Adding upstream version 115.7.0esr.upstream/115.7.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2854 insertions, 0 deletions

diff --git a/js/src/vm/NativeObject.cpp b/js/src/vm/NativeObject.cpp
new file mode 100644
index 0000000000..a9fd8916ec
--- /dev/null
+++ b/js/src/vm/NativeObject.cpp
@@ -0,0 +1,2854 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/NativeObject-inl.h"
+
+#include "mozilla/Casting.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/Maybe.h"
+
+#include <algorithm>
+#include <iterator>
+
+#include "gc/MaybeRooted.h"
+#include "gc/StableCellHasher.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/friend/StackLimits.h"    // js::AutoCheckRecursionLimit
+#include "js/Value.h"
+#include "vm/EqualityOperations.h"  // js::SameValue
+#include "vm/GetterSetter.h"        // js::GetterSetter
+#include "vm/Interpreter.h"         // js::CallGetter, js::CallSetter
+#include "vm/PlainObject.h"         // js::PlainObject
+#include "vm/TypedArrayObject.h"
+
+#ifdef ENABLE_RECORD_TUPLE
+#  include "builtin/RecordObject.h"
+#  include "builtin/TupleObject.h"
+#  include "vm/RecordTupleShared.h"
+#endif
+
+#include "gc/Nursery-inl.h"
+#include "vm/JSObject-inl.h"
+#include "vm/Shape-inl.h"
+
+using namespace js;
+
+using JS::AutoCheckCannotGC;
+using mozilla::CheckedInt;
+using mozilla::PodCopy;
+using mozilla::RoundUpPow2;
+
+struct EmptyObjectElements {
+  const ObjectElements emptyElementsHeader;
+
+  // Add an extra (unused) Value to make sure an out-of-bounds index when
+  // masked (resulting in index 0) accesses valid memory.
+  const Value val;
+
+ public:
+  constexpr EmptyObjectElements()
+      : emptyElementsHeader(0, 0), val(UndefinedValue()) {}
+  explicit constexpr EmptyObjectElements(ObjectElements::SharedMemory shmem)
+      : emptyElementsHeader(0, 0, shmem), val(UndefinedValue()) {}
+};
+
+static constexpr EmptyObjectElements emptyElementsHeader;
+
+/* Objects with no elements share one empty set of elements. */
+HeapSlot* const js::emptyObjectElements = reinterpret_cast<HeapSlot*>(
+    uintptr_t(&emptyElementsHeader) + sizeof(ObjectElements));
+
+static constexpr EmptyObjectElements emptyElementsHeaderShared(
+    ObjectElements::SharedMemory::IsShared);
+
+/* Objects with no elements share one empty set of elements. */
+HeapSlot* const js::emptyObjectElementsShared = reinterpret_cast<HeapSlot*>(
+    uintptr_t(&emptyElementsHeaderShared) + sizeof(ObjectElements));
+
+struct EmptyObjectSlots : public ObjectSlots {
+  explicit constexpr EmptyObjectSlots(size_t dictionarySlotSpan)
+      : ObjectSlots(0, dictionarySlotSpan, NoUniqueIdInSharedEmptySlots) {}
+};
+
+static constexpr EmptyObjectSlots emptyObjectSlotsHeaders[17] = {
+    EmptyObjectSlots(0),  EmptyObjectSlots(1),  EmptyObjectSlots(2),
+    EmptyObjectSlots(3),  EmptyObjectSlots(4),  EmptyObjectSlots(5),
+    EmptyObjectSlots(6),  EmptyObjectSlots(7),  EmptyObjectSlots(8),
+    EmptyObjectSlots(9),  EmptyObjectSlots(10), EmptyObjectSlots(11),
+    EmptyObjectSlots(12), EmptyObjectSlots(13), EmptyObjectSlots(14),
+    EmptyObjectSlots(15), EmptyObjectSlots(16)};
+
+static_assert(std::size(emptyObjectSlotsHeaders) ==
+              NativeObject::MAX_FIXED_SLOTS + 1);
+
+HeapSlot* const js::emptyObjectSlotsForDictionaryObject[17] = {
+    emptyObjectSlotsHeaders[0].slots(),  emptyObjectSlotsHeaders[1].slots(),
+    emptyObjectSlotsHeaders[2].slots(),  emptyObjectSlotsHeaders[3].slots(),
+    emptyObjectSlotsHeaders[4].slots(),  emptyObjectSlotsHeaders[5].slots(),
+    emptyObjectSlotsHeaders[6].slots(),  emptyObjectSlotsHeaders[7].slots(),
+    emptyObjectSlotsHeaders[8].slots(),  emptyObjectSlotsHeaders[9].slots(),
+    emptyObjectSlotsHeaders[10].slots(), emptyObjectSlotsHeaders[11].slots(),
+    emptyObjectSlotsHeaders[12].slots(), emptyObjectSlotsHeaders[13].slots(),
+    emptyObjectSlotsHeaders[14].slots(), emptyObjectSlotsHeaders[15].slots(),
+    emptyObjectSlotsHeaders[16].slots()};
+
+static_assert(std::size(emptyObjectSlotsForDictionaryObject) ==
+              NativeObject::MAX_FIXED_SLOTS + 1);
+
+HeapSlot* const js::emptyObjectSlots = emptyObjectSlotsForDictionaryObject[0];
+
+#ifdef DEBUG
+
+bool NativeObject::canHaveNonEmptyElements() {
+  return !this->is<TypedArrayObject>();
+}
+
+#endif  // DEBUG
+
+/* static */
+void ObjectElements::PrepareForPreventExtensions(JSContext* cx,
+                                                 NativeObject* obj) {
+  if (!obj->hasEmptyElements()) {
+    obj->shrinkCapacityToInitializedLength(cx);
+  }
+
+  // shrinkCapacityToInitializedLength ensures there are no shifted elements.
+  MOZ_ASSERT(obj->getElementsHeader()->numShiftedElements() == 0);
+}
+
+/* static */
+void ObjectElements::PreventExtensions(NativeObject* obj) {
+  MOZ_ASSERT(!obj->isExtensible());
+  MOZ_ASSERT(obj->getElementsHeader()->numShiftedElements() == 0);
+  MOZ_ASSERT(obj->getDenseInitializedLength() == obj->getDenseCapacity());
+
+  if (!obj->hasEmptyElements()) {
+    obj->getElementsHeader()->setNotExtensible();
+  }
+}
+
+/* static */
+bool ObjectElements::FreezeOrSeal(JSContext* cx, Handle<NativeObject*> obj,
+                                  IntegrityLevel level) {
+  MOZ_ASSERT_IF(level == IntegrityLevel::Frozen && obj->is<ArrayObject>(),
+                !obj->as<ArrayObject>().lengthIsWritable());
+  MOZ_ASSERT(!obj->isExtensible());
+  MOZ_ASSERT(obj->getElementsHeader()->numShiftedElements() == 0);
+
+  if (obj->hasEmptyElements() || obj->denseElementsAreFrozen()) {
+    return true;
+  }
+
+  if (level == IntegrityLevel::Frozen) {
+    if (!JSObject::setFlag(cx, obj, ObjectFlag::FrozenElements)) {
+      return false;
+    }
+  }
+
+  if (!obj->denseElementsAreSealed()) {
+    obj->getElementsHeader()->seal();
+  }
+
+  if (level == IntegrityLevel::Frozen) {
+    obj->getElementsHeader()->freeze();
+  }
+
+  return true;
+}
+
+#ifdef DEBUG
+static mozilla::Atomic<bool, mozilla::Relaxed> gShapeConsistencyChecksEnabled(
+    false);
+
+/* static */
+void js::NativeObject::enableShapeConsistencyChecks() {
+  gShapeConsistencyChecksEnabled = true;
+}
+
+void js::NativeObject::checkShapeConsistency() {
+  if (!gShapeConsistencyChecksEnabled) {
+    return;
+  }
+
+  MOZ_ASSERT(is<NativeObject>());
+
+  if (PropMap* map = shape()->propMap()) {
+    map->checkConsistency(this);
+  } else {
+    MOZ_ASSERT(shape()->propMapLength() == 0);
+  }
+}
+#endif
+
+#ifdef DEBUG
+
+bool js::NativeObject::slotInRange(uint32_t slot,
+                                   SentinelAllowed sentinel) const {
+  MOZ_ASSERT(!gc::IsForwarded(shape()));
+  uint32_t capacity = numFixedSlots() + numDynamicSlots();
+  if (sentinel == SENTINEL_ALLOWED) {
+    return slot <= capacity;
+  }
+  return slot < capacity;
+}
+
+bool js::NativeObject::slotIsFixed(uint32_t slot) const {
+  // We call numFixedSlotsMaybeForwarded() to allow reading slots of
+  // associated objects in trace hooks that may be called during a moving GC.
+  return slot < numFixedSlotsMaybeForwarded();
+}
+
+bool js::NativeObject::isNumFixedSlots(uint32_t nfixed) const {
+  // We call numFixedSlotsMaybeForwarded() to allow reading slots of
+  // associated objects in trace hooks that may be called during a moving GC.
+  return nfixed == numFixedSlotsMaybeForwarded();
+}
+
+uint32_t js::NativeObject::outOfLineNumDynamicSlots() const {
+  return numDynamicSlots();
+}
+#endif /* DEBUG */
+
+mozilla::Maybe<PropertyInfo> js::NativeObject::lookup(JSContext* cx, jsid id) {
+  MOZ_ASSERT(is<NativeObject>());
+  uint32_t index;
+  if (PropMap* map = shape()->lookup(cx, id, &index)) {
+    return mozilla::Some(map->getPropertyInfo(index));
+  }
+  return mozilla::Nothing();
+}
+
+mozilla::Maybe<PropertyInfo> js::NativeObject::lookupPure(jsid id) {
+  MOZ_ASSERT(is<NativeObject>());
+  uint32_t index;
+  if (PropMap* map = shape()->lookupPure(id, &index)) {
+    return mozilla::Some(map->getPropertyInfo(index));
+  }
+  return mozilla::Nothing();
+}
+
+bool NativeObject::setUniqueId(JSContext* cx, uint64_t uid) {
+  MOZ_ASSERT(!hasUniqueId());
+  MOZ_ASSERT(!gc::HasUniqueId(this));
+
+  return setOrUpdateUniqueId(cx, uid);
+}
+
+bool NativeObject::setOrUpdateUniqueId(JSContext* cx, uint64_t uid) {
+  if (!hasDynamicSlots() && !allocateSlots(cx, 0)) {
+    return false;
+  }
+
+  getSlotsHeader()->setUniqueId(uid);
+
+  return true;
+}
+
+bool NativeObject::growSlots(JSContext* cx, uint32_t oldCapacity,
+                             uint32_t newCapacity) {
+  MOZ_ASSERT(newCapacity > oldCapacity);
+
+  /*
+   * Slot capacities are determined by the span of allocated objects. Due to
+   * the limited number of bits to store shape slots, object growth is
+   * throttled well before the slot capacity can overflow.
+   */
+  NativeObject::slotsSizeMustNotOverflow();
+  MOZ_ASSERT(newCapacity <= MAX_SLOTS_COUNT);
+
+  if (!hasDynamicSlots()) {
+    return allocateSlots(cx, newCapacity);
+  }
+
+  uint64_t uid = maybeUniqueId();
+
+  uint32_t newAllocated = ObjectSlots::allocCount(newCapacity);
+
+  uint32_t dictionarySpan = getSlotsHeader()->dictionarySlotSpan();
+
+  uint32_t oldAllocated = ObjectSlots::allocCount(oldCapacity);
+
+  ObjectSlots* oldHeaderSlots = ObjectSlots::fromSlots(slots_);
+  MOZ_ASSERT(oldHeaderSlots->capacity() == oldCapacity);
+
+  HeapSlot* allocation = ReallocateObjectBuffer<HeapSlot>(
+      cx, this, reinterpret_cast<HeapSlot*>(oldHeaderSlots), oldAllocated,
+      newAllocated);
+  if (!allocation) {
+    return false; /* Leave slots at its old size. */
+  }
+
+  auto* newHeaderSlots =
+      new (allocation) ObjectSlots(newCapacity, dictionarySpan, uid);
+  slots_ = newHeaderSlots->slots();
+
+  Debug_SetSlotRangeToCrashOnTouch(slots_ + oldCapacity,
+                                   newCapacity - oldCapacity);
+
+  RemoveCellMemory(this, ObjectSlots::allocSize(oldCapacity),
+                   MemoryUse::ObjectSlots);
+  AddCellMemory(this, ObjectSlots::allocSize(newCapacity),
+                MemoryUse::ObjectSlots);
+
+  MOZ_ASSERT(hasDynamicSlots());
+  return true;
+}
+
+bool NativeObject::growSlotsForNewSlot(JSContext* cx, uint32_t numFixed,
+                                       uint32_t slot) {
+  MOZ_ASSERT(slotSpan() == slot);
+  MOZ_ASSERT(shape()->numFixedSlots() == numFixed);
+  MOZ_ASSERT(slot >= numFixed);
+
+  uint32_t newCapacity = calculateDynamicSlots(numFixed, slot + 1, getClass());
+
+  uint32_t oldCapacity = numDynamicSlots();
+  MOZ_ASSERT(oldCapacity < newCapacity);
+
+  return growSlots(cx, oldCapacity, newCapacity);
+}
+
+bool NativeObject::allocateInitialSlots(JSContext* cx, uint32_t capacity) {
+  uint32_t count = ObjectSlots::allocCount(capacity);
+  HeapSlot* allocation = AllocateObjectBuffer<HeapSlot>(cx, this, count);
+  if (!allocation) {
+    // The new object will be unreachable, but we still have to make it safe
+    // for finalization. Also we must check for it during GC compartment
+    // checks (see IsPartiallyInitializedObject).
+    initEmptyDynamicSlots();
+    return false;
+  }
+
+  auto* headerSlots = new (allocation)
+      ObjectSlots(capacity, 0, ObjectSlots::NoUniqueIdInDynamicSlots);
+  slots_ = headerSlots->slots();
+
+  Debug_SetSlotRangeToCrashOnTouch(slots_, capacity);
+
+  if (!IsInsideNursery(this)) {
+    AddCellMemory(this, ObjectSlots::allocSize(capacity),
+                  MemoryUse::ObjectSlots);
+  }
+
+  MOZ_ASSERT(hasDynamicSlots());
+  return true;
+}
+
+bool NativeObject::allocateSlots(JSContext* cx, uint32_t newCapacity) {
+  MOZ_ASSERT(!hasUniqueId());
+  MOZ_ASSERT(!hasDynamicSlots());
+
+  uint32_t newAllocated = ObjectSlots::allocCount(newCapacity);
+
+  uint32_t dictionarySpan = getSlotsHeader()->dictionarySlotSpan();
+
+  HeapSlot* allocation = AllocateObjectBuffer<HeapSlot>(cx, this, newAllocated);
+  if (!allocation) {
+    return false;
+  }
+
+  auto* newHeaderSlots = new (allocation) ObjectSlots(
+      newCapacity, dictionarySpan, ObjectSlots::NoUniqueIdInDynamicSlots);
+  slots_ = newHeaderSlots->slots();
+
+  Debug_SetSlotRangeToCrashOnTouch(slots_, newCapacity);
+
+  AddCellMemory(this, ObjectSlots::allocSize(newCapacity),
+                MemoryUse::ObjectSlots);
+
+  MOZ_ASSERT(hasDynamicSlots());
+  return true;
+}
+
+/* static */
+bool NativeObject::growSlotsPure(JSContext* cx, NativeObject* obj,
+                                 uint32_t newCapacity) {
+  // IC code calls this directly.
+  AutoUnsafeCallWithABI unsafe;
+
+  if (!obj->growSlots(cx, obj->numDynamicSlots(), newCapacity)) {
+    cx->recoverFromOutOfMemory();
+    return false;
+  }
+
+  return true;
+}
+
+/* static */
+bool NativeObject::addDenseElementPure(JSContext* cx, NativeObject* obj) {
+  // IC code calls this directly.
+  AutoUnsafeCallWithABI unsafe;
+
+  MOZ_ASSERT(obj->getDenseInitializedLength() == obj->getDenseCapacity());
+  MOZ_ASSERT(obj->isExtensible());
+  MOZ_ASSERT(!obj->isIndexed());
+  MOZ_ASSERT(!obj->is<TypedArrayObject>());
+  MOZ_ASSERT_IF(obj->is<ArrayObject>(),
+                obj->as<ArrayObject>().lengthIsWritable());
+
+  // growElements will report OOM also if the number of dense elements will
+  // exceed MAX_DENSE_ELEMENTS_COUNT. See goodElementsAllocationAmount.
+  uint32_t oldCapacity = obj->getDenseCapacity();
+  if (MOZ_UNLIKELY(!obj->growElements(cx, oldCapacity + 1))) {
+    cx->recoverFromOutOfMemory();
+    return false;
+  }
+
+  MOZ_ASSERT(obj->getDenseCapacity() > oldCapacity);
+  MOZ_ASSERT(obj->getDenseCapacity() <= MAX_DENSE_ELEMENTS_COUNT);
+  return true;
+}
+
+static inline void FreeSlots(JSContext* cx, NativeObject* obj,
+                             ObjectSlots* slots, size_t nbytes) {
+  // Note: this is called when shrinking slots, not from the finalizer.
+  MOZ_ASSERT(cx->isMainThreadContext());
+
+  if (obj->isTenured()) {
+    MOZ_ASSERT(!cx->nursery().isInside(slots));
+    js_free(slots);
+  } else {
+    cx->nursery().freeBuffer(slots, nbytes);
+  }
+}
+
+void NativeObject::shrinkSlots(JSContext* cx, uint32_t oldCapacity,
+                               uint32_t newCapacity) {
+  MOZ_ASSERT(hasDynamicSlots());
+  MOZ_ASSERT(newCapacity < oldCapacity);
+  MOZ_ASSERT(oldCapacity == getSlotsHeader()->capacity());
+
+  ObjectSlots* oldHeaderSlots = ObjectSlots::fromSlots(slots_);
+  MOZ_ASSERT(oldHeaderSlots->capacity() == oldCapacity);
+
+  uint64_t uid = maybeUniqueId();
+
+  uint32_t oldAllocated = ObjectSlots::allocCount(oldCapacity);
+
+  if (newCapacity == 0 && uid == 0) {
+    size_t nbytes = ObjectSlots::allocSize(oldCapacity);
+    RemoveCellMemory(this, nbytes, MemoryUse::ObjectSlots);
+    FreeSlots(cx, this, oldHeaderSlots, nbytes);
+    // dictionarySlotSpan is initialized to the correct value by the callers.
+    setEmptyDynamicSlots(0);
+    return;
+  }
+
+  MOZ_ASSERT_IF(!is<ArrayObject>() && !hasUniqueId(),
+                newCapacity >= SLOT_CAPACITY_MIN);
+
+  uint32_t dictionarySpan = getSlotsHeader()->dictionarySlotSpan();
+
+  uint32_t newAllocated = ObjectSlots::allocCount(newCapacity);
+
+  HeapSlot* allocation = ReallocateObjectBuffer<HeapSlot>(
+      cx, this, reinterpret_cast<HeapSlot*>(oldHeaderSlots), oldAllocated,
+      newAllocated);
+  if (!allocation) {
+    // It's possible for realloc to fail when shrinking an allocation. In this
+    // case we continue using the original allocation but still update the
+    // capacity to the new requested capacity, which is smaller than the actual
+    // capacity.
+    cx->recoverFromOutOfMemory();
+    allocation = reinterpret_cast<HeapSlot*>(getSlotsHeader());
+  }
+
+  RemoveCellMemory(this, ObjectSlots::allocSize(oldCapacity),
+                   MemoryUse::ObjectSlots);
+  AddCellMemory(this, ObjectSlots::allocSize(newCapacity),
+                MemoryUse::ObjectSlots);
+
+  auto* newHeaderSlots =
+      new (allocation) ObjectSlots(newCapacity, dictionarySpan, uid);
+  slots_ = newHeaderSlots->slots();
+}
+
+void NativeObject::initFixedElements(gc::AllocKind kind, uint32_t length) {
+  uint32_t capacity =
+      gc::GetGCKindSlots(kind) - ObjectElements::VALUES_PER_HEADER;
+
+  setFixedElements();
+  new (getElementsHeader()) ObjectElements(capacity, length);
+  getElementsHeader()->flags |= ObjectElements::FIXED;
+
+  MOZ_ASSERT(hasFixedElements());
+}
+
+bool NativeObject::willBeSparseElements(uint32_t requiredCapacity,
+                                        uint32_t newElementsHint) {
+  MOZ_ASSERT(is<NativeObject>());
+  MOZ_ASSERT(requiredCapacity > MIN_SPARSE_INDEX);
+
+  uint32_t cap = getDenseCapacity();
+  MOZ_ASSERT(requiredCapacity >= cap);
+
+  if (requiredCapacity > MAX_DENSE_ELEMENTS_COUNT) {
+    return true;
+  }
+
+  uint32_t minimalDenseCount = requiredCapacity / SPARSE_DENSITY_RATIO;
+  if (newElementsHint >= minimalDenseCount) {
+    return false;
+  }
+  minimalDenseCount -= newElementsHint;
+
+  if (minimalDenseCount > cap) {
+    return true;
+  }
+
+  uint32_t len = getDenseInitializedLength();
+  const Value* elems = getDenseElements();
+  for (uint32_t i = 0; i < len; i++) {
+    if (!elems[i].isMagic(JS_ELEMENTS_HOLE) && !--minimalDenseCount) {
+      return false;
+    }
+  }
+  return true;
+}
+
+/* static */
+DenseElementResult NativeObject::maybeDensifySparseElements(
+    JSContext* cx, Handle<NativeObject*> obj) {
+  /*
+   * Wait until after the object goes into dictionary mode, which must happen
+   * when sparsely packing any array with more than MIN_SPARSE_INDEX elements
+   * (see PropertyTree::MAX_HEIGHT).
+   */
+  if (!obj->inDictionaryMode()) {
+    return DenseElementResult::Incomplete;
+  }
+
+  /*
+   * Only measure the number of indexed properties every log(n) times when
+   * populating the object.
+   */
+  uint32_t slotSpan = obj->slotSpan();
+  if (slotSpan != RoundUpPow2(slotSpan)) {
+    return DenseElementResult::Incomplete;
+  }
+
+  /* Watch for conditions under which an object's elements cannot be dense. */
+  if (!obj->isExtensible()) {
+    return DenseElementResult::Incomplete;
+  }
+
+  /*
+   * The indexes in the object need to be sufficiently dense before they can
+   * be converted to dense mode.
+   */
+  uint32_t numDenseElements = 0;
+  uint32_t newInitializedLength = 0;
+
+  for (ShapePropertyIter<NoGC> iter(obj->shape()); !iter.done(); iter++) {
+    uint32_t index;
+    if (!IdIsIndex(iter->key(), &index)) {
+      continue;
+    }
+    if (iter->flags() != PropertyFlags::defaultDataPropFlags) {
+      // For simplicity, only densify the object if all indexed properties can
+      // be converted to dense elements.
+      return DenseElementResult::Incomplete;
+    }
+    MOZ_ASSERT(iter->isDataProperty());
+    numDenseElements++;
+    newInitializedLength = std::max(newInitializedLength, index + 1);
+  }
+
+  if (numDenseElements * SPARSE_DENSITY_RATIO < newInitializedLength) {
+    return DenseElementResult::Incomplete;
+  }
+
+  if (newInitializedLength > MAX_DENSE_ELEMENTS_COUNT) {
+    return DenseElementResult::Incomplete;
+  }
+
+  /*
+   * This object meets all necessary restrictions, convert all indexed
+   * properties into dense elements.
+   */
+
+  if (newInitializedLength > obj->getDenseCapacity()) {
+    if (!obj->growElements(cx, newInitializedLength)) {
+      return DenseElementResult::Failure;
+    }
+  }
+
+  obj->ensureDenseInitializedLength(newInitializedLength, 0);
+
+  if (obj->compartment()->objectMaybeInIteration(obj)) {
+    // Mark the densified elements as maybe-in-iteration. See also the comment
+    // in GetIterator.
+    obj->markDenseElementsMaybeInIteration();
+  }
+
+  if (!NativeObject::densifySparseElements(cx, obj)) {
+    return DenseElementResult::Failure;
+  }
+
+  return DenseElementResult::Success;
+}
+
+void NativeObject::moveShiftedElements() {
+  MOZ_ASSERT(isExtensible());
+
+  ObjectElements* header = getElementsHeader();
+  uint32_t numShifted = header->numShiftedElements();
+  MOZ_ASSERT(numShifted > 0);
+
+  uint32_t initLength = header->initializedLength;
+
+  ObjectElements* newHeader =
+      static_cast<ObjectElements*>(getUnshiftedElementsHeader());
+  memmove(newHeader, header, sizeof(ObjectElements));
+
+  newHeader->clearShiftedElements();
+  newHeader->capacity += numShifted;
+  elements_ = newHeader->elements();
+
+  // To move the elements, temporarily update initializedLength to include
+  // the shifted elements.
+  newHeader->initializedLength += numShifted;
+
+  // Move the elements. Initialize to |undefined| to ensure pre-barriers
+  // don't see garbage.
+  for (size_t i = 0; i < numShifted; i++) {
+    initDenseElement(i, UndefinedValue());
+  }
+  moveDenseElements(0, numShifted, initLength);
+
+  // Restore the initialized length. We use setDenseInitializedLength to
+  // make sure prepareElementRangeForOverwrite is called on the shifted
+  // elements.
+  setDenseInitializedLength(initLength);
+}
+
+void NativeObject::maybeMoveShiftedElements() {
+  MOZ_ASSERT(isExtensible());
+
+  ObjectElements* header = getElementsHeader();
+  MOZ_ASSERT(header->numShiftedElements() > 0);
+
+  // Move the elements if less than a third of the allocated space is in use.
+  if (header->capacity < header->numAllocatedElements() / 3) {
+    moveShiftedElements();
+  }
+}
+
+bool NativeObject::tryUnshiftDenseElements(uint32_t count) {
+  MOZ_ASSERT(isExtensible());
+  MOZ_ASSERT(count > 0);
+
+  ObjectElements* header = getElementsHeader();
+  uint32_t numShifted = header->numShiftedElements();
+
+  if (count > numShifted) {
+    // We need more elements than are easily available. Try to make space
+    // for more elements than we need (and shift the remaining ones) so
+    // that unshifting more elements later will be fast.
+
+    // Don't bother reserving elements if the number of elements is small.
+    // Note that there's no technical reason for using this particular
+    // limit.
+    if (header->initializedLength <= 10 ||
+        header->hasNonwritableArrayLength() ||
+        MOZ_UNLIKELY(count > ObjectElements::MaxShiftedElements)) {
+      return false;
+    }
+
+    MOZ_ASSERT(header->capacity >= header->initializedLength);
+    uint32_t unusedCapacity = header->capacity - header->initializedLength;
+
+    // Determine toShift, the number of extra elements we want to make
+    // available.
+    uint32_t toShift = count - numShifted;
+    MOZ_ASSERT(toShift <= ObjectElements::MaxShiftedElements,
+               "count <= MaxShiftedElements so toShift <= MaxShiftedElements");
+
+    // Give up if we need to allocate more elements.
+    if (toShift > unusedCapacity) {
+      return false;
+    }
+
+    // Move more elements than we need, so that other unshift calls will be
+    // fast. We just have to make sure we don't exceed unusedCapacity.
+    toShift = std::min(toShift + unusedCapacity / 2, unusedCapacity);
+
+    // Ensure |numShifted + toShift| does not exceed MaxShiftedElements.
+    if (numShifted + toShift > ObjectElements::MaxShiftedElements) {
+      toShift = ObjectElements::MaxShiftedElements - numShifted;
+    }
+
+    MOZ_ASSERT(count <= numShifted + toShift);
+    MOZ_ASSERT(numShifted + toShift <= ObjectElements::MaxShiftedElements);
+    MOZ_ASSERT(toShift <= unusedCapacity);
+
+    // Now move/unshift the elements.
+    uint32_t initLen = header->initializedLength;
+    setDenseInitializedLength(initLen + toShift);
+    for (uint32_t i = 0; i < toShift; i++) {
+      initDenseElement(initLen + i, UndefinedValue());
+    }
+    moveDenseElements(toShift, 0, initLen);
+
+    // Shift the elements we just prepended.
+    shiftDenseElementsUnchecked(toShift);
+
+    // We can now fall-through to the fast path below.
+    header = getElementsHeader();
+    MOZ_ASSERT(header->numShiftedElements() == numShifted + toShift);
+
+    numShifted = header->numShiftedElements();
+    MOZ_ASSERT(count <= numShifted);
+  }
+
+  elements_ -= count;
+  ObjectElements* newHeader = getElementsHeader();
+  memmove(newHeader, header, sizeof(ObjectElements));
+
+  newHeader->unshiftShiftedElements(count);
+
+  // Initialize to |undefined| to ensure pre-barriers don't see garbage.
+  for (uint32_t i = 0; i < count; i++) {
+    initDenseElement(i, UndefinedValue());
+  }
+
+  return true;
+}
+
+// Given a requested capacity (in elements) and (potentially) the length of an
+// array for which elements are being allocated, compute an actual allocation
+// amount (in elements).  (Allocation amounts include space for an
+// ObjectElements instance, so a return value of |N| implies
+// |N - ObjectElements::VALUES_PER_HEADER| usable elements.)
+//
+// The requested/actual allocation distinction is meant to:
+//
+//   * preserve amortized O(N) time to add N elements;
+//   * minimize the number of unused elements beyond an array's length, and
+//   * provide at least ELEMENT_CAPACITY_MIN elements no matter what (so adding
+//     the first several elements to small arrays only needs one allocation).
+//
+// Note: the structure and behavior of this method follow along with
+// UnboxedArrayObject::chooseCapacityIndex. Changes to the allocation strategy
+// in one should generally be matched by the other.
+/* static */
+bool NativeObject::goodElementsAllocationAmount(JSContext* cx,
+                                                uint32_t reqCapacity,
+                                                uint32_t length,
+                                                uint32_t* goodAmount) {
+  if (reqCapacity > MAX_DENSE_ELEMENTS_COUNT) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  uint32_t reqAllocated = reqCapacity + ObjectElements::VALUES_PER_HEADER;
+
+  // Handle "small" requests primarily by doubling.
+  const uint32_t Mebi = 1 << 20;
+  if (reqAllocated < Mebi) {
+    uint32_t amount =
+        mozilla::AssertedCast<uint32_t>(RoundUpPow2(reqAllocated));
+
+    // If |amount| would be 2/3 or more of the array's length, adjust
+    // it (up or down) to be equal to the array's length.  This avoids
+    // allocating excess elements that aren't likely to be needed, either
+    // in this resizing or a subsequent one.  The 2/3 factor is chosen so
+    // that exceptional resizings will at most triple the capacity, as
+    // opposed to the usual doubling.
+    uint32_t goodCapacity = amount - ObjectElements::VALUES_PER_HEADER;
+    if (length >= reqCapacity && goodCapacity > (length / 3) * 2) {
+      amount = length + ObjectElements::VALUES_PER_HEADER;
+    }
+
+    if (amount < ELEMENT_CAPACITY_MIN) {
+      amount = ELEMENT_CAPACITY_MIN;
+    }
+
+    *goodAmount = amount;
+
+    return true;
+  }
+
+  // The almost-doubling above wastes a lot of space for larger bucket sizes.
+  // For large amounts, switch to bucket sizes that obey this formula:
+  //
+  //   count(n+1) = Math.ceil(count(n) * 1.125)
+  //
+  // where |count(n)| is the size of the nth bucket, measured in 2**20 slots.
+  // These bucket sizes still preserve amortized O(N) time to add N elements,
+  // just with a larger constant factor.
+  //
+  // The bucket size table below was generated with this JavaScript (and
+  // manual reformatting):
+  //
+  //   for (let n = 1, i = 0; i < 34; i++) {
+  //     print('0x' + (n * (1 << 20)).toString(16) + ', ');
+  //     n = Math.ceil(n * 1.125);
+  //   }
+  static constexpr uint32_t BigBuckets[] = {
+      0x100000,  0x200000,  0x300000,  0x400000,  0x500000,  0x600000,
+      0x700000,  0x800000,  0x900000,  0xb00000,  0xd00000,  0xf00000,
+      0x1100000, 0x1400000, 0x1700000, 0x1a00000, 0x1e00000, 0x2200000,
+      0x2700000, 0x2c00000, 0x3200000, 0x3900000, 0x4100000, 0x4a00000,
+      0x5400000, 0x5f00000, 0x6b00000, 0x7900000, 0x8900000, 0x9b00000,
+      0xaf00000, 0xc500000, 0xde00000, 0xfa00000};
+  static_assert(BigBuckets[std::size(BigBuckets) - 1] <=
+                MAX_DENSE_ELEMENTS_ALLOCATION);
+
+  // Pick the first bucket that'll fit |reqAllocated|.
+  for (uint32_t b : BigBuckets) {
+    if (b >= reqAllocated) {
+      *goodAmount = b;
+      return true;
+    }
+  }
+
+  // Otherwise, return the maximum bucket size.
+  *goodAmount = MAX_DENSE_ELEMENTS_ALLOCATION;
+  return true;
+}
+
+bool NativeObject::growElements(JSContext* cx, uint32_t reqCapacity) {
+  MOZ_ASSERT(isExtensible());
+  MOZ_ASSERT(canHaveNonEmptyElements());
+
+  // If there are shifted elements, consider moving them first. If we don't
+  // move them here, the code below will include the shifted elements in the
+  // resize.
+  uint32_t numShifted = getElementsHeader()->numShiftedElements();
+  if (numShifted > 0) {
+    // If the number of elements is small, it's cheaper to just move them as
+    // it may avoid a malloc/realloc. Note that there's no technical reason
+    // for using this particular value, but it works well in real-world use
+    // cases.
+    static const size_t MaxElementsToMoveEagerly = 20;
+
+    if (getElementsHeader()->initializedLength <= MaxElementsToMoveEagerly) {
+      moveShiftedElements();
+    } else {
+      maybeMoveShiftedElements();
+    }
+    if (getDenseCapacity() >= reqCapacity) {
+      return true;
+    }
+    // moveShiftedElements() may have changed the number of shifted elements;
+    // update `numShifted` accordingly.
+    numShifted = getElementsHeader()->numShiftedElements();
+
+    // If |reqCapacity + numShifted| overflows, we just move all shifted
+    // elements to avoid the problem.
+    CheckedInt<uint32_t> checkedReqCapacity(reqCapacity);
+    checkedReqCapacity += numShifted;
+    if (MOZ_UNLIKELY(!checkedReqCapacity.isValid())) {
+      moveShiftedElements();
+      numShifted = 0;
+    }
+  }
+
+  uint32_t oldCapacity = getDenseCapacity();
+  MOZ_ASSERT(oldCapacity < reqCapacity);
+
+  uint32_t newAllocated = 0;
+  if (is<ArrayObject>() && !as<ArrayObject>().lengthIsWritable()) {
+    // Preserve the |capacity <= length| invariant for arrays with
+    // non-writable length.  See also js::ArraySetLength which initially
+    // enforces this requirement.
+    MOZ_ASSERT(reqCapacity <= as<ArrayObject>().length());
+    // Adding to reqCapacity must not overflow uint32_t.
+    MOZ_ASSERT(reqCapacity <= MAX_DENSE_ELEMENTS_COUNT);
+
+    // Then, add the header and shifted elements sizes to the new capacity
+    // to get the overall amount to allocate.
+    newAllocated = reqCapacity + numShifted + ObjectElements::VALUES_PER_HEADER;
+  } else {
+    // For arrays with writable length, and all non-Array objects, call
+    // `NativeObject::goodElementsAllocationAmount()` to determine the
+    // amount to allocate from the the requested capacity and existing length.
+    if (!goodElementsAllocationAmount(cx, reqCapacity + numShifted,
+                                      getElementsHeader()->length,
+                                      &newAllocated)) {
+      return false;
+    }
+  }
+
+  // newAllocated now contains the size of the buffer we need to allocate;
+  // subtract off the header and shifted elements size to get the new capacity
+  uint32_t newCapacity =
+      newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
+  // If the new capacity isn't strictly greater than the old capacity, then this
+  // method shouldn't have been called; if the new capacity doesn't satisfy
+  // what was requested, then one of the calculations above must have been
+  // wrong.
+  MOZ_ASSERT(newCapacity > oldCapacity && newCapacity >= reqCapacity);
+
+  // If newCapacity exceeds MAX_DENSE_ELEMENTS_COUNT, the array should become
+  // sparse.
+  MOZ_ASSERT(newCapacity <= MAX_DENSE_ELEMENTS_COUNT);
+
+  uint32_t initlen = getDenseInitializedLength();
+
+  HeapSlot* oldHeaderSlots =
+      reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
+  HeapSlot* newHeaderSlots;
+  uint32_t oldAllocated = 0;
+  if (hasDynamicElements()) {
+    // If the object has dynamic elements, then we might be able to resize the
+    // buffer in-place.
+
+    // First, check that adding to oldCapacity won't overflow uint32_t
+    MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
+    // Then, add the header and shifted elements sizes to get the overall size
+    // of the existing buffer
+    oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;
+
+    // Finally, try to resize the buffer.
+    newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(
+        cx, this, oldHeaderSlots, oldAllocated, newAllocated);
+    if (!newHeaderSlots) {
+      return false;  // If the resizing failed, then we leave elements at its
+                     // old size.
+    }
+  } else {
+    // If the object has fixed elements, then we always need to allocate a new
+    // buffer, because if we've reached this code, then the requested capacity
+    // is greater than the existing inline space available within the object
+    newHeaderSlots = AllocateObjectBuffer<HeapSlot>(cx, this, newAllocated);
+    if (!newHeaderSlots) {
+      return false;  // Leave elements at its old size.
+    }
+
+    // Copy the initialized elements into the new buffer,
+    PodCopy(newHeaderSlots, oldHeaderSlots,
+            ObjectElements::VALUES_PER_HEADER + initlen + numShifted);
+  }
+
+  // If the object already had dynamic elements, then we have to account
+  // for freeing the old elements buffer.
+  if (oldAllocated) {
+    RemoveCellMemory(this, oldAllocated * sizeof(HeapSlot),
+                     MemoryUse::ObjectElements);
+  }
+
+  ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
+  // Update the elements pointer to point to the new elements buffer.
+  elements_ = newheader->elements() + numShifted;
+
+  // Clear the "fixed elements" flag, because if this code has been reached,
+  // this object now has dynamic elements.
+  getElementsHeader()->flags &= ~ObjectElements::FIXED;
+  getElementsHeader()->capacity = newCapacity;
+
+  // Poison the uninitialized portion of the new elements buffer.
+  Debug_SetSlotRangeToCrashOnTouch(elements_ + initlen, newCapacity - initlen);
+
+  // Account for allocating the new elements buffer.
+  AddCellMemory(this, newAllocated * sizeof(HeapSlot),
+                MemoryUse::ObjectElements);
+
+  return true;
+}
+
+void NativeObject::shrinkElements(JSContext* cx, uint32_t reqCapacity) {
+  MOZ_ASSERT(canHaveNonEmptyElements());
+  MOZ_ASSERT(reqCapacity >= getDenseInitializedLength());
+
+  if (!hasDynamicElements()) {
+    return;
+  }
+
+  // If we have shifted elements, consider moving them.
+  uint32_t numShifted = getElementsHeader()->numShiftedElements();
+  if (numShifted > 0) {
+    maybeMoveShiftedElements();
+    numShifted = getElementsHeader()->numShiftedElements();
+  }
+
+  uint32_t oldCapacity = getDenseCapacity();
+  MOZ_ASSERT(reqCapacity < oldCapacity);
+
+  uint32_t newAllocated = 0;
+  MOZ_ALWAYS_TRUE(goodElementsAllocationAmount(cx, reqCapacity + numShifted, 0,
+                                               &newAllocated));
+  MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
+
+  uint32_t oldAllocated =
+      oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;
+  if (newAllocated == oldAllocated) {
+    return;  // Leave elements at its old size.
+  }
+
+  MOZ_ASSERT(newAllocated > ObjectElements::VALUES_PER_HEADER);
+  uint32_t newCapacity =
+      newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
+  MOZ_ASSERT(newCapacity <= MAX_DENSE_ELEMENTS_COUNT);
+
+  HeapSlot* oldHeaderSlots =
+      reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
+  HeapSlot* newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(
+      cx, this, oldHeaderSlots, oldAllocated, newAllocated);
+  if (!newHeaderSlots) {
+    cx->recoverFromOutOfMemory();
+    return;  // Leave elements at its old size.
+  }
+
+  RemoveCellMemory(this, oldAllocated * sizeof(HeapSlot),
+                   MemoryUse::ObjectElements);
+
+  ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
+  elements_ = newheader->elements() + numShifted;
+  getElementsHeader()->capacity = newCapacity;
+
+  AddCellMemory(this, newAllocated * sizeof(HeapSlot),
+                MemoryUse::ObjectElements);
+}
+
+void NativeObject::shrinkCapacityToInitializedLength(JSContext* cx) {
+  // When an array's length becomes non-writable, writes to indexes greater
+  // greater than or equal to the length don't change the array.  We handle this
+  // with a check for non-writable length in most places. But in JIT code every
+  // check counts -- so we piggyback the check on the already-required range
+  // check for |index < capacity| by making capacity of arrays with non-writable
+  // length never exceed the length. This mechanism is also used when an object
+  // becomes non-extensible.
+
+  if (getElementsHeader()->numShiftedElements() > 0) {
+    moveShiftedElements();
+  }
+
+  ObjectElements* header = getElementsHeader();
+  uint32_t len = header->initializedLength;
+  MOZ_ASSERT(header->capacity >= len);
+  if (header->capacity == len) {
+    return;
+  }
+
+  shrinkElements(cx, len);
+
+  header = getElementsHeader();
+  uint32_t oldAllocated = header->numAllocatedElements();
+  header->capacity = len;
+
+  // The size of the memory allocation hasn't changed but we lose the actual
+  // capacity information. Make the associated size match the updated capacity.
+  if (!hasFixedElements()) {
+    uint32_t newAllocated = header->numAllocatedElements();
+    RemoveCellMemory(this, oldAllocated * sizeof(HeapSlot),
+                     MemoryUse::ObjectElements);
+    AddCellMemory(this, newAllocated * sizeof(HeapSlot),
+                  MemoryUse::ObjectElements);
+  }
+}
+
+/* static */
+bool NativeObject::allocDictionarySlot(JSContext* cx, Handle<NativeObject*> obj,
+                                       uint32_t* slotp) {
+  MOZ_ASSERT(obj->inDictionaryMode());
+
+  uint32_t slotSpan = obj->slotSpan();
+  MOZ_ASSERT(slotSpan >= JSSLOT_FREE(obj->getClass()));
+
+  // Try to pull a free slot from the slot-number free list.
+  DictionaryPropMap* map = obj->dictionaryShape()->propMap();
+  uint32_t last = map->freeList();
+  if (last != SHAPE_INVALID_SLOT) {
+#ifdef DEBUG
+    MOZ_ASSERT(last < slotSpan);
+    uint32_t next = obj->getSlot(last).toPrivateUint32();
+    MOZ_ASSERT_IF(next != SHAPE_INVALID_SLOT, next < slotSpan);
+#endif
+    *slotp = last;
+    const Value& vref = obj->getSlot(last);
+    map->setFreeList(vref.toPrivateUint32());
+    obj->setSlot(last, UndefinedValue());
+    return true;
+  }
+
+  if (MOZ_UNLIKELY(slotSpan >= SHAPE_MAXIMUM_SLOT)) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  *slotp = slotSpan;
+
+  uint32_t numFixed = obj->numFixedSlots();
+  if (slotSpan < numFixed) {
+    obj->initFixedSlot(slotSpan, UndefinedValue());
+    obj->setDictionaryModeSlotSpan(slotSpan + 1);
+    return true;
+  }
+
+  uint32_t dynamicSlotIndex = slotSpan - numFixed;
+  if (dynamicSlotIndex >= obj->numDynamicSlots()) {
+    if (MOZ_UNLIKELY(!obj->growSlotsForNewSlot(cx, numFixed, slotSpan))) {
+      return false;
+    }
+  }
+  obj->initDynamicSlot(numFixed, slotSpan, UndefinedValue());
+  obj->setDictionaryModeSlotSpan(slotSpan + 1);
+  return true;
+}
+
+void NativeObject::freeDictionarySlot(uint32_t slot) {
+  MOZ_ASSERT(inDictionaryMode());
+  MOZ_ASSERT(slot < slotSpan());
+
+  DictionaryPropMap* map = dictionaryShape()->propMap();
+  uint32_t last = map->freeList();
+
+  // Can't afford to check the whole free list, but let's check the head.
+  MOZ_ASSERT_IF(last != SHAPE_INVALID_SLOT, last < slotSpan() && last != slot);
+
+  // Place all freed slots other than reserved slots (bug 595230) on the
+  // dictionary's free list.
+  if (JSSLOT_FREE(getClass()) <= slot) {
+    MOZ_ASSERT_IF(last != SHAPE_INVALID_SLOT, last < slotSpan());
+    setSlot(slot, PrivateUint32Value(last));
+    map->setFreeList(slot);
+  } else {
+    setSlot(slot, UndefinedValue());
+  }
+}
+
+template <AllowGC allowGC>
+bool js::NativeLookupOwnProperty(
+    JSContext* cx, typename MaybeRooted<NativeObject*, allowGC>::HandleType obj,
+    typename MaybeRooted<jsid, allowGC>::HandleType id, PropertyResult* propp) {
+  return NativeLookupOwnPropertyInline<allowGC>(cx, obj, id, propp);
+}
+
+template bool js::NativeLookupOwnProperty<CanGC>(JSContext* cx,
+                                                 Handle<NativeObject*> obj,
+                                                 HandleId id,
+                                                 PropertyResult* propp);
+
+template bool js::NativeLookupOwnProperty<NoGC>(JSContext* cx,
+                                                NativeObject* const& obj,
+                                                const jsid& id,
+                                                PropertyResult* propp);
+
+/*** [[DefineOwnProperty]] **************************************************/
+
+static bool CallJSAddPropertyOp(JSContext* cx, JSAddPropertyOp op,
+                                HandleObject obj, HandleId id, HandleValue v) {
+  AutoCheckRecursionLimit recursion(cx);
+  if (!recursion.check(cx)) {
+    return false;
+  }
+
+  cx->check(obj, id, v);
+  return op(cx, obj, id, v);
+}
+
+static MOZ_ALWAYS_INLINE bool CallAddPropertyHook(JSContext* cx,
+                                                  Handle<NativeObject*> obj,
+                                                  HandleId id,
+                                                  HandleValue value) {
+  JSAddPropertyOp addProperty = obj->getClass()->getAddProperty();
+  if (MOZ_UNLIKELY(addProperty)) {
+    MOZ_ASSERT(!cx->isHelperThreadContext());
+
+    if (!CallJSAddPropertyOp(cx, addProperty, obj, id, value)) {
+      NativeObject::removeProperty(cx, obj, id);
+      return false;
+    }
+  }
+  return true;
+}
+
+static MOZ_ALWAYS_INLINE bool CallAddPropertyHookDense(
+    JSContext* cx, Handle<NativeObject*> obj, uint32_t index,
+    HandleValue value) {
+  // Inline addProperty for array objects.
+  if (obj->is<ArrayObject>()) {
+    ArrayObject* arr = &obj->as<ArrayObject>();
+    uint32_t length = arr->length();
+    if (index >= length) {
+      arr->setLength(index + 1);
+    }
+    return true;
+  }
+
+  JSAddPropertyOp addProperty = obj->getClass()->getAddProperty();
+  if (MOZ_UNLIKELY(addProperty)) {
+    MOZ_ASSERT(!cx->isHelperThreadContext());
+
+    RootedId id(cx, PropertyKey::Int(index));
+    if (!CallJSAddPropertyOp(cx, addProperty, obj, id, value)) {
+      obj->setDenseElementHole(index);
+      return false;
+    }
+  }
+  return true;
+}
+
+/**
+ * Determines whether a write to the given element on |arr| should fail
+ * because |arr| has a non-writable length, and writing that element would
+ * increase the length of the array.
+ */
+static bool WouldDefinePastNonwritableLength(ArrayObject* arr, uint32_t index) {
+  return !arr->lengthIsWritable() && index >= arr->length();
+}
+
+static bool ChangeProperty(JSContext* cx, Handle<NativeObject*> obj,
+                           HandleId id, HandleObject getter,
+                           HandleObject setter, PropertyFlags flags,
+                           PropertyResult* existing, uint32_t* slotOut) {
+  MOZ_ASSERT(existing);
+
+  Rooted<GetterSetter*> gs(cx);
+
+  // If we're redefining a getter/setter property but the getter and setter
+  // objects are still the same, use the existing GetterSetter.
+  if (existing->isNativeProperty()) {
+    PropertyInfo prop = existing->propertyInfo();
+    if (prop.isAccessorProperty()) {
+      GetterSetter* current = obj->getGetterSetter(prop);
+      if (current->getter() == getter && current->setter() == setter) {
+        gs = current;
+      }
+    }
+  }
+
+  if (!gs) {
+    gs = GetterSetter::create(cx, getter, setter);
+    if (!gs) {
+      return false;
+    }
+  }
+
+  if (existing->isNativeProperty()) {
+    if (!NativeObject::changeProperty(cx, obj, id, flags, slotOut)) {
+      return false;
+    }
+  } else {
+    if (!NativeObject::addProperty(cx, obj, id, flags, slotOut)) {
+      return false;
+    }
+  }
+
+  obj->setSlot(*slotOut, PrivateGCThingValue(gs));
+  return true;
+}
+
+static PropertyFlags ComputePropertyFlags(const PropertyDescriptor& desc) {
+  desc.assertComplete();
+
+  PropertyFlags flags;
+  flags.setFlag(PropertyFlag::Configurable, desc.configurable());
+  flags.setFlag(PropertyFlag::Enumerable, desc.enumerable());
+
+  if (desc.isDataDescriptor()) {
+    flags.setFlag(PropertyFlag::Writable, desc.writable());
+  } else {
+    MOZ_ASSERT(desc.isAccessorDescriptor());
+    flags.setFlag(PropertyFlag::AccessorProperty);
+  }
+
+  return flags;
+}
+
+// Whether we're adding a new property or changing an existing property (this
+// can be either a property stored in the shape tree or a dense element).
+enum class IsAddOrChange { Add, Change };
+
+template <IsAddOrChange AddOrChange>
+static MOZ_ALWAYS_INLINE bool AddOrChangeProperty(
+    JSContext* cx, Handle<NativeObject*> obj, HandleId id,
+    Handle<PropertyDescriptor> desc, PropertyResult* existing = nullptr) {
+  desc.assertComplete();
+
+#ifdef DEBUG
+  if constexpr (AddOrChange == IsAddOrChange::Add) {
+    MOZ_ASSERT(existing == nullptr);
+    MOZ_ASSERT(!obj->containsPure(id));
+  } else {
+    static_assert(AddOrChange == IsAddOrChange::Change);
+    MOZ_ASSERT(existing);
+    MOZ_ASSERT(existing->isNativeProperty() || existing->isDenseElement());
+  }
+#endif
+
+  // Use dense storage for indexed properties where possible: when we have an
+  // integer key with default property attributes and are either adding a new
+  // property or changing a dense element.
+  PropertyFlags flags = ComputePropertyFlags(desc);
+  if (id.isInt() && flags == PropertyFlags::defaultDataPropFlags &&
+      (AddOrChange == IsAddOrChange::Add || existing->isDenseElement())) {
+    MOZ_ASSERT(!desc.isAccessorDescriptor());
+    MOZ_ASSERT(!obj->is<TypedArrayObject>());
+    uint32_t index = id.toInt();
+    DenseElementResult edResult = obj->ensureDenseElements(cx, index, 1);
+    if (edResult == DenseElementResult::Failure) {
+      return false;
+    }
+    if (edResult == DenseElementResult::Success) {
+      obj->setDenseElement(index, desc.value());
+      if (!CallAddPropertyHookDense(cx, obj, index, desc.value())) {
+        return false;
+      }
+      return true;
+    }
+  }
+
+  uint32_t slot;
+  if constexpr (AddOrChange == IsAddOrChange::Add) {
+    if (desc.isAccessorDescriptor()) {
+      Rooted<GetterSetter*> gs(
+          cx, GetterSetter::create(cx, desc.getter(), desc.setter()));
+      if (!gs) {
+        return false;
+      }
+      if (!NativeObject::addProperty(cx, obj, id, flags, &slot)) {
+        return false;
+      }
+      obj->initSlot(slot, PrivateGCThingValue(gs));
+    } else {
+      if (!NativeObject::addProperty(cx, obj, id, flags, &slot)) {
+        return false;
+      }
+      obj->initSlot(slot, desc.value());
+    }
+  } else {
+    if (desc.isAccessorDescriptor()) {
+      if (!ChangeProperty(cx, obj, id, desc.getter(), desc.setter(), flags,
+                          existing, &slot)) {
+        return false;
+      }
+    } else {
+      if (existing->isNativeProperty()) {
+        if (!NativeObject::changeProperty(cx, obj, id, flags, &slot)) {
+          return false;
+        }
+      } else {
+        if (!NativeObject::addProperty(cx, obj, id, flags, &slot)) {
+          return false;
+        }
+      }
+      obj->setSlot(slot, desc.value());
+    }
+  }
+
+  MOZ_ASSERT(slot < obj->slotSpan());
+
+  // Clear any existing dense index after adding a sparse indexed property,
+  // and investigate converting the object to dense indexes.
+  if (id.isInt()) {
+    uint32_t index = id.toInt();
+    if constexpr (AddOrChange == IsAddOrChange::Add) {
+      MOZ_ASSERT(!obj->containsDenseElement(index));
+    } else {
+      obj->removeDenseElementForSparseIndex(index);
+    }
+    // Only try to densify sparse elements if the property we just added/changed
+    // is in the last slot. This avoids a perf cliff in pathological cases: in
+    // maybeDensifySparseElements we densify if the slot span is a power-of-two,
+    // but if we get slots from the free list, the slot span will stay the same
+    // until the free list is empty. This means we'd get quadratic behavior by
+    // trying to densify for each sparse element we add. See bug 1782487.
+    if (slot == obj->slotSpan() - 1) {
+      DenseElementResult edResult =
+          NativeObject::maybeDensifySparseElements(cx, obj);
+      if (edResult == DenseElementResult::Failure) {
+        return false;
+      }
+      if (edResult == DenseElementResult::Success) {
+        MOZ_ASSERT(!desc.isAccessorDescriptor());
+        return CallAddPropertyHookDense(cx, obj, index, desc.value());
+      }
+    }
+  }
+
+  if (desc.isDataDescriptor()) {
+    return CallAddPropertyHook(cx, obj, id, desc.value());
+  }
+
+  return CallAddPropertyHook(cx, obj, id, UndefinedHandleValue);
+}
+
+// Versions of AddOrChangeProperty optimized for adding a plain data property.
+// This function doesn't handle integer ids as we may have to store them in
+// dense elements.
+static MOZ_ALWAYS_INLINE bool AddDataProperty(JSContext* cx,
+                                              Handle<NativeObject*> obj,
+                                              HandleId id, HandleValue v) {
+  MOZ_ASSERT(!id.isInt());
+
+  uint32_t slot;
+  if (!NativeObject::addProperty(cx, obj, id,
+                                 PropertyFlags::defaultDataPropFlags, &slot)) {
+    return false;
+  }
+
+  obj->initSlot(slot, v);
+
+  return CallAddPropertyHook(cx, obj, id, v);
+}
+
+bool js::AddSlotAndCallAddPropHook(JSContext* cx, Handle<NativeObject*> obj,
+                                   HandleValue v, Handle<Shape*> newShape) {
+  MOZ_ASSERT(obj->getClass()->getAddProperty());
+  MOZ_ASSERT(newShape->asShared().lastProperty().isDataProperty());
+
+  RootedId id(cx, newShape->asShared().lastProperty().key());
+  MOZ_ASSERT(!id.isInt());
+
+  uint32_t slot = newShape->asShared().lastProperty().slot();
+  if (!obj->setShapeAndAddNewSlot(cx, &newShape->asShared(), slot)) {
+    return false;
+  }
+  obj->initSlot(slot, v);
+
+  return CallAddPropertyHook(cx, obj, id, v);
+}
+
+static bool IsAccessorDescriptor(const PropertyResult& prop) {
+  if (prop.isNativeProperty()) {
+    return prop.propertyInfo().isAccessorProperty();
+  }
+
+  MOZ_ASSERT(prop.isDenseElement() || prop.isTypedArrayElement());
+  return false;
+}
+
+static bool IsDataDescriptor(const PropertyResult& prop) {
+  return !IsAccessorDescriptor(prop);
+}
+
+static bool GetCustomDataProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                  MutableHandleValue vp);
+
+static bool GetExistingDataProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                    HandleId id, const PropertyResult& prop,
+                                    MutableHandleValue vp) {
+  if (prop.isDenseElement()) {
+    vp.set(obj->getDenseElement(prop.denseElementIndex()));
+    return true;
+  }
+  if (prop.isTypedArrayElement()) {
+    size_t idx = prop.typedArrayElementIndex();
+    return obj->as<TypedArrayObject>().getElement<CanGC>(cx, idx, vp);
+  }
+
+  PropertyInfo propInfo = prop.propertyInfo();
+  if (propInfo.isDataProperty()) {
+    vp.set(obj->getSlot(propInfo.slot()));
+    return true;
+  }
+
+  MOZ_ASSERT(!cx->isHelperThreadContext());
+  MOZ_RELEASE_ASSERT(propInfo.isCustomDataProperty());
+  return GetCustomDataProperty(cx, obj, id, vp);
+}
+
+/*
+ * If desc is redundant with an existing own property obj[id], then set
+ * |*redundant = true| and return true.
+ */
+static bool DefinePropertyIsRedundant(JSContext* cx, Handle<NativeObject*> obj,
+                                      HandleId id, const PropertyResult& prop,
+                                      JS::PropertyAttributes attrs,
+                                      Handle<PropertyDescriptor> desc,
+                                      bool* redundant) {
+  *redundant = false;
+
+  if (desc.hasConfigurable() && desc.configurable() != attrs.configurable()) {
+    return true;
+  }
+  if (desc.hasEnumerable() && desc.enumerable() != attrs.enumerable()) {
+    return true;
+  }
+  if (desc.isDataDescriptor()) {
+    if (IsAccessorDescriptor(prop)) {
+      return true;
+    }
+    if (desc.hasWritable() && desc.writable() != attrs.writable()) {
+      return true;
+    }
+    if (desc.hasValue()) {
+      // Get the current value of the existing property.
+      RootedValue currentValue(cx);
+      if (!GetExistingDataProperty(cx, obj, id, prop, &currentValue)) {
+        return false;
+      }
+
+      // Don't call SameValue here to ensure we properly update distinct
+      // NaN values.
+      if (desc.value() != currentValue) {
+        return true;
+      }
+    }
+
+    // Check for custom data properties for ArrayObject/ArgumentsObject.
+    // PropertyDescriptor can't represent these properties so they're never
+    // redundant.
+    if (prop.isNativeProperty() && prop.propertyInfo().isCustomDataProperty()) {
+      return true;
+    }
+  } else if (desc.isAccessorDescriptor()) {
+    if (!prop.isNativeProperty()) {
+      return true;
+    }
+    PropertyInfo propInfo = prop.propertyInfo();
+    if (desc.hasGetter() && (!propInfo.isAccessorProperty() ||
+                             desc.getter() != obj->getGetter(propInfo))) {
+      return true;
+    }
+    if (desc.hasSetter() && (!propInfo.isAccessorProperty() ||
+                             desc.setter() != obj->getSetter(propInfo))) {
+      return true;
+    }
+  }
+
+  *redundant = true;
+  return true;
+}
+
+bool js::NativeDefineProperty(JSContext* cx, Handle<NativeObject*> obj,
+                              HandleId id, Handle<PropertyDescriptor> desc_,
+                              ObjectOpResult& result) {
+  desc_.assertValid();
+
+  // Section numbers and step numbers below refer to ES2018, draft rev
+  // 540b827fccf6122a984be99ab9af7be20e3b5562.
+  //
+  // This function aims to implement 9.1.6 [[DefineOwnProperty]] as well as
+  // the [[DefineOwnProperty]] methods described in 9.4.2.1 (arrays), 9.4.4.2
+  // (arguments), and 9.4.5.3 (typed array views).
+
+  // Dispense with custom behavior of exotic native objects first.
+  if (obj->is<ArrayObject>()) {
+    // 9.4.2.1 step 2. Redefining an array's length is very special.
+    Rooted<ArrayObject*> arr(cx, &obj->as<ArrayObject>());
+    if (id == NameToId(cx->names().length)) {
+      // 9.1.6.3 ValidateAndApplyPropertyDescriptor, step 7.a.
+      if (desc_.isAccessorDescriptor()) {
+        return result.fail(JSMSG_CANT_REDEFINE_PROP);
+      }
+
+      MOZ_ASSERT(!cx->isHelperThreadContext());
+      return ArraySetLength(cx, arr, id, desc_, result);
+    }
+
+    // 9.4.2.1 step 3. Don't extend a fixed-length array.
+    uint32_t index;
+    if (IdIsIndex(id, &index)) {
+      if (WouldDefinePastNonwritableLength(arr, index)) {
+        return result.fail(JSMSG_CANT_DEFINE_PAST_ARRAY_LENGTH);
+      }
+    }
+  } else if (obj->is<TypedArrayObject>()) {
+    // 9.4.5.3 step 3. Indexed properties of typed arrays are special.
+    if (mozilla::Maybe<uint64_t> index = ToTypedArrayIndex(id)) {
+      MOZ_ASSERT(!cx->isHelperThreadContext());
+      Rooted<TypedArrayObject*> tobj(cx, &obj->as<TypedArrayObject>());
+      return DefineTypedArrayElement(cx, tobj, index.value(), desc_, result);
+    }
+  } else if (obj->is<ArgumentsObject>()) {
+    Rooted<ArgumentsObject*> argsobj(cx, &obj->as<ArgumentsObject>());
+    if (id.isAtom(cx->names().length)) {
+      // Either we are resolving the .length property on this object,
+      // or redefining it. In the latter case only, we must reify the
+      // property.
+      if (!desc_.resolving()) {
+        if (!ArgumentsObject::reifyLength(cx, argsobj)) {
+          return false;
+        }
+      }
+    } else if (id.isAtom(cx->names().callee) &&
+               argsobj->is<MappedArgumentsObject>()) {
+      // Do same thing as .length for .callee on MappedArgumentsObject.
+      if (!desc_.resolving()) {
+        Rooted<MappedArgumentsObject*> mapped(
+            cx, &argsobj->as<MappedArgumentsObject>());
+        if (!MappedArgumentsObject::reifyCallee(cx, mapped)) {
+          return false;
+        }
+      }
+    } else if (id.isWellKnownSymbol(JS::SymbolCode::iterator)) {
+      // Do same thing as .length for [@@iterator].
+      if (!desc_.resolving()) {
+        if (!ArgumentsObject::reifyIterator(cx, argsobj)) {
+          return false;
+        }
+      }
+    } else if (id.isInt()) {
+      if (!desc_.resolving()) {
+        argsobj->markElementOverridden();
+      }
+    }
+  }
+
+  // 9.1.6.1 OrdinaryDefineOwnProperty step 1.
+  PropertyResult prop;
+  if (desc_.resolving()) {
+    // We are being called from a resolve or enumerate hook to reify a
+    // lazily-resolved property. To avoid reentering the resolve hook and
+    // recursing forever, skip the resolve hook when doing this lookup.
+    if (!NativeLookupOwnPropertyNoResolve(cx, obj, id, &prop)) {
+      return false;
+    }
+  } else {
+    if (!NativeLookupOwnProperty<CanGC>(cx, obj, id, &prop)) {
+      return false;
+    }
+  }
+
+  // From this point, the step numbers refer to
+  // 9.1.6.3, ValidateAndApplyPropertyDescriptor.
+  // Step 1 is a redundant assertion.
+
+  // Filling in desc: Here we make a copy of the desc_ argument. We will turn
+  // it into a complete descriptor before updating obj. The spec algorithm
+  // does not explicitly do this, but the end result is the same. Search for
+  // "fill in" below for places where the filling-in actually occurs.
+  Rooted<PropertyDescriptor> desc(cx, desc_);
+
+  // Step 2.
+  if (prop.isNotFound()) {
+    // Note: We are sharing the property definition machinery with private
+    //       fields. Private fields may be added to non-extensible objects.
+    if (!obj->isExtensible() && !id.isPrivateName() &&
+        // R&T wrappers are non-extensible, but we still want to be able to
+        // lazily resolve their properties. We can special-case them to
+        // allow doing so.
+        IF_RECORD_TUPLE(
+            !(IsExtendedPrimitiveWrapper(*obj) && desc_.resolving()), true)) {
+      return result.fail(JSMSG_CANT_DEFINE_PROP_OBJECT_NOT_EXTENSIBLE);
+    }
+
+    // Fill in missing desc fields with defaults.
+    CompletePropertyDescriptor(&desc);
+
+    if (!AddOrChangeProperty<IsAddOrChange::Add>(cx, obj, id, desc)) {
+      return false;
+    }
+    return result.succeed();
+  }
+
+  // Step 3 and 7.a.i.3, 8.a.iii, 10 (partially). Prop might not actually
+  // have a real shape, e.g. in the case of typed array elements,
+  // GetPropertyAttributes is used to paper-over that difference.
+  JS::PropertyAttributes attrs = GetPropertyAttributes(obj, prop);
+  bool redundant;
+  if (!DefinePropertyIsRedundant(cx, obj, id, prop, attrs, desc, &redundant)) {
+    return false;
+  }
+  if (redundant) {
+    return result.succeed();
+  }
+
+  // Step 4.
+  if (!attrs.configurable()) {
+    if (desc.hasConfigurable() && desc.configurable()) {
+      return result.fail(JSMSG_CANT_REDEFINE_PROP);
+    }
+    if (desc.hasEnumerable() && desc.enumerable() != attrs.enumerable()) {
+      return result.fail(JSMSG_CANT_REDEFINE_PROP);
+    }
+  }
+
+  // Fill in desc.[[Configurable]] and desc.[[Enumerable]] if missing.
+  if (!desc.hasConfigurable()) {
+    desc.setConfigurable(attrs.configurable());
+  }
+  if (!desc.hasEnumerable()) {
+    desc.setEnumerable(attrs.enumerable());
+  }
+
+  // Steps 5-8.
+  if (desc.isGenericDescriptor()) {
+    // Step 5. No further validation is required.
+
+    // Fill in desc. A generic descriptor has none of these fields, so copy
+    // everything from shape.
+    MOZ_ASSERT(!desc.hasValue());
+    MOZ_ASSERT(!desc.hasWritable());
+    MOZ_ASSERT(!desc.hasGetter());
+    MOZ_ASSERT(!desc.hasSetter());
+    if (IsDataDescriptor(prop)) {
+      RootedValue currentValue(cx);
+      if (!GetExistingDataProperty(cx, obj, id, prop, &currentValue)) {
+        return false;
+      }
+      desc.setValue(currentValue);
+      desc.setWritable(attrs.writable());
+    } else {
+      PropertyInfo propInfo = prop.propertyInfo();
+      desc.setGetter(obj->getGetter(propInfo));
+      desc.setSetter(obj->getSetter(propInfo));
+    }
+  } else if (desc.isDataDescriptor() != IsDataDescriptor(prop)) {
+    // Step 6.
+    if (!attrs.configurable()) {
+      return result.fail(JSMSG_CANT_REDEFINE_PROP);
+    }
+
+    // Fill in desc fields with default values (steps 6.b.i and 6.c.i).
+    CompletePropertyDescriptor(&desc);
+  } else if (desc.isDataDescriptor()) {
+    // Step 7.
+    bool frozen = !attrs.configurable() && !attrs.writable();
+
+    // Step 7.a.i.1.
+    if (frozen && desc.hasWritable() && desc.writable()) {
+      return result.fail(JSMSG_CANT_REDEFINE_PROP);
+    }
+
+    if (frozen || !desc.hasValue()) {
+      RootedValue currentValue(cx);
+      if (!GetExistingDataProperty(cx, obj, id, prop, &currentValue)) {
+        return false;
+      }
+
+      if (!desc.hasValue()) {
+        // Fill in desc.[[Value]].
+        desc.setValue(currentValue);
+      } else {
+        // Step 7.a.i.2.
+        bool same;
+        MOZ_ASSERT(!cx->isHelperThreadContext());
+        if (!SameValue(cx, desc.value(), currentValue, &same)) {
+          return false;
+        }
+        if (!same) {
+          return result.fail(JSMSG_CANT_REDEFINE_PROP);
+        }
+      }
+    }
+
+    // Step 7.a.i.3.
+    if (frozen) {
+      return result.succeed();
+    }
+
+    // Fill in desc.[[Writable]].
+    if (!desc.hasWritable()) {
+      desc.setWritable(attrs.writable());
+    }
+  } else {
+    // Step 8.
+    PropertyInfo propInfo = prop.propertyInfo();
+    MOZ_ASSERT(propInfo.isAccessorProperty());
+    MOZ_ASSERT(desc.isAccessorDescriptor());
+
+    // The spec says to use SameValue, but since the values in
+    // question are objects, we can just compare pointers.
+    if (desc.hasSetter()) {
+      // Step 8.a.i.
+      if (!attrs.configurable() && desc.setter() != obj->getSetter(propInfo)) {
+        return result.fail(JSMSG_CANT_REDEFINE_PROP);
+      }
+    } else {
+      // Fill in desc.[[Set]] from shape.
+      desc.setSetter(obj->getSetter(propInfo));
+    }
+    if (desc.hasGetter()) {
+      // Step 8.a.ii.
+      if (!attrs.configurable() && desc.getter() != obj->getGetter(propInfo)) {
+        return result.fail(JSMSG_CANT_REDEFINE_PROP);
+      }
+    } else {
+      // Fill in desc.[[Get]] from shape.
+      desc.setGetter(obj->getGetter(propInfo));
+    }
+
+    // Step 8.a.iii (Omitted).
+  }
+
+  // Step 9.
+  if (!AddOrChangeProperty<IsAddOrChange::Change>(cx, obj, id, desc, &prop)) {
+    return false;
+  }
+
+  // Step 10.
+  return result.succeed();
+}
+
+bool js::NativeDefineDataProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                  HandleId id, HandleValue value,
+                                  unsigned attrs, ObjectOpResult& result) {
+  Rooted<PropertyDescriptor> desc(cx, PropertyDescriptor::Data(value, attrs));
+  return NativeDefineProperty(cx, obj, id, desc, result);
+}
+
+bool js::NativeDefineAccessorProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                      HandleId id, HandleObject getter,
+                                      HandleObject setter, unsigned attrs) {
+  Rooted<PropertyDescriptor> desc(
+      cx, PropertyDescriptor::Accessor(
+              getter ? mozilla::Some(getter) : mozilla::Nothing(),
+              setter ? mozilla::Some(setter) : mozilla::Nothing(), attrs));
+
+  ObjectOpResult result;
+  if (!NativeDefineProperty(cx, obj, id, desc, result)) {
+    return false;
+  }
+
+  if (!result) {
+    // Off-thread callers should not get here: they must call this
+    // function only with known-valid arguments. Populating a new
+    // PlainObject with configurable properties is fine.
+    MOZ_ASSERT(!cx->isHelperThreadContext());
+    result.reportError(cx, obj, id);
+    return false;
+  }
+
+  return true;
+}
+
+bool js::NativeDefineDataProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                  HandleId id, HandleValue value,
+                                  unsigned attrs) {
+  ObjectOpResult result;
+  if (!NativeDefineDataProperty(cx, obj, id, value, attrs, result)) {
+    return false;
+  }
+  if (!result) {
+    // Off-thread callers should not get here: they must call this
+    // function only with known-valid arguments. Populating a new
+    // PlainObject with configurable properties is fine.
+    MOZ_ASSERT(!cx->isHelperThreadContext());
+    result.reportError(cx, obj, id);
+    return false;
+  }
+  return true;
+}
+
+bool js::NativeDefineDataProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                  PropertyName* name, HandleValue value,
+                                  unsigned attrs) {
+  RootedId id(cx, NameToId(name));
+  return NativeDefineDataProperty(cx, obj, id, value, attrs);
+}
+
+static bool DefineNonexistentProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                      HandleId id, HandleValue v,
+                                      ObjectOpResult& result) {
+  // Optimized NativeDefineProperty() version for known absent properties.
+
+  // Dispense with custom behavior of exotic native objects first.
+  if (obj->is<ArrayObject>()) {
+    // Array's length property is non-configurable, so we shouldn't
+    // encounter it in this function.
+    MOZ_ASSERT(id != NameToId(cx->names().length));
+
+    // 9.4.2.1 step 3. Don't extend a fixed-length array.
+    uint32_t index;
+    if (IdIsIndex(id, &index)) {
+      if (WouldDefinePastNonwritableLength(&obj->as<ArrayObject>(), index)) {
+        return result.fail(JSMSG_CANT_DEFINE_PAST_ARRAY_LENGTH);
+      }
+    }
+  } else if (obj->is<TypedArrayObject>()) {
+    // 9.4.5.5 step 2. Indexed properties of typed arrays are special.
+    if (mozilla::Maybe<uint64_t> index = ToTypedArrayIndex(id)) {
+      // This method is only called for non-existent properties, which
+      // means any absent indexed property must be out of range.
+      MOZ_ASSERT(index.value() >= obj->as<TypedArrayObject>().length());
+
+      // The following steps refer to 9.4.5.11 IntegerIndexedElementSet.
+
+      // Step 1 is enforced by the caller.
+
+      // Steps 2-3.
+      // We still need to call ToNumber or ToBigInt, because of its
+      // possible side effects.
+      if (!obj->as<TypedArrayObject>().convertForSideEffect(cx, v)) {
+        return false;
+      }
+
+      // Step 4 (nothing to do, the index is out of range).
+
+      // Step 5.
+      return result.succeed();
+    }
+  } else if (obj->is<ArgumentsObject>()) {
+    // If this method is called with either |length| or |@@iterator|, the
+    // property was previously deleted and hence should already be marked
+    // as overridden.
+    MOZ_ASSERT_IF(id.isAtom(cx->names().length),
+                  obj->as<ArgumentsObject>().hasOverriddenLength());
+    MOZ_ASSERT_IF(id.isWellKnownSymbol(JS::SymbolCode::iterator),
+                  obj->as<ArgumentsObject>().hasOverriddenIterator());
+
+    // We still need to mark any element properties as overridden.
+    if (id.isInt()) {
+      obj->as<ArgumentsObject>().markElementOverridden();
+    }
+  }
+
+#ifdef DEBUG
+  PropertyResult prop;
+  if (!NativeLookupOwnPropertyNoResolve(cx, obj, id, &prop)) {
+    return false;
+  }
+  MOZ_ASSERT(prop.isNotFound(), "didn't expect to find an existing property");
+#endif
+
+  // 9.1.6.3, ValidateAndApplyPropertyDescriptor.
+  // Step 1 is a redundant assertion, step 3 and later don't apply here.
+
+  // Step 2.
+  if (!obj->isExtensible()) {
+    return result.fail(JSMSG_CANT_DEFINE_PROP_OBJECT_NOT_EXTENSIBLE);
+  }
+
+  if (id.isInt()) {
+    // This might be a dense element. Use AddOrChangeProperty as it knows
+    // how to deal with that.
+    Rooted<PropertyDescriptor> desc(
+        cx, PropertyDescriptor::Data(v, {JS::PropertyAttribute::Configurable,
+                                         JS::PropertyAttribute::Enumerable,
+                                         JS::PropertyAttribute::Writable}));
+    if (!AddOrChangeProperty<IsAddOrChange::Add>(cx, obj, id, desc)) {
+      return false;
+    }
+  } else {
+    if (!AddDataProperty(cx, obj, id, v)) {
+      return false;
+    }
+  }
+
+  return result.succeed();
+}
+
+bool js::AddOrUpdateSparseElementHelper(JSContext* cx,
+                                        Handle<NativeObject*> obj,
+                                        int32_t int_id, HandleValue v,
+                                        bool strict) {
+  MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<PlainObject>());
+
+  // This helper doesn't handle the case where the index is a dense element.
+  MOZ_ASSERT(int_id >= 0);
+  MOZ_ASSERT(!obj->containsDenseElement(int_id));
+
+  MOZ_ASSERT(PropertyKey::fitsInInt(int_id));
+  RootedId id(cx, PropertyKey::Int(int_id));
+
+  // First decide if this is an add or an update. Because the IC guards have
+  // already ensured this exists exterior to the dense array range, and the
+  // prototype checks have ensured there are no indexes on the prototype, we
+  // can use the shape lineage to find the element if it exists:
+  uint32_t index;
+  PropMap* map = obj->shape()->lookup(cx, id, &index);
+
+  // If we didn't find the property, we're on the add path: delegate to
+  // AddOrChangeProperty. This will add either a sparse element or a dense
+  // element.
+  if (map == nullptr) {
+    Rooted<PropertyDescriptor> desc(
+        cx, PropertyDescriptor::Data(v, {JS::PropertyAttribute::Configurable,
+                                         JS::PropertyAttribute::Enumerable,
+                                         JS::PropertyAttribute::Writable}));
+    return AddOrChangeProperty<IsAddOrChange::Add>(cx, obj, id, desc);
+  }
+
+  // At this point we're updating a property: See SetExistingProperty.
+  PropertyInfo prop = map->getPropertyInfo(index);
+  if (prop.isDataProperty() && prop.writable()) {
+    obj->setSlot(prop.slot(), v);
+    return true;
+  }
+
+  // We don't know exactly what this object looks like, hit the slowpath.
+  RootedValue receiver(cx, ObjectValue(*obj));
+  JS::ObjectOpResult result;
+  return SetProperty(cx, obj, id, v, receiver, result) &&
+         result.checkStrictModeError(cx, obj, id, strict);
+}
+
+/*** [[HasProperty]] ********************************************************/
+
+// ES6 draft rev31 9.1.7.1 OrdinaryHasProperty
+bool js::NativeHasProperty(JSContext* cx, Handle<NativeObject*> obj,
+                           HandleId id, bool* foundp) {
+  Rooted<NativeObject*> pobj(cx, obj);
+  PropertyResult prop;
+
+  // This loop isn't explicit in the spec algorithm. See the comment on step
+  // 7.a. below.
+  for (;;) {
+    // Steps 2-3.
+    if (!NativeLookupOwnPropertyInline<CanGC>(cx, pobj, id, &prop)) {
+      return false;
+    }
+
+    // Step 4.
+    if (prop.isFound()) {
+      *foundp = true;
+      return true;
+    }
+
+    // Step 5-6.
+    JSObject* proto = pobj->staticPrototype();
+
+    // Step 8.
+    // As a side-effect of NativeLookupOwnPropertyInline, we may determine that
+    // a property is not found and the proto chain should not be searched. This
+    // can occur for:
+    //  - Out-of-range numeric properties of a TypedArrayObject
+    //  - Recursive resolve hooks (which is expected when they try to set the
+    //    property being resolved).
+    if (!proto || prop.shouldIgnoreProtoChain()) {
+      *foundp = false;
+      return true;
+    }
+
+    // Step 7.a. If the prototype is also native, this step is a
+    // recursive tail call, and we don't need to go through all the
+    // plumbing of HasProperty; the top of the loop is where
+    // we're going to end up anyway. But if pobj is non-native,
+    // that optimization would be incorrect.
+    if (!proto->is<NativeObject>()) {
+      RootedObject protoRoot(cx, proto);
+      return HasProperty(cx, protoRoot, id, foundp);
+    }
+
+    pobj = &proto->as<NativeObject>();
+  }
+}
+
+/*** [[GetOwnPropertyDescriptor]] *******************************************/
+
+bool js::NativeGetOwnPropertyDescriptor(
+    JSContext* cx, Handle<NativeObject*> obj, HandleId id,
+    MutableHandle<mozilla::Maybe<PropertyDescriptor>> desc) {
+  PropertyResult prop;
+  if (!NativeLookupOwnProperty<CanGC>(cx, obj, id, &prop)) {
+    return false;
+  }
+  if (prop.isNotFound()) {
+    desc.reset();
+    return true;
+  }
+
+  if (prop.isNativeProperty() && prop.propertyInfo().isAccessorProperty()) {
+    PropertyInfo propInfo = prop.propertyInfo();
+    desc.set(mozilla::Some(PropertyDescriptor::Accessor(
+        obj->getGetter(propInfo), obj->getSetter(propInfo),
+        propInfo.propAttributes())));
+    return true;
+  }
+
+  RootedValue value(cx);
+  if (!GetExistingDataProperty(cx, obj, id, prop, &value)) {
+    return false;
+  }
+
+  JS::PropertyAttributes attrs = GetPropertyAttributes(obj, prop);
+  desc.set(mozilla::Some(PropertyDescriptor::Data(value, attrs)));
+  return true;
+}
+
+/*** [[Get]] ****************************************************************/
+
+static bool GetCustomDataProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                  MutableHandleValue vp) {
+  cx->check(obj, id, vp);
+
+  const JSClass* clasp = obj->getClass();
+  if (clasp == &ArrayObject::class_) {
+    if (!ArrayLengthGetter(cx, obj, id, vp)) {
+      return false;
+    }
+  } else if (clasp == &MappedArgumentsObject::class_) {
+    if (!MappedArgGetter(cx, obj, id, vp)) {
+      return false;
+    }
+  } else {
+    MOZ_RELEASE_ASSERT(clasp == &UnmappedArgumentsObject::class_);
+    if (!UnmappedArgGetter(cx, obj, id, vp)) {
+      return false;
+    }
+  }
+
+  cx->check(vp);
+  return true;
+}
+
+static inline bool CallGetter(JSContext* cx, Handle<NativeObject*> obj,
+                              HandleValue receiver, HandleId id,
+                              PropertyInfo prop, MutableHandleValue vp) {
+  MOZ_ASSERT(!prop.isDataProperty());
+
+  if (prop.isAccessorProperty()) {
+    RootedValue getter(cx, obj->getGetterValue(prop));
+    return js::CallGetter(cx, receiver, getter, vp);
+  }
+
+  MOZ_ASSERT(prop.isCustomDataProperty());
+
+  return GetCustomDataProperty(cx, obj, id, vp);
+}
+
+template <AllowGC allowGC>
+static MOZ_ALWAYS_INLINE bool GetExistingProperty(
+    JSContext* cx, typename MaybeRooted<Value, allowGC>::HandleType receiver,
+    typename MaybeRooted<NativeObject*, allowGC>::HandleType obj,
+    typename MaybeRooted<jsid, allowGC>::HandleType id, PropertyInfo prop,
+    typename MaybeRooted<Value, allowGC>::MutableHandleType vp) {
+  if (prop.isDataProperty()) {
+    vp.set(obj->getSlot(prop.slot()));
+    return true;
+  }
+
+  vp.setUndefined();
+
+  if (!prop.isCustomDataProperty() && !obj->hasGetter(prop)) {
+    return true;
+  }
+
+  if constexpr (!allowGC) {
+    return false;
+  } else {
+    return CallGetter(cx, obj, receiver, id, prop, vp);
+  }
+}
+
+bool js::NativeGetExistingProperty(JSContext* cx, HandleObject receiver,
+                                   Handle<NativeObject*> obj, HandleId id,
+                                   PropertyInfo prop, MutableHandleValue vp) {
+  RootedValue receiverValue(cx, ObjectValue(*receiver));
+  return GetExistingProperty<CanGC>(cx, receiverValue, obj, id, prop, vp);
+}
+
+enum IsNameLookup { NotNameLookup = false, NameLookup = true };
+
+/*
+ * Finish getting the property `receiver[id]` after looking at every object on
+ * the prototype chain and not finding any such property.
+ *
+ * Per the spec, this should just set the result to `undefined` and call it a
+ * day. However this function also runs when we're evaluating an
+ * expression that's an Identifier (that is, an unqualified name lookup),
+ * so we need to figure out if that's what's happening and throw
+ * a ReferenceError if so.
+ */
+static bool GetNonexistentProperty(JSContext* cx, HandleId id,
+                                   IsNameLookup nameLookup,
+                                   MutableHandleValue vp) {
+  vp.setUndefined();
+
+  // If we are doing a name lookup, this is a ReferenceError.
+  if (nameLookup) {
+    ReportIsNotDefined(cx, id);
+    return false;
+  }
+
+  // Otherwise, just return |undefined|.
+  return true;
+}
+
+// The NoGC version of GetNonexistentProperty, present only to make types line
+// up.
+bool GetNonexistentProperty(JSContext* cx, const jsid& id,
+                            IsNameLookup nameLookup,
+                            FakeMutableHandle<Value> vp) {
+  return false;
+}
+
+static inline bool GeneralizedGetProperty(JSContext* cx, HandleObject obj,
+                                          HandleId id, HandleValue receiver,
+                                          IsNameLookup nameLookup,
+                                          MutableHandleValue vp) {
+  AutoCheckRecursionLimit recursion(cx);
+  if (!recursion.check(cx)) {
+    return false;
+  }
+  if (nameLookup) {
+    // When nameLookup is true, GetProperty implements ES6 rev 34 (2015 Feb
+    // 20) 8.1.1.2.6 GetBindingValue, with step 3 (the call to HasProperty)
+    // and step 6 (the call to Get) fused so that only a single lookup is
+    // needed.
+    //
+    // If we get here, we've reached a non-native object. Fall back on the
+    // algorithm as specified, with two separate lookups. (Note that we
+    // throw ReferenceErrors regardless of strictness, technically a bug.)
+
+    bool found;
+    if (!HasProperty(cx, obj, id, &found)) {
+      return false;
+    }
+    if (!found) {
+      ReportIsNotDefined(cx, id);
+      return false;
+    }
+  }
+
+  return GetProperty(cx, obj, receiver, id, vp);
+}
+
+static inline bool GeneralizedGetProperty(JSContext* cx, JSObject* obj, jsid id,
+                                          const Value& receiver,
+                                          IsNameLookup nameLookup,
+                                          FakeMutableHandle<Value> vp) {
+  AutoCheckRecursionLimit recursion(cx);
+  if (!recursion.checkDontReport(cx)) {
+    return false;
+  }
+  if (nameLookup) {
+    return false;
+  }
+  return GetPropertyNoGC(cx, obj, receiver, id, vp.address());
+}
+
+bool js::GetSparseElementHelper(JSContext* cx, Handle<NativeObject*> obj,
+                                int32_t int_id, MutableHandleValue result) {
+  MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<PlainObject>());
+
+  // This helper doesn't handle the case where the index is a dense element.
+  MOZ_ASSERT(int_id >= 0);
+  MOZ_ASSERT(!obj->containsDenseElement(int_id));
+
+  // Indexed properties can not exist on the prototype chain.
+  MOZ_ASSERT(!PrototypeMayHaveIndexedProperties(obj));
+
+  MOZ_ASSERT(PropertyKey::fitsInInt(int_id));
+  RootedId id(cx, PropertyKey::Int(int_id));
+
+  uint32_t index;
+  PropMap* map = obj->shape()->lookup(cx, id, &index);
+  if (!map) {
+    // Property not found, return directly.
+    result.setUndefined();
+    return true;
+  }
+
+  PropertyInfo prop = map->getPropertyInfo(index);
+  RootedValue receiver(cx, ObjectValue(*obj));
+  return GetExistingProperty<CanGC>(cx, receiver, obj, id, prop, result);
+}
+
+template <AllowGC allowGC>
+static MOZ_ALWAYS_INLINE bool NativeGetPropertyInline(
+    JSContext* cx, typename MaybeRooted<NativeObject*, allowGC>::HandleType obj,
+    typename MaybeRooted<Value, allowGC>::HandleType receiver,
+    typename MaybeRooted<jsid, allowGC>::HandleType id, IsNameLookup nameLookup,
+    typename MaybeRooted<Value, allowGC>::MutableHandleType vp) {
+  typename MaybeRooted<NativeObject*, allowGC>::RootType pobj(cx, obj);
+  PropertyResult prop;
+
+  // This loop isn't explicit in the spec algorithm. See the comment on step
+  // 4.d below.
+  for (;;) {
+    // Steps 2-3.
+    if (!NativeLookupOwnPropertyInline<allowGC>(cx, pobj, id, &prop)) {
+      return false;
+    }
+
+    if (prop.isFound()) {
+      // Steps 5-8. Special case for dense elements because
+      // GetExistingProperty doesn't support those.
+      if (prop.isDenseElement()) {
+        vp.set(pobj->getDenseElement(prop.denseElementIndex()));
+        return true;
+      }
+      if (prop.isTypedArrayElement()) {
+        size_t idx = prop.typedArrayElementIndex();
+        auto* tarr = &pobj->template as<TypedArrayObject>();
+        return tarr->template getElement<allowGC>(cx, idx, vp);
+      }
+
+      return GetExistingProperty<allowGC>(cx, receiver, pobj, id,
+                                          prop.propertyInfo(), vp);
+    }
+
+    // Steps 4.a-b.
+    JSObject* proto = pobj->staticPrototype();
+
+    // Step 4.c. The spec algorithm simply returns undefined if proto is
+    // null, but see the comment on GetNonexistentProperty.
+    if (!proto || prop.shouldIgnoreProtoChain()) {
+      return GetNonexistentProperty(cx, id, nameLookup, vp);
+    }
+
+    // Step 4.d. If the prototype is also native, this step is a
+    // recursive tail call, and we don't need to go through all the
+    // plumbing of JSObject::getGeneric; the top of the loop is where
+    // we're going to end up anyway. But if pobj is non-native,
+    // that optimization would be incorrect.
+    if (proto->getOpsGetProperty()) {
+      RootedObject protoRoot(cx, proto);
+      return GeneralizedGetProperty(cx, protoRoot, id, receiver, nameLookup,
+                                    vp);
+    }
+
+    pobj = &proto->as<NativeObject>();
+  }
+}
+
+bool js::NativeGetProperty(JSContext* cx, Handle<NativeObject*> obj,
+                           HandleValue receiver, HandleId id,
+                           MutableHandleValue vp) {
+  return NativeGetPropertyInline<CanGC>(cx, obj, receiver, id, NotNameLookup,
+                                        vp);
+}
+
+bool js::NativeGetPropertyNoGC(JSContext* cx, NativeObject* obj,
+                               const Value& receiver, jsid id, Value* vp) {
+  AutoAssertNoPendingException noexc(cx);
+  return NativeGetPropertyInline<NoGC>(cx, obj, receiver, id, NotNameLookup,
+                                       vp);
+}
+
+bool js::NativeGetElement(JSContext* cx, Handle<NativeObject*> obj,
+                          HandleValue receiver, int32_t index,
+                          MutableHandleValue vp) {
+  RootedId id(cx);
+
+  if (MOZ_LIKELY(index >= 0)) {
+    if (!IndexToId(cx, index, &id)) {
+      return false;
+    }
+  } else {
+    RootedValue indexVal(cx, Int32Value(index));
+    if (!PrimitiveValueToId<CanGC>(cx, indexVal, &id)) {
+      return false;
+    }
+  }
+  return NativeGetProperty(cx, obj, receiver, id, vp);
+}
+
+bool js::GetNameBoundInEnvironment(JSContext* cx, HandleObject envArg,
+                                   HandleId id, MutableHandleValue vp) {
+  // Manually unwrap 'with' environments to prevent looking up @@unscopables
+  // twice.
+  //
+  // This is unfortunate because internally, the engine does not distinguish
+  // HasProperty from HasBinding: both are implemented as a HasPropertyOp
+  // hook on a WithEnvironmentObject.
+  //
+  // In the case of attempting to get the value of a binding already looked up
+  // via JSOp::BindName, calling HasProperty on the WithEnvironmentObject is
+  // equivalent to calling HasBinding a second time. This results in the
+  // incorrect behavior of performing the @@unscopables check again.
+  RootedObject env(cx, MaybeUnwrapWithEnvironment(envArg));
+  RootedValue receiver(cx, ObjectValue(*env));
+  if (env->getOpsGetProperty()) {
+    return GeneralizedGetProperty(cx, env, id, receiver, NameLookup, vp);
+  }
+  return NativeGetPropertyInline<CanGC>(cx, env.as<NativeObject>(), receiver,
+                                        id, NameLookup, vp);
+}
+
+/*** [[Set]] ****************************************************************/
+
+static bool SetCustomDataProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                  HandleValue v, ObjectOpResult& result) {
+  cx->check(obj, id, v);
+
+  const JSClass* clasp = obj->getClass();
+  if (clasp == &ArrayObject::class_) {
+    return ArrayLengthSetter(cx, obj, id, v, result);
+  }
+  if (clasp == &MappedArgumentsObject::class_) {
+    return MappedArgSetter(cx, obj, id, v, result);
+  }
+  MOZ_RELEASE_ASSERT(clasp == &UnmappedArgumentsObject::class_);
+  return UnmappedArgSetter(cx, obj, id, v, result);
+}
+
+static bool MaybeReportUndeclaredVarAssignment(JSContext* cx, HandleId id) {
+  {
+    jsbytecode* pc;
+    JSScript* script =
+        cx->currentScript(&pc, JSContext::AllowCrossRealm::Allow);
+    if (!script) {
+      return true;
+    }
+
+    if (!IsStrictSetPC(pc)) {
+      return true;
+    }
+  }
+
+  UniqueChars bytes =
+      IdToPrintableUTF8(cx, id, IdToPrintableBehavior::IdIsIdentifier);
+  if (!bytes) {
+    return false;
+  }
+  JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_UNDECLARED_VAR,
+                           bytes.get());
+  return false;
+}
+
+/*
+ * Finish assignment to a shapeful data property of a native object obj. This
+ * conforms to no standard and there is a lot of legacy baggage here.
+ */
+static bool NativeSetExistingDataProperty(JSContext* cx,
+                                          Handle<NativeObject*> obj,
+                                          HandleId id, PropertyInfo prop,
+                                          HandleValue v,
+                                          ObjectOpResult& result) {
+  MOZ_ASSERT(obj->is<NativeObject>());
+  MOZ_ASSERT(prop.isDataDescriptor());
+
+  if (prop.isDataProperty()) {
+    // The common path. Standard data property.
+    obj->setSlot(prop.slot(), v);
+    return result.succeed();
+  }
+
+  MOZ_ASSERT(prop.isCustomDataProperty());
+  MOZ_ASSERT(!obj->is<WithEnvironmentObject>());  // See bug 1128681.
+
+  return SetCustomDataProperty(cx, obj, id, v, result);
+}
+
+/*
+ * When a [[Set]] operation finds no existing property with the given id
+ * or finds a writable data property on the prototype chain, we end up here.
+ * Finish the [[Set]] by defining a new property on receiver.
+ *
+ * This implements ES6 draft rev 28, 9.1.9 [[Set]] steps 5.b-f, but it
+ * is really old code and there are a few barnacles.
+ */
+bool js::SetPropertyByDefining(JSContext* cx, HandleId id, HandleValue v,
+                               HandleValue receiverValue,
+                               ObjectOpResult& result) {
+  // Step 5.b.
+  if (!receiverValue.isObject()) {
+    return result.fail(JSMSG_SET_NON_OBJECT_RECEIVER);
+  }
+  RootedObject receiver(cx, &receiverValue.toObject());
+
+  bool existing;
+  {
+    // Steps 5.c-d.
+    Rooted<mozilla::Maybe<PropertyDescriptor>> desc(cx);
+    if (!GetOwnPropertyDescriptor(cx, receiver, id, &desc)) {
+      return false;
+    }
+
+    existing = desc.isSome();
+
+    // Step 5.e.
+    if (existing) {
+      // Step 5.e.i.
+      if (desc->isAccessorDescriptor()) {
+        return result.fail(JSMSG_OVERWRITING_ACCESSOR);
+      }
+
+      // Step 5.e.ii.
+      if (!desc->writable()) {
+        return result.fail(JSMSG_READ_ONLY);
+      }
+    }
+  }
+
+  // Steps 5.e.iii-iv. and 5.f.i. Define the new data property.
+  Rooted<PropertyDescriptor> desc(cx);
+  if (existing) {
+    desc = PropertyDescriptor::Empty();
+    desc.setValue(v);
+  } else {
+    desc = PropertyDescriptor::Data(v, {JS::PropertyAttribute::Configurable,
+                                        JS::PropertyAttribute::Enumerable,
+                                        JS::PropertyAttribute::Writable});
+  }
+  return DefineProperty(cx, receiver, id, desc, result);
+}
+
+// When setting |id| for |receiver| and |obj| has no property for id, continue
+// the search up the prototype chain.
+bool js::SetPropertyOnProto(JSContext* cx, HandleObject obj, HandleId id,
+                            HandleValue v, HandleValue receiver,
+                            ObjectOpResult& result) {
+  MOZ_ASSERT(!obj->is<ProxyObject>());
+
+  RootedObject proto(cx, obj->staticPrototype());
+  if (proto) {
+    return SetProperty(cx, proto, id, v, receiver, result);
+  }
+
+  return SetPropertyByDefining(cx, id, v, receiver, result);
+}
+
+/*
+ * Implement "the rest of" assignment to a property when no property
+ * receiver[id] was found anywhere on the prototype chain.
+ *
+ * FIXME: This should be updated to follow ES6 draft rev 28, section 9.1.9,
+ * steps 4.d.i and 5.
+ */
+template <QualifiedBool IsQualified>
+static bool SetNonexistentProperty(JSContext* cx, Handle<NativeObject*> obj,
+                                   HandleId id, HandleValue v,
+                                   HandleValue receiver,
+                                   ObjectOpResult& result) {
+  if (!IsQualified && receiver.isObject() &&
+      receiver.toObject().isUnqualifiedVarObj()) {
+    if (!MaybeReportUndeclaredVarAssignment(cx, id)) {
+      return false;
+    }
+  }
+
+  // Pure optimization for the common case. There's no point performing the
+  // lookup in step 5.c again, as our caller just did it for us.
+  if (IsQualified && receiver.isObject() && obj == &receiver.toObject()) {
+    // Ensure that a custom GetOwnPropertyOp, if present, doesn't
+    // introduce additional properties which weren't previously found by
+    // LookupOwnProperty.
+#ifdef DEBUG
+    if (GetOwnPropertyOp op = obj->getOpsGetOwnPropertyDescriptor()) {
+      Rooted<mozilla::Maybe<PropertyDescriptor>> desc(cx);
+      if (!op(cx, obj, id, &desc)) {
+        return false;
+      }
+      MOZ_ASSERT(desc.isNothing());
+    }
+#endif
+
+    // Step 5.e. Define the new data property.
+    if (DefinePropertyOp op = obj->getOpsDefineProperty()) {
+      MOZ_ASSERT(!cx->isHelperThreadContext());
+
+      Rooted<PropertyDescriptor> desc(
+          cx, PropertyDescriptor::Data(v, {JS::PropertyAttribute::Configurable,
+                                           JS::PropertyAttribute::Enumerable,
+                                           JS::PropertyAttribute::Writable}));
+      return op(cx, obj, id, desc, result);
+    }
+
+    return DefineNonexistentProperty(cx, obj, id, v, result);
+  }
+
+  return SetPropertyByDefining(cx, id, v, receiver, result);
+}
+
+// Set an existing own property obj[index] that's a dense element.
+static bool SetDenseElement(JSContext* cx, Handle<NativeObject*> obj,
+                            uint32_t index, HandleValue v,
+                            ObjectOpResult& result) {
+  MOZ_ASSERT(!obj->is<TypedArrayObject>());
+  MOZ_ASSERT(obj->containsDenseElement(index));
+
+  obj->setDenseElement(index, v);
+  return result.succeed();
+}
+
+/*
+ * Finish the assignment `receiver[id] = v` when an existing property (shape)
+ * has been found on a native object (pobj). This implements ES6 draft rev 32
+ * (2015 Feb 2) 9.1.9 steps 5 and 6.
+ *
+ * It is necessary to pass both id and shape because shape could be an implicit
+ * dense or typed array element (i.e. not actually a pointer to a Shape).
+ */
+static bool SetExistingProperty(JSContext* cx, HandleId id, HandleValue v,
+                                HandleValue receiver,
+                                Handle<NativeObject*> pobj,
+                                const PropertyResult& prop,
+                                ObjectOpResult& result) {
+  // Step 5 for dense elements.
+  if (prop.isDenseElement() || prop.isTypedArrayElement()) {
+    // Step 5.a.
+    if (pobj->denseElementsAreFrozen()) {
+      return result.fail(JSMSG_READ_ONLY);
+    }
+
+    // Pure optimization for the common case:
+    if (receiver.isObject() && pobj == &receiver.toObject()) {
+      if (prop.isTypedArrayElement()) {
+        Rooted<TypedArrayObject*> tobj(cx, &pobj->as<TypedArrayObject>());
+        size_t idx = prop.typedArrayElementIndex();
+        return SetTypedArrayElement(cx, tobj, idx, v, result);
+      }
+
+      return SetDenseElement(cx, pobj, prop.denseElementIndex(), v, result);
+    }
+
+    // Steps 5.b-f.
+    return SetPropertyByDefining(cx, id, v, receiver, result);
+  }
+
+  // Step 5 for all other properties.
+  PropertyInfo propInfo = prop.propertyInfo();
+  if (propInfo.isDataDescriptor()) {
+    // Step 5.a.
+    if (!propInfo.writable()) {
+      return result.fail(JSMSG_READ_ONLY);
+    }
+
+    // steps 5.c-f.
+    if (receiver.isObject() && pobj == &receiver.toObject()) {
+      // Pure optimization for the common case. There's no point performing
+      // the lookup in step 5.c again, as our caller just did it for us. The
+      // result is |shapeProp|.
+
+      // Steps 5.e.i-ii.
+      return NativeSetExistingDataProperty(cx, pobj, id, propInfo, v, result);
+    }
+
+    // Shadow pobj[id] by defining a new data property receiver[id].
+    // Delegate everything to SetPropertyByDefining.
+    return SetPropertyByDefining(cx, id, v, receiver, result);
+  }
+
+  // Steps 6-11.
+  MOZ_ASSERT(propInfo.isAccessorProperty());
+
+  JSObject* setterObject = pobj->getSetter(propInfo);
+  if (!setterObject) {
+    return result.fail(JSMSG_GETTER_ONLY);
+  }
+
+  RootedValue setter(cx, ObjectValue(*setterObject));
+  if (!js::CallSetter(cx, receiver, setter, v)) {
+    return false;
+  }
+
+  return result.succeed();
+}
+
+template <QualifiedBool IsQualified>
+bool js::NativeSetProperty(JSContext* cx, Handle<NativeObject*> obj,
+                           HandleId id, HandleValue v, HandleValue receiver,
+                           ObjectOpResult& result) {
+  // Step numbers below reference ES6 rev 27 9.1.9, the [[Set]] internal
+  // method for ordinary objects. We substitute our own names for these names
+  // used in the spec: O -> pobj, P -> id, ownDesc -> shape.
+  PropertyResult prop;
+  Rooted<NativeObject*> pobj(cx, obj);
+
+  // This loop isn't explicit in the spec algorithm. See the comment on step
+  // 4.c.i below. (There's a very similar loop in the NativeGetProperty
+  // implementation, but unfortunately not similar enough to common up.)
+  //
+  // We're intentionally not spec-compliant for TypedArrays:
+  // When |pobj| is a TypedArray and |id| is a TypedArray index, we should
+  // ignore |receiver| and instead always try to set the property on |pobj|.
+  // Bug 1502889 showed that this behavior isn't web-compatible. This issue is
+  // also reported at <https://github.com/tc39/ecma262/issues/1541>.
+  for (;;) {
+    // Steps 2-3.
+    if (!NativeLookupOwnPropertyInline<CanGC>(cx, pobj, id, &prop)) {
+      return false;
+    }
+
+    if (prop.isFound()) {
+      // Steps 5-6.
+      return SetExistingProperty(cx, id, v, receiver, pobj, prop, result);
+    }
+
+    // Steps 4.a-b.
+    // As a side-effect of NativeLookupOwnPropertyInline, we may determine that
+    // a property is not found and the proto chain should not be searched. This
+    // can occur for:
+    //  - Out-of-range numeric properties of a TypedArrayObject
+    //  - Recursive resolve hooks (which is expected when they try to set the
+    //    property being resolved).
+    JSObject* proto = pobj->staticPrototype();
+    if (!proto || prop.shouldIgnoreProtoChain()) {
+      // Step 4.d.i (and step 5).
+      return SetNonexistentProperty<IsQualified>(cx, obj, id, v, receiver,
+                                                 result);
+    }
+
+    // Step 4.c.i. If the prototype is also native, this step is a
+    // recursive tail call, and we don't need to go through all the
+    // plumbing of SetProperty; the top of the loop is where we're going to
+    // end up anyway. But if pobj is non-native, that optimization would be
+    // incorrect.
+    if (!proto->is<NativeObject>()) {
+      // Unqualified assignments are not specified to go through [[Set]]
+      // at all, but they do go through this function. So check for
+      // unqualified assignment to a nonexistent global (a strict error).
+      RootedObject protoRoot(cx, proto);
+      if (!IsQualified) {
+        bool found;
+        if (!HasProperty(cx, protoRoot, id, &found)) {
+          return false;
+        }
+        if (!found) {
+          return SetNonexistentProperty<IsQualified>(cx, obj, id, v, receiver,
+                                                     result);
+        }
+      }
+
+      return SetProperty(cx, protoRoot, id, v, receiver, result);
+    }
+    pobj = &proto->as<NativeObject>();
+  }
+}
+
+template bool js::NativeSetProperty<Qualified>(JSContext* cx,
+                                               Handle<NativeObject*> obj,
+                                               HandleId id, HandleValue value,
+                                               HandleValue receiver,
+                                               ObjectOpResult& result);
+
+template bool js::NativeSetProperty<Unqualified>(JSContext* cx,
+                                                 Handle<NativeObject*> obj,
+                                                 HandleId id, HandleValue value,
+                                                 HandleValue receiver,
+                                                 ObjectOpResult& result);
+
+bool js::NativeSetElement(JSContext* cx, Handle<NativeObject*> obj,
+                          uint32_t index, HandleValue v, HandleValue receiver,
+                          ObjectOpResult& result) {
+  RootedId id(cx);
+  if (!IndexToId(cx, index, &id)) {
+    return false;
+  }
+  return NativeSetProperty<Qualified>(cx, obj, id, v, receiver, result);
+}
+
+/*** [[Delete]] *************************************************************/
+
+static bool CallJSDeletePropertyOp(JSContext* cx, JSDeletePropertyOp op,
+                                   HandleObject receiver, HandleId id,
+                                   ObjectOpResult& result) {
+  AutoCheckRecursionLimit recursion(cx);
+  if (!recursion.check(cx)) {
+    return false;
+  }
+
+  cx->check(receiver, id);
+  if (op) {
+    return op(cx, receiver, id, result);
+  }
+  return result.succeed();
+}
+
+// ES6 draft rev31 9.1.10 [[Delete]]
+bool js::NativeDeleteProperty(JSContext* cx, Handle<NativeObject*> obj,
+                              HandleId id, ObjectOpResult& result) {
+#ifdef ENABLE_RECORD_TUPLE
+  MOZ_ASSERT(!js::IsExtendedPrimitive(*obj));
+#endif
+
+  // Steps 2-3.
+  PropertyResult prop;
+  if (!NativeLookupOwnProperty<CanGC>(cx, obj, id, &prop)) {
+    return false;
+  }
+
+  // Step 4.
+  if (prop.isNotFound()) {
+    // If no property call the class's delProperty hook, passing succeeded
+    // as the result parameter. This always succeeds when there is no hook.
+    return CallJSDeletePropertyOp(cx, obj->getClass()->getDelProperty(), obj,
+                                  id, result);
+  }
+
+  // Step 6. Non-configurable property.
+  if (!GetPropertyAttributes(obj, prop).configurable()) {
+    return result.failCantDelete();
+  }
+
+  // Typed array elements are configurable, but can't be deleted.
+  if (prop.isTypedArrayElement()) {
+    return result.failCantDelete();
+  }
+
+  if (!CallJSDeletePropertyOp(cx, obj->getClass()->getDelProperty(), obj, id,
+                              result)) {
+    return false;
+  }
+  if (!result) {
+    return true;
+  }
+
+  // Step 5.
+  if (prop.isDenseElement()) {
+    obj->setDenseElementHole(prop.denseElementIndex());
+  } else {
+    if (!NativeObject::removeProperty(cx, obj, id)) {
+      return false;
+    }
+  }
+
+  return SuppressDeletedProperty(cx, obj, id);
+}
+
+bool js::CopyDataPropertiesNative(JSContext* cx, Handle<PlainObject*> target,
+                                  Handle<NativeObject*> from,
+                                  Handle<PlainObject*> excludedItems,
+                                  bool* optimized) {
+#ifdef ENABLE_RECORD_TUPLE
+  MOZ_ASSERT(!js::IsExtendedPrimitive(*target));
+#endif
+
+  *optimized = false;
+
+  // Don't use the fast path if |from| may have extra indexed or lazy
+  // properties.
+  if (from->getDenseInitializedLength() > 0 || from->isIndexed() ||
+      from->is<TypedArrayObject>() ||
+      IF_RECORD_TUPLE(from->is<RecordObject>() || from->is<TupleObject>(),
+                      false) ||
+      from->getClass()->getNewEnumerate() || from->getClass()->getEnumerate()) {
+    return true;
+  }
+
+  // Collect all enumerable data properties.
+  Rooted<PropertyInfoWithKeyVector> props(cx, PropertyInfoWithKeyVector(cx));
+
+  Rooted<NativeShape*> fromShape(cx, from->shape());
+  for (ShapePropertyIter<NoGC> iter(fromShape); !iter.done(); iter++) {
+    jsid id = iter->key();
+    MOZ_ASSERT(!id.isInt());
+
+    if (!iter->enumerable()) {
+      continue;
+    }
+    if (excludedItems && excludedItems->contains(cx, id)) {
+      continue;
+    }
+
+    // Don't use the fast path if |from| contains non-data properties.
+    //
+    // This enables two optimizations:
+    // 1. We don't need to handle the case when accessors modify |from|.
+    // 2. String and symbol properties can be added in one go.
+    if (!iter->isDataProperty()) {
+      return true;
+    }
+
+    if (!props.append(*iter)) {
+      return false;
+    }
+  }
+
+  *optimized = true;
+
+  // If |target| contains no own properties, we can directly call
+  // AddDataPropertyNonPrototype.
+  const bool targetHadNoOwnProperties = target->empty();
+
+  RootedId key(cx);
+  RootedValue value(cx);
+  for (size_t i = props.length(); i > 0; i--) {
+    PropertyInfoWithKey prop = props[i - 1];
+    MOZ_ASSERT(prop.isDataProperty());
+    MOZ_ASSERT(prop.enumerable());
+
+    key = prop.key();
+    MOZ_ASSERT(!key.isInt());
+
+    MOZ_ASSERT(from->is<NativeObject>());
+    MOZ_ASSERT(from->shape() == fromShape);
+
+    value = from->getSlot(prop.slot());
+    if (targetHadNoOwnProperties) {
+      MOZ_ASSERT(!target->containsPure(key),
+                 "didn't expect to find an existing property");
+
+      if (!AddDataPropertyToPlainObject(cx, target, key, value)) {
+        return false;
+      }
+    } else {
+      if (!NativeDefineDataProperty(cx, target, key, value, JSPROP_ENUMERATE)) {
+        return false;
+      }
+    }
+  }
+
+  return true;
+}