JSArray.cpp 47.3 KB
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/*
 *  Copyright (C) 1999-2000 Harri Porten (porten@kde.org)
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 *  Copyright (C) 2003, 2007, 2008, 2009 Apple Inc. All rights reserved.
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 *  Copyright (C) 2003 Peter Kelly (pmk@post.com)
 *  Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser 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
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *
 */

#include "config.h"
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#include "JSArray.h"
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#include "ArrayPrototype.h"
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#include "CachedCall.h"
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#include "Error.h"
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#include "Executable.h"
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#include "PropertyNameArray.h"
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#include <wtf/AVLTree.h>
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#include <wtf/Assertions.h>
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#include <wtf/OwnPtr.h>
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#include <Operations.h>
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using namespace std;
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using namespace WTF;
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namespace JSC {
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ASSERT_CLASS_FITS_IN_CELL(JSArray);

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// Overview of JSArray
//
// Properties of JSArray objects may be stored in one of three locations:
//   * The regular JSObject property map.
//   * A storage vector.
//   * A sparse map of array entries.
//
// Properties with non-numeric identifiers, with identifiers that are not representable
// as an unsigned integer, or where the value is greater than  MAX_ARRAY_INDEX
// (specifically, this is only one property - the value 0xFFFFFFFFU as an unsigned 32-bit
// integer) are not considered array indices and will be stored in the JSObject property map.
//
// All properties with a numeric identifer, representable as an unsigned integer i,
// where (i <= MAX_ARRAY_INDEX), are an array index and will be stored in either the
// storage vector or the sparse map.  An array index i will be handled in the following
// fashion:
//
//   * Where (i < MIN_SPARSE_ARRAY_INDEX) the value will be stored in the storage vector.
//   * Where (MIN_SPARSE_ARRAY_INDEX <= i <= MAX_STORAGE_VECTOR_INDEX) the value will either
//     be stored in the storage vector or in the sparse array, depending on the density of
//     data that would be stored in the vector (a vector being used where at least
//     (1 / minDensityMultiplier) of the entries would be populated).
//   * Where (MAX_STORAGE_VECTOR_INDEX < i <= MAX_ARRAY_INDEX) the value will always be stored
//     in the sparse array.

// The definition of MAX_STORAGE_VECTOR_LENGTH is dependant on the definition storageSize
// function below - the MAX_STORAGE_VECTOR_LENGTH limit is defined such that the storage
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// size calculation cannot overflow.  (sizeof(ArrayStorage) - sizeof(JSValue)) +
// (vectorLength * sizeof(JSValue)) must be <= 0xFFFFFFFFU (which is maximum value of size_t).
#define MAX_STORAGE_VECTOR_LENGTH static_cast<unsigned>((0xFFFFFFFFU - (sizeof(ArrayStorage) - sizeof(JSValue))) / sizeof(JSValue))
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// These values have to be macros to be used in max() and min() without introducing
// a PIC branch in Mach-O binaries, see <rdar://problem/5971391>.
#define MIN_SPARSE_ARRAY_INDEX 10000U
#define MAX_STORAGE_VECTOR_INDEX (MAX_STORAGE_VECTOR_LENGTH - 1)
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// 0xFFFFFFFF is a bit weird -- is not an array index even though it's an integer.
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#define MAX_ARRAY_INDEX 0xFFFFFFFEU
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// The value BASE_VECTOR_LEN is the maximum number of vector elements we'll allocate
// for an array that was created with a sepcified length (e.g. a = new Array(123))
#define BASE_VECTOR_LEN 4U
    
// The upper bound to the size we'll grow a zero length array when the first element
// is added.
#define FIRST_VECTOR_GROW 4U

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// Our policy for when to use a vector and when to use a sparse map.
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// For all array indices under MIN_SPARSE_ARRAY_INDEX, we always use a vector.
// When indices greater than MIN_SPARSE_ARRAY_INDEX are involved, we use a vector
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// as long as it is 1/8 full. If more sparse than that, we use a map.
static const unsigned minDensityMultiplier = 8;

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const ClassInfo JSArray::s_info = {"Array", &JSNonFinalObject::s_info, 0, 0, CREATE_METHOD_TABLE(JSArray)};
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// We keep track of the size of the last array after it was grown.  We use this
// as a simple heuristic for as the value to grow the next array from size 0.
// This value is capped by the constant FIRST_VECTOR_GROW defined above.
static unsigned lastArraySize = 0;

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static inline size_t storageSize(unsigned vectorLength)
{
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    ASSERT(vectorLength <= MAX_STORAGE_VECTOR_LENGTH);

    // MAX_STORAGE_VECTOR_LENGTH is defined such that provided (vectorLength <= MAX_STORAGE_VECTOR_LENGTH)
    // - as asserted above - the following calculation cannot overflow.
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    size_t size = (sizeof(ArrayStorage) - sizeof(JSValue)) + (vectorLength * sizeof(JSValue));
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    // Assertion to detect integer overflow in previous calculation (should not be possible, provided that
    // MAX_STORAGE_VECTOR_LENGTH is correctly defined).
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    ASSERT(((size - (sizeof(ArrayStorage) - sizeof(JSValue))) / sizeof(JSValue) == vectorLength) && (size >= (sizeof(ArrayStorage) - sizeof(JSValue))));
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    return size;
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}

static inline bool isDenseEnoughForVector(unsigned length, unsigned numValues)
{
    return length / minDensityMultiplier <= numValues;
}

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#if !CHECK_ARRAY_CONSISTENCY

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inline void JSArray::checkConsistency(ConsistencyCheckType)
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{
}

#endif

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JSArray::JSArray(VPtrStealingHackType)
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    : JSNonFinalObject(VPtrStealingHack)
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{
}

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JSArray::JSArray(JSGlobalData& globalData, Structure* structure)
    : JSNonFinalObject(globalData, structure)
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{
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}

void JSArray::finishCreation(JSGlobalData& globalData)
{
    Base::finishCreation(globalData);
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    ASSERT(inherits(&s_info));

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    unsigned initialCapacity = 0;

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    m_storage = static_cast<ArrayStorage*>(fastZeroedMalloc(storageSize(initialCapacity)));
    m_storage->m_allocBase = m_storage;
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    m_indexBias = 0;
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    m_vectorLength = initialCapacity;
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    checkConsistency();
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(0));
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}

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void JSArray::finishCreation(JSGlobalData& globalData, unsigned initialLength, ArrayCreationMode creationMode)
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{
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    Base::finishCreation(globalData);
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    ASSERT(inherits(&s_info));

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    unsigned initialCapacity;
    if (creationMode == CreateCompact)
        initialCapacity = initialLength;
    else
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        initialCapacity = min(BASE_VECTOR_LEN, MIN_SPARSE_ARRAY_INDEX);
    
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    m_storage = static_cast<ArrayStorage*>(fastMalloc(storageSize(initialCapacity)));
    m_storage->m_allocBase = m_storage;
    m_storage->m_length = initialLength;
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    m_indexBias = 0;
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    m_vectorLength = initialCapacity;
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    m_storage->m_sparseValueMap = 0;
    m_storage->subclassData = 0;
    m_storage->reportedMapCapacity = 0;
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    if (creationMode == CreateCompact) {
#if CHECK_ARRAY_CONSISTENCY
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        m_storage->m_inCompactInitialization = !!initialCapacity;
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#endif
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        m_storage->m_length = 0;
        m_storage->m_numValuesInVector = initialCapacity;
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    } else {
#if CHECK_ARRAY_CONSISTENCY
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        storage->m_inCompactInitialization = false;
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#endif
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        m_storage->m_length = initialLength;
        m_storage->m_numValuesInVector = 0;
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        WriteBarrier<Unknown>* vector = m_storage->m_vector;
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        for (size_t i = 0; i < initialCapacity; ++i)
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            vector[i].clear();
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    }
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    checkConsistency();
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(initialCapacity));
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}

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void JSArray::finishCreation(JSGlobalData& globalData, const ArgList& list)
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{
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    Base::finishCreation(globalData);
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    ASSERT(inherits(&s_info));

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    unsigned initialCapacity = list.size();
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    unsigned initialStorage;
    
    // If the ArgList is empty, allocate space for 3 entries.  This value empirically
    // works well for benchmarks.
    if (!initialCapacity)
        initialStorage = 3;
    else
        initialStorage = initialCapacity;
    
    m_storage = static_cast<ArrayStorage*>(fastMalloc(storageSize(initialStorage)));
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    m_storage->m_allocBase = m_storage;
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    m_indexBias = 0;
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    m_storage->m_length = initialCapacity;
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    m_vectorLength = initialStorage;
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    m_storage->m_numValuesInVector = initialCapacity;
    m_storage->m_sparseValueMap = 0;
    m_storage->subclassData = 0;
    m_storage->reportedMapCapacity = 0;
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#if CHECK_ARRAY_CONSISTENCY
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    m_storage->m_inCompactInitialization = false;
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#endif
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    size_t i = 0;
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    WriteBarrier<Unknown>* vector = m_storage->m_vector;
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    ArgList::const_iterator end = list.end();
    for (ArgList::const_iterator it = list.begin(); it != end; ++it, ++i)
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        vector[i].set(globalData, this, *it);
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    for (; i < initialStorage; i++)
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        vector[i].clear();
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    checkConsistency();
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(initialStorage));
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}

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JSArray::~JSArray()
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{
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    ASSERT(vptr() == JSGlobalData::jsArrayVPtr);
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    checkConsistency(DestructorConsistencyCheck);

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    delete m_storage->m_sparseValueMap;
    fastFree(m_storage->m_allocBase);
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}

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bool JSArray::getOwnPropertySlotByIndex(JSCell* cell, ExecState* exec, unsigned i, PropertySlot& slot)
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{
    JSArray* thisObject = static_cast<JSArray*>(cell);
    ArrayStorage* storage = thisObject->m_storage;
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    if (i >= storage->m_length) {
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        if (i > MAX_ARRAY_INDEX)
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            return thisObject->methodTable()->getOwnPropertySlot(thisObject, exec, Identifier::from(exec, i), slot);
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        return false;
    }

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    if (i < thisObject->m_vectorLength) {
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        JSValue value = storage->m_vector[i].get();
        if (value) {
            slot.setValue(value);
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            return true;
        }
    } else if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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        if (i >= MIN_SPARSE_ARRAY_INDEX) {
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            SparseArrayValueMap::iterator it = map->find(i);
            if (it != map->end()) {
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                slot.setValue(it->second.get());
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                return true;
            }
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        }
    }

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    return JSObject::getOwnPropertySlot(thisObject, exec, Identifier::from(exec, i), slot);
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}

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bool JSArray::getOwnPropertySlot(JSCell* cell, ExecState* exec, const Identifier& propertyName, PropertySlot& slot)
{
    JSArray* thisObject = static_cast<JSArray*>(cell);
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    if (propertyName == exec->propertyNames().length) {
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        slot.setValue(jsNumber(thisObject->length()));
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        return true;
    }

    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex)
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        return JSArray::getOwnPropertySlotByIndex(thisObject, exec, i, slot);
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    return JSObject::getOwnPropertySlot(thisObject, exec, propertyName, slot);
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}

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bool JSArray::getOwnPropertyDescriptor(ExecState* exec, const Identifier& propertyName, PropertyDescriptor& descriptor)
{
    if (propertyName == exec->propertyNames().length) {
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        descriptor.setDescriptor(jsNumber(length()), DontDelete | DontEnum);
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        return true;
    }
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    ArrayStorage* storage = m_storage;
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex) {
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        if (i >= storage->m_length)
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            return false;
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        if (i < m_vectorLength) {
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            WriteBarrier<Unknown>& value = storage->m_vector[i];
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            if (value) {
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                descriptor.setDescriptor(value.get(), 0);
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                return true;
            }
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        } else if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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            if (i >= MIN_SPARSE_ARRAY_INDEX) {
                SparseArrayValueMap::iterator it = map->find(i);
                if (it != map->end()) {
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                    descriptor.setDescriptor(it->second.get(), 0);
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                    return true;
                }
            }
        }
    }
    return JSObject::getOwnPropertyDescriptor(exec, propertyName, descriptor);
}

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// ECMA 15.4.5.1
void JSArray::put(JSCell* cell, ExecState* exec, const Identifier& propertyName, JSValue value, PutPropertySlot& slot)
{
    JSArray* thisObject = static_cast<JSArray*>(cell);
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex) {
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        putByIndex(thisObject, exec, i, value);
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        return;
    }

    if (propertyName == exec->propertyNames().length) {
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        unsigned newLength = value.toUInt32(exec);
        if (value.toNumber(exec) != static_cast<double>(newLength)) {
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            throwError(exec, createRangeError(exec, "Invalid array length"));
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            return;
        }
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        thisObject->setLength(newLength);
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        return;
    }

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    JSObject::put(thisObject, exec, propertyName, value, slot);
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}

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void JSArray::putByIndex(JSCell* cell, ExecState* exec, unsigned i, JSValue value)
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{
    JSArray* thisObject = static_cast<JSArray*>(cell);
    thisObject->checkConsistency();

    ArrayStorage* storage = thisObject->m_storage;
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    unsigned length = storage->m_length;
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    if (i >= length && i <= MAX_ARRAY_INDEX) {
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        length = i + 1;
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        storage->m_length = length;
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    }

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    if (i < thisObject->m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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        if (valueSlot) {
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            valueSlot.set(exec->globalData(), thisObject, value);
            thisObject->checkConsistency();
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            return;
        }
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        valueSlot.set(exec->globalData(), thisObject, value);
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        ++storage->m_numValuesInVector;
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        thisObject->checkConsistency();
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        return;
    }

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    thisObject->putSlowCase(exec, i, value);
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}

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NEVER_INLINE void JSArray::putSlowCase(ExecState* exec, unsigned i, JSValue value)
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{
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    ArrayStorage* storage = m_storage;
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    SparseArrayValueMap* map = storage->m_sparseValueMap;
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    if (i >= MIN_SPARSE_ARRAY_INDEX) {
        if (i > MAX_ARRAY_INDEX) {
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            PutPropertySlot slot;
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            methodTable()->put(this, exec, Identifier::from(exec, i), value, slot);
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            return;
        }

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        // We miss some cases where we could compact the storage, such as a large array that is being filled from the end
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        // (which will only be compacted as we reach indices that are less than MIN_SPARSE_ARRAY_INDEX) - but this makes the check much faster.
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        if ((i > MAX_STORAGE_VECTOR_INDEX) || !isDenseEnoughForVector(i + 1, storage->m_numValuesInVector + 1)) {
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            if (!map) {
                map = new SparseArrayValueMap;
                storage->m_sparseValueMap = map;
            }
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            WriteBarrier<Unknown> temp;
            pair<SparseArrayValueMap::iterator, bool> result = map->add(i, temp);
            result.first->second.set(exec->globalData(), this, value);
            if (!result.second) // pre-existing entry
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                return;

            size_t capacity = map->capacity();
            if (capacity != storage->reportedMapCapacity) {
                Heap::heap(this)->reportExtraMemoryCost((capacity - storage->reportedMapCapacity) * (sizeof(unsigned) + sizeof(JSValue)));
                storage->reportedMapCapacity = capacity;
            }
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            return;
        }
    }

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    // We have decided that we'll put the new item into the vector.
    // Fast case is when there is no sparse map, so we can increase the vector size without moving values from it.
    if (!map || map->isEmpty()) {
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        if (increaseVectorLength(i + 1)) {
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            storage = m_storage;
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            storage->m_vector[i].set(exec->globalData(), this, value);
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            ++storage->m_numValuesInVector;
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            checkConsistency();
        } else
            throwOutOfMemoryError(exec);
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        return;
    }

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    // Decide how many values it would be best to move from the map.
    unsigned newNumValuesInVector = storage->m_numValuesInVector + 1;
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    unsigned newVectorLength = getNewVectorLength(i + 1);
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    for (unsigned j = max(m_vectorLength, MIN_SPARSE_ARRAY_INDEX); j < newVectorLength; ++j)
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        newNumValuesInVector += map->contains(j);
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    if (i >= MIN_SPARSE_ARRAY_INDEX)
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        newNumValuesInVector -= map->contains(i);
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    if (isDenseEnoughForVector(newVectorLength, newNumValuesInVector)) {
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        unsigned needLength = max(i + 1, storage->m_length);
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        unsigned proposedNewNumValuesInVector = newNumValuesInVector;
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        // If newVectorLength is already the maximum - MAX_STORAGE_VECTOR_LENGTH - then do not attempt to grow any further.
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        while ((newVectorLength < needLength) && (newVectorLength < MAX_STORAGE_VECTOR_LENGTH)) {
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            unsigned proposedNewVectorLength = getNewVectorLength(newVectorLength + 1);
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            for (unsigned j = max(newVectorLength, MIN_SPARSE_ARRAY_INDEX); j < proposedNewVectorLength; ++j)
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                proposedNewNumValuesInVector += map->contains(j);
            if (!isDenseEnoughForVector(proposedNewVectorLength, proposedNewNumValuesInVector))
                break;
            newVectorLength = proposedNewVectorLength;
            newNumValuesInVector = proposedNewNumValuesInVector;
        }
    }

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    void* baseStorage = storage->m_allocBase;
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    if (!tryFastRealloc(baseStorage, storageSize(newVectorLength + m_indexBias)).getValue(baseStorage)) {
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        throwOutOfMemoryError(exec);
        return;
    }
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    m_storage = reinterpret_cast_ptr<ArrayStorage*>(static_cast<char*>(baseStorage) + m_indexBias * sizeof(JSValue));
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    m_storage->m_allocBase = baseStorage;
    storage = m_storage;
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    unsigned vectorLength = m_vectorLength;
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    WriteBarrier<Unknown>* vector = storage->m_vector;
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    if (newNumValuesInVector == storage->m_numValuesInVector + 1) {
        for (unsigned j = vectorLength; j < newVectorLength; ++j)
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            vector[j].clear();
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        if (i > MIN_SPARSE_ARRAY_INDEX)
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            map->remove(i);
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    } else {
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        for (unsigned j = vectorLength; j < max(vectorLength, MIN_SPARSE_ARRAY_INDEX); ++j)
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            vector[j].clear();
        JSGlobalData& globalData = exec->globalData();
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        for (unsigned j = max(vectorLength, MIN_SPARSE_ARRAY_INDEX); j < newVectorLength; ++j)
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            vector[j].set(globalData, this, map->take(j).get());
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    }

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    ASSERT(i < newVectorLength);
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    m_vectorLength = newVectorLength;
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    storage->m_numValuesInVector = newNumValuesInVector;
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    storage->m_vector[i].set(exec->globalData(), this, value);
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    checkConsistency();
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(newVectorLength) - storageSize(vectorLength));
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}

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bool JSArray::deleteProperty(JSCell* cell, ExecState* exec, const Identifier& propertyName)
{
    JSArray* thisObject = static_cast<JSArray*>(cell);
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex)
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        return thisObject->methodTable()->deletePropertyByIndex(thisObject, exec, i);
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    if (propertyName == exec->propertyNames().length)
        return false;

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    return JSObject::deleteProperty(thisObject, exec, propertyName);
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}

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bool JSArray::deletePropertyByIndex(JSCell* cell, ExecState* exec, unsigned i)
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{
    JSArray* thisObject = static_cast<JSArray*>(cell);
    thisObject->checkConsistency();

    ArrayStorage* storage = thisObject->m_storage;
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    if (i < thisObject->m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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        if (!valueSlot) {
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            thisObject->checkConsistency();
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            return false;
        }
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        valueSlot.clear();
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        --storage->m_numValuesInVector;
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        thisObject->checkConsistency();
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        return true;
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    }

    if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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        if (i >= MIN_SPARSE_ARRAY_INDEX) {
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            SparseArrayValueMap::iterator it = map->find(i);
            if (it != map->end()) {
                map->remove(it);
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                thisObject->checkConsistency();
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                return true;
            }
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        }
    }

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    thisObject->checkConsistency();
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    if (i > MAX_ARRAY_INDEX)
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        return thisObject->methodTable()->deleteProperty(thisObject, exec, Identifier::from(exec, i));
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    return false;
}

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void JSArray::getOwnPropertyNames(JSObject* object, ExecState* exec, PropertyNameArray& propertyNames, EnumerationMode mode)
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{
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    JSArray* thisObject = static_cast<JSArray*>(object);
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    // FIXME: Filling PropertyNameArray with an identifier for every integer
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    // is incredibly inefficient for large arrays. We need a different approach,
    // which almost certainly means a different structure for PropertyNameArray.
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    ArrayStorage* storage = thisObject->m_storage;
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    unsigned usedVectorLength = min(storage->m_length, thisObject->m_vectorLength);
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    for (unsigned i = 0; i < usedVectorLength; ++i) {
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        if (storage->m_vector[i])
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            propertyNames.add(Identifier::from(exec, i));
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    }

    if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
        SparseArrayValueMap::iterator end = map->end();
        for (SparseArrayValueMap::iterator it = map->begin(); it != end; ++it)
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            propertyNames.add(Identifier::from(exec, it->first));
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    }
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    if (mode == IncludeDontEnumProperties)
        propertyNames.add(exec->propertyNames().length);

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    JSObject::getOwnPropertyNames(thisObject, exec, propertyNames, mode);
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}

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ALWAYS_INLINE unsigned JSArray::getNewVectorLength(unsigned desiredLength)
{
    ASSERT(desiredLength <= MAX_STORAGE_VECTOR_LENGTH);

    unsigned increasedLength;
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    unsigned maxInitLength = min(m_storage->m_length, 100000U);
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    if (desiredLength < maxInitLength)
        increasedLength = maxInitLength;
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    else if (!m_vectorLength)
        increasedLength = max(desiredLength, lastArraySize);
    else {
        // Mathematically equivalent to:
        //   increasedLength = (newLength * 3 + 1) / 2;
        // or:
        //   increasedLength = (unsigned)ceil(newLength * 1.5));
        // This form is not prone to internal overflow.
        increasedLength = desiredLength + (desiredLength >> 1) + (desiredLength & 1);
    }

    ASSERT(increasedLength >= desiredLength);

    lastArraySize = min(increasedLength, FIRST_VECTOR_GROW);

    return min(increasedLength, MAX_STORAGE_VECTOR_LENGTH);
}

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bool JSArray::increaseVectorLength(unsigned newLength)
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{
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    // This function leaves the array in an internally inconsistent state, because it does not move any values from sparse value map
    // to the vector. Callers have to account for that, because they can do it more efficiently.

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    ArrayStorage* storage = m_storage;
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    unsigned vectorLength = m_vectorLength;
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    ASSERT(newLength > vectorLength);
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    ASSERT(newLength <= MAX_STORAGE_VECTOR_INDEX);
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    unsigned newVectorLength = getNewVectorLength(newLength);
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    void* baseStorage = storage->m_allocBase;
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    if (!tryFastRealloc(baseStorage, storageSize(newVectorLength + m_indexBias)).getValue(baseStorage))
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        return false;
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    storage = m_storage = reinterpret_cast_ptr<ArrayStorage*>(static_cast<char*>(baseStorage) + m_indexBias * sizeof(JSValue));
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    m_storage->m_allocBase = baseStorage;
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    WriteBarrier<Unknown>* vector = storage->m_vector;
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    for (unsigned i = vectorLength; i < newVectorLength; ++i)
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        vector[i].clear();
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    m_vectorLength = newVectorLength;
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(newVectorLength) - storageSize(vectorLength));
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    return true;
}
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bool JSArray::increaseVectorPrefixLength(unsigned newLength)
{
    // This function leaves the array in an internally inconsistent state, because it does not move any values from sparse value map
    // to the vector. Callers have to account for that, because they can do it more efficiently.
    
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    ArrayStorage* storage = m_storage;
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    unsigned vectorLength = m_vectorLength;
    ASSERT(newLength > vectorLength);
    ASSERT(newLength <= MAX_STORAGE_VECTOR_INDEX);
    unsigned newVectorLength = getNewVectorLength(newLength);
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    void* newBaseStorage = fastMalloc(storageSize(newVectorLength + m_indexBias));
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    if (!newBaseStorage)
        return false;
    
    m_indexBias += newVectorLength - newLength;
    
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    m_storage = reinterpret_cast_ptr<ArrayStorage*>(static_cast<char*>(newBaseStorage) + m_indexBias * sizeof(JSValue));
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    memcpy(m_storage, storage, storageSize(0));
    memcpy(&m_storage->m_vector[newLength - m_vectorLength], &storage->m_vector[0], vectorLength * sizeof(JSValue));
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    m_storage->m_allocBase = newBaseStorage;
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    m_vectorLength = newLength;
    
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    fastFree(storage->m_allocBase);
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    ASSERT(newLength > vectorLength);
    unsigned delta = newLength - vectorLength;
    for (unsigned i = 0; i < delta; i++)
        m_storage->m_vector[i].clear();
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    Heap::heap(this)->reportExtraMemoryCost(storageSize(newVectorLength) - storageSize(vectorLength));
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    return true;
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}
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void JSArray::setLength(unsigned newLength)
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{
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    ArrayStorage* storage = m_storage;
    
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#if CHECK_ARRAY_CONSISTENCY
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    if (!storage->m_inCompactInitialization)
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        checkConsistency();
    else
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        storage->m_inCompactInitialization = false;
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#endif
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    unsigned length = storage->m_length;
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    if (newLength < length) {
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        unsigned usedVectorLength = min(length, m_vectorLength);
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        for (unsigned i = newLength; i < usedVectorLength; ++i) {
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            WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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            bool hadValue = valueSlot;
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            valueSlot.clear();
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            storage->m_numValuesInVector -= hadValue;
        }

        if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
            SparseArrayValueMap copy = *map;
            SparseArrayValueMap::iterator end = copy.end();
            for (SparseArrayValueMap::iterator it = copy.begin(); it != end; ++it) {
                if (it->first >= newLength)
                    map->remove(it->first);
            }
            if (map->isEmpty()) {
                delete map;
                storage->m_sparseValueMap = 0;
            }
        }
    }
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    storage->m_length = newLength;
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    checkConsistency();
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}

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JSValue JSArray::pop()
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{
    checkConsistency();

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    ArrayStorage* storage = m_storage;
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    unsigned length = storage->m_length;
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    if (!length)
        return jsUndefined();

    --length;

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    JSValue result;
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    if (length < m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[length];
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        if (valueSlot) {
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            --storage->m_numValuesInVector;
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            result = valueSlot.get();
            valueSlot.clear();
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        } else
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            result = jsUndefined();
    } else {
        result = jsUndefined();
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        if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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            SparseArrayValueMap::iterator it = map->find(length);
            if (it != map->end()) {
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                result = it->second.get();
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                map->remove(it);
                if (map->isEmpty()) {
                    delete map;
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                    storage->m_sparseValueMap = 0;
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                }
            }
        }
    }

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    storage->m_length = length;
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    checkConsistency();

    return result;
}

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void JSArray::push(ExecState* exec, JSValue value)
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{
    checkConsistency();
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    ArrayStorage* storage = m_storage;
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    if (UNLIKELY(storage->m_length == 0xFFFFFFFFu)) {
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        methodTable()->putByIndex(this, exec, storage->m_length, value);
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        throwError(exec, createRangeError(exec, "Invalid array length"));
        return;
    }

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    if (storage->m_length < m_vectorLength) {
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        storage->m_vector[storage->m_length].set(exec->globalData(), this, value);
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        ++storage->m_numValuesInVector;
        ++storage->m_length;
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        checkConsistency();
        return;
    }

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    if (storage->m_length < MIN_SPARSE_ARRAY_INDEX) {
        SparseArrayValueMap* map = storage->m_sparseValueMap;
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        if (!map || map->isEmpty()) {
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            if (increaseVectorLength(storage->m_length + 1)) {
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                storage = m_storage;
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                storage->m_vector[storage->m_length].set(exec->globalData(), this, value);
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                ++storage->m_numValuesInVector;
                ++storage->m_length;
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                checkConsistency();
                return;
            }
            checkConsistency();
            throwOutOfMemoryError(exec);
            return;
        }
    }

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    putSlowCase(exec, storage->m_length++, value);
}

void JSArray::shiftCount(ExecState* exec, int count)
{
    ASSERT(count > 0);
    
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    ArrayStorage* storage = m_storage;
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    unsigned oldLength = storage->m_length;
    
    if (!oldLength)
        return;
    
    if (oldLength != storage->m_numValuesInVector) {
        // If m_length and m_numValuesInVector aren't the same, we have a sparse vector
        // which means we need to go through each entry looking for the the "empty"
        // slots and then fill them with possible properties.  See ECMA spec.
        // 15.4.4.9 steps 11 through 13.
        for (unsigned i = count; i < oldLength; ++i) {
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            if ((i >= m_vectorLength) || (!m_storage->m_vector[i])) {
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                PropertySlot slot(this);
                JSValue p = prototype();
                if ((!p.isNull()) && (asObject(p)->getPropertySlot(exec, i, slot)))
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                    methodTable()->putByIndex(this, exec, i, slot.getValue(exec, i));
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            }
        }

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        storage = m_storage; // The put() above could have grown the vector and realloc'ed storage.
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        // Need to decrement numValuesInvector based on number of real entries
        for (unsigned i = 0; i < (unsigned)count; ++i)
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            if ((i < m_vectorLength) && (storage->m_vector[i]))
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                --storage->m_numValuesInVector;
    } else
        storage->m_numValuesInVector -= count;
    
    storage->m_length -= count;
    
    if (m_vectorLength) {
        count = min(m_vectorLength, (unsigned)count);
        
        m_vectorLength -= count;
        
        if (m_vectorLength) {
            char* newBaseStorage = reinterpret_cast<char*>(storage) + count * sizeof(JSValue);
            memmove(newBaseStorage, storage, storageSize(0));
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            m_storage = reinterpret_cast_ptr<ArrayStorage*>(newBaseStorage);
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            m_indexBias += count;
        }
    }
}
    
void JSArray::unshiftCount(ExecState* exec, int count)
{
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    ArrayStorage* storage = m_storage;
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    ASSERT(m_indexBias >= 0);
    ASSERT(count >= 0);
    
    unsigned length = storage->m_length;
    
    if (length != storage->m_numValuesInVector) {
        // If m_length and m_numValuesInVector aren't the same, we have a sparse vector
        // which means we need to go through each entry looking for the the "empty"
        // slots and then fill them with possible properties.  See ECMA spec.
        // 15.4.4.13 steps 8 through 10.
        for (unsigned i = 0; i < length; ++i) {
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            if ((i >= m_vectorLength) || (!m_storage->m_vector[i])) {
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                PropertySlot slot(this);
                JSValue p = prototype();
                if ((!p.isNull()) && (asObject(p)->getPropertySlot(exec, i, slot)))
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                    methodTable()->putByIndex(this, exec, i, slot.getValue(exec, i));
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            }
        }
    }
    
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    storage = m_storage; // The put() above could have grown the vector and realloc'ed storage.
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    if (m_indexBias >= count) {
        m_indexBias -= count;
        char* newBaseStorage = reinterpret_cast<char*>(storage) - count * sizeof(JSValue);
        memmove(newBaseStorage, storage, storageSize(0));
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        m_storage = reinterpret_cast_ptr<ArrayStorage*>(newBaseStorage);
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        m_vectorLength += count;
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    } else if (!increaseVectorPrefixLength(m_vectorLength + count)) {
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        throwOutOfMemoryError(exec);
        return;
    }
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    WriteBarrier<Unknown>* vector = m_storage->m_vector;
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    for (int i = 0; i < count; i++)
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        vector[i].clear();
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