = JSRopeString::create(m_vm, asString(m_strings.at(0)), asString(m_strings.at(1)), asString(m_strings.at(2)));
	ASSERT(string->length() == m_length);
	m_strings.clear();
	m_strings.append(string);
	}

	void JSString::dumpToStream(const JSCell* cell, PrintStream& out)
	{
	VM& vm = cell->vm();
	const JSString* thisObject = jsCast<const JSString*>(cell);
	out.printf("<%p, %s, [%u], ", thisObject, thisObject->className(vm), thisObject->length());
	uintptr_t pointer = thisObject->m_fiber;
	if (pointer & isRopeInPointer) {
	if (pointer & JSRopeString::isSubstringInPointer)
	out.printf("[substring]");
	else
	out.printf("[rope]");
	} else {
	if (WTF::StringImpl* ourImpl = bitwise_cast<StringImpl*>(pointer)) {
	if (ourImpl->is8Bit())
	out.printf("[8 %p]", ourImpl->characters8());
	else
	out.printf("[16 %p]", ourImpl->characters16());
	}
	}
	out.printf(">");
	}

	bool JSString::equalSlowCase(JSGlobalObject* globalObject, JSString* other) const
	{
	VM& vm = globalObject->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);
	String str1 = value(globalObject);
	RETURN_IF_EXCEPTION(scope, false);
	String str2 = other->value(globalObject);
	RETURN_IF_EXCEPTION(scope, false);
	return WTF::equal(str1.impl(), str2.impl());
	}

	size_t JSString::estimatedSize(JSCell* cell, VM& vm)
	{
	JSString* thisObject = asString(cell);
	uintptr_t pointer = thisObject->m_fiber;
	if (pointer & isRopeInPointer)
	return Base::estimatedSize(cell, vm);
	return Base::estimatedSize(cell, vm) + bitwise_cast<StringImpl*>(pointer)->costDuringGC();
	}

	void JSString::visitChildren(JSCell* cell, SlotVisitor& visitor)
	{
	JSString* thisObject = asString(cell);
	ASSERT_GC_OBJECT_INHERITS(thisObject, info());
	Base::visitChildren(thisObject, visitor);

	uintptr_t pointer = thisObject->m_fiber;
	if (pointer & isRopeInPointer) {
	if (pointer & JSRopeString::isSubstringInPointer) {
	visitor.appendUnbarriered(static_cast<JSRopeString*>(thisObject)->fiber1());
	return;
	}
	for (unsigned index = 0; index < JSRopeString::s_maxInternalRopeLength; ++index) {
	JSString* fiber = nullptr;
	switch (index) {
	case 0:
	fiber = bitwise_cast<JSString*>(pointer & JSRopeString::stringMask);
	break;
	case 1:
	fiber = static_cast<JSRopeString*>(thisObject)->fiber1();
	break;
	case 2:
	fiber = static_cast<JSRopeString*>(thisObject)->fiber2();
	break;
	default:
	ASSERT_NOT_REACHED();
	return;
	}
	if (!fiber)
	break;
	visitor.appendUnbarriered(fiber);
	}
	return;
	}
	if (StringImpl* impl = bitwise_cast<StringImpl*>(pointer))
	visitor.reportExtraMemoryVisited(impl->costDuringGC());
	}

	static constexpr unsigned maxLengthForOnStackResolve = 2048;

	void JSRopeString::resolveRopeInternal8(LChar* buffer) const
	{
	if (isSubstring()) {
	StringImpl::copyCharacters(buffer, substringBase()->valueInternal().characters8() + substringOffset(), length());
	return;
	}

	resolveRopeInternalNoSubstring(buffer);
	}

	void JSRopeString::resolveRopeInternal16(UChar* buffer) const
	{
	if (isSubstring()) {
	StringImpl::copyCharacters(
	buffer, substringBase()->valueInternal().characters16() + substringOffset(), length());
	return;
	}

	resolveRopeInternalNoSubstring(buffer);
	}

	template<typename CharacterType>
	void JSRopeString::resolveRopeInternalNoSubstring(CharacterType* buffer) const
	{
	for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i) {
	if (fiber(i)->isRope()) {
	resolveRopeSlowCase(buffer);
	return;
	}
	}

	CharacterType* position = buffer;
	for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i) {
	const StringImpl& fiberString = *fiber(i)->valueInternal().impl();
	unsigned length = fiberString.length();
	if (fiberString.is8Bit())
	StringImpl::copyCharacters(position, fiberString.characters8(), length);
	else
	StringImpl::copyCharacters(position, fiberString.characters16(), length);
	position += length;
	}
	ASSERT((buffer + length()) == position);
	}

	AtomString JSRopeString::resolveRopeToAtomString(JSGlobalObject* globalObject) const
	{
	VM& vm = globalObject->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);

	if (length() > maxLengthForOnStackResolve) {
	scope.release();
	return resolveRopeWithFunction(globalObject, [&] (Ref<StringImpl>&& newImpl) {
	return AtomStringImpl::add(newImpl.ptr());
	});
	}

	if (is8Bit()) {
	LChar buffer[maxLengthForOnStackResolve];
	resolveRopeInternal8(buffer);
	convertToNonRope(AtomStringImpl::add(buffer, length()));
	} else {
	UChar buffer[maxLengthForOnStackResolve];
	resolveRopeInternal16(buffer);
	convertToNonRope(AtomStringImpl::add(buffer, length()));
	}

	// If we resolved a string that didn't previously exist, notify the heap that we've grown.
	if (valueInternal().impl()->hasOneRef())
	vm.heap.reportExtraMemoryAllocated(valueInternal().impl()->cost());
	return valueInternal();
	}

	inline void JSRopeString::convertToNonRope(String&& string) const
	{
	// Concurrent compiler threads can access String held by JSString. So we always emit
	// store-store barrier here to ensure concurrent compiler threads see initialized String.
	ASSERT(JSString::isRope());
	WTF::storeStoreFence();
	new (&uninitializedValueInternal()) String(WTFMove(string));
	static_assert(sizeof(String) == sizeof(RefPtr<StringImpl>), "JSString's String initialization must be done in one pointer move.");
	// We do not clear the trailing fibers and length information (fiber1 and fiber2) because we could be reading the length concurrently.
	ASSERT(!JSString::isRope());
	}

	RefPtr<AtomStringImpl> JSRopeString::resolveRopeToExistingAtomString(JSGlobalObject* globalObject) const
	{
	VM& vm = globalObject->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);

	if (length() > maxLengthForOnStackResolve) {
	RefPtr<AtomStringImpl> existingAtomString;
	resolveRopeWithFunction(globalObject, [&] (Ref<StringImpl>&& newImpl) -> Ref<StringImpl> {
	existingAtomString = AtomStringImpl::lookUp(newImpl.ptr());
	if (existingAtomString)
	return makeRef(*existingAtomString);
	return WTFMove(newImpl);
	});
	RETURN_IF_EXCEPTION(scope, nullptr);
	return existingAtomString;
	}

	if (is8Bit()) {
	LChar buffer[maxLengthForOnStackResolve];
	resolveRopeInternal8(buffer);
	if (RefPtr<AtomStringImpl> existingAtomString = AtomStringImpl::lookUp(buffer, length())) {
	convertToNonRope(*existingAtomString);
	return existingAtomString;
	}
	} else {
	UChar buffer[maxLengthForOnStackResolve];
	resolveRopeInternal16(buffer);
	if (RefPtr<AtomStringImpl> existingAtomString = AtomStringImpl::lookUp(buffer, length())) {
	convertToNonRope(*existingAtomString);
	return existingAtomString;
	}
	}

	return nullptr;
	}

	template<typename Function>
	const String& JSRopeString::resolveRopeWithFunction(JSGlobalObject* nullOrGlobalObjectForOOM, Function&& function) const
	{
	ASSERT(isRope());

	VM& vm = this->vm();
	if (isSubstring()) {
	ASSERT(!substringBase()->isRope());
	auto newImpl = substringBase()->valueInternal().substringSharingImpl(substringOffset(), length());
	convertToNonRope(function(newImpl.releaseImpl().releaseNonNull()));
	return valueInternal();
	}

	if (is8Bit()) {
	LChar* buffer;
	auto newImpl = StringImpl::tryCreateUninitialized(length(), buffer);
	if (!newImpl) {
	outOfMemory(nullOrGlobalObjectForOOM);
	return nullString();
	}
	vm.heap.reportExtraMemoryAllocated(newImpl->cost());

	resolveRopeInternalNoSubstring(buffer);
	convertToNonRope(function(newImpl.releaseNonNull()));
	return valueInternal();
	}

	UChar* buffer;
	auto newImpl = StringImpl::tryCreateUninitialized(length(), buffer);
	if (!newImpl) {
	outOfMemory(nullOrGlobalObjectForOOM);
	return nullString();
	}
	vm.heap.reportExtraMemoryAllocated(newImpl->cost());

	resolveRopeInternalNoSubstring(buffer);
	convertToNonRope(function(newImpl.releaseNonNull()));
	return valueInternal();
	}

	const String& JSRopeString::resolveRope(JSGlobalObject* nullOrGlobalObjectForOOM) const
	{
	return resolveRopeWithFunction(nullOrGlobalObjectForOOM, [] (Ref<StringImpl>&& newImpl) {
	return WTFMove(newImpl);
	});
	}

	// Overview: These functions convert a JSString from holding a string in rope form
	// down to a simple String representation. It does so by building up the string
	// backwards, since we want to avoid recursion, we expect that the tree structure
	// representing the rope is likely imbalanced with more nodes down the left side
	// (since appending to the string is likely more common) - and as such resolving
	// in this fashion should minimize work queue size. (If we built the queue forwards
	// we would likely have to place all of the constituent StringImpls into the
	// Vector before performing any concatenation, but by working backwards we likely
	// only fill the queue with the number of substrings at any given level in a
	// rope-of-ropes.)
	template<typename CharacterType>
	void JSRopeString::resolveRopeSlowCase(CharacterType* buffer) const
	{
	CharacterType* position = buffer + length(); // We will be working backwards over the rope.
	Vector<JSString*, 32, UnsafeVectorOverflow> workQueue; // These strings are kept alive by the parent rope, so using a Vector is OK.

	for (size_t i = 0; i < s_maxInternalRopeLength && fiber(i); ++i)
	workQueue.append(fiber(i));

	while (!workQueue.isEmpty()) {
	JSString* currentFiber = workQueue.last();
	workQueue.removeLast();

	if (currentFiber->isRope()) {
	JSRopeString* currentFiberAsRope = static_cast<JSRopeString*>(currentFiber);
	if (currentFiberAsRope->isSubstring()) {
	ASSERT(!currentFiberAsRope->substringBase()->isRope());
	StringImpl* string = static_cast<StringImpl*>(
	currentFiberAsRope->substringBase()->valueInternal().impl());
	unsigned offset = currentFiberAsRope->substringOffset();
	unsigned length = currentFiberAsRope->length();
	position -= length;
	if (string->is8Bit())
	StringImpl::copyCharacters(position, string->characters8() + offset, length);
	else
	StringImpl::copyCharacters(position, string->characters16() + offset, length);
	continue;
	}
	for (size_t i = 0; i < s_maxInternalRopeLength && currentFiberAsRope->fiber(i); ++i)
	workQueue.append(currentFiberAsRope->fiber(i));
	continue;
	}

	StringImpl* string = static_cast<StringImpl*>(currentFiber->valueInternal().impl());
	unsigned length = string->length();
	position -= length;
	if (string->is8Bit())
	StringImpl::copyCharacters(position, string->characters8(), length);
	else
	StringImpl::copyCharacters(position, string->characters16(), length);
	}

	ASSERT(buffer == position);
	}

	void JSRopeString::outOfMemory(JSGlobalObject* nullOrGlobalObjectForOOM) const
	{
	ASSERT(isRope());
	if (nullOrGlobalObjectForOOM) {
	VM& vm = nullOrGlobalObjectForOOM->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);
	throwOutOfMemoryError(nullOrGlobalObjectForOOM, scope);
	}
	}

	JSValue JSString::toPrimitive(JSGlobalObject*, PreferredPrimitiveType) const
	{
	return const_cast<JSString*>(this);
	}

	bool JSString::getPrimitiveNumber(JSGlobalObject* globalObject, double& number, JSValue& result) const
	{
	VM& vm = globalObject->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);
	StringView view = unsafeView(globalObject);
	RETURN_IF_EXCEPTION(scope, false);
	result = this;
	number = jsToNumber(view);
	return false;
	}

	double JSString::toNumber(JSGlobalObject* globalObject) const
	{
	VM& vm = globalObject->vm();
	auto scope = DECLARE_THROW_SCOPE(vm);
	StringView view = unsafeView(globalObject);
	RETURN_IF_EXCEPTION(scope, 0);
	return jsToNumber(view);
	}

	inline StringObject* StringObject::create(VM& vm, JSGlobalObject* globalObject, JSString* string)
	{
	StringObject* object = new (NotNull, allocateCell<StringObject>(vm.heap)) StringObject(vm, globalObject->stringObjectStructure());
	object->finishCreation(vm, string);
	return object;
	}

	JSObject* JSString::toObject(JSGlobalObject* globalObject) const
	{
	return StringObject::create(globalObject->vm(), globalObject, const_cast<JSString*>(this));
	}

	JSValue JSString::toThis(JSCell* cell, JSGlobalObject* globalObject, ECMAMode ecmaMode)
	{
	if (ecmaMode == StrictMode)
	return cell;
	return StringObject::create(globalObject->vm(), globalObject, asString(cell));
	}

	bool JSString::getStringPropertyDescriptor(JSGlobalObject* globalObject, PropertyName propertyName, PropertyDescriptor& descriptor)
	{
	VM& vm = globalObject->vm();
	if (propertyName == vm.propertyNames->length) {
	descriptor.setDescriptor(jsNumber(length()), PropertyAttribute::DontEnum \| PropertyAttribute::DontDelete \| PropertyAttribute::ReadOnly);
	return true;
	}

	Optional<uint32_t> index = parseIndex(propertyName);
	if (index && index.value() < length()) {
	descriptor.setDescriptor(getIndex(globalObject, index.value()), PropertyAttribute::DontDelete \| PropertyAttribute::ReadOnly);
	return true;
	}

	return false;
	}

	JSString* jsStringWithCacheSlowCase(VM& vm, StringImpl& stringImpl)
	{
	if (JSString* string = vm.stringCache.get(&stringImpl))
	return string;

	JSString* string = jsString(vm, String(stringImpl));
	vm.lastCachedString.set(vm, string);
	return string;
	}

	} // namespace JSC