xref: /dports/www/qt5-webengine/qtwebengine-everywhere-src-5.15.2/src/3rdparty/chromium/v8/src/objects/elements.cc

// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/objects/elements.h"

#include "src/common/message-template.h"
#include "src/execution/arguments.h"
#include "src/execution/frames.h"
#include "src/execution/isolate-inl.h"
#include "src/execution/protectors-inl.h"
#include "src/heap/factory.h"
#include "src/heap/heap-inl.h"  // For MaxNumberToStringCacheSize.
#include "src/heap/heap-write-barrier-inl.h"
#include "src/numbers/conversions.h"
#include "src/objects/arguments-inl.h"
#include "src/objects/hash-table-inl.h"
#include "src/objects/js-array-buffer-inl.h"
#include "src/objects/js-array-inl.h"
#include "src/objects/keys.h"
#include "src/objects/objects-inl.h"
#include "src/objects/slots-atomic-inl.h"
#include "src/objects/slots.h"
#include "src/utils/utils.h"

// Each concrete ElementsAccessor can handle exactly one ElementsKind,
// several abstract ElementsAccessor classes are used to allow sharing
// common code.
//
// Inheritance hierarchy:
// - ElementsAccessorBase                        (abstract)
//   - FastElementsAccessor                      (abstract)
//     - FastSmiOrObjectElementsAccessor
//       - FastPackedSmiElementsAccessor
//       - FastHoleySmiElementsAccessor
//       - FastPackedObjectElementsAccessor
//       - FastNonextensibleObjectElementsAccessor: template
//         - FastPackedNonextensibleObjectElementsAccessor
//         - FastHoleyNonextensibleObjectElementsAccessor
//       - FastSealedObjectElementsAccessor: template
//         - FastPackedSealedObjectElementsAccessor
//         - FastHoleySealedObjectElementsAccessor
//       - FastFrozenObjectElementsAccessor: template
//         - FastPackedFrozenObjectElementsAccessor
//         - FastHoleyFrozenObjectElementsAccessor
//       - FastHoleyObjectElementsAccessor
//     - FastDoubleElementsAccessor
//       - FastPackedDoubleElementsAccessor
//       - FastHoleyDoubleElementsAccessor
//   - TypedElementsAccessor: template, with instantiations:
//     - Uint8ElementsAccessor
//     - Int8ElementsAccessor
//     - Uint16ElementsAccessor
//     - Int16ElementsAccessor
//     - Uint32ElementsAccessor
//     - Int32ElementsAccessor
//     - Float32ElementsAccessor
//     - Float64ElementsAccessor
//     - Uint8ClampedElementsAccessor
//     - BigUint64ElementsAccessor
//     - BigInt64ElementsAccessor
//   - DictionaryElementsAccessor
//   - SloppyArgumentsElementsAccessor
//     - FastSloppyArgumentsElementsAccessor
//     - SlowSloppyArgumentsElementsAccessor
//   - StringWrapperElementsAccessor
//     - FastStringWrapperElementsAccessor
//     - SlowStringWrapperElementsAccessor

namespace v8 {
namespace internal {

namespace {

#define RETURN_NOTHING_IF_NOT_SUCCESSFUL(call) \
  do {                                         \
    if (!(call)) return Nothing<bool>();       \
  } while (false)

#define RETURN_FAILURE_IF_NOT_SUCCESSFUL(call)          \
  do {                                                  \
    ExceptionStatus status_enum_result = (call);        \
    if (!status_enum_result) return status_enum_result; \
  } while (false)

static const int kPackedSizeNotKnown = -1;

enum Where { AT_START, AT_END };

// First argument in list is the accessor class, the second argument is the
// accessor ElementsKind, and the third is the backing store class.  Use the
// fast element handler for smi-only arrays.  The implementation is currently
// identical.  Note that the order must match that of the ElementsKind enum for
// the |accessor_array[]| below to work.
#define ELEMENTS_LIST(V)                                                      \
  V(FastPackedSmiElementsAccessor, PACKED_SMI_ELEMENTS, FixedArray)           \
  V(FastHoleySmiElementsAccessor, HOLEY_SMI_ELEMENTS, FixedArray)             \
  V(FastPackedObjectElementsAccessor, PACKED_ELEMENTS, FixedArray)            \
  V(FastHoleyObjectElementsAccessor, HOLEY_ELEMENTS, FixedArray)              \
  V(FastPackedDoubleElementsAccessor, PACKED_DOUBLE_ELEMENTS,                 \
    FixedDoubleArray)                                                         \
  V(FastHoleyDoubleElementsAccessor, HOLEY_DOUBLE_ELEMENTS, FixedDoubleArray) \
  V(FastPackedNonextensibleObjectElementsAccessor,                            \
    PACKED_NONEXTENSIBLE_ELEMENTS, FixedArray)                                \
  V(FastHoleyNonextensibleObjectElementsAccessor,                             \
    HOLEY_NONEXTENSIBLE_ELEMENTS, FixedArray)                                 \
  V(FastPackedSealedObjectElementsAccessor, PACKED_SEALED_ELEMENTS,           \
    FixedArray)                                                               \
  V(FastHoleySealedObjectElementsAccessor, HOLEY_SEALED_ELEMENTS, FixedArray) \
  V(FastPackedFrozenObjectElementsAccessor, PACKED_FROZEN_ELEMENTS,           \
    FixedArray)                                                               \
  V(FastHoleyFrozenObjectElementsAccessor, HOLEY_FROZEN_ELEMENTS, FixedArray) \
  V(DictionaryElementsAccessor, DICTIONARY_ELEMENTS, NumberDictionary)        \
  V(FastSloppyArgumentsElementsAccessor, FAST_SLOPPY_ARGUMENTS_ELEMENTS,      \
    FixedArray)                                                               \
  V(SlowSloppyArgumentsElementsAccessor, SLOW_SLOPPY_ARGUMENTS_ELEMENTS,      \
    FixedArray)                                                               \
  V(FastStringWrapperElementsAccessor, FAST_STRING_WRAPPER_ELEMENTS,          \
    FixedArray)                                                               \
  V(SlowStringWrapperElementsAccessor, SLOW_STRING_WRAPPER_ELEMENTS,          \
    FixedArray)                                                               \
  V(Uint8ElementsAccessor, UINT8_ELEMENTS, ByteArray)                         \
  V(Int8ElementsAccessor, INT8_ELEMENTS, ByteArray)                           \
  V(Uint16ElementsAccessor, UINT16_ELEMENTS, ByteArray)                       \
  V(Int16ElementsAccessor, INT16_ELEMENTS, ByteArray)                         \
  V(Uint32ElementsAccessor, UINT32_ELEMENTS, ByteArray)                       \
  V(Int32ElementsAccessor, INT32_ELEMENTS, ByteArray)                         \
  V(Float32ElementsAccessor, FLOAT32_ELEMENTS, ByteArray)                     \
  V(Float64ElementsAccessor, FLOAT64_ELEMENTS, ByteArray)                     \
  V(Uint8ClampedElementsAccessor, UINT8_CLAMPED_ELEMENTS, ByteArray)          \
  V(BigUint64ElementsAccessor, BIGUINT64_ELEMENTS, ByteArray)                 \
  V(BigInt64ElementsAccessor, BIGINT64_ELEMENTS, ByteArray)

template <ElementsKind Kind>
class ElementsKindTraits {
 public:
  using BackingStore = FixedArrayBase;
};

#define ELEMENTS_TRAITS(Class, KindParam, Store)    \
  template <>                                       \
  class ElementsKindTraits<KindParam> {             \
   public: /* NOLINT */                             \
    static constexpr ElementsKind Kind = KindParam; \
    using BackingStore = Store;                     \
  };                                                \
  constexpr ElementsKind ElementsKindTraits<KindParam>::Kind;
ELEMENTS_LIST(ELEMENTS_TRAITS)
#undef ELEMENTS_TRAITS

V8_WARN_UNUSED_RESULT
MaybeHandle<Object> ThrowArrayLengthRangeError(Isolate* isolate) {
  THROW_NEW_ERROR(isolate, NewRangeError(MessageTemplate::kInvalidArrayLength),
                  Object);
}

WriteBarrierMode GetWriteBarrierMode(ElementsKind kind) {
  if (IsSmiElementsKind(kind)) return SKIP_WRITE_BARRIER;
  if (IsDoubleElementsKind(kind)) return SKIP_WRITE_BARRIER;
  return UPDATE_WRITE_BARRIER;
}

// If kCopyToEndAndInitializeToHole is specified as the copy_size to
// CopyElements, it copies all of elements from source after source_start to
// destination array, padding any remaining uninitialized elements in the
// destination array with the hole.
constexpr int kCopyToEndAndInitializeToHole = -1;

void CopyObjectToObjectElements(Isolate* isolate, FixedArrayBase from_base,
                                ElementsKind from_kind, uint32_t from_start,
                                FixedArrayBase to_base, ElementsKind to_kind,
                                uint32_t to_start, int raw_copy_size) {
  ReadOnlyRoots roots(isolate);
  DCHECK(to_base.map() != roots.fixed_cow_array_map());
  DisallowHeapAllocation no_allocation;
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size =
        Min(from_base.length() - from_start, to_base.length() - to_start);
    int start = to_start + copy_size;
    int length = to_base.length() - start;
    if (length > 0) {
      MemsetTagged(FixedArray::cast(to_base).RawFieldOfElementAt(start),
                   roots.the_hole_value(), length);
    }
  }
  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;
  FixedArray from = FixedArray::cast(from_base);
  FixedArray to = FixedArray::cast(to_base);
  DCHECK(IsSmiOrObjectElementsKind(from_kind));
  DCHECK(IsSmiOrObjectElementsKind(to_kind));

  WriteBarrierMode write_barrier_mode =
      (IsObjectElementsKind(from_kind) && IsObjectElementsKind(to_kind))
          ? UPDATE_WRITE_BARRIER
          : SKIP_WRITE_BARRIER;
  to.CopyElements(isolate, to_start, from, from_start, copy_size,
                  write_barrier_mode);
}

void CopyDictionaryToObjectElements(Isolate* isolate, FixedArrayBase from_base,
                                    uint32_t from_start, FixedArrayBase to_base,
                                    ElementsKind to_kind, uint32_t to_start,
                                    int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  NumberDictionary from = NumberDictionary::cast(from_base);
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size = from.max_number_key() + 1 - from_start;
    int start = to_start + copy_size;
    int length = to_base.length() - start;
    if (length > 0) {
      MemsetTagged(FixedArray::cast(to_base).RawFieldOfElementAt(start),
                   ReadOnlyRoots(isolate).the_hole_value(), length);
    }
  }
  DCHECK(to_base != from_base);
  DCHECK(IsSmiOrObjectElementsKind(to_kind));
  if (copy_size == 0) return;
  FixedArray to = FixedArray::cast(to_base);
  uint32_t to_length = to.length();
  if (to_start + copy_size > to_length) {
    copy_size = to_length - to_start;
  }
  WriteBarrierMode write_barrier_mode = GetWriteBarrierMode(to_kind);
  for (int i = 0; i < copy_size; i++) {
    InternalIndex entry = from.FindEntry(isolate, i + from_start);
    if (entry.is_found()) {
      Object value = from.ValueAt(entry);
      DCHECK(!value.IsTheHole(isolate));
      to.set(i + to_start, value, write_barrier_mode);
    } else {
      to.set_the_hole(isolate, i + to_start);
    }
  }
}

// NOTE: this method violates the handlified function signature convention:
// raw pointer parameters in the function that allocates.
// See ElementsAccessorBase::CopyElements() for details.
void CopyDoubleToObjectElements(Isolate* isolate, FixedArrayBase from_base,
                                uint32_t from_start, FixedArrayBase to_base,
                                uint32_t to_start, int raw_copy_size) {
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DisallowHeapAllocation no_allocation;
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size =
        Min(from_base.length() - from_start, to_base.length() - to_start);
    // Also initialize the area that will be copied over since HeapNumber
    // allocation below can cause an incremental marking step, requiring all
    // existing heap objects to be propertly initialized.
    int start = to_start;
    int length = to_base.length() - start;
    if (length > 0) {
      MemsetTagged(FixedArray::cast(to_base).RawFieldOfElementAt(start),
                   ReadOnlyRoots(isolate).the_hole_value(), length);
    }
  }

  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;

  // From here on, the code below could actually allocate. Therefore the raw
  // values are wrapped into handles.
  Handle<FixedDoubleArray> from(FixedDoubleArray::cast(from_base), isolate);
  Handle<FixedArray> to(FixedArray::cast(to_base), isolate);

  // Use an outer loop to not waste too much time on creating HandleScopes.
  // On the other hand we might overflow a single handle scope depending on
  // the copy_size.
  int offset = 0;
  while (offset < copy_size) {
    HandleScope scope(isolate);
    offset += 100;
    for (int i = offset - 100; i < offset && i < copy_size; ++i) {
      Handle<Object> value =
          FixedDoubleArray::get(*from, i + from_start, isolate);
      to->set(i + to_start, *value, UPDATE_WRITE_BARRIER);
    }
  }
}

void CopyDoubleToDoubleElements(FixedArrayBase from_base, uint32_t from_start,
                                FixedArrayBase to_base, uint32_t to_start,
                                int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size =
        Min(from_base.length() - from_start, to_base.length() - to_start);
    for (int i = to_start + copy_size; i < to_base.length(); ++i) {
      FixedDoubleArray::cast(to_base).set_the_hole(i);
    }
  }
  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;
  FixedDoubleArray from = FixedDoubleArray::cast(from_base);
  FixedDoubleArray to = FixedDoubleArray::cast(to_base);
  Address to_address = to.address() + FixedDoubleArray::kHeaderSize;
  Address from_address = from.address() + FixedDoubleArray::kHeaderSize;
  to_address += kDoubleSize * to_start;
  from_address += kDoubleSize * from_start;
#ifdef V8_COMPRESS_POINTERS
  // TODO(ishell, v8:8875): we use CopyTagged() in order to avoid unaligned
  // access to double values in the arrays. This will no longed be necessary
  // once the allocations alignment issue is fixed.
  int words_per_double = (kDoubleSize / kTaggedSize);
  CopyTagged(to_address, from_address,
             static_cast<size_t>(words_per_double * copy_size));
#else
  int words_per_double = (kDoubleSize / kSystemPointerSize);
  CopyWords(to_address, from_address,
            static_cast<size_t>(words_per_double * copy_size));
#endif
}

void CopySmiToDoubleElements(FixedArrayBase from_base, uint32_t from_start,
                             FixedArrayBase to_base, uint32_t to_start,
                             int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size = from_base.length() - from_start;
    for (int i = to_start + copy_size; i < to_base.length(); ++i) {
      FixedDoubleArray::cast(to_base).set_the_hole(i);
    }
  }
  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;
  FixedArray from = FixedArray::cast(from_base);
  FixedDoubleArray to = FixedDoubleArray::cast(to_base);
  Object the_hole = from.GetReadOnlyRoots().the_hole_value();
  for (uint32_t from_end = from_start + static_cast<uint32_t>(copy_size);
       from_start < from_end; from_start++, to_start++) {
    Object hole_or_smi = from.get(from_start);
    if (hole_or_smi == the_hole) {
      to.set_the_hole(to_start);
    } else {
      to.set(to_start, Smi::ToInt(hole_or_smi));
    }
  }
}

void CopyPackedSmiToDoubleElements(FixedArrayBase from_base,
                                   uint32_t from_start, FixedArrayBase to_base,
                                   uint32_t to_start, int packed_size,
                                   int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  int copy_size = raw_copy_size;
  uint32_t to_end;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size = packed_size - from_start;
    to_end = to_base.length();
    for (uint32_t i = to_start + copy_size; i < to_end; ++i) {
      FixedDoubleArray::cast(to_base).set_the_hole(i);
    }
  } else {
    to_end = to_start + static_cast<uint32_t>(copy_size);
  }
  DCHECK(static_cast<int>(to_end) <= to_base.length());
  DCHECK(packed_size >= 0 && packed_size <= copy_size);
  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;
  FixedArray from = FixedArray::cast(from_base);
  FixedDoubleArray to = FixedDoubleArray::cast(to_base);
  for (uint32_t from_end = from_start + static_cast<uint32_t>(packed_size);
       from_start < from_end; from_start++, to_start++) {
    Object smi = from.get(from_start);
    DCHECK(!smi.IsTheHole());
    to.set(to_start, Smi::ToInt(smi));
  }
}

void CopyObjectToDoubleElements(FixedArrayBase from_base, uint32_t from_start,
                                FixedArrayBase to_base, uint32_t to_start,
                                int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  int copy_size = raw_copy_size;
  if (raw_copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, raw_copy_size);
    copy_size = from_base.length() - from_start;
    for (int i = to_start + copy_size; i < to_base.length(); ++i) {
      FixedDoubleArray::cast(to_base).set_the_hole(i);
    }
  }
  DCHECK((copy_size + static_cast<int>(to_start)) <= to_base.length() &&
         (copy_size + static_cast<int>(from_start)) <= from_base.length());
  if (copy_size == 0) return;
  FixedArray from = FixedArray::cast(from_base);
  FixedDoubleArray to = FixedDoubleArray::cast(to_base);
  Object the_hole = from.GetReadOnlyRoots().the_hole_value();
  for (uint32_t from_end = from_start + copy_size; from_start < from_end;
       from_start++, to_start++) {
    Object hole_or_object = from.get(from_start);
    if (hole_or_object == the_hole) {
      to.set_the_hole(to_start);
    } else {
      to.set(to_start, hole_or_object.Number());
    }
  }
}

void CopyDictionaryToDoubleElements(Isolate* isolate, FixedArrayBase from_base,
                                    uint32_t from_start, FixedArrayBase to_base,
                                    uint32_t to_start, int raw_copy_size) {
  DisallowHeapAllocation no_allocation;
  NumberDictionary from = NumberDictionary::cast(from_base);
  int copy_size = raw_copy_size;
  if (copy_size < 0) {
    DCHECK_EQ(kCopyToEndAndInitializeToHole, copy_size);
    copy_size = from.max_number_key() + 1 - from_start;
    for (int i = to_start + copy_size; i < to_base.length(); ++i) {
      FixedDoubleArray::cast(to_base).set_the_hole(i);
    }
  }
  if (copy_size == 0) return;
  FixedDoubleArray to = FixedDoubleArray::cast(to_base);
  uint32_t to_length = to.length();
  if (to_start + copy_size > to_length) {
    copy_size = to_length - to_start;
  }
  for (int i = 0; i < copy_size; i++) {
    InternalIndex entry = from.FindEntry(isolate, i + from_start);
    if (entry.is_found()) {
      to.set(i + to_start, from.ValueAt(entry).Number());
    } else {
      to.set_the_hole(i + to_start);
    }
  }
}

void SortIndices(Isolate* isolate, Handle<FixedArray> indices,
                 uint32_t sort_size) {
  // Use AtomicSlot wrapper to ensure that std::sort uses atomic load and
  // store operations that are safe for concurrent marking.
  AtomicSlot start(indices->GetFirstElementAddress());
  AtomicSlot end(start + sort_size);
  std::sort(start, end, [isolate](Tagged_t elementA, Tagged_t elementB) {
#ifdef V8_COMPRESS_POINTERS
    Object a(DecompressTaggedAny(isolate, elementA));
    Object b(DecompressTaggedAny(isolate, elementB));
#else
    Object a(elementA);
    Object b(elementB);
#endif
    if (a.IsSmi() || !a.IsUndefined(isolate)) {
      if (!b.IsSmi() && b.IsUndefined(isolate)) {
        return true;
      }
      return a.Number() < b.Number();
    }
    return !b.IsSmi() && b.IsUndefined(isolate);
  });
  isolate->heap()->WriteBarrierForRange(*indices, ObjectSlot(start),
                                        ObjectSlot(end));
}

Maybe<bool> IncludesValueSlowPath(Isolate* isolate, Handle<JSObject> receiver,
                                  Handle<Object> value, size_t start_from,
                                  size_t length) {
  bool search_for_hole = value->IsUndefined(isolate);
  for (size_t k = start_from; k < length; ++k) {
    LookupIterator it(isolate, receiver, k);
    if (!it.IsFound()) {
      if (search_for_hole) return Just(true);
      continue;
    }
    Handle<Object> element_k;
    ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, element_k,
                                     Object::GetProperty(&it), Nothing<bool>());

    if (value->SameValueZero(*element_k)) return Just(true);
  }

  return Just(false);
}

Maybe<int64_t> IndexOfValueSlowPath(Isolate* isolate, Handle<JSObject> receiver,
                                    Handle<Object> value, size_t start_from,
                                    size_t length) {
  for (size_t k = start_from; k < length; ++k) {
    LookupIterator it(isolate, receiver, k);
    if (!it.IsFound()) {
      continue;
    }
    Handle<Object> element_k;
    ASSIGN_RETURN_ON_EXCEPTION_VALUE(
        isolate, element_k, Object::GetProperty(&it), Nothing<int64_t>());

    if (value->StrictEquals(*element_k)) return Just<int64_t>(k);
  }

  return Just<int64_t>(-1);
}

// The InternalElementsAccessor is a helper class to expose otherwise protected
// methods to its subclasses. Namely, we don't want to publicly expose methods
// that take an entry (instead of an index) as an argument.
class InternalElementsAccessor : public ElementsAccessor {
 public:
  InternalIndex GetEntryForIndex(Isolate* isolate, JSObject holder,
                                 FixedArrayBase backing_store,
                                 size_t index) override = 0;

  PropertyDetails GetDetails(JSObject holder, InternalIndex entry) override = 0;
};

// Base class for element handler implementations. Contains the
// the common logic for objects with different ElementsKinds.
// Subclasses must specialize method for which the element
// implementation differs from the base class implementation.
//
// This class is intended to be used in the following way:
//
//   class SomeElementsAccessor :
//       public ElementsAccessorBase<SomeElementsAccessor,
//                                   BackingStoreClass> {
//     ...
//   }
//
// This is an example of the Curiously Recurring Template Pattern (see
// http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern).  We use
// CRTP to guarantee aggressive compile time optimizations (i.e.  inlining and
// specialization of SomeElementsAccessor methods).
template <typename Subclass, typename ElementsTraitsParam>
class ElementsAccessorBase : public InternalElementsAccessor {
 public:
  ElementsAccessorBase() = default;

  using ElementsTraits = ElementsTraitsParam;
  using BackingStore = typename ElementsTraitsParam::BackingStore;

  static ElementsKind kind() { return ElementsTraits::Kind; }

  static void ValidateContents(JSObject holder, size_t length) {}

  static void ValidateImpl(JSObject holder) {
    FixedArrayBase fixed_array_base = holder.elements();
    if (!fixed_array_base.IsHeapObject()) return;
    // Arrays that have been shifted in place can't be verified.
    if (fixed_array_base.IsFreeSpaceOrFiller()) return;
    size_t length = 0;
    if (holder.IsJSArray()) {
      Object length_obj = JSArray::cast(holder).length();
      if (length_obj.IsSmi()) {
        length = Smi::ToInt(length_obj);
      }
    } else if (holder.IsJSTypedArray()) {
      length = JSTypedArray::cast(holder).length();
    } else {
      length = fixed_array_base.length();
    }
    Subclass::ValidateContents(holder, length);
  }

  void Validate(JSObject holder) final {
    DisallowHeapAllocation no_gc;
    Subclass::ValidateImpl(holder);
  }

  bool HasElement(JSObject holder, uint32_t index, FixedArrayBase backing_store,
                  PropertyFilter filter) final {
    return Subclass::HasElementImpl(holder.GetIsolate(), holder, index,
                                    backing_store, filter);
  }

  static bool HasElementImpl(Isolate* isolate, JSObject holder, size_t index,
                             FixedArrayBase backing_store,
                             PropertyFilter filter = ALL_PROPERTIES) {
    return Subclass::GetEntryForIndexImpl(isolate, holder, backing_store, index,
                                          filter)
        .is_found();
  }

  bool HasEntry(JSObject holder, InternalIndex entry) final {
    return Subclass::HasEntryImpl(holder.GetIsolate(), holder.elements(),
                                  entry);
  }

  static bool HasEntryImpl(Isolate* isolate, FixedArrayBase backing_store,
                           InternalIndex entry) {
    UNIMPLEMENTED();
  }

  bool HasAccessors(JSObject holder) final {
    return Subclass::HasAccessorsImpl(holder, holder.elements());
  }

  static bool HasAccessorsImpl(JSObject holder, FixedArrayBase backing_store) {
    return false;
  }

  Handle<Object> Get(Handle<JSObject> holder, InternalIndex entry) final {
    return Subclass::GetInternalImpl(holder, entry);
  }

  static Handle<Object> GetInternalImpl(Handle<JSObject> holder,
                                        InternalIndex entry) {
    return Subclass::GetImpl(holder->GetIsolate(), holder->elements(), entry);
  }

  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase backing_store,
                                InternalIndex entry) {
    return handle(BackingStore::cast(backing_store).get(entry.as_int()),
                  isolate);
  }

  void Set(Handle<JSObject> holder, InternalIndex entry, Object value) final {
    Subclass::SetImpl(holder, entry, value);
  }

  void Reconfigure(Handle<JSObject> object, Handle<FixedArrayBase> store,
                   InternalIndex entry, Handle<Object> value,
                   PropertyAttributes attributes) final {
    Subclass::ReconfigureImpl(object, store, entry, value, attributes);
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    UNREACHABLE();
  }

  void Add(Handle<JSObject> object, uint32_t index, Handle<Object> value,
           PropertyAttributes attributes, uint32_t new_capacity) final {
    Subclass::AddImpl(object, index, value, attributes, new_capacity);
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    UNREACHABLE();
  }

  uint32_t Push(Handle<JSArray> receiver, BuiltinArguments* args,
                uint32_t push_size) final {
    return Subclass::PushImpl(receiver, args, push_size);
  }

  static uint32_t PushImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                           uint32_t push_sized) {
    UNREACHABLE();
  }

  uint32_t Unshift(Handle<JSArray> receiver, BuiltinArguments* args,
                   uint32_t unshift_size) final {
    return Subclass::UnshiftImpl(receiver, args, unshift_size);
  }

  static uint32_t UnshiftImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                              uint32_t unshift_size) {
    UNREACHABLE();
  }

  Handle<Object> Pop(Handle<JSArray> receiver) final {
    return Subclass::PopImpl(receiver);
  }

  static Handle<Object> PopImpl(Handle<JSArray> receiver) { UNREACHABLE(); }

  Handle<Object> Shift(Handle<JSArray> receiver) final {
    return Subclass::ShiftImpl(receiver);
  }

  static Handle<Object> ShiftImpl(Handle<JSArray> receiver) { UNREACHABLE(); }

  void SetLength(Handle<JSArray> array, uint32_t length) final {
    Subclass::SetLengthImpl(array->GetIsolate(), array, length,
                            handle(array->elements(), array->GetIsolate()));
  }

  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    DCHECK(!array->SetLengthWouldNormalize(length));
    DCHECK(IsFastElementsKind(array->GetElementsKind()));
    uint32_t old_length = 0;
    CHECK(array->length().ToArrayIndex(&old_length));

    if (old_length < length) {
      ElementsKind kind = array->GetElementsKind();
      if (!IsHoleyElementsKind(kind)) {
        kind = GetHoleyElementsKind(kind);
        JSObject::TransitionElementsKind(array, kind);
      }
    }

    // Check whether the backing store should be shrunk.
    uint32_t capacity = backing_store->length();
    old_length = Min(old_length, capacity);
    if (length == 0) {
      array->initialize_elements();
    } else if (length <= capacity) {
      if (IsSmiOrObjectElementsKind(kind())) {
        JSObject::EnsureWritableFastElements(array);
        if (array->elements() != *backing_store) {
          backing_store = handle(array->elements(), isolate);
        }
      }
      if (2 * length + JSObject::kMinAddedElementsCapacity <= capacity) {
        // If more than half the elements won't be used, trim the array.
        // Do not trim from short arrays to prevent frequent trimming on
        // repeated pop operations.
        // Leave some space to allow for subsequent push operations.
        int elements_to_trim = length + 1 == old_length
                                   ? (capacity - length) / 2
                                   : capacity - length;
        isolate->heap()->RightTrimFixedArray(*backing_store, elements_to_trim);
        // Fill the non-trimmed elements with holes.
        BackingStore::cast(*backing_store)
            .FillWithHoles(length,
                           std::min(old_length, capacity - elements_to_trim));
      } else {
        // Otherwise, fill the unused tail with holes.
        BackingStore::cast(*backing_store).FillWithHoles(length, old_length);
      }
    } else {
      // Check whether the backing store should be expanded.
      capacity = Max(length, JSObject::NewElementsCapacity(capacity));
      Subclass::GrowCapacityAndConvertImpl(array, capacity);
    }

    array->set_length(Smi::FromInt(length));
    JSObject::ValidateElements(*array);
  }

  size_t NumberOfElements(JSObject receiver) final {
    return Subclass::NumberOfElementsImpl(receiver, receiver.elements());
  }

  static uint32_t NumberOfElementsImpl(JSObject receiver,
                                       FixedArrayBase backing_store) {
    UNREACHABLE();
  }

  static size_t GetMaxIndex(JSObject receiver, FixedArrayBase elements) {
    if (receiver.IsJSArray()) {
      DCHECK(JSArray::cast(receiver).length().IsSmi());
      return static_cast<uint32_t>(
          Smi::ToInt(JSArray::cast(receiver).length()));
    }
    return Subclass::GetCapacityImpl(receiver, elements);
  }

  static size_t GetMaxNumberOfEntries(JSObject receiver,
                                      FixedArrayBase elements) {
    return Subclass::GetMaxIndex(receiver, elements);
  }

  static Handle<FixedArrayBase> ConvertElementsWithCapacity(
      Handle<JSObject> object, Handle<FixedArrayBase> old_elements,
      ElementsKind from_kind, uint32_t capacity) {
    return ConvertElementsWithCapacity(object, old_elements, from_kind,
                                       capacity, 0, 0);
  }

  static Handle<FixedArrayBase> ConvertElementsWithCapacity(
      Handle<JSObject> object, Handle<FixedArrayBase> old_elements,
      ElementsKind from_kind, uint32_t capacity, uint32_t src_index,
      uint32_t dst_index) {
    Isolate* isolate = object->GetIsolate();
    Handle<FixedArrayBase> new_elements;
    if (IsDoubleElementsKind(kind())) {
      new_elements = isolate->factory()->NewFixedDoubleArray(capacity);
    } else {
      new_elements = isolate->factory()->NewUninitializedFixedArray(capacity);
    }

    int packed_size = kPackedSizeNotKnown;
    if (IsFastPackedElementsKind(from_kind) && object->IsJSArray()) {
      packed_size = Smi::ToInt(JSArray::cast(*object).length());
    }

    Subclass::CopyElementsImpl(isolate, *old_elements, src_index, *new_elements,
                               from_kind, dst_index, packed_size,
                               kCopyToEndAndInitializeToHole);

    return new_elements;
  }

  static void TransitionElementsKindImpl(Handle<JSObject> object,
                                         Handle<Map> to_map) {
    Isolate* isolate = object->GetIsolate();
    Handle<Map> from_map = handle(object->map(), isolate);
    ElementsKind from_kind = from_map->elements_kind();
    ElementsKind to_kind = to_map->elements_kind();
    if (IsHoleyElementsKind(from_kind)) {
      to_kind = GetHoleyElementsKind(to_kind);
    }
    if (from_kind != to_kind) {
      // This method should never be called for any other case.
      DCHECK(IsFastElementsKind(from_kind));
      DCHECK(IsFastElementsKind(to_kind));
      DCHECK_NE(TERMINAL_FAST_ELEMENTS_KIND, from_kind);

      Handle<FixedArrayBase> from_elements(object->elements(), isolate);
      if (object->elements() == ReadOnlyRoots(isolate).empty_fixed_array() ||
          IsDoubleElementsKind(from_kind) == IsDoubleElementsKind(to_kind)) {
        // No change is needed to the elements() buffer, the transition
        // only requires a map change.
        JSObject::MigrateToMap(isolate, object, to_map);
      } else {
        DCHECK(
            (IsSmiElementsKind(from_kind) && IsDoubleElementsKind(to_kind)) ||
            (IsDoubleElementsKind(from_kind) && IsObjectElementsKind(to_kind)));
        uint32_t capacity = static_cast<uint32_t>(object->elements().length());
        Handle<FixedArrayBase> elements = ConvertElementsWithCapacity(
            object, from_elements, from_kind, capacity);
        JSObject::SetMapAndElements(object, to_map, elements);
      }
      if (FLAG_trace_elements_transitions) {
        JSObject::PrintElementsTransition(stdout, object, from_kind,
                                          from_elements, to_kind,
                                          handle(object->elements(), isolate));
      }
    }
  }

  static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                         uint32_t capacity) {
    ElementsKind from_kind = object->GetElementsKind();
    if (IsSmiOrObjectElementsKind(from_kind)) {
      // Array optimizations rely on the prototype lookups of Array objects
      // always returning undefined. If there is a store to the initial
      // prototype object, make sure all of these optimizations are invalidated.
      object->GetIsolate()->UpdateNoElementsProtectorOnSetLength(object);
    }
    Handle<FixedArrayBase> old_elements(object->elements(),
                                        object->GetIsolate());
    // This method should only be called if there's a reason to update the
    // elements.
    DCHECK(IsDoubleElementsKind(from_kind) != IsDoubleElementsKind(kind()) ||
           IsDictionaryElementsKind(from_kind) ||
           static_cast<uint32_t>(old_elements->length()) < capacity);
    Subclass::BasicGrowCapacityAndConvertImpl(object, old_elements, from_kind,
                                              kind(), capacity);
  }

  static void BasicGrowCapacityAndConvertImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> old_elements,
      ElementsKind from_kind, ElementsKind to_kind, uint32_t capacity) {
    Handle<FixedArrayBase> elements =
        ConvertElementsWithCapacity(object, old_elements, from_kind, capacity);

    if (IsHoleyElementsKind(from_kind)) {
      to_kind = GetHoleyElementsKind(to_kind);
    }
    Handle<Map> new_map = JSObject::GetElementsTransitionMap(object, to_kind);
    JSObject::SetMapAndElements(object, new_map, elements);

    // Transition through the allocation site as well if present.
    JSObject::UpdateAllocationSite(object, to_kind);

    if (FLAG_trace_elements_transitions) {
      JSObject::PrintElementsTransition(stdout, object, from_kind, old_elements,
                                        to_kind, elements);
    }
  }

  void TransitionElementsKind(Handle<JSObject> object, Handle<Map> map) final {
    Subclass::TransitionElementsKindImpl(object, map);
  }

  void GrowCapacityAndConvert(Handle<JSObject> object,
                              uint32_t capacity) final {
    Subclass::GrowCapacityAndConvertImpl(object, capacity);
  }

  bool GrowCapacity(Handle<JSObject> object, uint32_t index) final {
    // This function is intended to be called from optimized code. We don't
    // want to trigger lazy deopts there, so refuse to handle cases that would.
    if (object->map().is_prototype_map() ||
        object->WouldConvertToSlowElements(index)) {
      return false;
    }
    Handle<FixedArrayBase> old_elements(object->elements(),
                                        object->GetIsolate());
    uint32_t new_capacity = JSObject::NewElementsCapacity(index + 1);
    DCHECK(static_cast<uint32_t>(old_elements->length()) < new_capacity);
    Handle<FixedArrayBase> elements =
        ConvertElementsWithCapacity(object, old_elements, kind(), new_capacity);

    DCHECK_EQ(object->GetElementsKind(), kind());
    // Transition through the allocation site as well if present.
    if (JSObject::UpdateAllocationSite<AllocationSiteUpdateMode::kCheckOnly>(
            object, kind())) {
      return false;
    }

    object->set_elements(*elements);
    return true;
  }

  void Delete(Handle<JSObject> obj, InternalIndex entry) final {
    Subclass::DeleteImpl(obj, entry);
  }

  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    UNREACHABLE();
  }

  void CopyElements(JSObject from_holder, uint32_t from_start,
                    ElementsKind from_kind, Handle<FixedArrayBase> to,
                    uint32_t to_start, int copy_size) final {
    int packed_size = kPackedSizeNotKnown;
    bool is_packed =
        IsFastPackedElementsKind(from_kind) && from_holder.IsJSArray();
    if (is_packed) {
      packed_size = Smi::ToInt(JSArray::cast(from_holder).length());
      if (copy_size >= 0 && packed_size > copy_size) {
        packed_size = copy_size;
      }
    }
    FixedArrayBase from = from_holder.elements();
    // NOTE: the Subclass::CopyElementsImpl() methods
    // violate the handlified function signature convention:
    // raw pointer parameters in the function that allocates. This is done
    // intentionally to avoid ArrayConcat() builtin performance degradation.
    //
    // Details: The idea is that allocations actually happen only in case of
    // copying from object with fast double elements to object with object
    // elements. In all the other cases there are no allocations performed and
    // handle creation causes noticeable performance degradation of the builtin.
    Subclass::CopyElementsImpl(from_holder.GetIsolate(), from, from_start, *to,
                               from_kind, to_start, packed_size, copy_size);
  }

  void CopyElements(Isolate* isolate, Handle<FixedArrayBase> source,
                    ElementsKind source_kind,
                    Handle<FixedArrayBase> destination, int size) override {
    Subclass::CopyElementsImpl(isolate, *source, 0, *destination, source_kind,
                               0, kPackedSizeNotKnown, size);
  }

  void CopyTypedArrayElementsSlice(JSTypedArray source,
                                   JSTypedArray destination, size_t start,
                                   size_t end) override {
    Subclass::CopyTypedArrayElementsSliceImpl(source, destination, start, end);
  }

  static void CopyTypedArrayElementsSliceImpl(JSTypedArray source,
                                              JSTypedArray destination,
                                              size_t start, size_t end) {
    UNREACHABLE();
  }

  Object CopyElements(Handle<Object> source, Handle<JSObject> destination,
                      size_t length, size_t offset) final {
    return Subclass::CopyElementsHandleImpl(source, destination, length,
                                            offset);
  }

  static Object CopyElementsHandleImpl(Handle<Object> source,
                                       Handle<JSObject> destination,
                                       size_t length, size_t offset) {
    UNREACHABLE();
  }

  Handle<NumberDictionary> Normalize(Handle<JSObject> object) final {
    return Subclass::NormalizeImpl(
        object, handle(object->elements(), object->GetIsolate()));
  }

  static Handle<NumberDictionary> NormalizeImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> elements) {
    UNREACHABLE();
  }

  Maybe<bool> CollectValuesOrEntries(Isolate* isolate, Handle<JSObject> object,
                                     Handle<FixedArray> values_or_entries,
                                     bool get_entries, int* nof_items,
                                     PropertyFilter filter) override {
    return Subclass::CollectValuesOrEntriesImpl(
        isolate, object, values_or_entries, get_entries, nof_items, filter);
  }

  static Maybe<bool> CollectValuesOrEntriesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArray> values_or_entries, bool get_entries, int* nof_items,
      PropertyFilter filter) {
    DCHECK_EQ(*nof_items, 0);
    KeyAccumulator accumulator(isolate, KeyCollectionMode::kOwnOnly,
                               ALL_PROPERTIES);
    RETURN_NOTHING_IF_NOT_SUCCESSFUL(Subclass::CollectElementIndicesImpl(
        object, handle(object->elements(), isolate), &accumulator));
    Handle<FixedArray> keys = accumulator.GetKeys();

    int count = 0;
    int i = 0;
    ElementsKind original_elements_kind = object->GetElementsKind();

    for (; i < keys->length(); ++i) {
      Handle<Object> key(keys->get(i), isolate);
      uint32_t index;
      if (!key->ToUint32(&index)) continue;

      DCHECK_EQ(object->GetElementsKind(), original_elements_kind);
      InternalIndex entry = Subclass::GetEntryForIndexImpl(
          isolate, *object, object->elements(), index, filter);
      if (entry.is_not_found()) continue;
      PropertyDetails details = Subclass::GetDetailsImpl(*object, entry);

      Handle<Object> value;
      if (details.kind() == kData) {
        value = Subclass::GetInternalImpl(object, entry);
      } else {
        // This might modify the elements and/or change the elements kind.
        LookupIterator it(isolate, object, index, LookupIterator::OWN);
        ASSIGN_RETURN_ON_EXCEPTION_VALUE(
            isolate, value, Object::GetProperty(&it), Nothing<bool>());
      }
      if (get_entries) value = MakeEntryPair(isolate, index, value);
      values_or_entries->set(count++, *value);
      if (object->GetElementsKind() != original_elements_kind) break;
    }

    // Slow path caused by changes in elements kind during iteration.
    for (; i < keys->length(); i++) {
      Handle<Object> key(keys->get(i), isolate);
      uint32_t index;
      if (!key->ToUint32(&index)) continue;

      if (filter & ONLY_ENUMERABLE) {
        InternalElementsAccessor* accessor =
            reinterpret_cast<InternalElementsAccessor*>(
                object->GetElementsAccessor());
        InternalIndex entry = accessor->GetEntryForIndex(
            isolate, *object, object->elements(), index);
        if (entry.is_not_found()) continue;
        PropertyDetails details = accessor->GetDetails(*object, entry);
        if (!details.IsEnumerable()) continue;
      }

      Handle<Object> value;
      LookupIterator it(isolate, object, index, LookupIterator::OWN);
      ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, value, Object::GetProperty(&it),
                                       Nothing<bool>());

      if (get_entries) value = MakeEntryPair(isolate, index, value);
      values_or_entries->set(count++, *value);
    }

    *nof_items = count;
    return Just(true);
  }

  V8_WARN_UNUSED_RESULT ExceptionStatus CollectElementIndices(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      KeyAccumulator* keys) final {
    if (keys->filter() & ONLY_ALL_CAN_READ) return ExceptionStatus::kSuccess;
    return Subclass::CollectElementIndicesImpl(object, backing_store, keys);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus CollectElementIndicesImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      KeyAccumulator* keys) {
    DCHECK_NE(DICTIONARY_ELEMENTS, kind());
    // Non-dictionary elements can't have all-can-read accessors.
    size_t length = Subclass::GetMaxIndex(*object, *backing_store);
    PropertyFilter filter = keys->filter();
    Isolate* isolate = keys->isolate();
    Factory* factory = isolate->factory();
    for (size_t i = 0; i < length; i++) {
      if (Subclass::HasElementImpl(isolate, *object, i, *backing_store,
                                   filter)) {
        RETURN_FAILURE_IF_NOT_SUCCESSFUL(
            keys->AddKey(factory->NewNumberFromSize(i)));
      }
    }
    return ExceptionStatus::kSuccess;
  }

  static Handle<FixedArray> DirectCollectElementIndicesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArrayBase> backing_store, GetKeysConversion convert,
      PropertyFilter filter, Handle<FixedArray> list, uint32_t* nof_indices,
      uint32_t insertion_index = 0) {
    size_t length = Subclass::GetMaxIndex(*object, *backing_store);
    uint32_t const kMaxStringTableEntries =
        isolate->heap()->MaxNumberToStringCacheSize();
    for (size_t i = 0; i < length; i++) {
      if (Subclass::HasElementImpl(isolate, *object, i, *backing_store,
                                   filter)) {
        if (convert == GetKeysConversion::kConvertToString) {
          bool use_cache = i < kMaxStringTableEntries;
          Handle<String> index_string =
              isolate->factory()->SizeToString(i, use_cache);
          list->set(insertion_index, *index_string);
        } else {
          Handle<Object> number = isolate->factory()->NewNumberFromSize(i);
          list->set(insertion_index, *number);
        }
        insertion_index++;
      }
    }
    *nof_indices = insertion_index;
    return list;
  }

  MaybeHandle<FixedArray> PrependElementIndices(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      Handle<FixedArray> keys, GetKeysConversion convert,
      PropertyFilter filter) final {
    return Subclass::PrependElementIndicesImpl(object, backing_store, keys,
                                               convert, filter);
  }

  static MaybeHandle<FixedArray> PrependElementIndicesImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      Handle<FixedArray> keys, GetKeysConversion convert,
      PropertyFilter filter) {
    Isolate* isolate = object->GetIsolate();
    uint32_t nof_property_keys = keys->length();
    size_t initial_list_length =
        Subclass::GetMaxNumberOfEntries(*object, *backing_store);

    if (initial_list_length > FixedArray::kMaxLength - nof_property_keys) {
      return isolate->Throw<FixedArray>(isolate->factory()->NewRangeError(
          MessageTemplate::kInvalidArrayLength));
    }
    initial_list_length += nof_property_keys;

    // Collect the element indices into a new list.
    DCHECK_LE(initial_list_length, std::numeric_limits<int>::max());
    MaybeHandle<FixedArray> raw_array = isolate->factory()->TryNewFixedArray(
        static_cast<int>(initial_list_length));
    Handle<FixedArray> combined_keys;

    // If we have a holey backing store try to precisely estimate the backing
    // store size as a last emergency measure if we cannot allocate the big
    // array.
    if (!raw_array.ToHandle(&combined_keys)) {
      if (IsHoleyOrDictionaryElementsKind(kind())) {
        // If we overestimate the result list size we might end up in the
        // large-object space which doesn't free memory on shrinking the list.
        // Hence we try to estimate the final size for holey backing stores more
        // precisely here.
        initial_list_length =
            Subclass::NumberOfElementsImpl(*object, *backing_store);
        initial_list_length += nof_property_keys;
      }
      DCHECK_LE(initial_list_length, std::numeric_limits<int>::max());
      combined_keys = isolate->factory()->NewFixedArray(
          static_cast<int>(initial_list_length));
    }

    uint32_t nof_indices = 0;
    bool needs_sorting = IsDictionaryElementsKind(kind()) ||
                         IsSloppyArgumentsElementsKind(kind());
    combined_keys = Subclass::DirectCollectElementIndicesImpl(
        isolate, object, backing_store,
        needs_sorting ? GetKeysConversion::kKeepNumbers : convert, filter,
        combined_keys, &nof_indices);

    if (needs_sorting) {
      SortIndices(isolate, combined_keys, nof_indices);
      // Indices from dictionary elements should only be converted after
      // sorting.
      if (convert == GetKeysConversion::kConvertToString) {
        for (uint32_t i = 0; i < nof_indices; i++) {
          Handle<Object> index_string = isolate->factory()->Uint32ToString(
              combined_keys->get(i).Number());
          combined_keys->set(i, *index_string);
        }
      }
    }

    // Copy over the passed-in property keys.
    CopyObjectToObjectElements(isolate, *keys, PACKED_ELEMENTS, 0,
                               *combined_keys, PACKED_ELEMENTS, nof_indices,
                               nof_property_keys);

    // For holey elements and arguments we might have to shrink the collected
    // keys since the estimates might be off.
    if (IsHoleyOrDictionaryElementsKind(kind()) ||
        IsSloppyArgumentsElementsKind(kind())) {
      // Shrink combined_keys to the final size.
      int final_size = nof_indices + nof_property_keys;
      DCHECK_LE(final_size, combined_keys->length());
      return FixedArray::ShrinkOrEmpty(isolate, combined_keys, final_size);
    }

    return combined_keys;
  }

  V8_WARN_UNUSED_RESULT ExceptionStatus AddElementsToKeyAccumulator(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) final {
    return Subclass::AddElementsToKeyAccumulatorImpl(receiver, accumulator,
                                                     convert);
  }

  static uint32_t GetCapacityImpl(JSObject holder,
                                  FixedArrayBase backing_store) {
    return backing_store.length();
  }

  size_t GetCapacity(JSObject holder, FixedArrayBase backing_store) final {
    return Subclass::GetCapacityImpl(holder, backing_store);
  }

  static Object FillImpl(Handle<JSObject> receiver, Handle<Object> obj_value,
                         size_t start, size_t end) {
    UNREACHABLE();
  }

  Object Fill(Handle<JSObject> receiver, Handle<Object> obj_value, size_t start,
              size_t end) override {
    return Subclass::FillImpl(receiver, obj_value, start, end);
  }

  static Maybe<bool> IncludesValueImpl(Isolate* isolate,
                                       Handle<JSObject> receiver,
                                       Handle<Object> value, size_t start_from,
                                       size_t length) {
    return IncludesValueSlowPath(isolate, receiver, value, start_from, length);
  }

  Maybe<bool> IncludesValue(Isolate* isolate, Handle<JSObject> receiver,
                            Handle<Object> value, size_t start_from,
                            size_t length) final {
    return Subclass::IncludesValueImpl(isolate, receiver, value, start_from,
                                       length);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> receiver,
                                         Handle<Object> value,
                                         size_t start_from, size_t length) {
    return IndexOfValueSlowPath(isolate, receiver, value, start_from, length);
  }

  Maybe<int64_t> IndexOfValue(Isolate* isolate, Handle<JSObject> receiver,
                              Handle<Object> value, size_t start_from,
                              size_t length) final {
    return Subclass::IndexOfValueImpl(isolate, receiver, value, start_from,
                                      length);
  }

  static Maybe<int64_t> LastIndexOfValueImpl(Handle<JSObject> receiver,
                                             Handle<Object> value,
                                             size_t start_from) {
    UNREACHABLE();
  }

  Maybe<int64_t> LastIndexOfValue(Handle<JSObject> receiver,
                                  Handle<Object> value,
                                  size_t start_from) final {
    return Subclass::LastIndexOfValueImpl(receiver, value, start_from);
  }

  static void ReverseImpl(JSObject receiver) { UNREACHABLE(); }

  void Reverse(JSObject receiver) final { Subclass::ReverseImpl(receiver); }

  static InternalIndex GetEntryForIndexImpl(Isolate* isolate, JSObject holder,
                                            FixedArrayBase backing_store,
                                            size_t index,
                                            PropertyFilter filter) {
    DCHECK(IsFastElementsKind(kind()) ||
           IsAnyNonextensibleElementsKind(kind()));
    size_t length = Subclass::GetMaxIndex(holder, backing_store);
    if (IsHoleyElementsKindForRead(kind())) {
      DCHECK_IMPLIES(
          index < length,
          index <= static_cast<size_t>(std::numeric_limits<int>::max()));
      return index < length &&
                     !BackingStore::cast(backing_store)
                          .is_the_hole(isolate, static_cast<int>(index))
                 ? InternalIndex(index)
                 : InternalIndex::NotFound();
    } else {
      return index < length ? InternalIndex(index) : InternalIndex::NotFound();
    }
  }

  InternalIndex GetEntryForIndex(Isolate* isolate, JSObject holder,
                                 FixedArrayBase backing_store,
                                 size_t index) final {
    return Subclass::GetEntryForIndexImpl(isolate, holder, backing_store, index,
                                          ALL_PROPERTIES);
  }

  static PropertyDetails GetDetailsImpl(FixedArrayBase backing_store,
                                        InternalIndex entry) {
    return PropertyDetails(kData, NONE, PropertyCellType::kNoCell);
  }

  static PropertyDetails GetDetailsImpl(JSObject holder, InternalIndex entry) {
    return PropertyDetails(kData, NONE, PropertyCellType::kNoCell);
  }

  PropertyDetails GetDetails(JSObject holder, InternalIndex entry) final {
    return Subclass::GetDetailsImpl(holder, entry);
  }

  Handle<FixedArray> CreateListFromArrayLike(Isolate* isolate,
                                             Handle<JSObject> object,
                                             uint32_t length) final {
    return Subclass::CreateListFromArrayLikeImpl(isolate, object, length);
  }

  static Handle<FixedArray> CreateListFromArrayLikeImpl(Isolate* isolate,
                                                        Handle<JSObject> object,
                                                        uint32_t length) {
    UNREACHABLE();
  }

 private:
  DISALLOW_COPY_AND_ASSIGN(ElementsAccessorBase);
};

class DictionaryElementsAccessor
    : public ElementsAccessorBase<DictionaryElementsAccessor,
                                  ElementsKindTraits<DICTIONARY_ELEMENTS>> {
 public:
  static uint32_t GetMaxIndex(JSObject receiver, FixedArrayBase elements) {
    // We cannot properly estimate this for dictionaries.
    UNREACHABLE();
  }

  static uint32_t GetMaxNumberOfEntries(JSObject receiver,
                                        FixedArrayBase backing_store) {
    return NumberOfElementsImpl(receiver, backing_store);
  }

  static uint32_t NumberOfElementsImpl(JSObject receiver,
                                       FixedArrayBase backing_store) {
    NumberDictionary dict = NumberDictionary::cast(backing_store);
    return dict.NumberOfElements();
  }

  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    Handle<NumberDictionary> dict =
        Handle<NumberDictionary>::cast(backing_store);
    uint32_t old_length = 0;
    CHECK(array->length().ToArrayLength(&old_length));
    {
      DisallowHeapAllocation no_gc;
      ReadOnlyRoots roots(isolate);
      if (length < old_length) {
        if (dict->requires_slow_elements()) {
          // Find last non-deletable element in range of elements to be
          // deleted and adjust range accordingly.
          for (InternalIndex entry : dict->IterateEntries()) {
            Object index = dict->KeyAt(entry);
            if (dict->IsKey(roots, index)) {
              uint32_t number = static_cast<uint32_t>(index.Number());
              if (length <= number && number < old_length) {
                PropertyDetails details = dict->DetailsAt(entry);
                if (!details.IsConfigurable()) length = number + 1;
              }
            }
          }
        }

        if (length == 0) {
          // Flush the backing store.
          array->initialize_elements();
        } else {
          // Remove elements that should be deleted.
          int removed_entries = 0;
          for (InternalIndex entry : dict->IterateEntries()) {
            Object index = dict->KeyAt(entry);
            if (dict->IsKey(roots, index)) {
              uint32_t number = static_cast<uint32_t>(index.Number());
              if (length <= number && number < old_length) {
                dict->ClearEntry(entry);
                removed_entries++;
              }
            }
          }

          if (removed_entries > 0) {
            // Update the number of elements.
            dict->ElementsRemoved(removed_entries);
          }
        }
      }
    }

    Handle<Object> length_obj = isolate->factory()->NewNumberFromUint(length);
    array->set_length(*length_obj);
  }

  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    UNREACHABLE();
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    Handle<NumberDictionary> dict(NumberDictionary::cast(obj->elements()),
                                  obj->GetIsolate());
    dict = NumberDictionary::DeleteEntry(obj->GetIsolate(), dict, entry);
    obj->set_elements(*dict);
  }

  static bool HasAccessorsImpl(JSObject holder, FixedArrayBase backing_store) {
    DisallowHeapAllocation no_gc;
    NumberDictionary dict = NumberDictionary::cast(backing_store);
    if (!dict.requires_slow_elements()) return false;
    ReadOnlyRoots roots = holder.GetReadOnlyRoots();
    for (InternalIndex i : dict.IterateEntries()) {
      Object key = dict.KeyAt(i);
      if (!dict.IsKey(roots, key)) continue;
      PropertyDetails details = dict.DetailsAt(i);
      if (details.kind() == kAccessor) return true;
    }
    return false;
  }

  static Object GetRaw(FixedArrayBase store, InternalIndex entry) {
    NumberDictionary backing_store = NumberDictionary::cast(store);
    return backing_store.ValueAt(entry);
  }

  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase backing_store,
                                InternalIndex entry) {
    return handle(GetRaw(backing_store, entry), isolate);
  }

  static inline void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                             Object value) {
    SetImpl(holder->elements(), entry, value);
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value) {
    NumberDictionary::cast(backing_store).ValueAtPut(entry, value);
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    NumberDictionary dictionary = NumberDictionary::cast(*store);
    if (attributes != NONE) object->RequireSlowElements(dictionary);
    dictionary.ValueAtPut(entry, *value);
    PropertyDetails details = dictionary.DetailsAt(entry);
    details = PropertyDetails(kData, attributes, PropertyCellType::kNoCell,
                              details.dictionary_index());

    dictionary.DetailsAtPut(entry, details);
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    PropertyDetails details(kData, attributes, PropertyCellType::kNoCell);
    Handle<NumberDictionary> dictionary =
        object->HasFastElements() || object->HasFastStringWrapperElements()
            ? JSObject::NormalizeElements(object)
            : handle(NumberDictionary::cast(object->elements()),
                     object->GetIsolate());
    Handle<NumberDictionary> new_dictionary = NumberDictionary::Add(
        object->GetIsolate(), dictionary, index, value, details);
    new_dictionary->UpdateMaxNumberKey(index, object);
    if (attributes != NONE) object->RequireSlowElements(*new_dictionary);
    if (dictionary.is_identical_to(new_dictionary)) return;
    object->set_elements(*new_dictionary);
  }

  static bool HasEntryImpl(Isolate* isolate, FixedArrayBase store,
                           InternalIndex entry) {
    DisallowHeapAllocation no_gc;
    NumberDictionary dict = NumberDictionary::cast(store);
    Object index = dict.KeyAt(entry);
    return !index.IsTheHole(isolate);
  }

  static InternalIndex GetEntryForIndexImpl(Isolate* isolate, JSObject holder,
                                            FixedArrayBase store, size_t index,
                                            PropertyFilter filter) {
    DisallowHeapAllocation no_gc;
    NumberDictionary dictionary = NumberDictionary::cast(store);
    DCHECK_LE(index, std::numeric_limits<uint32_t>::max());
    InternalIndex entry =
        dictionary.FindEntry(isolate, static_cast<uint32_t>(index));
    if (entry.is_not_found()) return entry;

    if (filter != ALL_PROPERTIES) {
      PropertyDetails details = dictionary.DetailsAt(entry);
      PropertyAttributes attr = details.attributes();
      if ((attr & filter) != 0) return InternalIndex::NotFound();
    }
    return entry;
  }

  static PropertyDetails GetDetailsImpl(JSObject holder, InternalIndex entry) {
    return GetDetailsImpl(holder.elements(), entry);
  }

  static PropertyDetails GetDetailsImpl(FixedArrayBase backing_store,
                                        InternalIndex entry) {
    return NumberDictionary::cast(backing_store).DetailsAt(entry);
  }

  static uint32_t FilterKey(Handle<NumberDictionary> dictionary,
                            InternalIndex entry, Object raw_key,
                            PropertyFilter filter) {
    DCHECK(raw_key.IsNumber());
    DCHECK_LE(raw_key.Number(), kMaxUInt32);
    PropertyDetails details = dictionary->DetailsAt(entry);
    PropertyAttributes attr = details.attributes();
    if ((attr & filter) != 0) return kMaxUInt32;
    return static_cast<uint32_t>(raw_key.Number());
  }

  static uint32_t GetKeyForEntryImpl(Isolate* isolate,
                                     Handle<NumberDictionary> dictionary,
                                     InternalIndex entry,
                                     PropertyFilter filter) {
    DisallowHeapAllocation no_gc;
    Object raw_key = dictionary->KeyAt(entry);
    if (!dictionary->IsKey(ReadOnlyRoots(isolate), raw_key)) return kMaxUInt32;
    return FilterKey(dictionary, entry, raw_key, filter);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus CollectElementIndicesImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      KeyAccumulator* keys) {
    if (keys->filter() & SKIP_STRINGS) return ExceptionStatus::kSuccess;
    Isolate* isolate = keys->isolate();
    Handle<NumberDictionary> dictionary =
        Handle<NumberDictionary>::cast(backing_store);
    Handle<FixedArray> elements = isolate->factory()->NewFixedArray(
        GetMaxNumberOfEntries(*object, *backing_store));
    int insertion_index = 0;
    PropertyFilter filter = keys->filter();
    ReadOnlyRoots roots(isolate);
    for (InternalIndex i : dictionary->IterateEntries()) {
      AllowHeapAllocation allow_gc;
      Object raw_key = dictionary->KeyAt(i);
      if (!dictionary->IsKey(roots, raw_key)) continue;
      uint32_t key = FilterKey(dictionary, i, raw_key, filter);
      if (key == kMaxUInt32) {
        // This might allocate, but {raw_key} is not used afterwards.
        keys->AddShadowingKey(raw_key, &allow_gc);
        continue;
      }
      elements->set(insertion_index, raw_key);
      insertion_index++;
    }
    SortIndices(isolate, elements, insertion_index);
    for (int i = 0; i < insertion_index; i++) {
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(keys->AddKey(elements->get(i)));
    }
    return ExceptionStatus::kSuccess;
  }

  static Handle<FixedArray> DirectCollectElementIndicesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArrayBase> backing_store, GetKeysConversion convert,
      PropertyFilter filter, Handle<FixedArray> list, uint32_t* nof_indices,
      uint32_t insertion_index = 0) {
    if (filter & SKIP_STRINGS) return list;
    if (filter & ONLY_ALL_CAN_READ) return list;

    Handle<NumberDictionary> dictionary =
        Handle<NumberDictionary>::cast(backing_store);
    for (InternalIndex i : dictionary->IterateEntries()) {
      uint32_t key = GetKeyForEntryImpl(isolate, dictionary, i, filter);
      if (key == kMaxUInt32) continue;
      Handle<Object> index = isolate->factory()->NewNumberFromUint(key);
      list->set(insertion_index, *index);
      insertion_index++;
    }
    *nof_indices = insertion_index;
    return list;
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus AddElementsToKeyAccumulatorImpl(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) {
    Isolate* isolate = accumulator->isolate();
    Handle<NumberDictionary> dictionary(
        NumberDictionary::cast(receiver->elements()), isolate);
    ReadOnlyRoots roots(isolate);
    for (InternalIndex i : dictionary->IterateEntries()) {
      Object k = dictionary->KeyAt(i);
      if (!dictionary->IsKey(roots, k)) continue;
      Object value = dictionary->ValueAt(i);
      DCHECK(!value.IsTheHole(isolate));
      DCHECK(!value.IsAccessorPair());
      DCHECK(!value.IsAccessorInfo());
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(accumulator->AddKey(value, convert));
    }
    return ExceptionStatus::kSuccess;
  }

  static bool IncludesValueFastPath(Isolate* isolate, Handle<JSObject> receiver,
                                    Handle<Object> value, size_t start_from,
                                    size_t length, Maybe<bool>* result) {
    DisallowHeapAllocation no_gc;
    NumberDictionary dictionary = NumberDictionary::cast(receiver->elements());
    Object the_hole = ReadOnlyRoots(isolate).the_hole_value();
    Object undefined = ReadOnlyRoots(isolate).undefined_value();

    // Scan for accessor properties. If accessors are present, then elements
    // must be accessed in order via the slow path.
    bool found = false;
    for (InternalIndex i : dictionary.IterateEntries()) {
      Object k = dictionary.KeyAt(i);
      if (k == the_hole) continue;
      if (k == undefined) continue;

      uint32_t index;
      if (!k.ToArrayIndex(&index) || index < start_from || index >= length) {
        continue;
      }

      if (dictionary.DetailsAt(i).kind() == kAccessor) {
        // Restart from beginning in slow path, otherwise we may observably
        // access getters out of order
        return false;
      } else if (!found) {
        Object element_k = dictionary.ValueAt(i);
        if (value->SameValueZero(element_k)) found = true;
      }
    }

    *result = Just(found);
    return true;
  }

  static Maybe<bool> IncludesValueImpl(Isolate* isolate,
                                       Handle<JSObject> receiver,
                                       Handle<Object> value, size_t start_from,
                                       size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *receiver));
    bool search_for_hole = value->IsUndefined(isolate);

    if (!search_for_hole) {
      Maybe<bool> result = Nothing<bool>();
      if (DictionaryElementsAccessor::IncludesValueFastPath(
              isolate, receiver, value, start_from, length, &result)) {
        return result;
      }
    }
    ElementsKind original_elements_kind = receiver->GetElementsKind();
    USE(original_elements_kind);
    Handle<NumberDictionary> dictionary(
        NumberDictionary::cast(receiver->elements()), isolate);
    // Iterate through the entire range, as accessing elements out of order is
    // observable.
    for (size_t k = start_from; k < length; ++k) {
      DCHECK_EQ(receiver->GetElementsKind(), original_elements_kind);
      InternalIndex entry =
          dictionary->FindEntry(isolate, static_cast<uint32_t>(k));
      if (entry.is_not_found()) {
        if (search_for_hole) return Just(true);
        continue;
      }

      PropertyDetails details = GetDetailsImpl(*dictionary, entry);
      switch (details.kind()) {
        case kData: {
          Object element_k = dictionary->ValueAt(entry);
          if (value->SameValueZero(element_k)) return Just(true);
          break;
        }
        case kAccessor: {
          LookupIterator it(isolate, receiver, k,
                            LookupIterator::OWN_SKIP_INTERCEPTOR);
          DCHECK(it.IsFound());
          DCHECK_EQ(it.state(), LookupIterator::ACCESSOR);
          Handle<Object> element_k;

          ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, element_k,
                                           Object::GetPropertyWithAccessor(&it),
                                           Nothing<bool>());

          if (value->SameValueZero(*element_k)) return Just(true);

          // Bailout to slow path if elements on prototype changed
          if (!JSObject::PrototypeHasNoElements(isolate, *receiver)) {
            return IncludesValueSlowPath(isolate, receiver, value, k + 1,
                                         length);
          }

          // Continue if elements unchanged
          if (*dictionary == receiver->elements()) continue;

          // Otherwise, bailout or update elements

          // If switched to initial elements, return true if searching for
          // undefined, and false otherwise.
          if (receiver->map().GetInitialElements() == receiver->elements()) {
            return Just(search_for_hole);
          }

          // If switched to fast elements, continue with the correct accessor.
          if (receiver->GetElementsKind() != DICTIONARY_ELEMENTS) {
            ElementsAccessor* accessor = receiver->GetElementsAccessor();
            return accessor->IncludesValue(isolate, receiver, value, k + 1,
                                           length);
          }
          dictionary =
              handle(NumberDictionary::cast(receiver->elements()), isolate);
          break;
        }
      }
    }
    return Just(false);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> receiver,
                                         Handle<Object> value,
                                         size_t start_from, size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *receiver));

    ElementsKind original_elements_kind = receiver->GetElementsKind();
    USE(original_elements_kind);
    Handle<NumberDictionary> dictionary(
        NumberDictionary::cast(receiver->elements()), isolate);
    // Iterate through entire range, as accessing elements out of order is
    // observable.
    for (size_t k = start_from; k < length; ++k) {
      DCHECK_EQ(receiver->GetElementsKind(), original_elements_kind);
      DCHECK_LE(k, std::numeric_limits<uint32_t>::max());
      InternalIndex entry =
          dictionary->FindEntry(isolate, static_cast<uint32_t>(k));
      if (entry.is_not_found()) continue;

      PropertyDetails details =
          GetDetailsImpl(*dictionary, InternalIndex(entry));
      switch (details.kind()) {
        case kData: {
          Object element_k = dictionary->ValueAt(entry);
          if (value->StrictEquals(element_k)) {
            return Just<int64_t>(k);
          }
          break;
        }
        case kAccessor: {
          LookupIterator it(isolate, receiver, k,
                            LookupIterator::OWN_SKIP_INTERCEPTOR);
          DCHECK(it.IsFound());
          DCHECK_EQ(it.state(), LookupIterator::ACCESSOR);
          Handle<Object> element_k;

          ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, element_k,
                                           Object::GetPropertyWithAccessor(&it),
                                           Nothing<int64_t>());

          if (value->StrictEquals(*element_k)) return Just<int64_t>(k);

          // Bailout to slow path if elements on prototype changed.
          if (!JSObject::PrototypeHasNoElements(isolate, *receiver)) {
            return IndexOfValueSlowPath(isolate, receiver, value, k + 1,
                                        length);
          }

          // Continue if elements unchanged.
          if (*dictionary == receiver->elements()) continue;

          // Otherwise, bailout or update elements.
          if (receiver->GetElementsKind() != DICTIONARY_ELEMENTS) {
            // Otherwise, switch to slow path.
            return IndexOfValueSlowPath(isolate, receiver, value, k + 1,
                                        length);
          }
          dictionary =
              handle(NumberDictionary::cast(receiver->elements()), isolate);
          break;
        }
      }
    }
    return Just<int64_t>(-1);
  }

  static void ValidateContents(JSObject holder, size_t length) {
    DisallowHeapAllocation no_gc;
#if DEBUG
    DCHECK_EQ(holder.map().elements_kind(), DICTIONARY_ELEMENTS);
    if (!FLAG_enable_slow_asserts) return;
    ReadOnlyRoots roots = holder.GetReadOnlyRoots();
    NumberDictionary dictionary = NumberDictionary::cast(holder.elements());
    // Validate the requires_slow_elements and max_number_key values.
    bool requires_slow_elements = false;
    int max_key = 0;
    for (InternalIndex i : dictionary.IterateEntries()) {
      Object k;
      if (!dictionary.ToKey(roots, i, &k)) continue;
      DCHECK_LE(0.0, k.Number());
      if (k.Number() > NumberDictionary::kRequiresSlowElementsLimit) {
        requires_slow_elements = true;
      } else {
        max_key = Max(max_key, Smi::ToInt(k));
      }
    }
    if (requires_slow_elements) {
      DCHECK(dictionary.requires_slow_elements());
    } else if (!dictionary.requires_slow_elements()) {
      DCHECK_LE(max_key, dictionary.max_number_key());
    }
#endif
  }
};

// Super class for all fast element arrays.
template <typename Subclass, typename KindTraits>
class FastElementsAccessor : public ElementsAccessorBase<Subclass, KindTraits> {
 public:
  using BackingStore = typename KindTraits::BackingStore;

  static Handle<NumberDictionary> NormalizeImpl(Handle<JSObject> object,
                                                Handle<FixedArrayBase> store) {
    Isolate* isolate = object->GetIsolate();
    ElementsKind kind = Subclass::kind();

    // Ensure that notifications fire if the array or object prototypes are
    // normalizing.
    if (IsSmiOrObjectElementsKind(kind) ||
        kind == FAST_STRING_WRAPPER_ELEMENTS) {
      isolate->UpdateNoElementsProtectorOnNormalizeElements(object);
    }

    int capacity = object->GetFastElementsUsage();
    Handle<NumberDictionary> dictionary =
        NumberDictionary::New(isolate, capacity);

    PropertyDetails details = PropertyDetails::Empty();
    int j = 0;
    int max_number_key = -1;
    for (int i = 0; j < capacity; i++) {
      if (IsHoleyElementsKindForRead(kind)) {
        if (BackingStore::cast(*store).is_the_hole(isolate, i)) continue;
      }
      max_number_key = i;
      Handle<Object> value =
          Subclass::GetImpl(isolate, *store, InternalIndex(i));
      dictionary =
          NumberDictionary::Add(isolate, dictionary, i, value, details);
      j++;
    }

    if (max_number_key > 0) {
      dictionary->UpdateMaxNumberKey(static_cast<uint32_t>(max_number_key),
                                     object);
    }
    return dictionary;
  }

  static void DeleteAtEnd(Handle<JSObject> obj,
                          Handle<BackingStore> backing_store, uint32_t entry) {
    uint32_t length = static_cast<uint32_t>(backing_store->length());
    Isolate* isolate = obj->GetIsolate();
    for (; entry > 0; entry--) {
      if (!backing_store->is_the_hole(isolate, entry - 1)) break;
    }
    if (entry == 0) {
      FixedArray empty = ReadOnlyRoots(isolate).empty_fixed_array();
      // Dynamically ask for the elements kind here since we manually redirect
      // the operations for argument backing stores.
      if (obj->GetElementsKind() == FAST_SLOPPY_ARGUMENTS_ELEMENTS) {
        SloppyArgumentsElements::cast(obj->elements()).set_arguments(empty);
      } else {
        obj->set_elements(empty);
      }
      return;
    }

    isolate->heap()->RightTrimFixedArray(*backing_store, length - entry);
  }

  static void DeleteCommon(Handle<JSObject> obj, uint32_t entry,
                           Handle<FixedArrayBase> store) {
    DCHECK(obj->HasSmiOrObjectElements() || obj->HasDoubleElements() ||
           obj->HasNonextensibleElements() || obj->HasFastArgumentsElements() ||
           obj->HasFastStringWrapperElements());
    Handle<BackingStore> backing_store = Handle<BackingStore>::cast(store);
    if (!obj->IsJSArray() &&
        entry == static_cast<uint32_t>(store->length()) - 1) {
      DeleteAtEnd(obj, backing_store, entry);
      return;
    }

    Isolate* isolate = obj->GetIsolate();
    backing_store->set_the_hole(isolate, entry);

    // TODO(verwaest): Move this out of elements.cc.
    // If an old space backing store is larger than a certain size and
    // has too few used values, normalize it.
    const int kMinLengthForSparsenessCheck = 64;
    if (backing_store->length() < kMinLengthForSparsenessCheck) return;
    // TODO(ulan): Check if it works with young large objects.
    if (ObjectInYoungGeneration(*backing_store)) return;
    uint32_t length = 0;
    if (obj->IsJSArray()) {
      JSArray::cast(*obj).length().ToArrayLength(&length);
    } else {
      length = static_cast<uint32_t>(store->length());
    }

    // To avoid doing the check on every delete, use a counter-based heuristic.
    const int kLengthFraction = 16;
    // The above constant must be large enough to ensure that we check for
    // normalization frequently enough. At a minimum, it should be large
    // enough to reliably hit the "window" of remaining elements count where
    // normalization would be beneficial.
    STATIC_ASSERT(kLengthFraction >=
                  NumberDictionary::kEntrySize *
                      NumberDictionary::kPreferFastElementsSizeFactor);
    size_t current_counter = isolate->elements_deletion_counter();
    if (current_counter < length / kLengthFraction) {
      isolate->set_elements_deletion_counter(current_counter + 1);
      return;
    }
    // Reset the counter whenever the full check is performed.
    isolate->set_elements_deletion_counter(0);

    if (!obj->IsJSArray()) {
      uint32_t i;
      for (i = entry + 1; i < length; i++) {
        if (!backing_store->is_the_hole(isolate, i)) break;
      }
      if (i == length) {
        DeleteAtEnd(obj, backing_store, entry);
        return;
      }
    }
    int num_used = 0;
    for (int i = 0; i < backing_store->length(); ++i) {
      if (!backing_store->is_the_hole(isolate, i)) {
        ++num_used;
        // Bail out if a number dictionary wouldn't be able to save much space.
        if (NumberDictionary::kPreferFastElementsSizeFactor *
                NumberDictionary::ComputeCapacity(num_used) *
                NumberDictionary::kEntrySize >
            static_cast<uint32_t>(backing_store->length())) {
          return;
        }
      }
    }
    JSObject::NormalizeElements(obj);
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    Handle<NumberDictionary> dictionary = JSObject::NormalizeElements(object);
    entry = InternalIndex(
        dictionary->FindEntry(object->GetIsolate(), entry.as_uint32()));
    DictionaryElementsAccessor::ReconfigureImpl(object, dictionary, entry,
                                                value, attributes);
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    DCHECK_EQ(NONE, attributes);
    ElementsKind from_kind = object->GetElementsKind();
    ElementsKind to_kind = Subclass::kind();
    if (IsDictionaryElementsKind(from_kind) ||
        IsDoubleElementsKind(from_kind) != IsDoubleElementsKind(to_kind) ||
        Subclass::GetCapacityImpl(*object, object->elements()) !=
            new_capacity) {
      Subclass::GrowCapacityAndConvertImpl(object, new_capacity);
    } else {
      if (IsFastElementsKind(from_kind) && from_kind != to_kind) {
        JSObject::TransitionElementsKind(object, to_kind);
      }
      if (IsSmiOrObjectElementsKind(from_kind)) {
        DCHECK(IsSmiOrObjectElementsKind(to_kind));
        JSObject::EnsureWritableFastElements(object);
      }
    }
    Subclass::SetImpl(object, InternalIndex(index), *value);
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    ElementsKind kind = KindTraits::Kind;
    if (IsFastPackedElementsKind(kind) ||
        kind == PACKED_NONEXTENSIBLE_ELEMENTS) {
      JSObject::TransitionElementsKind(obj, GetHoleyElementsKind(kind));
    }
    if (IsSmiOrObjectElementsKind(KindTraits::Kind) ||
        IsNonextensibleElementsKind(kind)) {
      JSObject::EnsureWritableFastElements(obj);
    }
    DeleteCommon(obj, entry.as_uint32(),
                 handle(obj->elements(), obj->GetIsolate()));
  }

  static bool HasEntryImpl(Isolate* isolate, FixedArrayBase backing_store,
                           InternalIndex entry) {
    return !BackingStore::cast(backing_store)
                .is_the_hole(isolate, entry.as_int());
  }

  static uint32_t NumberOfElementsImpl(JSObject receiver,
                                       FixedArrayBase backing_store) {
    size_t max_index = Subclass::GetMaxIndex(receiver, backing_store);
    DCHECK_LE(max_index, std::numeric_limits<uint32_t>::max());
    if (IsFastPackedElementsKind(Subclass::kind())) {
      return static_cast<uint32_t>(max_index);
    }
    Isolate* isolate = receiver.GetIsolate();
    uint32_t count = 0;
    for (size_t i = 0; i < max_index; i++) {
      if (Subclass::HasEntryImpl(isolate, backing_store, InternalIndex(i))) {
        count++;
      }
    }
    return count;
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus AddElementsToKeyAccumulatorImpl(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) {
    Isolate* isolate = accumulator->isolate();
    Handle<FixedArrayBase> elements(receiver->elements(), isolate);
    size_t length = Subclass::GetMaxNumberOfEntries(*receiver, *elements);
    for (size_t i = 0; i < length; i++) {
      if (IsFastPackedElementsKind(KindTraits::Kind) ||
          HasEntryImpl(isolate, *elements, InternalIndex(i))) {
        RETURN_FAILURE_IF_NOT_SUCCESSFUL(accumulator->AddKey(
            Subclass::GetImpl(isolate, *elements, InternalIndex(i)), convert));
      }
    }
    return ExceptionStatus::kSuccess;
  }

  static void ValidateContents(JSObject holder, size_t length) {
#if DEBUG
    Isolate* isolate = holder.GetIsolate();
    Heap* heap = isolate->heap();
    FixedArrayBase elements = holder.elements();
    Map map = elements.map();
    if (IsSmiOrObjectElementsKind(KindTraits::Kind)) {
      DCHECK_NE(map, ReadOnlyRoots(heap).fixed_double_array_map());
    } else if (IsDoubleElementsKind(KindTraits::Kind)) {
      DCHECK_NE(map, ReadOnlyRoots(heap).fixed_cow_array_map());
      if (map == ReadOnlyRoots(heap).fixed_array_map()) DCHECK_EQ(0u, length);
    } else {
      UNREACHABLE();
    }
    if (length == 0u) return;  // nothing to do!
#if ENABLE_SLOW_DCHECKS
    DisallowHeapAllocation no_gc;
    BackingStore backing_store = BackingStore::cast(elements);
    DCHECK(length <= std::numeric_limits<int>::max());
    int length_int = static_cast<int>(length);
    if (IsSmiElementsKind(KindTraits::Kind)) {
      HandleScope scope(isolate);
      for (int i = 0; i < length_int; i++) {
        DCHECK(BackingStore::get(backing_store, i, isolate)->IsSmi() ||
               (IsHoleyElementsKind(KindTraits::Kind) &&
                backing_store.is_the_hole(isolate, i)));
      }
    } else if (KindTraits::Kind == PACKED_ELEMENTS ||
               KindTraits::Kind == PACKED_DOUBLE_ELEMENTS) {
      for (int i = 0; i < length_int; i++) {
        DCHECK(!backing_store.is_the_hole(isolate, i));
      }
    } else {
      DCHECK(IsHoleyElementsKind(KindTraits::Kind));
    }
#endif
#endif
  }

  static Handle<Object> PopImpl(Handle<JSArray> receiver) {
    return Subclass::RemoveElement(receiver, AT_END);
  }

  static Handle<Object> ShiftImpl(Handle<JSArray> receiver) {
    return Subclass::RemoveElement(receiver, AT_START);
  }

  static uint32_t PushImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                           uint32_t push_size) {
    Handle<FixedArrayBase> backing_store(receiver->elements(),
                                         receiver->GetIsolate());
    return Subclass::AddArguments(receiver, backing_store, args, push_size,
                                  AT_END);
  }

  static uint32_t UnshiftImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                              uint32_t unshift_size) {
    Handle<FixedArrayBase> backing_store(receiver->elements(),
                                         receiver->GetIsolate());
    return Subclass::AddArguments(receiver, backing_store, args, unshift_size,
                                  AT_START);
  }

  static void MoveElements(Isolate* isolate, Handle<JSArray> receiver,
                           Handle<FixedArrayBase> backing_store, int dst_index,
                           int src_index, int len, int hole_start,
                           int hole_end) {
    Handle<BackingStore> dst_elms = Handle<BackingStore>::cast(backing_store);
    if (len > JSArray::kMaxCopyElements && dst_index == 0 &&
        isolate->heap()->CanMoveObjectStart(*dst_elms)) {
      // Update all the copies of this backing_store handle.
      *dst_elms.location() =
          BackingStore::cast(
              isolate->heap()->LeftTrimFixedArray(*dst_elms, src_index))
              .ptr();
      receiver->set_elements(*dst_elms);
      // Adjust the hole offset as the array has been shrunk.
      hole_end -= src_index;
      DCHECK_LE(hole_start, backing_store->length());
      DCHECK_LE(hole_end, backing_store->length());
    } else if (len != 0) {
      WriteBarrierMode mode = GetWriteBarrierMode(KindTraits::Kind);
      dst_elms->MoveElements(isolate, dst_index, src_index, len, mode);
    }
    if (hole_start != hole_end) {
      dst_elms->FillWithHoles(hole_start, hole_end);
    }
  }

  static Object FillImpl(Handle<JSObject> receiver, Handle<Object> obj_value,
                         size_t start, size_t end) {
    // Ensure indexes are within array bounds
    DCHECK_LE(0, start);
    DCHECK_LE(start, end);

    // Make sure COW arrays are copied.
    if (IsSmiOrObjectElementsKind(Subclass::kind())) {
      JSObject::EnsureWritableFastElements(receiver);
    }

    // Make sure we have enough space.
    DCHECK_LE(end, std::numeric_limits<uint32_t>::max());
    if (end > Subclass::GetCapacityImpl(*receiver, receiver->elements())) {
      Subclass::GrowCapacityAndConvertImpl(receiver,
                                           static_cast<uint32_t>(end));
      CHECK_EQ(Subclass::kind(), receiver->GetElementsKind());
    }
    DCHECK_LE(end, Subclass::GetCapacityImpl(*receiver, receiver->elements()));

    for (size_t index = start; index < end; ++index) {
      Subclass::SetImpl(receiver, InternalIndex(index), *obj_value);
    }
    return *receiver;
  }

  static Maybe<bool> IncludesValueImpl(Isolate* isolate,
                                       Handle<JSObject> receiver,
                                       Handle<Object> search_value,
                                       size_t start_from, size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *receiver));
    DisallowHeapAllocation no_gc;
    FixedArrayBase elements_base = receiver->elements();
    Object the_hole = ReadOnlyRoots(isolate).the_hole_value();
    Object undefined = ReadOnlyRoots(isolate).undefined_value();
    Object value = *search_value;

    if (start_from >= length) return Just(false);

    // Elements beyond the capacity of the backing store treated as undefined.
    size_t elements_length = static_cast<size_t>(elements_base.length());
    if (value == undefined && elements_length < length) return Just(true);
    if (elements_length == 0) {
      DCHECK_NE(value, undefined);
      return Just(false);
    }

    length = std::min(elements_length, length);
    DCHECK_LE(length, std::numeric_limits<int>::max());

    if (!value.IsNumber()) {
      if (value == undefined) {
        // Search for `undefined` or The Hole. Even in the case of
        // PACKED_DOUBLE_ELEMENTS or PACKED_SMI_ELEMENTS, we might encounter The
        // Hole here, since the {length} used here can be larger than
        // JSArray::length.
        if (IsSmiOrObjectElementsKind(Subclass::kind()) ||
            IsAnyNonextensibleElementsKind(Subclass::kind())) {
          FixedArray elements = FixedArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            Object element_k = elements.get(static_cast<int>(k));

            if (element_k == the_hole || element_k == undefined) {
              return Just(true);
            }
          }
          return Just(false);
        } else {
          // Search for The Hole in HOLEY_DOUBLE_ELEMENTS or
          // PACKED_DOUBLE_ELEMENTS.
          DCHECK(IsDoubleElementsKind(Subclass::kind()));
          FixedDoubleArray elements =
              FixedDoubleArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            if (elements.is_the_hole(static_cast<int>(k))) return Just(true);
          }
          return Just(false);
        }
      } else if (!IsObjectElementsKind(Subclass::kind()) &&
                 !IsAnyNonextensibleElementsKind(Subclass::kind())) {
        // Search for non-number, non-Undefined value, with either
        // PACKED_SMI_ELEMENTS, PACKED_DOUBLE_ELEMENTS, HOLEY_SMI_ELEMENTS or
        // HOLEY_DOUBLE_ELEMENTS. Guaranteed to return false, since these
        // elements kinds can only contain Number values or undefined.
        return Just(false);
      } else {
        // Search for non-number, non-Undefined value with either
        // PACKED_ELEMENTS or HOLEY_ELEMENTS.
        DCHECK(IsObjectElementsKind(Subclass::kind()) ||
               IsAnyNonextensibleElementsKind(Subclass::kind()));
        FixedArray elements = FixedArray::cast(receiver->elements());

        for (size_t k = start_from; k < length; ++k) {
          Object element_k = elements.get(static_cast<int>(k));
          if (element_k == the_hole) continue;
          if (value.SameValueZero(element_k)) return Just(true);
        }
        return Just(false);
      }
    } else {
      if (!value.IsNaN()) {
        double search_value = value.Number();
        if (IsDoubleElementsKind(Subclass::kind())) {
          // Search for non-NaN Number in PACKED_DOUBLE_ELEMENTS or
          // HOLEY_DOUBLE_ELEMENTS --- Skip TheHole, and trust UCOMISD or
          // similar operation for result.
          FixedDoubleArray elements =
              FixedDoubleArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            if (elements.is_the_hole(static_cast<int>(k))) continue;
            if (elements.get_scalar(static_cast<int>(k)) == search_value) {
              return Just(true);
            }
          }
          return Just(false);
        } else {
          // Search for non-NaN Number in PACKED_ELEMENTS, HOLEY_ELEMENTS,
          // PACKED_SMI_ELEMENTS or HOLEY_SMI_ELEMENTS --- Skip non-Numbers,
          // and trust UCOMISD or similar operation for result
          FixedArray elements = FixedArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            Object element_k = elements.get(static_cast<int>(k));
            if (element_k.IsNumber() && element_k.Number() == search_value) {
              return Just(true);
            }
          }
          return Just(false);
        }
      } else {
        // Search for NaN --- NaN cannot be represented with Smi elements, so
        // abort if ElementsKind is PACKED_SMI_ELEMENTS or HOLEY_SMI_ELEMENTS
        if (IsSmiElementsKind(Subclass::kind())) return Just(false);

        if (IsDoubleElementsKind(Subclass::kind())) {
          // Search for NaN in PACKED_DOUBLE_ELEMENTS or
          // HOLEY_DOUBLE_ELEMENTS --- Skip The Hole and trust
          // std::isnan(elementK) for result
          FixedDoubleArray elements =
              FixedDoubleArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            if (elements.is_the_hole(static_cast<int>(k))) continue;
            if (std::isnan(elements.get_scalar(static_cast<int>(k)))) {
              return Just(true);
            }
          }
          return Just(false);
        } else {
          // Search for NaN in PACKED_ELEMENTS or HOLEY_ELEMENTS. Return true
          // if elementK->IsHeapNumber() && std::isnan(elementK->Number())
          DCHECK(IsObjectElementsKind(Subclass::kind()) ||
                 IsAnyNonextensibleElementsKind(Subclass::kind()));
          FixedArray elements = FixedArray::cast(receiver->elements());

          for (size_t k = start_from; k < length; ++k) {
            if (elements.get(static_cast<int>(k)).IsNaN()) return Just(true);
          }
          return Just(false);
        }
      }
    }
  }

  static Handle<FixedArray> CreateListFromArrayLikeImpl(Isolate* isolate,
                                                        Handle<JSObject> object,
                                                        uint32_t length) {
    Handle<FixedArray> result = isolate->factory()->NewFixedArray(length);
    Handle<FixedArrayBase> elements(object->elements(), isolate);
    for (uint32_t i = 0; i < length; i++) {
      InternalIndex entry(i);
      if (!Subclass::HasEntryImpl(isolate, *elements, entry)) continue;
      Handle<Object> value;
      value = Subclass::GetImpl(isolate, *elements, entry);
      if (value->IsName()) {
        value = isolate->factory()->InternalizeName(Handle<Name>::cast(value));
      }
      result->set(i, *value);
    }
    return result;
  }

  static Handle<Object> RemoveElement(Handle<JSArray> receiver,
                                      Where remove_position) {
    Isolate* isolate = receiver->GetIsolate();
    ElementsKind kind = KindTraits::Kind;
    if (IsSmiOrObjectElementsKind(kind)) {
      HandleScope scope(isolate);
      JSObject::EnsureWritableFastElements(receiver);
    }
    Handle<FixedArrayBase> backing_store(receiver->elements(), isolate);
    uint32_t length = static_cast<uint32_t>(Smi::ToInt(receiver->length()));
    DCHECK_GT(length, 0);
    int new_length = length - 1;
    int remove_index = remove_position == AT_START ? 0 : new_length;
    Handle<Object> result =
        Subclass::GetImpl(isolate, *backing_store, InternalIndex(remove_index));
    if (remove_position == AT_START) {
      Subclass::MoveElements(isolate, receiver, backing_store, 0, 1, new_length,
                             0, 0);
    }
    Subclass::SetLengthImpl(isolate, receiver, new_length, backing_store);

    if (IsHoleyElementsKind(kind) && result->IsTheHole(isolate)) {
      return isolate->factory()->undefined_value();
    }
    return result;
  }

  static uint32_t AddArguments(Handle<JSArray> receiver,
                               Handle<FixedArrayBase> backing_store,
                               BuiltinArguments* args, uint32_t add_size,
                               Where add_position) {
    uint32_t length = Smi::ToInt(receiver->length());
    DCHECK_LT(0, add_size);
    uint32_t elms_len = backing_store->length();
    // Check we do not overflow the new_length.
    DCHECK(add_size <= static_cast<uint32_t>(Smi::kMaxValue - length));
    uint32_t new_length = length + add_size;

    if (new_length > elms_len) {
      // New backing storage is needed.
      uint32_t capacity = JSObject::NewElementsCapacity(new_length);
      // If we add arguments to the start we have to shift the existing objects.
      int copy_dst_index = add_position == AT_START ? add_size : 0;
      // Copy over all objects to a new backing_store.
      backing_store = Subclass::ConvertElementsWithCapacity(
          receiver, backing_store, KindTraits::Kind, capacity, 0,
          copy_dst_index);
      receiver->set_elements(*backing_store);
    } else if (add_position == AT_START) {
      // If the backing store has enough capacity and we add elements to the
      // start we have to shift the existing objects.
      Isolate* isolate = receiver->GetIsolate();
      Subclass::MoveElements(isolate, receiver, backing_store, add_size, 0,
                             length, 0, 0);
    }

    int insertion_index = add_position == AT_START ? 0 : length;
    // Copy the arguments to the start.
    Subclass::CopyArguments(args, backing_store, add_size, 1, insertion_index);
    // Set the length.
    receiver->set_length(Smi::FromInt(new_length));
    return new_length;
  }

  static void CopyArguments(BuiltinArguments* args,
                            Handle<FixedArrayBase> dst_store,
                            uint32_t copy_size, uint32_t src_index,
                            uint32_t dst_index) {
    // Add the provided values.
    DisallowHeapAllocation no_gc;
    FixedArrayBase raw_backing_store = *dst_store;
    WriteBarrierMode mode = raw_backing_store.GetWriteBarrierMode(no_gc);
    for (uint32_t i = 0; i < copy_size; i++) {
      Object argument = (*args)[src_index + i];
      DCHECK(!argument.IsTheHole());
      Subclass::SetImpl(raw_backing_store, InternalIndex(dst_index + i),
                        argument, mode);
    }
  }
};

template <typename Subclass, typename KindTraits>
class FastSmiOrObjectElementsAccessor
    : public FastElementsAccessor<Subclass, KindTraits> {
 public:
  static inline void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                             Object value) {
    SetImpl(holder->elements(), entry, value);
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value) {
    FixedArray::cast(backing_store).set(entry.as_int(), value);
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value, WriteBarrierMode mode) {
    FixedArray::cast(backing_store).set(entry.as_int(), value, mode);
  }

  static Object GetRaw(FixedArray backing_store, InternalIndex entry) {
    return backing_store.get(entry.as_int());
  }

  // NOTE: this method violates the handlified function signature convention:
  // raw pointer parameters in the function that allocates.
  // See ElementsAccessor::CopyElements() for details.
  // This method could actually allocate if copying from double elements to
  // object elements.
  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    DisallowHeapAllocation no_gc;
    ElementsKind to_kind = KindTraits::Kind;
    switch (from_kind) {
      case PACKED_SMI_ELEMENTS:
      case HOLEY_SMI_ELEMENTS:
      case PACKED_ELEMENTS:
      case PACKED_FROZEN_ELEMENTS:
      case PACKED_SEALED_ELEMENTS:
      case PACKED_NONEXTENSIBLE_ELEMENTS:
      case HOLEY_ELEMENTS:
      case HOLEY_FROZEN_ELEMENTS:
      case HOLEY_SEALED_ELEMENTS:
      case HOLEY_NONEXTENSIBLE_ELEMENTS:
        CopyObjectToObjectElements(isolate, from, from_kind, from_start, to,
                                   to_kind, to_start, copy_size);
        break;
      case PACKED_DOUBLE_ELEMENTS:
      case HOLEY_DOUBLE_ELEMENTS: {
        AllowHeapAllocation allow_allocation;
        DCHECK(IsObjectElementsKind(to_kind));
        CopyDoubleToObjectElements(isolate, from, from_start, to, to_start,
                                   copy_size);
        break;
      }
      case DICTIONARY_ELEMENTS:
        CopyDictionaryToObjectElements(isolate, from, from_start, to, to_kind,
                                       to_start, copy_size);
        break;
      case FAST_SLOPPY_ARGUMENTS_ELEMENTS:
      case SLOW_SLOPPY_ARGUMENTS_ELEMENTS:
      case FAST_STRING_WRAPPER_ELEMENTS:
      case SLOW_STRING_WRAPPER_ELEMENTS:
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype) case TYPE##_ELEMENTS:
        TYPED_ARRAYS(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE
        // This function is currently only used for JSArrays with non-zero
        // length.
        UNREACHABLE();
      case NO_ELEMENTS:
        break;  // Nothing to do.
    }
  }

  static Maybe<bool> CollectValuesOrEntriesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArray> values_or_entries, bool get_entries, int* nof_items,
      PropertyFilter filter) {
    int count = 0;
    if (get_entries) {
      // Collecting entries needs to allocate, so this code must be handlified.
      Handle<FixedArray> elements(FixedArray::cast(object->elements()),
                                  isolate);
      uint32_t length = elements->length();
      for (uint32_t index = 0; index < length; ++index) {
        InternalIndex entry(index);
        if (!Subclass::HasEntryImpl(isolate, *elements, entry)) continue;
        Handle<Object> value = Subclass::GetImpl(isolate, *elements, entry);
        value = MakeEntryPair(isolate, index, value);
        values_or_entries->set(count++, *value);
      }
    } else {
      // No allocations here, so we can avoid handlification overhead.
      DisallowHeapAllocation no_gc;
      FixedArray elements = FixedArray::cast(object->elements());
      uint32_t length = elements.length();
      for (uint32_t index = 0; index < length; ++index) {
        InternalIndex entry(index);
        if (!Subclass::HasEntryImpl(isolate, elements, entry)) continue;
        Object value = GetRaw(elements, entry);
        values_or_entries->set(count++, value);
      }
    }
    *nof_items = count;
    return Just(true);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> receiver,
                                         Handle<Object> search_value,
                                         size_t start_from, size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *receiver));
    DisallowHeapAllocation no_gc;
    FixedArrayBase elements_base = receiver->elements();
    Object value = *search_value;

    if (start_from >= length) return Just<int64_t>(-1);

    length = std::min(static_cast<size_t>(elements_base.length()), length);

    // Only FAST_{,HOLEY_}ELEMENTS can store non-numbers.
    if (!value.IsNumber() && !IsObjectElementsKind(Subclass::kind()) &&
        !IsAnyNonextensibleElementsKind(Subclass::kind())) {
      return Just<int64_t>(-1);
    }
    // NaN can never be found by strict equality.
    if (value.IsNaN()) return Just<int64_t>(-1);

    // k can be greater than receiver->length() below, but it is bounded by
    // elements_base->length() so we never read out of bounds. This means that
    // elements->get(k) can return the hole, for which the StrictEquals will
    // always fail.
    FixedArray elements = FixedArray::cast(receiver->elements());
    STATIC_ASSERT(FixedArray::kMaxLength <=
                  std::numeric_limits<uint32_t>::max());
    for (size_t k = start_from; k < length; ++k) {
      if (value.StrictEquals(elements.get(static_cast<uint32_t>(k)))) {
        return Just<int64_t>(k);
      }
    }
    return Just<int64_t>(-1);
  }
};

class FastPackedSmiElementsAccessor
    : public FastSmiOrObjectElementsAccessor<
          FastPackedSmiElementsAccessor,
          ElementsKindTraits<PACKED_SMI_ELEMENTS>> {};

class FastHoleySmiElementsAccessor
    : public FastSmiOrObjectElementsAccessor<
          FastHoleySmiElementsAccessor,
          ElementsKindTraits<HOLEY_SMI_ELEMENTS>> {};

class FastPackedObjectElementsAccessor
    : public FastSmiOrObjectElementsAccessor<
          FastPackedObjectElementsAccessor,
          ElementsKindTraits<PACKED_ELEMENTS>> {};

template <typename Subclass, typename KindTraits>
class FastNonextensibleObjectElementsAccessor
    : public FastSmiOrObjectElementsAccessor<Subclass, KindTraits> {
 public:
  using BackingStore = typename KindTraits::BackingStore;

  static uint32_t PushImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                           uint32_t push_size) {
    UNREACHABLE();
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    UNREACHABLE();
  }

  // TODO(duongn): refactor this due to code duplication of sealed version.
  // Consider using JSObject::NormalizeElements(). Also consider follow the fast
  // element logic instead of changing to dictionary mode.
  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    uint32_t old_length = 0;
    CHECK(array->length().ToArrayIndex(&old_length));
    if (length == old_length) {
      // Do nothing.
      return;
    }

    // Transition to DICTIONARY_ELEMENTS.
    // Convert to dictionary mode.
    Handle<NumberDictionary> new_element_dictionary =
        old_length == 0 ? isolate->factory()->empty_slow_element_dictionary()
                        : array->GetElementsAccessor()->Normalize(array);

    // Migrate map.
    Handle<Map> new_map = Map::Copy(isolate, handle(array->map(), isolate),
                                    "SlowCopyForSetLengthImpl");
    new_map->set_is_extensible(false);
    new_map->set_elements_kind(DICTIONARY_ELEMENTS);
    JSObject::MigrateToMap(isolate, array, new_map);

    if (!new_element_dictionary.is_null()) {
      array->set_elements(*new_element_dictionary);
    }

    if (array->elements() !=
        ReadOnlyRoots(isolate).empty_slow_element_dictionary()) {
      Handle<NumberDictionary> dictionary(array->element_dictionary(), isolate);
      // Make sure we never go back to the fast case
      array->RequireSlowElements(*dictionary);
      JSObject::ApplyAttributesToDictionary(isolate, ReadOnlyRoots(isolate),
                                            dictionary,
                                            PropertyAttributes::NONE);
    }

    // Set length.
    Handle<FixedArrayBase> new_backing_store(array->elements(), isolate);
    DictionaryElementsAccessor::SetLengthImpl(isolate, array, length,
                                              new_backing_store);
  }
};

class FastPackedNonextensibleObjectElementsAccessor
    : public FastNonextensibleObjectElementsAccessor<
          FastPackedNonextensibleObjectElementsAccessor,
          ElementsKindTraits<PACKED_NONEXTENSIBLE_ELEMENTS>> {};

class FastHoleyNonextensibleObjectElementsAccessor
    : public FastNonextensibleObjectElementsAccessor<
          FastHoleyNonextensibleObjectElementsAccessor,
          ElementsKindTraits<HOLEY_NONEXTENSIBLE_ELEMENTS>> {};

template <typename Subclass, typename KindTraits>
class FastSealedObjectElementsAccessor
    : public FastSmiOrObjectElementsAccessor<Subclass, KindTraits> {
 public:
  using BackingStore = typename KindTraits::BackingStore;

  static Handle<Object> RemoveElement(Handle<JSArray> receiver,
                                      Where remove_position) {
    UNREACHABLE();
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    UNREACHABLE();
  }

  static void DeleteAtEnd(Handle<JSObject> obj,
                          Handle<BackingStore> backing_store, uint32_t entry) {
    UNREACHABLE();
  }

  static void DeleteCommon(Handle<JSObject> obj, uint32_t entry,
                           Handle<FixedArrayBase> store) {
    UNREACHABLE();
  }

  static Handle<Object> PopImpl(Handle<JSArray> receiver) { UNREACHABLE(); }

  static uint32_t PushImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                           uint32_t push_size) {
    UNREACHABLE();
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    UNREACHABLE();
  }

  // TODO(duongn): refactor this due to code duplication of nonextensible
  // version. Consider using JSObject::NormalizeElements(). Also consider follow
  // the fast element logic instead of changing to dictionary mode.
  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    uint32_t old_length = 0;
    CHECK(array->length().ToArrayIndex(&old_length));
    if (length == old_length) {
      // Do nothing.
      return;
    }

    // Transition to DICTIONARY_ELEMENTS.
    // Convert to dictionary mode
    Handle<NumberDictionary> new_element_dictionary =
        old_length == 0 ? isolate->factory()->empty_slow_element_dictionary()
                        : array->GetElementsAccessor()->Normalize(array);

    // Migrate map.
    Handle<Map> new_map = Map::Copy(isolate, handle(array->map(), isolate),
                                    "SlowCopyForSetLengthImpl");
    new_map->set_is_extensible(false);
    new_map->set_elements_kind(DICTIONARY_ELEMENTS);
    JSObject::MigrateToMap(isolate, array, new_map);

    if (!new_element_dictionary.is_null()) {
      array->set_elements(*new_element_dictionary);
    }

    if (array->elements() !=
        ReadOnlyRoots(isolate).empty_slow_element_dictionary()) {
      Handle<NumberDictionary> dictionary(array->element_dictionary(), isolate);
      // Make sure we never go back to the fast case
      array->RequireSlowElements(*dictionary);
      JSObject::ApplyAttributesToDictionary(isolate, ReadOnlyRoots(isolate),
                                            dictionary,
                                            PropertyAttributes::SEALED);
    }

    // Set length
    Handle<FixedArrayBase> new_backing_store(array->elements(), isolate);
    DictionaryElementsAccessor::SetLengthImpl(isolate, array, length,
                                              new_backing_store);
  }
};

class FastPackedSealedObjectElementsAccessor
    : public FastSealedObjectElementsAccessor<
          FastPackedSealedObjectElementsAccessor,
          ElementsKindTraits<PACKED_SEALED_ELEMENTS>> {};

class FastHoleySealedObjectElementsAccessor
    : public FastSealedObjectElementsAccessor<
          FastHoleySealedObjectElementsAccessor,
          ElementsKindTraits<HOLEY_SEALED_ELEMENTS>> {};

template <typename Subclass, typename KindTraits>
class FastFrozenObjectElementsAccessor
    : public FastSmiOrObjectElementsAccessor<Subclass, KindTraits> {
 public:
  using BackingStore = typename KindTraits::BackingStore;

  static inline void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                             Object value) {
    UNREACHABLE();
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value) {
    UNREACHABLE();
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value, WriteBarrierMode mode) {
    UNREACHABLE();
  }

  static Handle<Object> RemoveElement(Handle<JSArray> receiver,
                                      Where remove_position) {
    UNREACHABLE();
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    UNREACHABLE();
  }

  static void DeleteAtEnd(Handle<JSObject> obj,
                          Handle<BackingStore> backing_store, uint32_t entry) {
    UNREACHABLE();
  }

  static void DeleteCommon(Handle<JSObject> obj, uint32_t entry,
                           Handle<FixedArrayBase> store) {
    UNREACHABLE();
  }

  static Handle<Object> PopImpl(Handle<JSArray> receiver) { UNREACHABLE(); }

  static uint32_t PushImpl(Handle<JSArray> receiver, BuiltinArguments* args,
                           uint32_t push_size) {
    UNREACHABLE();
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    UNREACHABLE();
  }

  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    UNREACHABLE();
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    UNREACHABLE();
  }
};

class FastPackedFrozenObjectElementsAccessor
    : public FastFrozenObjectElementsAccessor<
          FastPackedFrozenObjectElementsAccessor,
          ElementsKindTraits<PACKED_FROZEN_ELEMENTS>> {};

class FastHoleyFrozenObjectElementsAccessor
    : public FastFrozenObjectElementsAccessor<
          FastHoleyFrozenObjectElementsAccessor,
          ElementsKindTraits<HOLEY_FROZEN_ELEMENTS>> {};

class FastHoleyObjectElementsAccessor
    : public FastSmiOrObjectElementsAccessor<
          FastHoleyObjectElementsAccessor, ElementsKindTraits<HOLEY_ELEMENTS>> {
};

template <typename Subclass, typename KindTraits>
class FastDoubleElementsAccessor
    : public FastElementsAccessor<Subclass, KindTraits> {
 public:
  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase backing_store,
                                InternalIndex entry) {
    return FixedDoubleArray::get(FixedDoubleArray::cast(backing_store),
                                 entry.as_int(), isolate);
  }

  static inline void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                             Object value) {
    SetImpl(holder->elements(), entry, value);
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value) {
    FixedDoubleArray::cast(backing_store).set(entry.as_int(), value.Number());
  }

  static inline void SetImpl(FixedArrayBase backing_store, InternalIndex entry,
                             Object value, WriteBarrierMode mode) {
    FixedDoubleArray::cast(backing_store).set(entry.as_int(), value.Number());
  }

  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    DisallowHeapAllocation no_allocation;
    switch (from_kind) {
      case PACKED_SMI_ELEMENTS:
        CopyPackedSmiToDoubleElements(from, from_start, to, to_start,
                                      packed_size, copy_size);
        break;
      case HOLEY_SMI_ELEMENTS:
        CopySmiToDoubleElements(from, from_start, to, to_start, copy_size);
        break;
      case PACKED_DOUBLE_ELEMENTS:
      case HOLEY_DOUBLE_ELEMENTS:
        CopyDoubleToDoubleElements(from, from_start, to, to_start, copy_size);
        break;
      case PACKED_ELEMENTS:
      case PACKED_FROZEN_ELEMENTS:
      case PACKED_SEALED_ELEMENTS:
      case PACKED_NONEXTENSIBLE_ELEMENTS:
      case HOLEY_ELEMENTS:
      case HOLEY_FROZEN_ELEMENTS:
      case HOLEY_SEALED_ELEMENTS:
      case HOLEY_NONEXTENSIBLE_ELEMENTS:
        CopyObjectToDoubleElements(from, from_start, to, to_start, copy_size);
        break;
      case DICTIONARY_ELEMENTS:
        CopyDictionaryToDoubleElements(isolate, from, from_start, to, to_start,
                                       copy_size);
        break;
      case FAST_SLOPPY_ARGUMENTS_ELEMENTS:
      case SLOW_SLOPPY_ARGUMENTS_ELEMENTS:
      case FAST_STRING_WRAPPER_ELEMENTS:
      case SLOW_STRING_WRAPPER_ELEMENTS:
      case NO_ELEMENTS:
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype) case TYPE##_ELEMENTS:
        TYPED_ARRAYS(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE
        // This function is currently only used for JSArrays with non-zero
        // length.
        UNREACHABLE();
    }
  }

  static Maybe<bool> CollectValuesOrEntriesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArray> values_or_entries, bool get_entries, int* nof_items,
      PropertyFilter filter) {
    Handle<FixedDoubleArray> elements(
        FixedDoubleArray::cast(object->elements()), isolate);
    int count = 0;
    uint32_t length = elements->length();
    for (uint32_t index = 0; index < length; ++index) {
      InternalIndex entry(index);
      if (!Subclass::HasEntryImpl(isolate, *elements, entry)) continue;
      Handle<Object> value = Subclass::GetImpl(isolate, *elements, entry);
      if (get_entries) {
        value = MakeEntryPair(isolate, index, value);
      }
      values_or_entries->set(count++, *value);
    }
    *nof_items = count;
    return Just(true);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> receiver,
                                         Handle<Object> search_value,
                                         size_t start_from, size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *receiver));
    DisallowHeapAllocation no_gc;
    FixedArrayBase elements_base = receiver->elements();
    Object value = *search_value;

    length = std::min(static_cast<size_t>(elements_base.length()), length);

    if (start_from >= length) return Just<int64_t>(-1);

    if (!value.IsNumber()) {
      return Just<int64_t>(-1);
    }
    if (value.IsNaN()) {
      return Just<int64_t>(-1);
    }
    double numeric_search_value = value.Number();
    FixedDoubleArray elements = FixedDoubleArray::cast(receiver->elements());

    STATIC_ASSERT(FixedDoubleArray::kMaxLength <=
                  std::numeric_limits<int>::max());
    for (size_t k = start_from; k < length; ++k) {
      int k_int = static_cast<int>(k);
      if (elements.is_the_hole(k_int)) {
        continue;
      }
      if (elements.get_scalar(k_int) == numeric_search_value) {
        return Just<int64_t>(k);
      }
    }
    return Just<int64_t>(-1);
  }
};

class FastPackedDoubleElementsAccessor
    : public FastDoubleElementsAccessor<
          FastPackedDoubleElementsAccessor,
          ElementsKindTraits<PACKED_DOUBLE_ELEMENTS>> {};

class FastHoleyDoubleElementsAccessor
    : public FastDoubleElementsAccessor<
          FastHoleyDoubleElementsAccessor,
          ElementsKindTraits<HOLEY_DOUBLE_ELEMENTS>> {};

// Super class for all external element arrays.
template <ElementsKind Kind, typename ElementType>
class TypedElementsAccessor
    : public ElementsAccessorBase<TypedElementsAccessor<Kind, ElementType>,
                                  ElementsKindTraits<Kind>> {
 public:
  using BackingStore = typename ElementsKindTraits<Kind>::BackingStore;
  using AccessorClass = TypedElementsAccessor<Kind, ElementType>;

  // Conversions from (other) scalar values.
  static ElementType FromScalar(int value) {
    return static_cast<ElementType>(value);
  }
  static ElementType FromScalar(uint32_t value) {
    return static_cast<ElementType>(value);
  }
  static ElementType FromScalar(double value) {
    return FromScalar(DoubleToInt32(value));
  }
  static ElementType FromScalar(int64_t value) { UNREACHABLE(); }
  static ElementType FromScalar(uint64_t value) { UNREACHABLE(); }

  // Conversions from objects / handles.
  static ElementType FromObject(Object value, bool* lossless = nullptr) {
    if (value.IsSmi()) {
      return FromScalar(Smi::ToInt(value));
    } else if (value.IsHeapNumber()) {
      return FromScalar(HeapNumber::cast(value).value());
    } else {
      // Clamp undefined here as well. All other types have been
      // converted to a number type further up in the call chain.
      DCHECK(value.IsUndefined());
      return FromScalar(Oddball::cast(value).to_number_raw());
    }
  }
  static ElementType FromHandle(Handle<Object> value,
                                bool* lossless = nullptr) {
    return FromObject(*value, lossless);
  }

  // Conversion of scalar value to handlified object.
  static Handle<Object> ToHandle(Isolate* isolate, ElementType value);

  static void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                      Object value) {
    Handle<JSTypedArray> typed_array = Handle<JSTypedArray>::cast(holder);
    DCHECK_LE(entry.raw_value(), typed_array->length());
    SetImpl(static_cast<ElementType*>(typed_array->DataPtr()),
            entry.raw_value(), FromObject(value));
  }

  static void SetImpl(ElementType* data_ptr, size_t entry, ElementType value) {
    // The JavaScript memory model allows for racy reads and writes to a
    // SharedArrayBuffer's backing store. ThreadSanitizer will catch these
    // racy accesses and warn about them, so we disable TSAN for these reads
    // and writes using annotations.
    //
    // We don't use relaxed atomics here, as it is not a requirement of the
    // JavaScript memory model to have tear-free reads of overlapping accesses,
    // and using relaxed atomics may introduce overhead.
    TSAN_ANNOTATE_IGNORE_WRITES_BEGIN;
    if (COMPRESS_POINTERS_BOOL && alignof(ElementType) > kTaggedSize) {
      // TODO(ishell, v8:8875): When pointer compression is enabled 8-byte size
      // fields (external pointers, doubles and BigInt data) are only
      // kTaggedSize aligned so we have to use unaligned pointer friendly way of
      // accessing them in order to avoid undefined behavior in C++ code.
      base::WriteUnalignedValue<ElementType>(
          reinterpret_cast<Address>(data_ptr + entry), value);
    } else {
      data_ptr[entry] = value;
    }
    TSAN_ANNOTATE_IGNORE_WRITES_END;
  }

  static Handle<Object> GetInternalImpl(Handle<JSObject> holder,
                                        InternalIndex entry) {
    Handle<JSTypedArray> typed_array = Handle<JSTypedArray>::cast(holder);
    Isolate* isolate = typed_array->GetIsolate();
    DCHECK_LE(entry.raw_value(), typed_array->length());
    DCHECK(!typed_array->WasDetached());
    ElementType elem = GetImpl(
        static_cast<ElementType*>(typed_array->DataPtr()), entry.raw_value());
    return ToHandle(isolate, elem);
  }

  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase backing_store,
                                InternalIndex entry) {
    UNREACHABLE();
  }

  static ElementType GetImpl(ElementType* data_ptr, size_t entry) {
    // The JavaScript memory model allows for racy reads and writes to a
    // SharedArrayBuffer's backing store. ThreadSanitizer will catch these
    // racy accesses and warn about them, so we disable TSAN for these reads
    // and writes using annotations.
    //
    // We don't use relaxed atomics here, as it is not a requirement of the
    // JavaScript memory model to have tear-free reads of overlapping accesses,
    // and using relaxed atomics may introduce overhead.
    TSAN_ANNOTATE_IGNORE_READS_BEGIN;
    ElementType result;
    if (COMPRESS_POINTERS_BOOL && alignof(ElementType) > kTaggedSize) {
      // TODO(ishell, v8:8875): When pointer compression is enabled 8-byte size
      // fields (external pointers, doubles and BigInt data) are only
      // kTaggedSize aligned so we have to use unaligned pointer friendly way of
      // accessing them in order to avoid undefined behavior in C++ code.
      result = base::ReadUnalignedValue<ElementType>(
          reinterpret_cast<Address>(data_ptr + entry));
    } else {
      result = data_ptr[entry];
    }
    TSAN_ANNOTATE_IGNORE_READS_END;
    return result;
  }

  static PropertyDetails GetDetailsImpl(JSObject holder, InternalIndex entry) {
    return PropertyDetails(kData, DONT_DELETE, PropertyCellType::kNoCell);
  }

  static PropertyDetails GetDetailsImpl(FixedArrayBase backing_store,
                                        InternalIndex entry) {
    return PropertyDetails(kData, DONT_DELETE, PropertyCellType::kNoCell);
  }

  static bool HasElementImpl(Isolate* isolate, JSObject holder, size_t index,
                             FixedArrayBase backing_store,
                             PropertyFilter filter) {
    return index < AccessorClass::GetCapacityImpl(holder, backing_store);
  }

  static bool HasAccessorsImpl(JSObject holder, FixedArrayBase backing_store) {
    return false;
  }

  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> backing_store) {
    // External arrays do not support changing their length.
    UNREACHABLE();
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    UNREACHABLE();
  }

  static InternalIndex GetEntryForIndexImpl(Isolate* isolate, JSObject holder,
                                            FixedArrayBase backing_store,
                                            size_t index,
                                            PropertyFilter filter) {
    return index < AccessorClass::GetCapacityImpl(holder, backing_store)
               ? InternalIndex(index)
               : InternalIndex::NotFound();
  }

  static size_t GetCapacityImpl(JSObject holder, FixedArrayBase backing_store) {
    JSTypedArray typed_array = JSTypedArray::cast(holder);
    if (typed_array.WasDetached()) return 0;
    return typed_array.length();
  }

  static size_t NumberOfElementsImpl(JSObject receiver,
                                     FixedArrayBase backing_store) {
    return AccessorClass::GetCapacityImpl(receiver, backing_store);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus AddElementsToKeyAccumulatorImpl(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) {
    Isolate* isolate = receiver->GetIsolate();
    Handle<FixedArrayBase> elements(receiver->elements(), isolate);
    size_t length = AccessorClass::GetCapacityImpl(*receiver, *elements);
    for (size_t i = 0; i < length; i++) {
      Handle<Object> value =
          AccessorClass::GetInternalImpl(receiver, InternalIndex(i));
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(accumulator->AddKey(value, convert));
    }
    return ExceptionStatus::kSuccess;
  }

  static Maybe<bool> CollectValuesOrEntriesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArray> values_or_entries, bool get_entries, int* nof_items,
      PropertyFilter filter) {
    int count = 0;
    if ((filter & ONLY_CONFIGURABLE) == 0) {
      Handle<FixedArrayBase> elements(object->elements(), isolate);
      size_t length = AccessorClass::GetCapacityImpl(*object, *elements);
      for (size_t index = 0; index < length; ++index) {
        Handle<Object> value =
            AccessorClass::GetInternalImpl(object, InternalIndex(index));
        if (get_entries) {
          value = MakeEntryPair(isolate, index, value);
        }
        values_or_entries->set(count++, *value);
      }
    }
    *nof_items = count;
    return Just(true);
  }

  static Object FillImpl(Handle<JSObject> receiver, Handle<Object> value,
                         size_t start, size_t end) {
    Handle<JSTypedArray> typed_array = Handle<JSTypedArray>::cast(receiver);
    DCHECK(!typed_array->WasDetached());
    DCHECK_LE(start, end);
    DCHECK_LE(end, typed_array->length());
    DisallowHeapAllocation no_gc;
    ElementType scalar = FromHandle(value);
    ElementType* data = static_cast<ElementType*>(typed_array->DataPtr());
    if (COMPRESS_POINTERS_BOOL && alignof(ElementType) > kTaggedSize) {
      // TODO(ishell, v8:8875): See UnalignedSlot<T> for details.
      std::fill(UnalignedSlot<ElementType>(data + start),
                UnalignedSlot<ElementType>(data + end), scalar);
    } else {
      std::fill(data + start, data + end, scalar);
    }
    return *typed_array;
  }

  static Maybe<bool> IncludesValueImpl(Isolate* isolate,
                                       Handle<JSObject> receiver,
                                       Handle<Object> value, size_t start_from,
                                       size_t length) {
    DisallowHeapAllocation no_gc;
    JSTypedArray typed_array = JSTypedArray::cast(*receiver);

    // TODO(caitp): return Just(false) here when implementing strict throwing on
    // detached views.
    if (typed_array.WasDetached()) {
      return Just(value->IsUndefined(isolate) && length > start_from);
    }

    if (value->IsUndefined(isolate) && length > typed_array.length()) {
      return Just(true);
    }

    // Prototype has no elements, and not searching for the hole --- limit
    // search to backing store length.
    if (typed_array.length() < length) {
      length = typed_array.length();
    }

    ElementType typed_search_value;
    ElementType* data_ptr =
        reinterpret_cast<ElementType*>(typed_array.DataPtr());
    if (Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS) {
      if (!value->IsBigInt()) return Just(false);
      bool lossless;
      typed_search_value = FromHandle(value, &lossless);
      if (!lossless) return Just(false);
    } else {
      if (!value->IsNumber()) return Just(false);
      double search_value = value->Number();
      if (!std::isfinite(search_value)) {
        // Integral types cannot represent +Inf or NaN.
        if (Kind < FLOAT32_ELEMENTS || Kind > FLOAT64_ELEMENTS) {
          return Just(false);
        }
        if (std::isnan(search_value)) {
          for (size_t k = start_from; k < length; ++k) {
            double elem_k =
                static_cast<double>(AccessorClass::GetImpl(data_ptr, k));
            if (std::isnan(elem_k)) return Just(true);
          }
          return Just(false);
        }
      } else if (search_value < std::numeric_limits<ElementType>::lowest() ||
                 search_value > std::numeric_limits<ElementType>::max()) {
        // Return false if value can't be represented in this space.
        return Just(false);
      }
      typed_search_value = static_cast<ElementType>(search_value);
      if (static_cast<double>(typed_search_value) != search_value) {
        return Just(false);  // Loss of precision.
      }
    }

    for (size_t k = start_from; k < length; ++k) {
      ElementType elem_k = AccessorClass::GetImpl(data_ptr, k);
      if (elem_k == typed_search_value) return Just(true);
    }
    return Just(false);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> receiver,
                                         Handle<Object> value,
                                         size_t start_from, size_t length) {
    DisallowHeapAllocation no_gc;
    JSTypedArray typed_array = JSTypedArray::cast(*receiver);

    if (typed_array.WasDetached()) return Just<int64_t>(-1);

    ElementType typed_search_value;

    ElementType* data_ptr =
        reinterpret_cast<ElementType*>(typed_array.DataPtr());
    if (Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS) {
      if (!value->IsBigInt()) return Just<int64_t>(-1);
      bool lossless;
      typed_search_value = FromHandle(value, &lossless);
      if (!lossless) return Just<int64_t>(-1);
    } else {
      if (!value->IsNumber()) return Just<int64_t>(-1);
      double search_value = value->Number();
      if (!std::isfinite(search_value)) {
        // Integral types cannot represent +Inf or NaN.
        if (Kind < FLOAT32_ELEMENTS || Kind > FLOAT64_ELEMENTS) {
          return Just<int64_t>(-1);
        }
        if (std::isnan(search_value)) {
          return Just<int64_t>(-1);
        }
      } else if (search_value < std::numeric_limits<ElementType>::lowest() ||
                 search_value > std::numeric_limits<ElementType>::max()) {
        // Return false if value can't be represented in this ElementsKind.
        return Just<int64_t>(-1);
      }
      typed_search_value = static_cast<ElementType>(search_value);
      if (static_cast<double>(typed_search_value) != search_value) {
        return Just<int64_t>(-1);  // Loss of precision.
      }
    }

    // Prototype has no elements, and not searching for the hole --- limit
    // search to backing store length.
    if (typed_array.length() < length) {
      length = typed_array.length();
    }

    for (size_t k = start_from; k < length; ++k) {
      ElementType elem_k = AccessorClass::GetImpl(data_ptr, k);
      if (elem_k == typed_search_value) return Just<int64_t>(k);
    }
    return Just<int64_t>(-1);
  }

  static Maybe<int64_t> LastIndexOfValueImpl(Handle<JSObject> receiver,
                                             Handle<Object> value,
                                             size_t start_from) {
    DisallowHeapAllocation no_gc;
    JSTypedArray typed_array = JSTypedArray::cast(*receiver);

    DCHECK(!typed_array.WasDetached());

    ElementType typed_search_value;

    ElementType* data_ptr =
        reinterpret_cast<ElementType*>(typed_array.DataPtr());
    if (Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS) {
      if (!value->IsBigInt()) return Just<int64_t>(-1);
      bool lossless;
      typed_search_value = FromHandle(value, &lossless);
      if (!lossless) return Just<int64_t>(-1);
    } else {
      if (!value->IsNumber()) return Just<int64_t>(-1);
      double search_value = value->Number();
      if (!std::isfinite(search_value)) {
        if (std::is_integral<ElementType>::value) {
          // Integral types cannot represent +Inf or NaN.
          return Just<int64_t>(-1);
        } else if (std::isnan(search_value)) {
          // Strict Equality Comparison of NaN is always false.
          return Just<int64_t>(-1);
        }
      } else if (search_value < std::numeric_limits<ElementType>::lowest() ||
                 search_value > std::numeric_limits<ElementType>::max()) {
        // Return -1 if value can't be represented in this ElementsKind.
        return Just<int64_t>(-1);
      }
      typed_search_value = static_cast<ElementType>(search_value);
      if (static_cast<double>(typed_search_value) != search_value) {
        return Just<int64_t>(-1);  // Loss of precision.
      }
    }

    DCHECK_LT(start_from, typed_array.length());
    size_t k = start_from;
    do {
      ElementType elem_k = AccessorClass::GetImpl(data_ptr, k);
      if (elem_k == typed_search_value) return Just<int64_t>(k);
    } while (k-- != 0);
    return Just<int64_t>(-1);
  }

  static void ReverseImpl(JSObject receiver) {
    DisallowHeapAllocation no_gc;
    JSTypedArray typed_array = JSTypedArray::cast(receiver);

    DCHECK(!typed_array.WasDetached());

    size_t len = typed_array.length();
    if (len == 0) return;

    ElementType* data = static_cast<ElementType*>(typed_array.DataPtr());
    if (COMPRESS_POINTERS_BOOL && alignof(ElementType) > kTaggedSize) {
      // TODO(ishell, v8:8875): See UnalignedSlot<T> for details.
      std::reverse(UnalignedSlot<ElementType>(data),
                   UnalignedSlot<ElementType>(data + len));
    } else {
      std::reverse(data, data + len);
    }
  }

  static Handle<FixedArray> CreateListFromArrayLikeImpl(Isolate* isolate,
                                                        Handle<JSObject> object,
                                                        uint32_t length) {
    Handle<JSTypedArray> typed_array = Handle<JSTypedArray>::cast(object);
    Handle<FixedArray> result = isolate->factory()->NewFixedArray(length);
    for (uint32_t i = 0; i < length; i++) {
      Handle<Object> value =
          AccessorClass::GetInternalImpl(typed_array, InternalIndex(i));
      result->set(i, *value);
    }
    return result;
  }

  static void CopyTypedArrayElementsSliceImpl(JSTypedArray source,
                                              JSTypedArray destination,
                                              size_t start, size_t end) {
    DisallowHeapAllocation no_gc;
    DCHECK_EQ(destination.GetElementsKind(), AccessorClass::kind());
    CHECK(!source.WasDetached());
    CHECK(!destination.WasDetached());
    DCHECK_LE(start, end);
    DCHECK_LE(end, source.length());
    size_t count = end - start;
    DCHECK_LE(count, destination.length());
    ElementType* dest_data = static_cast<ElementType*>(destination.DataPtr());
    switch (source.GetElementsKind()) {
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype)                            \
  case TYPE##_ELEMENTS: {                                                    \
    ctype* source_data = reinterpret_cast<ctype*>(source.DataPtr()) + start; \
    CopyBetweenBackingStores<TYPE##_ELEMENTS, ctype>(source_data, dest_data, \
                                                     count);                 \
    break;                                                                   \
  }
      TYPED_ARRAYS(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE
      default:
        UNREACHABLE();
        break;
    }
  }

  static bool HasSimpleRepresentation(ExternalArrayType type) {
    return !(type == kExternalFloat32Array || type == kExternalFloat64Array ||
             type == kExternalUint8ClampedArray);
  }

  template <ElementsKind SourceKind, typename SourceElementType>
  static void CopyBetweenBackingStores(SourceElementType* source_data_ptr,
                                       ElementType* dest_data_ptr,
                                       size_t length) {
    DisallowHeapAllocation no_gc;
    for (size_t i = 0; i < length; i++) {
      // We use scalar accessors to avoid boxing/unboxing, so there are no
      // allocations.
      SourceElementType source_elem =
          TypedElementsAccessor<SourceKind, SourceElementType>::GetImpl(
              source_data_ptr, i);
      ElementType dest_elem = FromScalar(source_elem);
      SetImpl(dest_data_ptr, i, dest_elem);
    }
  }

  static void CopyElementsFromTypedArray(JSTypedArray source,
                                         JSTypedArray destination,
                                         size_t length, size_t offset) {
    // The source is a typed array, so we know we don't need to do ToNumber
    // side-effects, as the source elements will always be a number.
    DisallowHeapAllocation no_gc;

    CHECK(!source.WasDetached());
    CHECK(!destination.WasDetached());

    DCHECK_LE(offset, destination.length());
    DCHECK_LE(length, destination.length() - offset);
    DCHECK_LE(length, source.length());

    ExternalArrayType source_type = source.type();
    ExternalArrayType destination_type = destination.type();

    bool same_type = source_type == destination_type;
    bool same_size = source.element_size() == destination.element_size();
    bool both_are_simple = HasSimpleRepresentation(source_type) &&
                           HasSimpleRepresentation(destination_type);

    uint8_t* source_data = static_cast<uint8_t*>(source.DataPtr());
    uint8_t* dest_data = static_cast<uint8_t*>(destination.DataPtr());
    size_t source_byte_length = source.byte_length();
    size_t dest_byte_length = destination.byte_length();

    // We can simply copy the backing store if the types are the same, or if
    // we are converting e.g. Uint8 <-> Int8, as the binary representation
    // will be the same. This is not the case for floats or clamped Uint8,
    // which have special conversion operations.
    if (same_type || (same_size && both_are_simple)) {
      size_t element_size = source.element_size();
      std::memmove(dest_data + offset * element_size, source_data,
                   length * element_size);
    } else {
      std::unique_ptr<uint8_t[]> cloned_source_elements;

      // If the typedarrays are overlapped, clone the source.
      if (dest_data + dest_byte_length > source_data &&
          source_data + source_byte_length > dest_data) {
        cloned_source_elements.reset(new uint8_t[source_byte_length]);
        std::memcpy(cloned_source_elements.get(), source_data,
                    source_byte_length);
        source_data = cloned_source_elements.get();
      }

      switch (source.GetElementsKind()) {
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype)                    \
  case TYPE##_ELEMENTS:                                              \
    CopyBetweenBackingStores<TYPE##_ELEMENTS, ctype>(                \
        reinterpret_cast<ctype*>(source_data),                       \
        reinterpret_cast<ElementType*>(dest_data) + offset, length); \
    break;
        TYPED_ARRAYS(TYPED_ARRAY_CASE)
        default:
          UNREACHABLE();
          break;
      }
#undef TYPED_ARRAY_CASE
    }
  }

  static bool HoleyPrototypeLookupRequired(Isolate* isolate, Context context,
                                           JSArray source) {
    DisallowHeapAllocation no_gc;
    DisallowJavascriptExecution no_js(isolate);

#ifdef V8_ENABLE_FORCE_SLOW_PATH
    if (isolate->force_slow_path()) return true;
#endif

    Object source_proto = source.map().prototype();

    // Null prototypes are OK - we don't need to do prototype chain lookups on
    // them.
    if (source_proto.IsNull(isolate)) return false;
    if (source_proto.IsJSProxy()) return true;
    if (!context.native_context().is_initial_array_prototype(
            JSObject::cast(source_proto))) {
      return true;
    }

    return !Protectors::IsNoElementsIntact(isolate);
  }

  static bool TryCopyElementsFastNumber(Context context, JSArray source,
                                        JSTypedArray destination, size_t length,
                                        size_t offset) {
    if (Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS) return false;
    Isolate* isolate = source.GetIsolate();
    DisallowHeapAllocation no_gc;
    DisallowJavascriptExecution no_js(isolate);

    CHECK(!destination.WasDetached());

    size_t current_length;
    DCHECK(source.length().IsNumber() &&
           TryNumberToSize(source.length(), &current_length) &&
           length <= current_length);
    USE(current_length);

    size_t dest_length = destination.length();
    DCHECK(length + offset <= dest_length);
    USE(dest_length);

    ElementsKind kind = source.GetElementsKind();

    // When we find the hole, we normally have to look up the element on the
    // prototype chain, which is not handled here and we return false instead.
    // When the array has the original array prototype, and that prototype has
    // not been changed in a way that would affect lookups, we can just convert
    // the hole into undefined.
    if (HoleyPrototypeLookupRequired(isolate, context, source)) return false;

    Oddball undefined = ReadOnlyRoots(isolate).undefined_value();
    ElementType* dest_data =
        reinterpret_cast<ElementType*>(destination.DataPtr()) + offset;

    // Fast-path for packed Smi kind.
    if (kind == PACKED_SMI_ELEMENTS) {
      FixedArray source_store = FixedArray::cast(source.elements());

      for (size_t i = 0; i < length; i++) {
        Object elem = source_store.get(static_cast<int>(i));
        SetImpl(dest_data, i, FromScalar(Smi::ToInt(elem)));
      }
      return true;
    } else if (kind == HOLEY_SMI_ELEMENTS) {
      FixedArray source_store = FixedArray::cast(source.elements());
      for (size_t i = 0; i < length; i++) {
        if (source_store.is_the_hole(isolate, static_cast<int>(i))) {
          SetImpl(dest_data, i, FromObject(undefined));
        } else {
          Object elem = source_store.get(static_cast<int>(i));
          SetImpl(dest_data, i, FromScalar(Smi::ToInt(elem)));
        }
      }
      return true;
    } else if (kind == PACKED_DOUBLE_ELEMENTS) {
      // Fast-path for packed double kind. We avoid boxing and then immediately
      // unboxing the double here by using get_scalar.
      FixedDoubleArray source_store = FixedDoubleArray::cast(source.elements());

      for (size_t i = 0; i < length; i++) {
        // Use the from_double conversion for this specific TypedArray type,
        // rather than relying on C++ to convert elem.
        double elem = source_store.get_scalar(static_cast<int>(i));
        SetImpl(dest_data, i, FromScalar(elem));
      }
      return true;
    } else if (kind == HOLEY_DOUBLE_ELEMENTS) {
      FixedDoubleArray source_store = FixedDoubleArray::cast(source.elements());
      for (size_t i = 0; i < length; i++) {
        if (source_store.is_the_hole(static_cast<int>(i))) {
          SetImpl(dest_data, i, FromObject(undefined));
        } else {
          double elem = source_store.get_scalar(static_cast<int>(i));
          SetImpl(dest_data, i, FromScalar(elem));
        }
      }
      return true;
    }
    return false;
  }

  static Object CopyElementsHandleSlow(Handle<Object> source,
                                       Handle<JSTypedArray> destination,
                                       size_t length, size_t offset) {
    Isolate* isolate = destination->GetIsolate();
    for (size_t i = 0; i < length; i++) {
      Handle<Object> elem;
      LookupIterator it(isolate, source, i);
      ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, elem,
                                         Object::GetProperty(&it));
      if (Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS) {
        ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, elem,
                                           BigInt::FromObject(isolate, elem));
      } else {
        ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, elem,
                                           Object::ToNumber(isolate, elem));
      }

      if (V8_UNLIKELY(destination->WasDetached())) {
        const char* op = "set";
        const MessageTemplate message = MessageTemplate::kDetachedOperation;
        Handle<String> operation =
            isolate->factory()->NewStringFromAsciiChecked(op);
        THROW_NEW_ERROR_RETURN_FAILURE(isolate,
                                       NewTypeError(message, operation));
      }
      // The spec says we store the length, then get each element, so we don't
      // need to check changes to length.
      SetImpl(destination, InternalIndex(offset + i), *elem);
    }
    return *isolate->factory()->undefined_value();
  }

  // This doesn't guarantee that the destination array will be completely
  // filled. The caller must do this by passing a source with equal length, if
  // that is required.
  static Object CopyElementsHandleImpl(Handle<Object> source,
                                       Handle<JSObject> destination,
                                       size_t length, size_t offset) {
    Isolate* isolate = destination->GetIsolate();
    Handle<JSTypedArray> destination_ta =
        Handle<JSTypedArray>::cast(destination);
    DCHECK_LE(offset + length, destination_ta->length());
    CHECK(!destination_ta->WasDetached());

    if (length == 0) return *isolate->factory()->undefined_value();

    // All conversions from TypedArrays can be done without allocation.
    if (source->IsJSTypedArray()) {
      Handle<JSTypedArray> source_ta = Handle<JSTypedArray>::cast(source);
      ElementsKind source_kind = source_ta->GetElementsKind();
      bool source_is_bigint =
          source_kind == BIGINT64_ELEMENTS || source_kind == BIGUINT64_ELEMENTS;
      bool target_is_bigint =
          Kind == BIGINT64_ELEMENTS || Kind == BIGUINT64_ELEMENTS;
      // If we have to copy more elements than we have in the source, we need to
      // do special handling and conversion; that happens in the slow case.
      if (source_is_bigint == target_is_bigint && !source_ta->WasDetached() &&
          length + offset <= source_ta->length()) {
        CopyElementsFromTypedArray(*source_ta, *destination_ta, length, offset);
        return *isolate->factory()->undefined_value();
      }
    } else if (source->IsJSArray()) {
      // Fast cases for packed numbers kinds where we don't need to allocate.
      Handle<JSArray> source_js_array = Handle<JSArray>::cast(source);
      size_t current_length;
      DCHECK(source_js_array->length().IsNumber());
      if (TryNumberToSize(source_js_array->length(), &current_length) &&
          length <= current_length) {
        Handle<JSArray> source_array = Handle<JSArray>::cast(source);
        if (TryCopyElementsFastNumber(isolate->context(), *source_array,
                                      *destination_ta, length, offset)) {
          return *isolate->factory()->undefined_value();
        }
      }
    }
    // Final generic case that handles prototype chain lookups, getters, proxies
    // and observable side effects via valueOf, etc.
    return CopyElementsHandleSlow(source, destination_ta, length, offset);
  }
};

// static
template <>
Handle<Object> TypedElementsAccessor<INT8_ELEMENTS, int8_t>::ToHandle(
    Isolate* isolate, int8_t value) {
  return handle(Smi::FromInt(value), isolate);
}

// static
template <>
Handle<Object> TypedElementsAccessor<UINT8_ELEMENTS, uint8_t>::ToHandle(
    Isolate* isolate, uint8_t value) {
  return handle(Smi::FromInt(value), isolate);
}

// static
template <>
Handle<Object> TypedElementsAccessor<INT16_ELEMENTS, int16_t>::ToHandle(
    Isolate* isolate, int16_t value) {
  return handle(Smi::FromInt(value), isolate);
}

// static
template <>
Handle<Object> TypedElementsAccessor<UINT16_ELEMENTS, uint16_t>::ToHandle(
    Isolate* isolate, uint16_t value) {
  return handle(Smi::FromInt(value), isolate);
}

// static
template <>
Handle<Object> TypedElementsAccessor<INT32_ELEMENTS, int32_t>::ToHandle(
    Isolate* isolate, int32_t value) {
  return isolate->factory()->NewNumberFromInt(value);
}

// static
template <>
Handle<Object> TypedElementsAccessor<UINT32_ELEMENTS, uint32_t>::ToHandle(
    Isolate* isolate, uint32_t value) {
  return isolate->factory()->NewNumberFromUint(value);
}

// static
template <>
float TypedElementsAccessor<FLOAT32_ELEMENTS, float>::FromScalar(double value) {
  return DoubleToFloat32(value);
}

// static
template <>
Handle<Object> TypedElementsAccessor<FLOAT32_ELEMENTS, float>::ToHandle(
    Isolate* isolate, float value) {
  return isolate->factory()->NewNumber(value);
}

// static
template <>
double TypedElementsAccessor<FLOAT64_ELEMENTS, double>::FromScalar(
    double value) {
  return value;
}

// static
template <>
Handle<Object> TypedElementsAccessor<FLOAT64_ELEMENTS, double>::ToHandle(
    Isolate* isolate, double value) {
  return isolate->factory()->NewNumber(value);
}

// static
template <>
uint8_t TypedElementsAccessor<UINT8_CLAMPED_ELEMENTS, uint8_t>::FromScalar(
    int value) {
  if (value < 0x00) return 0x00;
  if (value > 0xFF) return 0xFF;
  return static_cast<uint8_t>(value);
}

// static
template <>
uint8_t TypedElementsAccessor<UINT8_CLAMPED_ELEMENTS, uint8_t>::FromScalar(
    uint32_t value) {
  // We need this special case for Uint32 -> Uint8Clamped, because the highest
  // Uint32 values will be negative as an int, clamping to 0, rather than 255.
  if (value > 0xFF) return 0xFF;
  return static_cast<uint8_t>(value);
}

// static
template <>
uint8_t TypedElementsAccessor<UINT8_CLAMPED_ELEMENTS, uint8_t>::FromScalar(
    double value) {
  // Handle NaNs and less than zero values which clamp to zero.
  if (!(value > 0)) return 0;
  if (value > 0xFF) return 0xFF;
  return static_cast<uint8_t>(lrint(value));
}

// static
template <>
Handle<Object> TypedElementsAccessor<UINT8_CLAMPED_ELEMENTS, uint8_t>::ToHandle(
    Isolate* isolate, uint8_t value) {
  return handle(Smi::FromInt(value), isolate);
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromScalar(
    int value) {
  UNREACHABLE();
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromScalar(
    uint32_t value) {
  UNREACHABLE();
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromScalar(
    double value) {
  UNREACHABLE();
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromScalar(
    int64_t value) {
  return value;
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromScalar(
    uint64_t value) {
  return static_cast<int64_t>(value);
}

// static
template <>
int64_t TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::FromObject(
    Object value, bool* lossless) {
  return BigInt::cast(value).AsInt64(lossless);
}

// static
template <>
Handle<Object> TypedElementsAccessor<BIGINT64_ELEMENTS, int64_t>::ToHandle(
    Isolate* isolate, int64_t value) {
  return BigInt::FromInt64(isolate, value);
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromScalar(
    int value) {
  UNREACHABLE();
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromScalar(
    uint32_t value) {
  UNREACHABLE();
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromScalar(
    double value) {
  UNREACHABLE();
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromScalar(
    int64_t value) {
  return static_cast<uint64_t>(value);
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromScalar(
    uint64_t value) {
  return value;
}

// static
template <>
uint64_t TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::FromObject(
    Object value, bool* lossless) {
  return BigInt::cast(value).AsUint64(lossless);
}

// static
template <>
Handle<Object> TypedElementsAccessor<BIGUINT64_ELEMENTS, uint64_t>::ToHandle(
    Isolate* isolate, uint64_t value) {
  return BigInt::FromUint64(isolate, value);
}

#define FIXED_ELEMENTS_ACCESSOR(Type, type, TYPE, ctype) \
  using Type##ElementsAccessor = TypedElementsAccessor<TYPE##_ELEMENTS, ctype>;
TYPED_ARRAYS(FIXED_ELEMENTS_ACCESSOR)
#undef FIXED_ELEMENTS_ACCESSOR

template <typename Subclass, typename ArgumentsAccessor, typename KindTraits>
class SloppyArgumentsElementsAccessor
    : public ElementsAccessorBase<Subclass, KindTraits> {
 public:
  static void ConvertArgumentsStoreResult(
      Handle<SloppyArgumentsElements> elements, Handle<Object> result) {
    UNREACHABLE();
  }

  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase parameters,
                                InternalIndex entry) {
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(parameters), isolate);
    uint32_t length = elements->parameter_map_length();
    if (entry.as_uint32() < length) {
      // Read context mapped entry.
      DisallowHeapAllocation no_gc;
      Object probe = elements->get_mapped_entry(entry.as_uint32());
      DCHECK(!probe.IsTheHole(isolate));
      Context context = elements->context();
      int context_entry = Smi::ToInt(probe);
      DCHECK(!context.get(context_entry).IsTheHole(isolate));
      return handle(context.get(context_entry), isolate);
    } else {
      // Entry is not context mapped, defer to the arguments.
      Handle<Object> result = ArgumentsAccessor::GetImpl(
          isolate, elements->arguments(), entry.adjust_down(length));
      return Subclass::ConvertArgumentsStoreResult(isolate, elements, result);
    }
  }

  static void TransitionElementsKindImpl(Handle<JSObject> object,
                                         Handle<Map> map) {
    UNREACHABLE();
  }

  static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                         uint32_t capacity) {
    UNREACHABLE();
  }

  static inline void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                             Object value) {
    SetImpl(holder->elements(), entry, value);
  }

  static inline void SetImpl(FixedArrayBase store, InternalIndex entry,
                             Object value) {
    SloppyArgumentsElements elements = SloppyArgumentsElements::cast(store);
    uint32_t length = elements.parameter_map_length();
    if (entry.as_uint32() < length) {
      // Store context mapped entry.
      DisallowHeapAllocation no_gc;
      Object probe = elements.get_mapped_entry(entry.as_uint32());
      DCHECK(!probe.IsTheHole());
      Context context = elements.context();
      int context_entry = Smi::ToInt(probe);
      DCHECK(!context.get(context_entry).IsTheHole());
      context.set(context_entry, value);
    } else {
      //  Entry is not context mapped defer to arguments.
      FixedArray arguments = elements.arguments();
      Object current =
          ArgumentsAccessor::GetRaw(arguments, entry.adjust_down(length));
      if (current.IsAliasedArgumentsEntry()) {
        AliasedArgumentsEntry alias = AliasedArgumentsEntry::cast(current);
        Context context = elements.context();
        int context_entry = alias.aliased_context_slot();
        DCHECK(!context.get(context_entry).IsTheHole());
        context.set(context_entry, value);
      } else {
        ArgumentsAccessor::SetImpl(arguments, entry.adjust_down(length), value);
      }
    }
  }

  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
                            uint32_t length,
                            Handle<FixedArrayBase> parameter_map) {
    // Sloppy arguments objects are not arrays.
    UNREACHABLE();
  }

  static uint32_t GetCapacityImpl(JSObject holder, FixedArrayBase store) {
    SloppyArgumentsElements elements = SloppyArgumentsElements::cast(store);
    FixedArray arguments = elements.arguments();
    return elements.parameter_map_length() +
           ArgumentsAccessor::GetCapacityImpl(holder, arguments);
  }

  static uint32_t GetMaxNumberOfEntries(JSObject holder,
                                        FixedArrayBase backing_store) {
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(backing_store);
    FixedArrayBase arguments = elements.arguments();
    size_t max_entries =
        ArgumentsAccessor::GetMaxNumberOfEntries(holder, arguments);
    DCHECK_LE(max_entries, std::numeric_limits<uint32_t>::max());
    return elements.parameter_map_length() + static_cast<uint32_t>(max_entries);
  }

  static uint32_t NumberOfElementsImpl(JSObject receiver,
                                       FixedArrayBase backing_store) {
    Isolate* isolate = receiver.GetIsolate();
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(backing_store);
    FixedArrayBase arguments = elements.arguments();
    uint32_t nof_elements = 0;
    uint32_t length = elements.parameter_map_length();
    for (uint32_t index = 0; index < length; index++) {
      if (HasParameterMapArg(isolate, elements, index)) nof_elements++;
    }
    return nof_elements +
           ArgumentsAccessor::NumberOfElementsImpl(receiver, arguments);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus AddElementsToKeyAccumulatorImpl(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) {
    Isolate* isolate = accumulator->isolate();
    Handle<FixedArrayBase> elements(receiver->elements(), isolate);
    uint32_t length = GetCapacityImpl(*receiver, *elements);
    for (uint32_t index = 0; index < length; index++) {
      InternalIndex entry(index);
      if (!HasEntryImpl(isolate, *elements, entry)) continue;
      Handle<Object> value = GetImpl(isolate, *elements, entry);
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(accumulator->AddKey(value, convert));
    }
    return ExceptionStatus::kSuccess;
  }

  static bool HasEntryImpl(Isolate* isolate, FixedArrayBase parameters,
                           InternalIndex entry) {
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(parameters);
    uint32_t length = elements.parameter_map_length();
    if (entry.raw_value() < length) {
      return HasParameterMapArg(isolate, elements, entry.raw_value());
    }
    FixedArrayBase arguments = elements.arguments();
    return ArgumentsAccessor::HasEntryImpl(isolate, arguments,
                                           entry.adjust_down(length));
  }

  static bool HasAccessorsImpl(JSObject holder, FixedArrayBase backing_store) {
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(backing_store);
    FixedArray arguments = elements.arguments();
    return ArgumentsAccessor::HasAccessorsImpl(holder, arguments);
  }

  static InternalIndex GetEntryForIndexImpl(Isolate* isolate, JSObject holder,
                                            FixedArrayBase parameters,
                                            size_t index,
                                            PropertyFilter filter) {
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(parameters);
    if (HasParameterMapArg(isolate, elements, index)) {
      return InternalIndex(index);
    }
    FixedArray arguments = elements.arguments();
    InternalIndex entry = ArgumentsAccessor::GetEntryForIndexImpl(
        isolate, holder, arguments, index, filter);
    if (entry.is_not_found()) return entry;
    // Arguments entries could overlap with the dictionary entries, hence offset
    // them by the number of context mapped entries.
    return entry.adjust_up(elements.parameter_map_length());
  }

  static PropertyDetails GetDetailsImpl(JSObject holder, InternalIndex entry) {
    SloppyArgumentsElements elements =
        SloppyArgumentsElements::cast(holder.elements());
    uint32_t length = elements.parameter_map_length();
    if (entry.as_uint32() < length) {
      return PropertyDetails(kData, NONE, PropertyCellType::kNoCell);
    }
    FixedArray arguments = elements.arguments();
    return ArgumentsAccessor::GetDetailsImpl(arguments,
                                             entry.adjust_down(length));
  }

  static bool HasParameterMapArg(Isolate* isolate,
                                 SloppyArgumentsElements elements,
                                 size_t index) {
    uint32_t length = elements.parameter_map_length();
    if (index >= length) return false;
    return !elements.get_mapped_entry(static_cast<uint32_t>(index))
                .IsTheHole(isolate);
  }

  static void DeleteImpl(Handle<JSObject> obj, InternalIndex entry) {
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(obj->elements()), obj->GetIsolate());
    uint32_t length = elements->parameter_map_length();
    InternalIndex delete_or_entry = entry;
    if (entry.as_uint32() < length) {
      delete_or_entry = InternalIndex::NotFound();
    }
    Subclass::SloppyDeleteImpl(obj, elements, delete_or_entry);
    // SloppyDeleteImpl allocates a new dictionary elements store. For making
    // heap verification happy we postpone clearing out the mapped entry.
    if (entry.as_uint32() < length) {
      elements->set_mapped_entry(entry.as_uint32(),
                                 obj->GetReadOnlyRoots().the_hole_value());
    }
  }

  static void SloppyDeleteImpl(Handle<JSObject> obj,
                               Handle<SloppyArgumentsElements> elements,
                               InternalIndex entry) {
    // Implemented in subclasses.
    UNREACHABLE();
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus CollectElementIndicesImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      KeyAccumulator* keys) {
    Isolate* isolate = keys->isolate();
    uint32_t nof_indices = 0;
    Handle<FixedArray> indices = isolate->factory()->NewFixedArray(
        GetCapacityImpl(*object, *backing_store));
    DirectCollectElementIndicesImpl(isolate, object, backing_store,
                                    GetKeysConversion::kKeepNumbers,
                                    ENUMERABLE_STRINGS, indices, &nof_indices);
    SortIndices(isolate, indices, nof_indices);
    for (uint32_t i = 0; i < nof_indices; i++) {
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(keys->AddKey(indices->get(i)));
    }
    return ExceptionStatus::kSuccess;
  }

  static Handle<FixedArray> DirectCollectElementIndicesImpl(
      Isolate* isolate, Handle<JSObject> object,
      Handle<FixedArrayBase> backing_store, GetKeysConversion convert,
      PropertyFilter filter, Handle<FixedArray> list, uint32_t* nof_indices,
      uint32_t insertion_index = 0) {
    Handle<SloppyArgumentsElements> elements =
        Handle<SloppyArgumentsElements>::cast(backing_store);
    uint32_t length = elements->parameter_map_length();

    for (uint32_t i = 0; i < length; ++i) {
      if (elements->get_mapped_entry(i).IsTheHole(isolate)) continue;
      if (convert == GetKeysConversion::kConvertToString) {
        Handle<String> index_string = isolate->factory()->Uint32ToString(i);
        list->set(insertion_index, *index_string);
      } else {
        list->set(insertion_index, Smi::FromInt(i));
      }
      insertion_index++;
    }

    Handle<FixedArray> store(elements->arguments(), isolate);
    return ArgumentsAccessor::DirectCollectElementIndicesImpl(
        isolate, object, store, convert, filter, list, nof_indices,
        insertion_index);
  }

  static Maybe<bool> IncludesValueImpl(Isolate* isolate,
                                       Handle<JSObject> object,
                                       Handle<Object> value, size_t start_from,
                                       size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *object));
    Handle<Map> original_map(object->map(), isolate);
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(object->elements()), isolate);
    bool search_for_hole = value->IsUndefined(isolate);

    for (size_t k = start_from; k < length; ++k) {
      DCHECK_EQ(object->map(), *original_map);
      InternalIndex entry =
          GetEntryForIndexImpl(isolate, *object, *elements, k, ALL_PROPERTIES);
      if (entry.is_not_found()) {
        if (search_for_hole) return Just(true);
        continue;
      }

      Handle<Object> element_k = Subclass::GetImpl(isolate, *elements, entry);

      if (element_k->IsAccessorPair()) {
        LookupIterator it(isolate, object, k, LookupIterator::OWN);
        DCHECK(it.IsFound());
        DCHECK_EQ(it.state(), LookupIterator::ACCESSOR);
        ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, element_k,
                                         Object::GetPropertyWithAccessor(&it),
                                         Nothing<bool>());

        if (value->SameValueZero(*element_k)) return Just(true);

        if (object->map() != *original_map) {
          // Some mutation occurred in accessor. Abort "fast" path
          return IncludesValueSlowPath(isolate, object, value, k + 1, length);
        }
      } else if (value->SameValueZero(*element_k)) {
        return Just(true);
      }
    }
    return Just(false);
  }

  static Maybe<int64_t> IndexOfValueImpl(Isolate* isolate,
                                         Handle<JSObject> object,
                                         Handle<Object> value,
                                         size_t start_from, size_t length) {
    DCHECK(JSObject::PrototypeHasNoElements(isolate, *object));
    Handle<Map> original_map(object->map(), isolate);
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(object->elements()), isolate);

    for (size_t k = start_from; k < length; ++k) {
      DCHECK_EQ(object->map(), *original_map);
      InternalIndex entry =
          GetEntryForIndexImpl(isolate, *object, *elements, k, ALL_PROPERTIES);
      if (entry.is_not_found()) {
        continue;
      }

      Handle<Object> element_k = Subclass::GetImpl(isolate, *elements, entry);

      if (element_k->IsAccessorPair()) {
        LookupIterator it(isolate, object, k, LookupIterator::OWN);
        DCHECK(it.IsFound());
        DCHECK_EQ(it.state(), LookupIterator::ACCESSOR);
        ASSIGN_RETURN_ON_EXCEPTION_VALUE(isolate, element_k,
                                         Object::GetPropertyWithAccessor(&it),
                                         Nothing<int64_t>());

        if (value->StrictEquals(*element_k)) {
          return Just<int64_t>(k);
        }

        if (object->map() != *original_map) {
          // Some mutation occurred in accessor. Abort "fast" path.
          return IndexOfValueSlowPath(isolate, object, value, k + 1, length);
        }
      } else if (value->StrictEquals(*element_k)) {
        return Just<int64_t>(k);
      }
    }
    return Just<int64_t>(-1);
  }
};

class SlowSloppyArgumentsElementsAccessor
    : public SloppyArgumentsElementsAccessor<
          SlowSloppyArgumentsElementsAccessor, DictionaryElementsAccessor,
          ElementsKindTraits<SLOW_SLOPPY_ARGUMENTS_ELEMENTS>> {
 public:
  static Handle<Object> ConvertArgumentsStoreResult(
      Isolate* isolate, Handle<SloppyArgumentsElements> elements,
      Handle<Object> result) {
    // Elements of the arguments object in slow mode might be slow aliases.
    if (result->IsAliasedArgumentsEntry()) {
      DisallowHeapAllocation no_gc;
      AliasedArgumentsEntry alias = AliasedArgumentsEntry::cast(*result);
      Context context = elements->context();
      int context_entry = alias.aliased_context_slot();
      DCHECK(!context.get(context_entry).IsTheHole(isolate));
      return handle(context.get(context_entry), isolate);
    }
    return result;
  }
  static void SloppyDeleteImpl(Handle<JSObject> obj,
                               Handle<SloppyArgumentsElements> elements,
                               InternalIndex entry) {
    // No need to delete a context mapped entry from the arguments elements.
    if (entry.is_not_found()) return;
    Isolate* isolate = obj->GetIsolate();
    Handle<NumberDictionary> dict(NumberDictionary::cast(elements->arguments()),
                                  isolate);
    uint32_t length = elements->parameter_map_length();
    dict =
        NumberDictionary::DeleteEntry(isolate, dict, entry.adjust_down(length));
    elements->set_arguments(*dict);
  }
  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    Isolate* isolate = object->GetIsolate();
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(object->elements()), isolate);
    Handle<FixedArrayBase> old_arguments(
        FixedArrayBase::cast(elements->arguments()), isolate);
    Handle<NumberDictionary> dictionary =
        old_arguments->IsNumberDictionary()
            ? Handle<NumberDictionary>::cast(old_arguments)
            : JSObject::NormalizeElements(object);
    PropertyDetails details(kData, attributes, PropertyCellType::kNoCell);
    Handle<NumberDictionary> new_dictionary =
        NumberDictionary::Add(isolate, dictionary, index, value, details);
    if (attributes != NONE) object->RequireSlowElements(*new_dictionary);
    if (*dictionary != *new_dictionary) {
      elements->set_arguments(*new_dictionary);
    }
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    Isolate* isolate = object->GetIsolate();
    Handle<SloppyArgumentsElements> elements =
        Handle<SloppyArgumentsElements>::cast(store);
    uint32_t length = elements->parameter_map_length();
    if (entry.as_uint32() < length) {
      Object probe = elements->get_mapped_entry(entry.as_uint32());
      DCHECK(!probe.IsTheHole(isolate));
      Context context = elements->context();
      int context_entry = Smi::ToInt(probe);
      DCHECK(!context.get(context_entry).IsTheHole(isolate));
      context.set(context_entry, *value);

      // Redefining attributes of an aliased element destroys fast aliasing.
      elements->set_mapped_entry(entry.as_uint32(),
                                 ReadOnlyRoots(isolate).the_hole_value());
      // For elements that are still writable we re-establish slow aliasing.
      if ((attributes & READ_ONLY) == 0) {
        value = isolate->factory()->NewAliasedArgumentsEntry(context_entry);
      }

      PropertyDetails details(kData, attributes, PropertyCellType::kNoCell);
      Handle<NumberDictionary> arguments(
          NumberDictionary::cast(elements->arguments()), isolate);
      arguments = NumberDictionary::Add(isolate, arguments, entry.as_uint32(),
                                        value, details);
      // If the attributes were NONE, we would have called set rather than
      // reconfigure.
      DCHECK_NE(NONE, attributes);
      object->RequireSlowElements(*arguments);
      elements->set_arguments(*arguments);
    } else {
      Handle<FixedArrayBase> arguments(elements->arguments(), isolate);
      DictionaryElementsAccessor::ReconfigureImpl(
          object, arguments, entry.adjust_down(length), value, attributes);
    }
  }
};

class FastSloppyArgumentsElementsAccessor
    : public SloppyArgumentsElementsAccessor<
          FastSloppyArgumentsElementsAccessor, FastHoleyObjectElementsAccessor,
          ElementsKindTraits<FAST_SLOPPY_ARGUMENTS_ELEMENTS>> {
 public:
  static Handle<Object> ConvertArgumentsStoreResult(
      Isolate* isolate, Handle<SloppyArgumentsElements> paramtere_map,
      Handle<Object> result) {
    DCHECK(!result->IsAliasedArgumentsEntry());
    return result;
  }

  static Handle<FixedArray> GetArguments(Isolate* isolate,
                                         FixedArrayBase store) {
    SloppyArgumentsElements elements = SloppyArgumentsElements::cast(store);
    return Handle<FixedArray>(elements.arguments(), isolate);
  }

  static Handle<NumberDictionary> NormalizeImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> elements) {
    Handle<FixedArray> arguments =
        GetArguments(object->GetIsolate(), *elements);
    return FastHoleyObjectElementsAccessor::NormalizeImpl(object, arguments);
  }

  static Handle<NumberDictionary> NormalizeArgumentsElements(
      Handle<JSObject> object, Handle<SloppyArgumentsElements> elements,
      InternalIndex* entry) {
    Handle<NumberDictionary> dictionary = JSObject::NormalizeElements(object);
    elements->set_arguments(*dictionary);
    // kMaxUInt32 indicates that a context mapped element got deleted. In this
    // case we only normalize the elements (aka. migrate to SLOW_SLOPPY).
    if (entry->is_not_found()) return dictionary;
    uint32_t length = elements->parameter_map_length();
    if (entry->as_uint32() >= length) {
      *entry =
          dictionary
              ->FindEntry(object->GetIsolate(), entry->as_uint32() - length)
              .adjust_up(length);
    }
    return dictionary;
  }

  static void SloppyDeleteImpl(Handle<JSObject> obj,
                               Handle<SloppyArgumentsElements> elements,
                               InternalIndex entry) {
    // Always normalize element on deleting an entry.
    NormalizeArgumentsElements(obj, elements, &entry);
    SlowSloppyArgumentsElementsAccessor::SloppyDeleteImpl(obj, elements, entry);
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    DCHECK_EQ(NONE, attributes);
    Isolate* isolate = object->GetIsolate();
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(object->elements()), isolate);
    Handle<FixedArray> old_arguments(elements->arguments(), isolate);
    if (old_arguments->IsNumberDictionary() ||
        static_cast<uint32_t>(old_arguments->length()) < new_capacity) {
      GrowCapacityAndConvertImpl(object, new_capacity);
    }
    FixedArray arguments = elements->arguments();
    // For fast holey objects, the entry equals the index. The code above made
    // sure that there's enough space to store the value. We cannot convert
    // index to entry explicitly since the slot still contains the hole, so the
    // current EntryForIndex would indicate that it is "absent" by returning
    // kMaxUInt32.
    FastHoleyObjectElementsAccessor::SetImpl(arguments, InternalIndex(index),
                                             *value);
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    DCHECK_EQ(object->elements(), *store);
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(*store), object->GetIsolate());
    NormalizeArgumentsElements(object, elements, &entry);
    SlowSloppyArgumentsElementsAccessor::ReconfigureImpl(object, store, entry,
                                                         value, attributes);
  }

  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    DCHECK(!to.IsNumberDictionary());
    if (from_kind == SLOW_SLOPPY_ARGUMENTS_ELEMENTS) {
      CopyDictionaryToObjectElements(isolate, from, from_start, to,
                                     HOLEY_ELEMENTS, to_start, copy_size);
    } else {
      DCHECK_EQ(FAST_SLOPPY_ARGUMENTS_ELEMENTS, from_kind);
      CopyObjectToObjectElements(isolate, from, HOLEY_ELEMENTS, from_start, to,
                                 HOLEY_ELEMENTS, to_start, copy_size);
    }
  }

  static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                         uint32_t capacity) {
    Isolate* isolate = object->GetIsolate();
    Handle<SloppyArgumentsElements> elements(
        SloppyArgumentsElements::cast(object->elements()), isolate);
    Handle<FixedArray> old_arguments(FixedArray::cast(elements->arguments()),
                                     isolate);
    ElementsKind from_kind = object->GetElementsKind();
    // This method should only be called if there's a reason to update the
    // elements.
    DCHECK(from_kind == SLOW_SLOPPY_ARGUMENTS_ELEMENTS ||
           static_cast<uint32_t>(old_arguments->length()) < capacity);
    Handle<FixedArrayBase> arguments =
        ConvertElementsWithCapacity(object, old_arguments, from_kind, capacity);
    Handle<Map> new_map = JSObject::GetElementsTransitionMap(
        object, FAST_SLOPPY_ARGUMENTS_ELEMENTS);
    JSObject::MigrateToMap(isolate, object, new_map);
    elements->set_arguments(FixedArray::cast(*arguments));
    JSObject::ValidateElements(*object);
  }
};

template <typename Subclass, typename BackingStoreAccessor, typename KindTraits>
class StringWrapperElementsAccessor
    : public ElementsAccessorBase<Subclass, KindTraits> {
 public:
  static Handle<Object> GetInternalImpl(Handle<JSObject> holder,
                                        InternalIndex entry) {
    return GetImpl(holder, entry);
  }

  static Handle<Object> GetImpl(Handle<JSObject> holder, InternalIndex entry) {
    Isolate* isolate = holder->GetIsolate();
    Handle<String> string(GetString(*holder), isolate);
    uint32_t length = static_cast<uint32_t>(string->length());
    if (entry.as_uint32() < length) {
      return isolate->factory()->LookupSingleCharacterStringFromCode(
          String::Flatten(isolate, string)->Get(entry.as_int()));
    }
    return BackingStoreAccessor::GetImpl(isolate, holder->elements(),
                                         entry.adjust_down(length));
  }

  static Handle<Object> GetImpl(Isolate* isolate, FixedArrayBase elements,
                                InternalIndex entry) {
    UNREACHABLE();
  }

  static PropertyDetails GetDetailsImpl(JSObject holder, InternalIndex entry) {
    uint32_t length = static_cast<uint32_t>(GetString(holder).length());
    if (entry.as_uint32() < length) {
      PropertyAttributes attributes =
          static_cast<PropertyAttributes>(READ_ONLY | DONT_DELETE);
      return PropertyDetails(kData, attributes, PropertyCellType::kNoCell);
    }
    return BackingStoreAccessor::GetDetailsImpl(holder,
                                                entry.adjust_down(length));
  }

  static InternalIndex GetEntryForIndexImpl(Isolate* isolate, JSObject holder,
                                            FixedArrayBase backing_store,
                                            size_t index,
                                            PropertyFilter filter) {
    uint32_t length = static_cast<uint32_t>(GetString(holder).length());
    if (index < length) return InternalIndex(index);
    InternalIndex backing_store_entry =
        BackingStoreAccessor::GetEntryForIndexImpl(
            isolate, holder, backing_store, index, filter);
    if (backing_store_entry.is_not_found()) return backing_store_entry;
    return backing_store_entry.adjust_up(length);
  }

  static void DeleteImpl(Handle<JSObject> holder, InternalIndex entry) {
    uint32_t length = static_cast<uint32_t>(GetString(*holder).length());
    if (entry.as_uint32() < length) {
      return;  // String contents can't be deleted.
    }
    BackingStoreAccessor::DeleteImpl(holder, entry.adjust_down(length));
  }

  static void SetImpl(Handle<JSObject> holder, InternalIndex entry,
                      Object value) {
    uint32_t length = static_cast<uint32_t>(GetString(*holder).length());
    if (entry.as_uint32() < length) {
      return;  // String contents are read-only.
    }
    BackingStoreAccessor::SetImpl(holder->elements(), entry.adjust_down(length),
                                  value);
  }

  static void AddImpl(Handle<JSObject> object, uint32_t index,
                      Handle<Object> value, PropertyAttributes attributes,
                      uint32_t new_capacity) {
    DCHECK(index >= static_cast<uint32_t>(GetString(*object).length()));
    // Explicitly grow fast backing stores if needed. Dictionaries know how to
    // extend their capacity themselves.
    if (KindTraits::Kind == FAST_STRING_WRAPPER_ELEMENTS &&
        (object->GetElementsKind() == SLOW_STRING_WRAPPER_ELEMENTS ||
         BackingStoreAccessor::GetCapacityImpl(*object, object->elements()) !=
             new_capacity)) {
      GrowCapacityAndConvertImpl(object, new_capacity);
    }
    BackingStoreAccessor::AddImpl(object, index, value, attributes,
                                  new_capacity);
  }

  static void ReconfigureImpl(Handle<JSObject> object,
                              Handle<FixedArrayBase> store, InternalIndex entry,
                              Handle<Object> value,
                              PropertyAttributes attributes) {
    uint32_t length = static_cast<uint32_t>(GetString(*object).length());
    if (entry.as_uint32() < length) {
      return;  // String contents can't be reconfigured.
    }
    BackingStoreAccessor::ReconfigureImpl(
        object, store, entry.adjust_down(length), value, attributes);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus AddElementsToKeyAccumulatorImpl(
      Handle<JSObject> receiver, KeyAccumulator* accumulator,
      AddKeyConversion convert) {
    Isolate* isolate = receiver->GetIsolate();
    Handle<String> string(GetString(*receiver), isolate);
    string = String::Flatten(isolate, string);
    uint32_t length = static_cast<uint32_t>(string->length());
    for (uint32_t i = 0; i < length; i++) {
      Handle<String> key =
          isolate->factory()->LookupSingleCharacterStringFromCode(
              string->Get(i));
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(accumulator->AddKey(key, convert));
    }
    return BackingStoreAccessor::AddElementsToKeyAccumulatorImpl(
        receiver, accumulator, convert);
  }

  V8_WARN_UNUSED_RESULT static ExceptionStatus CollectElementIndicesImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> backing_store,
      KeyAccumulator* keys) {
    uint32_t length = GetString(*object).length();
    Factory* factory = keys->isolate()->factory();
    for (uint32_t i = 0; i < length; i++) {
      RETURN_FAILURE_IF_NOT_SUCCESSFUL(
          keys->AddKey(factory->NewNumberFromUint(i)));
    }
    return BackingStoreAccessor::CollectElementIndicesImpl(object,
                                                           backing_store, keys);
  }

  static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                         uint32_t capacity) {
    Handle<FixedArrayBase> old_elements(object->elements(),
                                        object->GetIsolate());
    ElementsKind from_kind = object->GetElementsKind();
    if (from_kind == FAST_STRING_WRAPPER_ELEMENTS) {
      // The optimizing compiler relies on the prototype lookups of String
      // objects always returning undefined. If there's a store to the
      // initial String.prototype object, make sure all the optimizations
      // are invalidated.
      object->GetIsolate()->UpdateNoElementsProtectorOnSetLength(object);
    }
    // This method should only be called if there's a reason to update the
    // elements.
    DCHECK(from_kind == SLOW_STRING_WRAPPER_ELEMENTS ||
           static_cast<uint32_t>(old_elements->length()) < capacity);
    Subclass::BasicGrowCapacityAndConvertImpl(object, old_elements, from_kind,
                                              FAST_STRING_WRAPPER_ELEMENTS,
                                              capacity);
  }

  static void CopyElementsImpl(Isolate* isolate, FixedArrayBase from,
                               uint32_t from_start, FixedArrayBase to,
                               ElementsKind from_kind, uint32_t to_start,
                               int packed_size, int copy_size) {
    DCHECK(!to.IsNumberDictionary());
    if (from_kind == SLOW_STRING_WRAPPER_ELEMENTS) {
      CopyDictionaryToObjectElements(isolate, from, from_start, to,
                                     HOLEY_ELEMENTS, to_start, copy_size);
    } else {
      DCHECK_EQ(FAST_STRING_WRAPPER_ELEMENTS, from_kind);
      CopyObjectToObjectElements(isolate, from, HOLEY_ELEMENTS, from_start, to,
                                 HOLEY_ELEMENTS, to_start, copy_size);
    }
  }

  static uint32_t NumberOfElementsImpl(JSObject object,
                                       FixedArrayBase backing_store) {
    uint32_t length = GetString(object).length();
    return length +
           BackingStoreAccessor::NumberOfElementsImpl(object, backing_store);
  }

 private:
  static String GetString(JSObject holder) {
    DCHECK(holder.IsJSPrimitiveWrapper());
    JSPrimitiveWrapper js_value = JSPrimitiveWrapper::cast(holder);
    DCHECK(js_value.value().IsString());
    return String::cast(js_value.value());
  }
};

class FastStringWrapperElementsAccessor
    : public StringWrapperElementsAccessor<
          FastStringWrapperElementsAccessor, FastHoleyObjectElementsAccessor,
          ElementsKindTraits<FAST_STRING_WRAPPER_ELEMENTS>> {
 public:
  static Handle<NumberDictionary> NormalizeImpl(
      Handle<JSObject> object, Handle<FixedArrayBase> elements) {
    return FastHoleyObjectElementsAccessor::NormalizeImpl(object, elements);
  }
};

class SlowStringWrapperElementsAccessor
    : public StringWrapperElementsAccessor<
          SlowStringWrapperElementsAccessor, DictionaryElementsAccessor,
          ElementsKindTraits<SLOW_STRING_WRAPPER_ELEMENTS>> {
 public:
  static bool HasAccessorsImpl(JSObject holder, FixedArrayBase backing_store) {
    return DictionaryElementsAccessor::HasAccessorsImpl(holder, backing_store);
  }
};

}  // namespace

MaybeHandle<Object> ArrayConstructInitializeElements(
    Handle<JSArray> array, JavaScriptArguments* args) {
  if (args->length() == 0) {
    // Optimize the case where there are no parameters passed.
    JSArray::Initialize(array, JSArray::kPreallocatedArrayElements);
    return array;

  } else if (args->length() == 1 && args->at(0)->IsNumber()) {
    uint32_t length;
    if (!args->at(0)->ToArrayLength(&length)) {
      return ThrowArrayLengthRangeError(array->GetIsolate());
    }

    // Optimize the case where there is one argument and the argument is a small
    // smi.
    if (length > 0 && length < JSArray::kInitialMaxFastElementArray) {
      ElementsKind elements_kind = array->GetElementsKind();
      JSArray::Initialize(array, length, length);

      if (!IsHoleyElementsKind(elements_kind)) {
        elements_kind = GetHoleyElementsKind(elements_kind);
        JSObject::TransitionElementsKind(array, elements_kind);
      }
    } else if (length == 0) {
      JSArray::Initialize(array, JSArray::kPreallocatedArrayElements);
    } else {
      // Take the argument as the length.
      JSArray::Initialize(array, 0);
      JSArray::SetLength(array, length);
    }
    return array;
  }

  Factory* factory = array->GetIsolate()->factory();

  // Set length and elements on the array.
  int number_of_elements = args->length();
  JSObject::EnsureCanContainElements(array, args, number_of_elements,
                                     ALLOW_CONVERTED_DOUBLE_ELEMENTS);

  // Allocate an appropriately typed elements array.
  ElementsKind elements_kind = array->GetElementsKind();
  Handle<FixedArrayBase> elms;
  if (IsDoubleElementsKind(elements_kind)) {
    elms = Handle<FixedArrayBase>::cast(
        factory->NewFixedDoubleArray(number_of_elements));
  } else {
    elms = Handle<FixedArrayBase>::cast(
        factory->NewFixedArrayWithHoles(number_of_elements));
  }

  // Fill in the content
  switch (elements_kind) {
    case HOLEY_SMI_ELEMENTS:
    case PACKED_SMI_ELEMENTS: {
      Handle<FixedArray> smi_elms = Handle<FixedArray>::cast(elms);
      for (int entry = 0; entry < number_of_elements; entry++) {
        smi_elms->set(entry, (*args)[entry], SKIP_WRITE_BARRIER);
      }
      break;
    }
    case HOLEY_ELEMENTS:
    case PACKED_ELEMENTS: {
      DisallowHeapAllocation no_gc;
      WriteBarrierMode mode = elms->GetWriteBarrierMode(no_gc);
      Handle<FixedArray> object_elms = Handle<FixedArray>::cast(elms);
      for (int entry = 0; entry < number_of_elements; entry++) {
        object_elms->set(entry, (*args)[entry], mode);
      }
      break;
    }
    case HOLEY_DOUBLE_ELEMENTS:
    case PACKED_DOUBLE_ELEMENTS: {
      Handle<FixedDoubleArray> double_elms =
          Handle<FixedDoubleArray>::cast(elms);
      for (int entry = 0; entry < number_of_elements; entry++) {
        double_elms->set(entry, (*args)[entry].Number());
      }
      break;
    }
    default:
      UNREACHABLE();
  }

  array->set_elements(*elms);
  array->set_length(Smi::FromInt(number_of_elements));
  return array;
}

void CopyFastNumberJSArrayElementsToTypedArray(Address raw_context,
                                               Address raw_source,
                                               Address raw_destination,
                                               uintptr_t length,
                                               uintptr_t offset) {
  Context context = Context::cast(Object(raw_context));
  JSArray source = JSArray::cast(Object(raw_source));
  JSTypedArray destination = JSTypedArray::cast(Object(raw_destination));

  switch (destination.GetElementsKind()) {
#define TYPED_ARRAYS_CASE(Type, type, TYPE, ctype)           \
  case TYPE##_ELEMENTS:                                      \
    CHECK(Type##ElementsAccessor::TryCopyElementsFastNumber( \
        context, source, destination, length, offset));      \
    break;
    TYPED_ARRAYS(TYPED_ARRAYS_CASE)
#undef TYPED_ARRAYS_CASE
    default:
      UNREACHABLE();
  }
}

void CopyTypedArrayElementsToTypedArray(Address raw_source,
                                        Address raw_destination,
                                        uintptr_t length, uintptr_t offset) {
  JSTypedArray source = JSTypedArray::cast(Object(raw_source));
  JSTypedArray destination = JSTypedArray::cast(Object(raw_destination));

  switch (destination.GetElementsKind()) {
#define TYPED_ARRAYS_CASE(Type, type, TYPE, ctype)                          \
  case TYPE##_ELEMENTS:                                                     \
    Type##ElementsAccessor::CopyElementsFromTypedArray(source, destination, \
                                                       length, offset);     \
    break;
    TYPED_ARRAYS(TYPED_ARRAYS_CASE)
#undef TYPED_ARRAYS_CASE
    default:
      UNREACHABLE();
  }
}

void CopyTypedArrayElementsSlice(Address raw_source, Address raw_destination,
                                 uintptr_t start, uintptr_t end) {
  JSTypedArray source = JSTypedArray::cast(Object(raw_source));
  JSTypedArray destination = JSTypedArray::cast(Object(raw_destination));

  destination.GetElementsAccessor()->CopyTypedArrayElementsSlice(
      source, destination, start, end);
}

void ElementsAccessor::InitializeOncePerProcess() {
  static ElementsAccessor* accessor_array[] = {
#define ACCESSOR_ARRAY(Class, Kind, Store) new Class(),
      ELEMENTS_LIST(ACCESSOR_ARRAY)
#undef ACCESSOR_ARRAY
  };

  STATIC_ASSERT((sizeof(accessor_array) / sizeof(*accessor_array)) ==
                kElementsKindCount);

  elements_accessors_ = accessor_array;
}

void ElementsAccessor::TearDown() {
  if (elements_accessors_ == nullptr) return;
#define ACCESSOR_DELETE(Class, Kind, Store) delete elements_accessors_[Kind];
  ELEMENTS_LIST(ACCESSOR_DELETE)
#undef ACCESSOR_DELETE
  elements_accessors_ = nullptr;
}

Handle<JSArray> ElementsAccessor::Concat(Isolate* isolate,
                                         BuiltinArguments* args,
                                         uint32_t concat_size,
                                         uint32_t result_len) {
  ElementsKind result_elements_kind = GetInitialFastElementsKind();
  bool has_raw_doubles = false;
  {
    DisallowHeapAllocation no_gc;
    bool is_holey = false;
    for (uint32_t i = 0; i < concat_size; i++) {
      Object arg = (*args)[i];
      ElementsKind arg_kind = JSArray::cast(arg).GetElementsKind();
      has_raw_doubles = has_raw_doubles || IsDoubleElementsKind(arg_kind);
      is_holey = is_holey || IsHoleyElementsKind(arg_kind);
      result_elements_kind =
          GetMoreGeneralElementsKind(result_elements_kind, arg_kind);
    }
    if (is_holey) {
      result_elements_kind = GetHoleyElementsKind(result_elements_kind);
    }
  }

  // If a double array is concatted into a fast elements array, the fast
  // elements array needs to be initialized to contain proper holes, since
  // boxing doubles may cause incremental marking.
  bool requires_double_boxing =
      has_raw_doubles && !IsDoubleElementsKind(result_elements_kind);
  ArrayStorageAllocationMode mode = requires_double_boxing
                                        ? INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
                                        : DONT_INITIALIZE_ARRAY_ELEMENTS;
  Handle<JSArray> result_array = isolate->factory()->NewJSArray(
      result_elements_kind, result_len, result_len, mode);
  if (result_len == 0) return result_array;

  uint32_t insertion_index = 0;
  Handle<FixedArrayBase> storage(result_array->elements(), isolate);
  ElementsAccessor* accessor = ElementsAccessor::ForKind(result_elements_kind);
  for (uint32_t i = 0; i < concat_size; i++) {
    // It is crucial to keep |array| in a raw pointer form to avoid
    // performance degradation.
    JSArray array = JSArray::cast((*args)[i]);
    uint32_t len = 0;
    array.length().ToArrayLength(&len);
    if (len == 0) continue;
    ElementsKind from_kind = array.GetElementsKind();
    accessor->CopyElements(array, 0, from_kind, storage, insertion_index, len);
    insertion_index += len;
  }

  DCHECK_EQ(insertion_index, result_len);
  return result_array;
}

ElementsAccessor** ElementsAccessor::elements_accessors_ = nullptr;

#undef ELEMENTS_LIST
#undef RETURN_NOTHING_IF_NOT_SUCCESSFUL
#undef RETURN_FAILURE_IF_NOT_SUCCESSFUL
}  // namespace internal
}  // namespace v8