text: // Copyright 2019 the V8 project authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #include "src/objects/map.h"
  
  #include "src/execution/frames.h"
  #include "src/execution/isolate.h"
  #include "src/handles/handles-inl.h"
  #include "src/handles/maybe-handles.h"
  #include "src/heap/heap-write-barrier-inl.h"
  #include "src/init/bootstrapper.h"
  #include "src/logging/log.h"
  #include "src/logging/runtime-call-stats-scope.h"
  #include "src/objects/arguments-inl.h"
  #include "src/objects/descriptor-array.h"
  #include "src/objects/elements-kind.h"
  #include "src/objects/field-type.h"
  #include "src/objects/js-objects.h"
  #include "src/objects/map-updater.h"
  #include "src/objects/maybe-object.h"
  #include "src/objects/oddball.h"
  #include "src/objects/property.h"
  #include "src/objects/transitions-inl.h"
  #include "src/roots/roots.h"
  #include "src/utils/ostreams.h"
  #include "src/zone/zone-containers.h"
  
  namespace v8 {
  namespace internal {
  
  Map Map::GetPrototypeChainRootMap(Isolate* isolate) const {
    DisallowGarbageCollection no_alloc;
    if (IsJSReceiverMap()) {
      return *this;
    }
    int constructor_function_index = GetConstructorFunctionIndex();
    if (constructor_function_index != Map::kNoConstructorFunctionIndex) {
      Context native_context = isolate->context().native_context();
      JSFunction constructor_function =
          JSFunction::cast(native_context.get(constructor_function_index));
      return constructor_function.initial_map();
    }
    return ReadOnlyRoots(isolate).null_value().map();
  }
  
  // static
  base::Optional<JSFunction> Map::GetConstructorFunction(Map map,
                                                         Context native_context) {
    DisallowGarbageCollection no_gc;
    if (map.IsPrimitiveMap()) {
      int const constructor_function_index = map.GetConstructorFunctionIndex();
      if (constructor_function_index != kNoConstructorFunctionIndex) {
        return JSFunction::cast(native_context.get(constructor_function_index));
      }
    }
    return {};
  }
  
  Map Map::GetInstanceTypeMap(ReadOnlyRoots roots, InstanceType type) {
    Map map;
    switch (type) {
  #define MAKE_CASE(TYPE, Name, name) \
    case TYPE:                        \
      map = roots.name##_map();       \
      break;
      STRUCT_LIST(MAKE_CASE)
  #undef MAKE_CASE
  #define MAKE_CASE(TYPE, Name, name) \
    case TYPE:                        \
      map = roots.name##_map();       \
      break;
      TORQUE_DEFINED_INSTANCE_TYPE_LIST(MAKE_CASE)
  #undef MAKE_CASE
      default:
        UNREACHABLE();
    }
    return map;
  }
  
  VisitorId Map::GetVisitorId(Map map) {
    STATIC_ASSERT(kVisitorIdCount <= 256);
  
    const int instance_type = map.instance_type();
  
    if (instance_type < FIRST_NONSTRING_TYPE) {
      switch (instance_type & kStringRepresentationMask) {
        case kSeqStringTag:
          if ((instance_type & kStringEncodingMask) == kOneByteStringTag) {
            return kVisitSeqOneByteString;
          } else {
            return kVisitSeqTwoByteString;
          }
  
        case kConsStringTag:
          if (IsShortcutCandidate(instance_type)) {
            return kVisitShortcutCandidate;
          } else {
            return kVisitConsString;
          }
  
        case kSlicedStringTag:
          return kVisitSlicedString;
  
        case kExternalStringTag:
          return kVisitDataObject;
  
        case kThinStringTag:
          return kVisitThinString;
      }
      UNREACHABLE();
    }
  
    if (InstanceTypeChecker::IsJSApiObject(map.instance_type())) {
      return kVisitJSApiObject;
    }
  
    switch (instance_type) {
      case BYTE_ARRAY_TYPE:
        return kVisitByteArray;
  
      case BYTECODE_ARRAY_TYPE:
        return kVisitBytecodeArray;
  
      case FREE_SPACE_TYPE:
        return kVisitFreeSpace;
  
      case EMBEDDER_DATA_ARRAY_TYPE:
        return kVisitEmbedderDataArray;
  
      case OBJECT_BOILERPLATE_DESCRIPTION_TYPE:
      case NAME_TO_INDEX_HASH_TABLE_TYPE:
      case CLOSURE_FEEDBACK_CELL_ARRAY_TYPE:
      case HASH_TABLE_TYPE:
      case ORDERED_HASH_MAP_TYPE:
      case ORDERED_HASH_SET_TYPE:
      case ORDERED_NAME_DICTIONARY_TYPE:
      case NAME_DICTIONARY_TYPE:
      case GLOBAL_DICTIONARY_TYPE:
      case NUMBER_DICTIONARY_TYPE:
      case SIMPLE_NUMBER_DICTIONARY_TYPE:
      case SCRIPT_CONTEXT_TABLE_TYPE:
        return kVisitFixedArray;
  
      case AWAIT_CONTEXT_TYPE:
      case BLOCK_CONTEXT_TYPE:
      case CATCH_CONTEXT_TYPE:
      case DEBUG_EVALUATE_CONTEXT_TYPE:
      case EVAL_CONTEXT_TYPE:
      case FUNCTION_CONTEXT_TYPE:
      case MODULE_CONTEXT_TYPE:
      case SCRIPT_CONTEXT_TYPE:
      case WITH_CONTEXT_TYPE:
        return kVisitContext;
  
      case NATIVE_CONTEXT_TYPE:
        return kVisitNativeContext;
  
      case EPHEMERON_HASH_TABLE_TYPE:
        return kVisitEphemeronHashTable;
  
      case FIXED_DOUBLE_ARRAY_TYPE:
        return kVisitFixedDoubleArray;
  
      case PROPERTY_ARRAY_TYPE:
        return kVisitPropertyArray;
  
      case FEEDBACK_CELL_TYPE:
        return kVisitFeedbackCell;
  
      case FEEDBACK_METADATA_TYPE:
        return kVisitFeedbackMetadata;
  
      case MAP_TYPE:
        return kVisitMap;
  
      case CODE_TYPE:
        return kVisitCode;
  
      case CELL_TYPE:
        return kVisitCell;
  
      case PROPERTY_CELL_TYPE:
        return kVisitPropertyCell;
  
      case TRANSITION_ARRAY_TYPE:
        return kVisitTransitionArray;
  
      case JS_WEAK_MAP_TYPE:
      case JS_WEAK_SET_TYPE:
        return kVisitJSWeakCollection;
  
      case CALL_HANDLER_INFO_TYPE:
        return kVisitStruct;
  
      case JS_PROXY_TYPE:
        return kVisitStruct;
  
      case SYMBOL_TYPE:
        return kVisitSymbol;
  
      case JS_ARRAY_BUFFER_TYPE:
        return kVisitJSArrayBuffer;
  
      case JS_DATA_VIEW_TYPE:
        return kVisitJSDataView;
  
      case JS_FUNCTION_TYPE:
      case JS_CLASS_CONSTRUCTOR_TYPE:
      case JS_PROMISE_CONSTRUCTOR_TYPE:
      case JS_REG_EXP_CONSTRUCTOR_TYPE:
      case JS_ARRAY_CONSTRUCTOR_TYPE:
  #define TYPED_ARRAY_CONSTRUCTORS_SWITCH(Type, type, TYPE, Ctype) \
    case TYPE##_TYPED_ARRAY_CONSTRUCTOR_TYPE:
        TYPED_ARRAYS(TYPED_ARRAY_CONSTRUCTORS_SWITCH)
  #undef TYPED_ARRAY_CONSTRUCTORS_SWITCH
        return kVisitJSFunction;
  
      case JS_TYPED_ARRAY_TYPE:
        return kVisitJSTypedArray;
  
      case SMALL_ORDERED_HASH_MAP_TYPE:
        return kVisitSmallOrderedHashMap;
  
      case SMALL_ORDERED_HASH_SET_TYPE:
        return kVisitSmallOrderedHashSet;
  
      case SMALL_ORDERED_NAME_DICTIONARY_TYPE:
        return kVisitSmallOrderedNameDictionary;
  
      case SWISS_NAME_DICTIONARY_TYPE:
        return kVisitSwissNameDictionary;
  
      case CODE_DATA_CONTAINER_TYPE:
        return kVisitCodeDataContainer;
  
      case PREPARSE_DATA_TYPE:
        return kVisitPreparseData;
  
      case COVERAGE_INFO_TYPE:
        return kVisitCoverageInfo;
  
      case JS_ARGUMENTS_OBJECT_TYPE:
      case JS_ARRAY_ITERATOR_PROTOTYPE_TYPE:
      case JS_ARRAY_ITERATOR_TYPE:
      case JS_ARRAY_TYPE:
      case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
      case JS_ASYNC_FUNCTION_OBJECT_TYPE:
      case JS_ASYNC_GENERATOR_OBJECT_TYPE:
      case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
      case JS_DATE_TYPE:
      case JS_ERROR_TYPE:
      case JS_GENERATOR_OBJECT_TYPE:
      case JS_ITERATOR_PROTOTYPE_TYPE:
      case JS_MAP_ITERATOR_PROTOTYPE_TYPE:
      case JS_MAP_KEY_ITERATOR_TYPE:
      case JS_MAP_KEY_VALUE_ITERATOR_TYPE:
      case JS_MAP_TYPE:
      case JS_MAP_VALUE_ITERATOR_TYPE:
      case JS_MESSAGE_OBJECT_TYPE:
      case JS_MODULE_NAMESPACE_TYPE:
      case JS_OBJECT_PROTOTYPE_TYPE:
      case JS_OBJECT_TYPE:
      case JS_PRIMITIVE_WRAPPER_TYPE:
      case JS_PROMISE_PROTOTYPE_TYPE:
      case JS_PROMISE_TYPE:
      case JS_REG_EXP_PROTOTYPE_TYPE:
      case JS_REG_EXP_STRING_ITERATOR_TYPE:
      case JS_REG_EXP_TYPE:
      case JS_SET_ITERATOR_PROTOTYPE_TYPE:
      case JS_SET_KEY_VALUE_ITERATOR_TYPE:
      case JS_SET_PROTOTYPE_TYPE:
      case JS_SET_TYPE:
      case JS_SET_VALUE_ITERATOR_TYPE:
      case JS_SHADOW_REALM_TYPE:
      case JS_SHARED_STRUCT_TYPE:
      case JS_STRING_ITERATOR_PROTOTYPE_TYPE:
      case JS_STRING_ITERATOR_TYPE:
      case JS_TEMPORAL_CALENDAR_TYPE:
      case JS_TEMPORAL_DURATION_TYPE:
      case JS_TEMPORAL_INSTANT_TYPE:
      case JS_TEMPORAL_PLAIN_DATE_TYPE:
      case JS_TEMPORAL_PLAIN_DATE_TIME_TYPE:
      case JS_TEMPORAL_PLAIN_MONTH_DAY_TYPE:
      case JS_TEMPORAL_PLAIN_TIME_TYPE:
      case JS_TEMPORAL_PLAIN_YEAR_MONTH_TYPE:
      case JS_TEMPORAL_TIME_ZONE_TYPE:
      case JS_TEMPORAL_ZONED_DATE_TIME_TYPE:
      case JS_TYPED_ARRAY_PROTOTYPE_TYPE:
  #ifdef V8_INTL_SUPPORT
      case JS_V8_BREAK_ITERATOR_TYPE:
      case JS_COLLATOR_TYPE:
      case JS_DATE_TIME_FORMAT_TYPE:
      case JS_DISPLAY_NAMES_TYPE:
      case JS_LIST_FORMAT_TYPE:
      case JS_LOCALE_TYPE:
      case JS_NUMBER_FORMAT_TYPE:
      case JS_PLURAL_RULES_TYPE:
      case JS_RELATIVE_TIME_FORMAT_TYPE:
      case JS_SEGMENT_ITERATOR_TYPE:
      case JS_SEGMENTER_TYPE:
      case JS_SEGMENTS_TYPE:
  #endif  // V8_INTL_SUPPORT
  #if V8_ENABLE_WEBASSEMBLY
      case WASM_TAG_OBJECT_TYPE:
      case WASM_GLOBAL_OBJECT_TYPE:
      case WASM_MEMORY_OBJECT_TYPE:
      case WASM_MODULE_OBJECT_TYPE:
      case WASM_TABLE_OBJECT_TYPE:
      case WASM_VALUE_OBJECT_TYPE:
  #endif  // V8_ENABLE_WEBASSEMBLY
      case JS_BOUND_FUNCTION_TYPE: {
        const bool has_raw_data_fields =
            COMPRESS_POINTERS_BOOL && JSObject::GetEmbedderFieldCount(map) > 0;
        return has_raw_data_fields ? kVisitJSObject : kVisitJSObjectFast;
      }
      case JS_API_OBJECT_TYPE:
      case JS_GLOBAL_PROXY_TYPE:
      case JS_GLOBAL_OBJECT_TYPE:
      case JS_SPECIAL_API_OBJECT_TYPE:
        return kVisitJSApiObject;
  
      case JS_WEAK_REF_TYPE:
        return kVisitJSWeakRef;
  
      case WEAK_CELL_TYPE:
        return kVisitWeakCell;
  
      case JS_FINALIZATION_REGISTRY_TYPE:
        return kVisitJSFinalizationRegistry;
  
      case FILLER_TYPE:
      case FOREIGN_TYPE:
      case HEAP_NUMBER_TYPE:
        return kVisitDataObject;
  
      case BIGINT_TYPE:
        return kVisitBigInt;
  
      case ALLOCATION_SITE_TYPE:
        return kVisitAllocationSite;
  
  #define MAKE_STRUCT_CASE(TYPE, Name, name) case TYPE:
        STRUCT_LIST(MAKE_STRUCT_CASE)
  #undef MAKE_STRUCT_CASE
        if (instance_type == PROTOTYPE_INFO_TYPE) {
          return kVisitPrototypeInfo;
        }
  #if V8_ENABLE_WEBASSEMBLY
        if (instance_type == WASM_INDIRECT_FUNCTION_TABLE_TYPE) {
          return kVisitWasmIndirectFunctionTable;
        }
  #endif  // V8_ENABLE_WEBASSEMBLY
        return kVisitStruct;
  
      case LOAD_HANDLER_TYPE:
      case STORE_HANDLER_TYPE:
        return kVisitDataHandler;
  
      case SOURCE_TEXT_MODULE_TYPE:
        return kVisitSourceTextModule;
      case SYNTHETIC_MODULE_TYPE:
        return kVisitSyntheticModule;
  
  #if V8_ENABLE_WEBASSEMBLY
      case WASM_INSTANCE_OBJECT_TYPE:
        return kVisitWasmInstanceObject;
      case WASM_ARRAY_TYPE:
        return kVisitWasmArray;
      case WASM_STRUCT_TYPE:
        return kVisitWasmStruct;
      case WASM_TYPE_INFO_TYPE:
        return kVisitWasmTypeInfo;
      case WASM_INTERNAL_FUNCTION_TYPE:
        return kVisitWasmInternalFunction;
      case WASM_JS_FUNCTION_DATA_TYPE:
        return kVisitWasmJSFunctionData;
      case WASM_ON_FULFILLED_DATA_TYPE:
        return kVisitWasmOnFulfilledData;
      case WASM_API_FUNCTION_REF_TYPE:
        return kVisitWasmApiFunctionRef;
      case WASM_EXPORTED_FUNCTION_DATA_TYPE:
        return kVisitWasmExportedFunctionData;
      case WASM_CAPI_FUNCTION_DATA_TYPE:
        return kVisitWasmCapiFunctionData;
      case WASM_SUSPENDER_OBJECT_TYPE:
        return kVisitWasmSuspenderObject;
  #endif  // V8_ENABLE_WEBASSEMBLY
  
  #define MAKE_TQ_CASE(TYPE, Name) \
    case TYPE:                     \
      return kVisit##Name;
        TORQUE_INSTANCE_TYPE_TO_BODY_DESCRIPTOR_LIST(MAKE_TQ_CASE)
  #undef MAKE_TQ_CASE
  
      default:
        UNREACHABLE();
    }
  }
  
  // static
  MaybeObjectHandle Map::WrapFieldType(Isolate* isolate, Handle<FieldType> type) {
    if (type->IsClass()) {
      return MaybeObjectHandle::Weak(type->AsClass(), isolate);
    }
    return MaybeObjectHandle(type);
  }
  
  // static
  FieldType Map::UnwrapFieldType(MaybeObject wrapped_type) {
    if (wrapped_type->IsCleared()) {
      return FieldType::None();
    }
    HeapObject heap_object;
    if (wrapped_type->GetHeapObjectIfWeak(&heap_object)) {
      return FieldType::cast(heap_object);
    }
    return wrapped_type->cast<FieldType>();
  }
  
  MaybeHandle<Map> Map::CopyWithField(Isolate* isolate, Handle<Map> map,
                                      Handle<Name> name, Handle<FieldType> type,
                                      PropertyAttributes attributes,
                                      PropertyConstness constness,
                                      Representation representation,
                                      TransitionFlag flag) {
    DCHECK(map->instance_descriptors(isolate)
               .Search(*name, map->NumberOfOwnDescriptors())
               .is_not_found());
  
    // Ensure the descriptor array does not get too big.
    if (map->NumberOfOwnDescriptors() >= kMaxNumberOfDescriptors) {
      return MaybeHandle<Map>();
    }
  
    // Compute the new index for new field.
    int index = map->NextFreePropertyIndex();
  
    if (map->instance_type() == JS_CONTEXT_EXTENSION_OBJECT_TYPE) {
      constness = PropertyConstness::kMutable;
      representation = Representation::Tagged();
      type = FieldType::Any(isolate);
    } else {
      Map::GeneralizeIfCanHaveTransitionableFastElementsKind(
          isolate, map->instance_type(), &representation, &type);
    }
  
    MaybeObjectHandle wrapped_type = WrapFieldType(isolate, type);
  
    Descriptor d = Descriptor::DataField(name, index, attributes, constness,
                                         representation, wrapped_type);
    Handle<Map> new_map = Map::CopyAddDescriptor(isolate, map, &d, flag);
    new_map->AccountAddedPropertyField();
    return new_map;
  }
  
  MaybeHandle<Map> Map::CopyWithConstant(Isolate* isolate, Handle<Map> map,
                                         Handle<Name> name,
                                         Handle<Object> constant,
                                         PropertyAttributes attributes,
                                         TransitionFlag flag) {
    // Ensure the descriptor array does not get too big.
    if (map->NumberOfOwnDescriptors() >= kMaxNumberOfDescriptors) {
      return MaybeHandle<Map>();
    }
  
    Representation representation = constant->OptimalRepresentation(isolate);
    Handle<FieldType> type = constant->OptimalType(isolate, representation);
    return CopyWithField(isolate, map, name, type, attributes,
                         PropertyConstness::kConst, representation, flag);
  }
  
  bool Map::InstancesNeedRewriting(Map target, ConcurrencyMode cmode) const {
    int target_number_of_fields = target.NumberOfFields(cmode);
    int target_inobject = target.GetInObjectProperties();
    int target_unused = target.UnusedPropertyFields();
    int old_number_of_fields;
  
    return InstancesNeedRewriting(target, target_number_of_fields,
                                  target_inobject, target_unused,
                                  &old_number_of_fields, cmode);
  }
  
  bool Map::InstancesNeedRewriting(Map target, int target_number_of_fields,
                                   int target_inobject, int target_unused,
                                   int* old_number_of_fields,
                                   ConcurrencyMode cmode) const {
    // If fields were added (or removed), rewrite the instance.
    *old_number_of_fields = NumberOfFields(cmode);
    DCHECK(target_number_of_fields >= *old_number_of_fields);
    if (target_number_of_fields != *old_number_of_fields) return true;
  
    // If smi descriptors were replaced by double descriptors, rewrite.
    DescriptorArray old_desc = cmode == ConcurrencyMode::kConcurrent
                                   ? instance_descriptors(kAcquireLoad)
                                   : instance_descriptors();
    DescriptorArray new_desc = cmode == ConcurrencyMode::kConcurrent
                                   ? target.instance_descriptors(kAcquireLoad)
                                   : target.instance_descriptors();
    for (InternalIndex i : IterateOwnDescriptors()) {
      if (new_desc.GetDetails(i).representation().IsDouble() !=
          old_desc.GetDetails(i).representation().IsDouble()) {
        return true;
      }
    }
  
    // If no fields were added, and no inobject properties were removed, setting
    // the map is sufficient.
    if (target_inobject == GetInObjectProperties()) return false;
    // In-object slack tracking may have reduced the object size of the new map.
    // In that case, succeed if all existing fields were inobject, and they still
    // fit within the new inobject size.
    DCHECK(target_inobject < GetInObjectProperties());
    if (target_number_of_fields <= target_inobject) {
      DCHECK(target_number_of_fields + target_unused == target_inobject);
      return false;
    }
    // Otherwise, properties will need to be moved to the backing store.
    return true;
  }
  
  int Map::NumberOfFields(ConcurrencyMode cmode) const {
    DescriptorArray descriptors = cmode == ConcurrencyMode::kConcurrent
                                      ? instance_descriptors(kAcquireLoad)
                                      : instance_descriptors();
    int result = 0;
    for (InternalIndex i : IterateOwnDescriptors()) {
      if (descriptors.GetDetails(i).location() == PropertyLocation::kField)
        result++;
    }
    return result;
  }
  
  Map::FieldCounts Map::GetFieldCounts() const {
    DescriptorArray descriptors = instance_descriptors();
    int mutable_count = 0;
    int const_count = 0;
    for (InternalIndex i : IterateOwnDescriptors()) {
      PropertyDetails details = descriptors.GetDetails(i);
      if (details.location() == PropertyLocation::kField) {
        switch (details.constness()) {
          case PropertyConstness::kMutable:
            mutable_count++;
            break;
          case PropertyConstness::kConst:
            const_count++;
            break;
        }
      }
    }
    return FieldCounts(mutable_count, const_count);
  }
  
  bool Map::HasOutOfObjectProperties() const {
    return GetInObjectProperties() <
           NumberOfFields(ConcurrencyMode::kNotConcurrent);
  }
  
  void Map::DeprecateTransitionTree(Isolate* isolate) {
    if (is_deprecated()) return;
    TransitionsAccessor transitions(isolate, *this);
    int num_transitions = transitions.NumberOfTransitions();
    for (int i = 0; i < num_transitions; ++i) {
      transitions.GetTarget(i).DeprecateTransitionTree(isolate);
    }
    DCHECK(!constructor_or_back_pointer().IsFunctionTemplateInfo());
    DCHECK(CanBeDeprecated());
    set_is_deprecated(true);
    if (FLAG_log_maps) {
      LOG(isolate, MapEvent("Deprecate", handle(*this, isolate), Handle<Map>()));
    }
    dependent_code().DeoptimizeDependentCodeGroup(
        isolate, DependentCode::kTransitionGroup);
    NotifyLeafMapLayoutChange(isolate);
  }
  
  // Installs |new_descriptors| over the current instance_descriptors to ensure
  // proper sharing of descriptor arrays.
  void Map::ReplaceDescriptors(Isolate* isolate,
                               DescriptorArray new_descriptors) {
    // Don't overwrite the empty descriptor array or initial map's descriptors.
    if (NumberOfOwnDescriptors() == 0 ||
        GetBackPointer(isolate).IsUndefined(isolate)) {
      return;
    }
  
    DescriptorArray to_replace = instance_descriptors(isolate);
    // Replace descriptors by new_descriptors in all maps that share it. The old
    // descriptors will not be trimmed in the mark-compactor, we need to mark
    // all its elements.
    Map current = *this;
  #ifndef V8_DISABLE_WRITE_BARRIERS
    WriteBarrier::Marking(to_replace, to_replace.number_of_descriptors());
  #endif
    while (current.instance_descriptors(isolate) == to_replace) {
      Object next = current.GetBackPointer(isolate);
      if (next.IsUndefined(isolate)) break;  // Stop overwriting at initial map.
      current.SetEnumLength(kInvalidEnumCacheSentinel);
      current.UpdateDescriptors(isolate, new_descriptors,
                                current.NumberOfOwnDescriptors());
      current = Map::cast(next);
    }
    set_owns_descriptors(false);
  }
  
  Map Map::FindRootMap(Isolate* isolate) const {
    DisallowGarbageCollection no_gc;
    Map result = *this;
    PtrComprCageBase cage_base(isolate);
    while (true) {
      Object back = result.GetBackPointer(cage_base);
      if (back.IsUndefined(isolate)) {
        // Initial map must not contain descriptors in the descriptors array
        // that do not belong to the map.
        DCHECK_LE(result.NumberOfOwnDescriptors(),
                  result.instance_descriptors(cage_base, kRelaxedLoad)
                      .number_of_descriptors());
        return result;
      }
      result = Map::cast(back);
    }
  }
  
  Map Map::FindFieldOwner(Isolate* isolate, InternalIndex descriptor) const {
    DisallowGarbageCollection no_gc;
    DCHECK_EQ(PropertyLocation::kField,
              instance_descriptors(isolate, kRelaxedLoad)
                  .GetDetails(descriptor)
                  .location());
    Map result = *this;
    while (true) {
      Object back = result.GetBackPointer(isolate);
      if (back.IsUndefined(isolate)) break;
      const Map parent = Map::cast(back);
      if (parent.NumberOfOwnDescriptors() <= descriptor.as_int()) break;
      result = parent;
    }
    return result;
  }
  
  namespace {
  
  Map SearchMigrationTarget(Isolate* isolate, Map old_map) {
    DisallowGarbageCollection no_gc;
  
    Map target = old_map;
    do {
      target = TransitionsAccessor(isolate, target).GetMigrationTarget();
    } while (!target.is_null() && target.is_deprecated());
    if (target.is_null()) return Map();
  
    // TODO(ishell): if this validation ever become a bottleneck consider adding a
    // bit to the Map telling whether it contains fields whose field types may be
    // cleared.
    // TODO(ishell): revisit handling of cleared field types in
    // TryReplayPropertyTransitions() and consider checking the target map's field
    // types instead of old_map's types.
    // Go to slow map updating if the old_map has fast properties with cleared
    // field types.
    DescriptorArray old_descriptors = old_map.instance_descriptors(isolate);
    for (InternalIndex i : old_map.IterateOwnDescriptors()) {
      PropertyDetails old_details = old_descriptors.GetDetails(i);
      if (old_details.location() == PropertyLocation::kField &&
          old_details.kind() == PropertyKind::kData) {
        FieldType old_type = old_descriptors.GetFieldType(i);
        if (Map::FieldTypeIsCleared(old_details.representation(), old_type)) {
          return Map();
        }
      }
    }
  
    SLOW_DCHECK(MapUpdater::TryUpdateNoLock(
                    isolate, old_map, ConcurrencyMode::kNotConcurrent) == target);
    return target;
  }
  }  // namespace
  
  // static
  MaybeHandle<Map> Map::TryUpdate(Isolate* isolate, Handle<Map> old_map) {
    DisallowGarbageCollection no_gc;
    DisallowDeoptimization no_deoptimization(isolate);
  
    if (!old_map->is_deprecated()) return old_map;
  
    if (FLAG_fast_map_update) {
      Map target_map = SearchMigrationTarget(isolate, *old_map);
      if (!target_map.is_null()) {
        return handle(target_map, isolate);
      }
    }
  
    base::Optional<Map> new_map = MapUpdater::TryUpdateNoLock(
        isolate, *old_map, ConcurrencyMode::kNotConcurrent);
    if (!new_map.has_value()) return MaybeHandle<Map>();
    if (FLAG_fast_map_update) {
      TransitionsAccessor::SetMigrationTarget(isolate, old_map, new_map.value());
    }
    return handle(new_map.value(), isolate);
  }
  
  Map Map::TryReplayPropertyTransitions(Isolate* isolate, Map old_map,
                                        ConcurrencyMode cmode) {
    DisallowGarbageCollection no_gc;
  
    const bool is_concurrent = cmode == ConcurrencyMode::kConcurrent;
    const int root_nof = NumberOfOwnDescriptors();
    const int old_nof = old_map.NumberOfOwnDescriptors();
    // TODO(jgruber,chromium:1239009): The main thread should use non-atomic
    // reads, but this currently leads to odd behavior (see the linked bug).
    // Investigate and fix this properly. Also below and in called functions.
    DescriptorArray old_descriptors =
        old_map.instance_descriptors(isolate, kAcquireLoad);
  
    Map new_map = *this;
    for (InternalIndex i : InternalIndex::Range(root_nof, old_nof)) {
      PropertyDetails old_details = old_descriptors.GetDetails(i);
      Map transition =
          TransitionsAccessor(isolate, new_map, is_concurrent)
              .SearchTransition(old_descriptors.GetKey(i), old_details.kind(),
                                old_details.attributes());
      if (transition.is_null()) return Map();
      new_map = transition;
      DescriptorArray new_descriptors =
          new_map.instance_descriptors(isolate, kAcquireLoad);
  
      PropertyDetails new_details = new_descriptors.GetDetails(i);
      DCHECK_EQ(old_details.kind(), new_details.kind());
      DCHECK_EQ(old_details.attributes(), new_details.attributes());
      if (!IsGeneralizableTo(old_details.constness(), new_details.constness())) {
        return Map();
      }
      DCHECK(IsGeneralizableTo(old_details.location(), new_details.location()));
      if (!old_details.representation().fits_into(new_details.representation())) {
        return Map();
      }
      if (new_details.location() == PropertyLocation::kField) {
        if (new_details.kind() == PropertyKind::kData) {
          FieldType new_type = new_descriptors.GetFieldType(i);
          // Cleared field types need special treatment. They represent lost
          // knowledge, so we must first generalize the new_type to "Any".
          if (FieldTypeIsCleared(new_details.representation(), new_type)) {
            return Map();
          }
          DCHECK_EQ(PropertyKind::kData, old_details.kind());
          DCHECK_EQ(PropertyLocation::kField, old_details.location());
          FieldType old_type = old_descriptors.GetFieldType(i);
          if (FieldTypeIsCleared(old_details.representation(), old_type) ||
              !old_type.NowIs(new_type)) {
            return Map();
          }
        } else {
          DCHECK_EQ(PropertyKind::kAccessor, new_details.kind());
  #ifdef DEBUG
          FieldType new_type = new_descriptors.GetFieldType(i);
          DCHECK(new_type.IsAny());
  #endif
          UNREACHABLE();
        }
      } else {
        DCHECK_EQ(PropertyLocation::kDescriptor, new_details.location());
        if (old_details.location() == PropertyLocation::kField ||
            old_descriptors.GetStrongValue(i) !=
                new_descriptors.GetStrongValue(i)) {
          return Map();
        }
      }
    }
    if (new_map.NumberOfOwnDescriptors() != old_nof) return Map();
    return new_map;
  }
  
  // static
  Handle<Map> Map::Update(Isolate* isolate, Handle<Map> map) {
    if (!map->is_deprecated()) return map;
    if (FLAG_fast_map_update) {
      Map target_map = SearchMigrationTarget(isolate, *map);
      if (!target_map.is_null()) {
        return handle(target_map, isolate);
      }
    }
    MapUpdater mu(isolate, map);
    return mu.Update();
  }
  
  void Map::EnsureDescriptorSlack(Isolate* isolate, Handle<Map> map, int slack) {
    // Only supports adding slack to owned descriptors.
    DCHECK(map->owns_descriptors());
  
    Handle<DescriptorArray> descriptors(map->instance_descriptors(isolate),
                                        isolate);
    int old_size = map->NumberOfOwnDescriptors();
    if (slack <= descriptors->number_of_slack_descriptors()) return;
  
    Handle<DescriptorArray> new_descriptors =
        DescriptorArray::CopyUpTo(isolate, descriptors, old_size, slack);
  
    DisallowGarbageCollection no_gc;
    if (old_size == 0) {
      map->UpdateDescriptors(isolate, *new_descriptors,
                             map->NumberOfOwnDescriptors());
      return;
    }
  
    // If the source descriptors had an enum cache we copy it. This ensures
    // that the maps to which we push the new descriptor array back can rely
    // on a cache always being available once it is set. If the map has more
    // enumerated descriptors than available in the original cache, the cache
    // will be lazily replaced by the extended cache when needed.
    new_descriptors->CopyEnumCacheFrom(*descriptors);
  
    // Replace descriptors by new_descriptors in all maps that share it. The old
    // descriptors will not be trimmed in the mark-compactor, we need to mark
    // all its elements.
  #ifndef V8_DISABLE_WRITE_BARRIERS
    WriteBarrier::Marking(*descriptors, descriptors->number_of_descriptors());
  #endif
  
    // Update the descriptors from {map} (inclusive) until the initial map
    // (exclusive). In the case that {map} is the initial map, update it.
    map->UpdateDescriptors(isolate, *new_descriptors,
                           map->NumberOfOwnDescriptors());
    Object next = map->GetBackPointer();
    if (next.IsUndefined(isolate)) return;
  
    Map current = Map::cast(next);
    while (current.instance_descriptors(isolate) == *descriptors) {
      next = current.GetBackPointer();
      if (next.IsUndefined(isolate)) break;
      current.UpdateDescriptors(isolate, *new_descriptors,
                                current.NumberOfOwnDescriptors());
      current = Map::cast(next);
    }
  }
  
  // static
  Handle<Map> Map::GetObjectCreateMap(Isolate* isolate,
                                      Handle<HeapObject> prototype) {
    Handle<Map> map(isolate->native_context()->object_function().initial_map(),
                    isolate);
    if (map->prototype() == *prototype) return map;
    if (prototype->IsNull(isolate)) {
      return isolate->slow_object_with_null_prototype_map();
    }
    if (prototype->IsJSObject()) {
      Handle<JSObject> js_prototype = Handle<JSObject>::cast(prototype);
      if (!js_prototype->map().is_prototype_map()) {
        JSObject::OptimizeAsPrototype(js_prototype);
      }
      Handle<PrototypeInfo> info =
          Map::GetOrCreatePrototypeInfo(js_prototype, isolate);
      // TODO(verwaest): Use inobject slack tracking for this map.
      if (info->HasObjectCreateMap()) {
        map = handle(info->ObjectCreateMap(), isolate);
      } else {
        map = Map::CopyInitialMap(isolate, map);
        Map::SetPrototype(isolate, map, prototype);
        PrototypeInfo::SetObjectCreateMap(info, map);
      }
      return map;
    }
  
    return Map::TransitionToPrototype(isolate, map, prototype);
  }
  
  static bool ContainsMap(MapHandles const& maps, Map map) {
    DCHECK(!map.is_null());
    for (Handle<Map> current : maps) {
      if (!current.is_null() && *current == map) return true;
    }
    return false;
  }
  
  static bool HasElementsKind(MapHandles const& maps,
                              ElementsKind elements_kind) {
    for (Handle<Map> current : maps) {
      if (!current.is_null() && current->elements_kind() == elements_kind)
        return true;
    }
    return false;
  }
  
  Map Map::FindElementsKindTransitionedMap(Isolate* isolate,
                                           MapHandles const& candidates,
                                           ConcurrencyMode cmode) {
    DisallowGarbageCollection no_gc;
  
    if (IsDetached(isolate)) return Map();
  
    ElementsKind kind = elements_kind();
    bool is_packed = IsFastPackedElementsKind(kind);
  
    Map transition;
    if (IsTransitionableFastElementsKind(kind)) {
      // Check the state of the root map.
      Map root_map = FindRootMap(isolate);
      if (!EquivalentToForElementsKindTransition(root_map, cmode)) return Map();
      root_map = root_map.LookupElementsTransitionMap(isolate, kind, cmode);
      DCHECK(!root_map.is_null());
      // Starting from the next existing elements kind transition try to
      // replay the property transitions that does not involve instance rewriting
      // (ElementsTransitionAndStoreStub does not support that).
      for (root_map = root_map.ElementsTransitionMap(isolate, cmode);
           !root_map.is_null() && root_map.has_fast_elements();
           root_map = root_map.ElementsTransitionMap(isolate, cmode)) {
        // If root_map's elements kind doesn't match any of the elements kind in
        // the candidates there is no need to do any additional work.
        if (!HasElementsKind(candidates, root_map.elements_kind())) continue;
        Map current =
            root_map.TryReplayPropertyTransitions(isolate, *this, cmode);
        if (current.is_null()) continue;
        if (InstancesNeedRewriting(current, cmode)) continue;
  
        const bool current_is_packed =
            IsFastPackedElementsKind(current.elements_kind());
        if (ContainsMap(candidates, current) &&
            (is_packed || !current_is_packed)) {
          transition = current;
          is_packed = is_packed && current_is_packed;
        }
      }
    }
    return transition;
  }
  
  static Map FindClosestElementsTransition(Isolate* isolate, Map map,
                                           ElementsKind to_kind,
                                           ConcurrencyMode cmode) {
    DisallowGarbageCollection no_gc;
    // Ensure we are requested to search elements kind transition "near the root".
    DCHECK_EQ(map.FindRootMap(isolate).NumberOfOwnDescriptors(),
              map.NumberOfOwnDescriptors());
    Map current_map = map;
  
    ElementsKind kind = map.elements_kind();
    while (kind != to_kind) {
      Map next_map = current_map.ElementsTransitionMap(isolate, cmode);
      if (next_map.is_null()) return current_map;
      kind = next_map.elements_kind();
      current_map = next_map;
    }
  
    DCHECK_EQ(to_kind, current_map.elements_kind());
    return current_map;
  }
  
  Map Map::LookupElementsTransitionMap(Isolate* isolate, ElementsKind to_kind,
                                       ConcurrencyMode cmode) {
    Map to_map = FindClosestElementsTransition(isolate, *this, to_kind, cmode);
    if (to_map.elements_kind() == to_kind) return to_map;
    return Map();
  }
  
  bool Map::IsMapInArrayPrototypeChain(Isolate* isolate) const {
    if (isolate->initial_array_prototype()->map() == *this) {
      return true;
    }
  
    if (isolate->initial_object_prototype()->map() == *this) {
      return true;
    }
  
    return false;
  }
  
  Handle<Map> Map::TransitionElementsTo(Isolate* isolate, Handle<Map> map,
                                        ElementsKind to_kind) {
    ElementsKind from_kind = map->elements_kind();
    if (from_kind == to_kind) return map;
  
    Context native_context = isolate->context().native_context();
    if (from_kind == FAST_SLOPPY_ARGUMENTS_ELEMENTS) {
      if (*map == native_context.fast_aliased_arguments_map()) {
        DCHECK_EQ(SLOW_SLOPPY_ARGUMENTS_ELEMENTS, to_kind);
        return handle(native_context.slow_aliased_arguments_map(), isolate);
      }
    } else if (from_kind == SLOW_SLOPPY_ARGUMENTS_ELEMENTS) {
      if (*map == native_context.slow_aliased_arguments_map()) {
        DCHECK_EQ(FAST_SLOPPY_ARGUMENTS_ELEMENTS, to_kind);
        return handle(native_context.fast_aliased_arguments_map(), isolate);
      }
    } else if (IsFastElementsKind(from_kind) && IsFastElementsKind(to_kind)) {
      // Reuse map transitions for JSArrays.
      DisallowGarbageCollection no_gc;
      if (native_context.GetInitialJSArrayMap(from_kind) == *map) {
        Object maybe_transitioned_map =
            native_context.get(Context::ArrayMapIndex(to_kind));
        if (maybe_transitioned_map.IsMap()) {
          return handle(Map::cast(maybe_transitioned_map), isolate);
        }
      }
    }
  
    DCHECK(!map->IsUndefined(isolate));
    // Check if we can go back in the elements kind transition chain.
    if (IsHoleyElementsKind(from_kind) &&
        to_kind == GetPackedElementsKind(from_kind) &&
        map->GetBackPointer().IsMap() &&
        Map::cast(map->GetBackPointer()).elements_kind() == to_kind) {
      return handle(Map::cast(map->GetBackPointer()), isolate);
    }
  
    bool allow_store_transition = IsTransitionElementsKind(from_kind);
    // Only store fast element maps in ascending generality.
    if (IsFastElementsKind(to_kind)) {
      allow_store_transition =
          allow_store_transition && IsTransitionableFastElementsKind(from_kind) &&
          IsMoreGeneralElementsKindTransition(from_kind, to_kind);
    }
  
    if (!allow_store_transition) {
      return Map::CopyAsElementsKind(isolate, map, to_kind, OMIT_TRANSITION);
    }
  
    return MapUpdater{isolate, map}.ReconfigureElementsKind(to_kind);
  }
  
  static Handle<Map> AddMissingElementsTransitions(Isolate* isolate,
                                                   Handle<Map> map,
                                                   ElementsKind to_kind) {
    DCHECK(IsTransitionElementsKind(map->elements_kind()));
  
    Handle<Map> current_map = map;
  
    ElementsKind kind = map->elements_kind();
    TransitionFlag flag;
    if (map->IsDetached(isolate)) {
      flag = OMIT_TRANSITION;
    } else {
      flag = INSERT_TRANSITION;
      if (IsFastElementsKind(kind)) {
        while (kind != to_kind && !IsTerminalElementsKind(kind)) {
          kind = GetNextTransitionElementsKind(kind);
          current_map = Map::CopyAsElementsKind(isolate, current_map, kind, flag);
        }
      }
    }
  
    // In case we are exiting the fast elements kind system, just add the map in
    // the end.
    if (kind != to_kind) {
      current_map = Map::CopyAsElementsKind(isolate, current_map, to_kind, flag);
    }
  
    DCHECK(current_map->elements_kind() == to_kind);
    return current_map;
  }
  
  // static
  base::Optional<Map> Map::TryAsElementsKind(Isolate* isolate, Handle<Map> map,
                                             ElementsKind kind,
                                             ConcurrencyMode cmode) {
    Map closest_map = FindClosestElementsTransition(isolate, *map, kind, cmode);
    if (closest_map.elements_kind() != kind) return {};
    return closest_map;
  }
  
  // static
  Handle<Map> Map::AsElementsKind(Isolate* isolate, Handle<Map> map,
                                  ElementsKind kind) {
    Handle<Map> closest_map(
        FindClosestElementsTransition(isolate, *map, kind,
                                      ConcurrencyMode::kNotConcurrent),
        isolate);
  
    if (closest_map->elements_kind() == kind) {
      return closest_map;
    }
  
    return AddMissingElementsTransitions(isolate, closest_map, kind);
  }
  
  int Map::NumberOfEnumerableProperties() const {
    int result = 0;
    DescriptorArray descs = instance_descriptors(kRelaxedLoad);
    for (InternalIndex i : IterateOwnDescriptors()) {
      if ((descs.GetDetails(i).attributes() & ONLY_ENUMERABLE) == 0 &&
          !descs.GetKey(i).FilterKey(ENUMERABLE_STRINGS)) {
        result++;
      }
    }
    return result;
  }
  
  int Map::NextFreePropertyIndex() const {
    int number_of_own_descriptors = NumberOfOwnDescriptors();
    DescriptorArray descs = instance_descriptors(kRelaxedLoad);
    // Search properties backwards to find the last field.
    for (int i = number_of_own_descriptors - 1; i >= 0; --i) {
      PropertyDetails details = descs.GetDetails(InternalIndex(i));
      if (details.location() == PropertyLocation::kField) {
        return details.field_index() + details.field_width_in_words();
      }
    }
    return 0;
  }
  
  bool Map::OnlyHasSimpleProperties() const {
    // Wrapped string elements aren't explicitly stored in the elements backing
    // store, but are loaded indirectly from the underlying string.
    return !IsStringWrapperElementsKind(elements_kind()) &&
           !IsSpecialReceiverMap() && !is_dictionary_map();
  }
  
  bool Map::MayHaveReadOnlyElementsInPrototypeChain(Isolate* isolate) {
    for (PrototypeIterator iter(isolate, *this); !iter.IsAtEnd();
         iter.Advance()) {
      // Be conservative, don't look into any JSReceivers that may have custom
      // elements. For example, into JSProxies, String wrappers (which have have
      // non-configurable, non-writable elements), API objects, etc.
      if (iter.GetCurrent().map().IsCustomElementsReceiverMap()) return true;
  
      JSObject current = iter.GetCurrent<JSObject>();
      ElementsKind elements_kind = current.GetElementsKind(isolate);
      if (IsFrozenElementsKind(elements_kind)) return true;
  
      if (IsDictionaryElementsKind(elements_kind) &&
          current.element_dictionary(isolate).requires_slow_elements()) {
        return true;
      }
  
      if (IsSlowArgumentsElementsKind(elements_kind)) {
        SloppyArgumentsElements elements =
            SloppyArgumentsElements::cast(current.elements(isolate));
        Object arguments = elements.arguments();
        if (NumberDictionary::cast(arguments).requires_slow_elements()) {
          return true;
        }
      }
    }
  
    return false;
  }
  
  Handle<Map> Map::RawCopy(Isolate* isolate, Handle<Map> src_handle,
                           int instance_size, int inobject_properties) {
    Handle<Map> result = isolate->factory()->NewMap(
        src_handle->instance_type(), instance_size, TERMINAL_FAST_ELEMENTS_KIND,
        inobject_properties);
    // We have to set the bitfields before any potential GCs could happen because
    // heap verification might fail otherwise.
    {
      DisallowGarbageCollection no_gc;
      Map src = *src_handle;
      Map raw = *result;
      raw.set_constructor_or_back_pointer(src.GetConstructor());
      raw.set_bit_field(src.bit_field());
      raw.set_bit_field2(src.bit_field2());
      int new_bit_field3 = src.bit_field3();
      new_bit_field3 = Bits3::OwnsDescriptorsBit::update(new_bit_field3, true);
      new_bit_field3 =
          Bits3::NumberOfOwnDescriptorsBits::update(new_bit_field3, 0);
      new_bit_field3 = Bits3::EnumLengthBits::update(new_bit_field3,
                                                     kInvalidEnumCacheSentinel);
      new_bit_field3 = Bits3::IsDeprecatedBit::update(new_bit_field3, false);
      new_bit_field3 =
          Bits3::IsInRetainedMapListBit::update(new_bit_field3, false);
      if (!src.is_dictionary_map()) {
        new_bit_field3 = Bits3::IsUnstableBit::update(new_bit_field3, false);
      }
      // Same as bit_field comment above.
      raw.set_bit_field3(new_bit_field3);
      raw.clear_padding();
    }
    Handle<HeapObject> prototype(src_handle->prototype(), isolate);
    Map::SetPrototype(isolate, result, prototype);
    return result;
  }
  
  Handle<Map> Map::Normalize(Isolate* isolate, Handle<Map> fast_map,
                             ElementsKind new_elements_kind,
                             PropertyNormalizationMode mode, const char* reason) {
    DCHECK(!fast_map->is_dictionary_map());
  
    Handle<Object> maybe_cache(isolate->native_context()->normalized_map_cache(),
                               isolate);
    bool use_cache =
        !fast_map->is_prototype_map() && !maybe_cache->IsUndefined(isolate);
    Handle<NormalizedMapCache> cache;
    if (use_cache) cache = Handle<NormalizedMapCache>::cast(maybe_cache);
  
    Handle<Map> new_map;
    if (use_cache &&
        cache->Get(fast_map, new_elements_kind, mode).ToHandle(&new_map)) {
  #ifdef VERIFY_HEAP
      if (FLAG_verify_heap) new_map->DictionaryMapVerify(isolate);
  #endif
  #ifdef ENABLE_SLOW_DCHECKS
      if (FLAG_enable_slow_asserts) {
        // The cached map should match newly created normalized map bit-by-bit,
        // except for the code cache, which can contain some ICs which can be
        // applied to the shared map, dependent code and weak cell cache.
        Handle<Map> fresh = Map::CopyNormalized(isolate, fast_map, mode);
        fresh->set_elements_kind(new_elements_kind);
  
        STATIC_ASSERT(Map::kPrototypeValidityCellOffset ==
                      Map::kDependentCodeOffset + kTaggedSize);
        DCHECK_EQ(0, memcmp(reinterpret_cast<void*>(fresh->address()),
                            reinterpret_cast<void*>(new_map->address()),
                            Map::kBitField3Offset));
        // The IsInRetainedMapListBit might be different if the {new_map}
        // that we got from the {cache} was already embedded into optimized
        // code somewhere.
        // The IsMigrationTargetBit might be different if the {new_map} from
        // {cache} has already been marked as a migration target.
        constexpr int ignored_bit_field3_bits =
            Bits3::IsInRetainedMapListBit::kMask |
            Bits3::IsMigrationTargetBit::kMask;
        DCHECK_EQ(fresh->bit_field3() & ~ignored_bit_field3_bits,
                  new_map->bit_field3() & ~ignored_bit_field3_bits);
        int offset = Map::kBitField3Offset + kInt32Size;
        DCHECK_EQ(0, memcmp(reinterpret_cast<void*>(fresh->address() + offset),
                            reinterpret_cast<void*>(new_map->address() + offset),
                            Map::kDependentCodeOffset - offset));
        offset = Map::kPrototypeValidityCellOffset + kTaggedSize;
        if (new_map->is_prototype_map()) {
          // For prototype maps, the PrototypeInfo is not copied.
          STATIC_ASSERT(Map::kTransitionsOrPrototypeInfoOffset ==
                        Map::kPrototypeValidityCellOffset + kTaggedSize);
          offset = kTransitionsOrPrototypeInfoOffset + kTaggedSize;
          DCHECK_EQ(fresh->raw_transitions(),
                    MaybeObject::FromObject(Smi::zero()));
        }
        DCHECK_EQ(0, memcmp(reinterpret_cast<void*>(fresh->address() + offset),
                            reinterpret_cast<void*>(new_map->address() + offset),
                            Map::kSize - offset));
      }
  #endif
      if (FLAG_log_maps) {
        LOG(isolate, MapEvent("NormalizeCached", fast_map, new_map, reason));
      }
    } else {
      new_map = Map::CopyNormalized(isolate, fast_map, mode);
      new_map->set_elements_kind(new_elements_kind);
      if (use_cache) {
        cache->Set(fast_map, new_map);
        isolate->counters()->maps_normalized()->Increment();
      }
      if (FLAG_log_maps) {
        LOG(isolate, MapEvent("Normalize", fast_map, new_map, reason));
      }
    }
    fast_map->NotifyLeafMapLayoutChange(isolate);
    return new_map;
  }
  
  Handle<Map> Map::CopyNormalized(Isolate* isolate, Handle<Map> map,
                                  PropertyNormalizationMode mode) {
    int new_instance_size = map->instance_size();
    if (mode == CLEAR_INOBJECT_PROPERTIES) {
      new_instance_size -= map->GetInObjectProperties() * kTaggedSize;
    }
  
    Handle<Map> result = RawCopy(
        isolate, map, new_instance_size,
        mode == CLEAR_INOBJECT_PROPERTIES ? 0 : map->GetInObjectProperties());
    {
      DisallowGarbageCollection no_gc;
      Map raw = *result;
      // Clear the unused_property_fields explicitly as this field should not
      // be accessed for normalized maps.
      raw.SetInObjectUnusedPropertyFields(0);
      raw.set_is_dictionary_map(true);
      raw.set_is_migration_target(false);
      raw.set_may_have_interesting_symbols(true);
      raw.set_construction_counter(kNoSlackTracking);
    }
  
  #ifdef VERIFY_HEAP
    if (FLAG_verify_heap) result->DictionaryMapVerify(isolate);
  #endif
  
    return result;
  }
  
  // Return an immutable prototype exotic object version of the input map.
  // Never even try to cache it in the transition tree, as it is intended
  // for the global object and its prototype chain, and excluding it saves
  // memory on the map transition tree.
  
  // static
  Handle<Map> Map::TransitionToImmutableProto(Isolate* isolate, Handle<Map> map) {
    Handle<Map> new_map = Map::Copy(isolate, map, "ImmutablePrototype");
    new_map->set_is_immutable_proto(true);
    return new_map;
  }
  
  namespace {
  void EnsureInitialMap(Isolate* isolate, Handle<Map> map) {
  #ifdef DEBUG
    Object maybe_constructor = map->GetConstructor();
    DCHECK((maybe_constructor.IsJSFunction() &&
            *map == JSFunction::cast(maybe_constructor).initial_map()) ||
           // Below are the exceptions to the check above.
           // Strict function maps have Function as a constructor but the
           // Function's initial map is a sloppy function map.
           *map == *isolate->strict_function_map() ||
           *map == *isolate->strict_function_with_name_map() ||
           // Same holds for GeneratorFunction and its initial map.
           *map == *isolate->generator_function_map() ||
           *map == *isolate->generator_function_with_name_map() ||
           // AsyncFunction has Null as a constructor.
           *map == *isolate->async_function_map() ||
           *map == *isolate->async_function_with_name_map());
  #endif
    // Initial maps must not contain descriptors in the descriptors array
    // that do not belong to the map.
    DCHECK_EQ(map->NumberOfOwnDescriptors(),
              map->instance_descriptors(isolate).number_of_descriptors());
  }
  }  // namespace
  
  // static
  Handle<Map> Map::CopyInitialMapNormalized(Isolate* isolate, Handle<Map> map,
                                            PropertyNormalizationMode mode) {
    EnsureInitialMap(isolate, map);
    return CopyNormalized(isolate, map, mode);
  }
  
  // static
  Handle<Map> Map::CopyInitialMap(Isolate* isolate, Handle<Map> map,
                                  int instance_size, int inobject_properties,
                                  int unused_property_fields) {
    EnsureInitialMap(isolate, map);
  
    Handle<Map> result =
        RawCopy(isolate, map, instance_size, inobject_properties);
  
    // Please note instance_type and instance_size are set when allocated.
    result->SetInObjectUnusedPropertyFields(unused_property_fields);
  
    int number_of_own_descriptors = map->NumberOfOwnDescriptors();
    if (number_of_own_descriptors > 0) {
      // The copy will use the same descriptors array without ownership.
      DescriptorArray descriptors = map->instance_descriptors(isolate);
      result->set_owns_descriptors(false);
      result->UpdateDescriptors(isolate, descriptors, number_of_own_descriptors);
  
      DCHECK_EQ(result->NumberOfFields(ConcurrencyMode::kNotConcurrent),
                result->GetInObjectProperties() - result->UnusedPropertyFields());
    }
  
    return result;
  }
  
  Handle<Map> Map::CopyDropDescriptors(Isolate* isolate, Handle<Map> map) {
    Handle<Map> result =
        RawCopy(isolate, map, map->instance_size(),
                map->IsJSObjectMap() ? map->GetInObjectProperties() : 0);
  
    // Please note instance_type and instance_size are set when allocated.
    if (map->IsJSObjectMap()) {
      result->CopyUnusedPropertyFields(*map);
    }
    map->NotifyLeafMapLayoutChange(isolate);
    return result;
  }
  
  Handle<Map> Map::ShareDescriptor(Isolate* isolate, Handle<Map> map,
                                   Handle<DescriptorArray> descriptors,
                                   Descriptor* descriptor) {
    // Sanity check. This path is only to be taken if the map owns its descriptor
    // array, implying that its NumberOfOwnDescriptors equals the number of
    // descriptors in the descriptor array.
    DCHECK_EQ(map->NumberOfOwnDescriptors(),
              map->instance_descriptors(isolate).number_of_descriptors());
  
    Handle<Map> result = CopyDropDescriptors(isolate, map);
    Handle<Name> name = descriptor->GetKey();
  
    // Properly mark the {result} if the {name} is an "interesting symbol".
    if (name->IsInterestingSymbol()) {
      result->set_may_have_interesting_symbols(true);
    }
  
    // Ensure there's space for the new descriptor in the shared descriptor array.
    if (descriptors->number_of_slack_descriptors() == 0) {
      int old_size = descriptors->number_of_descriptors();
      if (old_size == 0) {
        descriptors = DescriptorArray::Allocate(isolate, 0, 1);
      } else {
        int slack = SlackForArraySize(old_size, kMaxNumberOfDescriptors);
        EnsureDescriptorSlack(isolate, map, slack);
        descriptors = handle(map->instance_descriptors(isolate), isolate);
      }
    }
  
    {
      DisallowGarbageCollection no_gc;
      descriptors->Append(descriptor);
      result->InitializeDescriptors(isolate, *descriptors);
    }
  
    DCHECK(result->NumberOfOwnDescriptors() == map->NumberOfOwnDescriptors() + 1);
    ConnectTransition(isolate, map, result, name, SIMPLE_PROPERTY_TRANSITION);
  
    return result;
  }
  
  void Map::ConnectTransition(Isolate* isolate, Handle<Map> parent,
                              Handle<Map> child, Handle<Name> name,
                              SimpleTransitionFlag flag) {
    DCHECK_IMPLIES(name->IsInterestingSymbol(),
                   child->may_have_interesting_symbols());
    DCHECK_IMPLIES(parent->may_have_interesting_symbols(),
                   child->may_have_interesting_symbols());
    if (!parent->GetBackPointer().IsUndefined(isolate)) {
      parent->set_owns_descriptors(false);
    } else if (!parent->IsDetached(isolate)) {
      // |parent| is initial map and it must not contain descriptors in the
      // descriptors array that do not belong to the map.
      DCHECK_EQ(parent->NumberOfOwnDescriptors(),
                parent->instance_descriptors(isolate).number_of_descriptors());
    }
    if (parent->IsDetached(isolate)) {
      DCHECK(child->IsDetached(isolate));
      if (FLAG_log_maps) {
        LOG(isolate, MapEvent("Transition", parent, child, "prototype", name));
      }
    } else {
      TransitionsAccessor::Insert(isolate, parent, name, child, flag);
      if (FLAG_log_maps) {
        LOG(isolate, MapEvent("Transition", parent, child, "", name));
      }
    }
  }
  
  Handle<Map> Map::CopyReplaceDescriptors(Isolate* isolate, Handle<Map> map,
                                          Handle<DescriptorArray> descriptors,
                                          TransitionFlag flag,
                                          MaybeHandle<Name> maybe_name,
                                          const char* reason,
                                          SimpleTransitionFlag simple_flag) {
    DCHECK(descriptors->IsSortedNoDuplicates());
  
    Handle<Map> result = CopyDropDescriptors(isolate, map);
    bool is_connected = false;
  
    // Properly mark the {result} if the {name} is an "interesting symbol".
    Handle<Name> name;
    if (maybe_name.ToHandle(&name) && name->IsInterestingSymbol()) {
      result->set_may_have_interesting_symbols(true);
    }
  
    if (map->is_prototype_map()) {
      result->InitializeDescriptors(isolate, *descriptors);
    } else {
      if (flag == INSERT_TRANSITION &&
          TransitionsAccessor::CanHaveMoreTransitions(isolate, map)) {
        result->InitializeDescriptors(isolate, *descriptors);
  
        DCHECK(!maybe_name.is_null());
        ConnectTransition(isolate, map, result, name, simple_flag);
        is_connected = true;
      } else {
        descriptors->GeneralizeAllFields();
        result->InitializeDescriptors(isolate, *descriptors);
      }
    }
    if (FLAG_log_maps && !is_connected) {
      LOG(isolate, MapEvent("ReplaceDescriptors", map, result, reason,
                            maybe_name.is_null() ? Handle<HeapObject>() : name));
    }
    return result;
  }
  
  // Creates transition tree starting from |split_map| and adding all descriptors
  // starting from descriptor with index |split_map|.NumberOfOwnDescriptors().
  // The way how it is done is tricky because of GC and special descriptors
  // marking logic.
  Handle<Map> Map::AddMissingTransitions(Isolate* isolate, Handle<Map> split_map,
                                         Handle<DescriptorArray> descriptors) {
    DCHECK(descriptors->IsSortedNoDuplicates());
    int split_nof = split_map->NumberOfOwnDescriptors();
    int nof_descriptors = descriptors->number_of_descriptors();
    DCHECK_LT(split_nof, nof_descriptors);
  
    // Start with creating last map which will own full descriptors array.
    // This is necessary to guarantee that GC will mark the whole descriptor
    // array if any of the allocations happening below fail.
    // Number of unused properties is temporarily incorrect and the layout
    // descriptor could unnecessarily be in slow mode but we will fix after
    // all the other intermediate maps are created.
    // Also the last map might have interesting symbols, we temporarily set
    // the flag and clear it right before the descriptors are installed. This
    // makes heap verification happy and ensures the flag ends up accurate.
    Handle<Map> last_map = CopyDropDescriptors(isolate, split_map);
    last_map->InitializeDescriptors(isolate, *descriptors);
    last_map->SetInObjectUnusedPropertyFields(0);
    last_map->set_may_have_interesting_symbols(true);
  
    // During creation of intermediate maps we violate descriptors sharing
    // invariant since the last map is not yet connected to the transition tree
    // we create here. But it is safe because GC never trims map's descriptors
    // if there are no dead transitions from that map and this is exactly the
    // case for all the intermediate maps we create here.
    Handle<Map> map = split_map;
    for (InternalIndex i : InternalIndex::Range(split_nof, nof_descriptors - 1)) {
      Handle<Map> new_map = CopyDropDescriptors(isolate, map);
      InstallDescriptors(isolate, map, new_map, i, descriptors);
  
      map = new_map;
    }
    map->NotifyLeafMapLayoutChange(isolate);
    last_map->set_may_have_interesting_symbols(false);
    InstallDescriptors(isolate, map, last_map, InternalIndex(nof_descriptors - 1),
                       descriptors);
    return last_map;
  }
  
  // Since this method is used to rewrite an existing transition tree, it can
  // always insert transitions without checking.
  void Map::InstallDescriptors(Isolate* isolate, Handle<Map> parent,
                               Handle<Map> child, InternalIndex new_descriptor,
                               Handle<DescriptorArray> descriptors) {
    DCHECK(descriptors->IsSortedNoDuplicates());
  
    child->SetInstanceDescriptors(isolate, *descriptors,
                                  new_descriptor.as_int() + 1);
    child->CopyUnusedPropertyFields(*parent);
    PropertyDetails details = descriptors->GetDetails(new_descriptor);
    if (details.location() == PropertyLocation::kField) {
      child->AccountAddedPropertyField();
    }
  
    Handle<Name> name = handle(descriptors->GetKey(new_descriptor), isolate);
    if (parent->may_have_interesting_symbols() || name->IsInterestingSymbol()) {
      child->set_may_have_interesting_symbols(true);
    }
    ConnectTransition(isolate, parent, child, name, SIMPLE_PROPERTY_TRANSITION);
  }
  
  Handle<Map> Map::CopyAsElementsKind(Isolate* isolate, Handle<Map> map,
                                      ElementsKind kind, TransitionFlag flag) {
    // Only certain objects are allowed to have non-terminal fast transitional
    // elements kinds.
    DCHECK(map->IsJSObjectMap());
    DCHECK_IMPLIES(
        !map->CanHaveFastTransitionableElementsKind(),
        IsDictionaryElementsKind(kind) || IsTerminalElementsKind(kind));
  
    Map maybe_elements_transition_map;
    if (flag == INSERT_TRANSITION) {
      // Ensure we are requested to add elements kind transition "near the root".
      DCHECK_EQ(map->FindRootMap(isolate).NumberOfOwnDescriptors(),
                map->NumberOfOwnDescriptors());
  
      maybe_elements_transition_map =
          map->ElementsTransitionMap(isolate, ConcurrencyMode::kNotConcurrent);
      DCHECK(
          maybe_elements_transition_map.is_null() ||
          (maybe_elements_transition_map.elements_kind() == DICTIONARY_ELEMENTS &&
           kind == DICTIONARY_ELEMENTS));
      DCHECK(!IsFastElementsKind(kind) ||
             IsMoreGeneralElementsKindTransition(map->elements_kind(), kind));
      DCHECK(kind != map->elements_kind());
    }
  
    bool insert_transition =
        flag == INSERT_TRANSITION &&
        TransitionsAccessor::CanHaveMoreTransitions(isolate, map) &&
        maybe_elements_transition_map.is_null();
  
    if (insert_transition) {
      Handle<Map> new_map = CopyForElementsTransition(isolate, map);
      new_map->set_elements_kind(kind);
  
      Handle<Name> name = isolate->factory()->elements_transition_symbol();
      ConnectTransition(isolate, map, new_map, name, SPECIAL_TRANSITION);
      return new_map;
    }
  
    // Create a new free-floating map only if we are not allowed to store it.
    Handle<Map> new_map = Copy(isolate, map, "CopyAsElementsKind");
    new_map->set_elements_kind(kind);
    return new_map;
  }
  
  Handle<Map> Map::AsLanguageMode(Isolate* isolate, Handle<Map> initial_map,
                                  Handle<SharedFunctionInfo> shared_info) {
    DCHECK(InstanceTypeChecker::IsJSFunction(initial_map->instance_type()));
    // Initial map for sloppy mode function is stored in the function
    // constructor. Initial maps for strict mode are cached as special transitions
    // using |strict_function_transition_symbol| as a key.
    if (is_sloppy(shared_info->language_mode())) return initial_map;
  
    Handle<Map> function_map(Map::cast(isolate->native_context()->get(
                                 shared_info->function_map_index())),
                             isolate);
  
    STATIC_ASSERT(LanguageModeSize == 2);
    DCHECK_EQ(LanguageMode::kStrict, shared_info->language_mode());
    Handle<Symbol> transition_symbol =
        isolate->factory()->strict_function_transition_symbol();
    MaybeHandle<Map> maybe_transition = TransitionsAccessor::SearchSpecial(
        isolate, initial_map, *transition_symbol);
    if (!maybe_transition.is_null()) {
      return maybe_transition.ToHandleChecked();
    }
    initial_map->NotifyLeafMapLayoutChange(isolate);
  
    // Create new map taking descriptors from the |function_map| and all
    // the other details from the |initial_map|.
    Handle<Map> map =
        Map::CopyInitialMap(isolate, function_map, initial_map->instance_size(),
                            initial_map->GetInObjectProperties(),
                            initial_map->UnusedPropertyFields());
    map->SetConstructor(initial_map->GetConstructor());
    map->set_prototype(initial_map->prototype());
    map->set_construction_counter(initial_map->construction_counter());
  
    if (TransitionsAccessor::CanHaveMoreTransitions(isolate, initial_map)) {
      Map::ConnectTransition(isolate, initial_map, map, transition_symbol,
                             SPECIAL_TRANSITION);
    }
    return map;
  }
  
  Handle<Map> Map::CopyForElementsTransition(Isolate* isolate, Handle<Map> map) {
    DCHECK(!map->IsDetached(isolate));
    Handle<Map> new_map = CopyDropDescriptors(isolate, map);
  
    if (map->owns_descriptors()) {
      // In case the map owned its own descriptors, share the descriptors and
      // transfer ownership to the new map.
      // The properties did not change, so reuse descriptors.
      map->set_owns_descriptors(false);
      new_map->InitializeDescriptors(isolate, map->instance_descriptors(isolate));
    } else {
      // In case the map did not own its own descriptors, a split is forced by
      // copying the map; creating a new descriptor array cell.
      Handle<DescriptorArray> descriptors(map->instance_descriptors(isolate),
                                          isolate);
      int number_of_own_descriptors = map->NumberOfOwnDescriptors();
      Handle<DescriptorArray> new_descriptors = DescriptorArray::CopyUpTo(
          isolate, descriptors, number_of_own_descriptors);
      new_map->InitializeDescriptors(isolate, *new_descriptors);
    }
    return new_map;
  }
  
  Handle<Map> Map::Copy(Isolate* isolate, Handle<Map> map, const char* reason) {
    Handle<DescriptorArray> descriptors(map->instance_descriptors(isolate),
                                        isolate);
    int number_of_own_descriptors = map->NumberOfOwnDescriptors();
    Handle<DescriptorArray> new_descriptors = DescriptorArray::CopyUpTo(
        isolate, descriptors, number_of_own_descriptors);
    return CopyReplaceDescriptors(isolate, map, new_descriptors, OMIT_TRANSITION,
                                  MaybeHandle<Name>(), reason,
                                  SPECIAL_TRANSITION);
  }
  
  Handle<Map> Map::Create(Isolate* isolate, int inobject_properties) {
    Handle<Map> copy_handle =
        Copy(isolate, handle(isolate->object_function()->initial_map(), isolate),
             "MapCreate");
    DisallowGarbageCollection no_gc;
    Map copy = *copy_handle;
  
    // Check that we do not overflow the instance size when adding the extra
    // inobject properties. If the instance size overflows, we allocate as many
    // properties as we can as inobject properties.
    if (inobject_properties > JSObject::kMaxInObjectProperties) {
      inobject_properties = JSObject::kMaxInObjectProperties;
    }
  
    int new_instance_size =
        JSObject::kHeaderSize + kTaggedSize * inobject_properties;
  
    // Adjust the map with the extra inobject properties.
    copy.set_instance_size(new_instance_size);
    copy.SetInObjectPropertiesStartInWords(JSObject::kHeaderSize / kTaggedSize);
    DCHECK_EQ(copy.GetInObjectProperties(), inobject_properties);
    copy.SetInObjectUnusedPropertyFields(inobject_properties);
    copy.set_visitor_id(Map::GetVisitorId(copy));
  
    return copy_handle;
  }
  
  Handle<Map> Map::CopyForPreventExtensions(
      Isolate* isolate, Handle<Map> map, PropertyAttributes attrs_to_add,
      Handle<Symbol> transition_marker, const char* reason,
      bool old_map_is_dictionary_elements_kind) {
    int num_descriptors = map->NumberOfOwnDescriptors();
    Handle<DescriptorArray> new_desc = DescriptorArray::CopyUpToAddAttributes(
        isolate, handle(map->instance_descriptors(isolate), isolate),
        num_descriptors, attrs_to_add);
    // Do not track transitions during bootstrapping.
    TransitionFlag flag =
        isolate->bootstrapper()->IsActive() ? OMIT_TRANSITION : INSERT_TRANSITION;
    Handle<Map> new_map =
        CopyReplaceDescriptors(isolate, map, new_desc, flag, transition_marker,
                               reason, SPECIAL_TRANSITION);
    new_map->set_is_extensible(false);
    if (!IsTypedArrayElementsKind(map->elements_kind())) {
      ElementsKind new_kind = IsStringWrapperElementsKind(map->elements_kind())
                                  ? SLOW_STRING_WRAPPER_ELEMENTS
                                  : DICTIONARY_ELEMENTS;
      if (FLAG_enable_sealed_frozen_elements_kind &&
          !old_map_is_dictionary_elements_kind) {
        switch (map->elements_kind()) {
          case PACKED_ELEMENTS:
            if (attrs_to_add == SEALED) {
              new_kind = PACKED_SEALED_ELEMENTS;
            } else if (attrs_to_add == FROZEN) {
              new_kind = PACKED_FROZEN_ELEMENTS;
            } else {
              new_kind = PACKED_NONEXTENSIBLE_ELEMENTS;
            }
            break;
          case PACKED_NONEXTENSIBLE_ELEMENTS:
            if (attrs_to_add == SEALED) {
              new_kind = PACKED_SEALED_ELEMENTS;
            } else if (attrs_to_add == FROZEN) {
              new_kind = PACKED_FROZEN_ELEMENTS;
            }
            break;
          case PACKED_SEALED_ELEMENTS:
            if (attrs_to_add == FROZEN) {
              new_kind = PACKED_FROZEN_ELEMENTS;
            }
            break;
          case HOLEY_ELEMENTS:
            if (attrs_to_add == SEALED) {
              new_kind = HOLEY_SEALED_ELEMENTS;
            } else if (attrs_to_add == FROZEN) {
              new_kind = HOLEY_FROZEN_ELEMENTS;
            } else {
              new_kind = HOLEY_NONEXTENSIBLE_ELEMENTS;
            }
            break;
          case HOLEY_NONEXTENSIBLE_ELEMENTS:
            if (attrs_to_add == SEALED) {
              new_kind = HOLEY_SEALED_ELEMENTS;
            } else if (attrs_to_add == FROZEN) {
              new_kind = HOLEY_FROZEN_ELEMENTS;
            }
            break;
          case HOLEY_SEALED_ELEMENTS:
            if (attrs_to_add == FROZEN) {
              new_kind = HOLEY_FROZEN_ELEMENTS;
            }
            break;
          default:
            break;
        }
      }
      new_map->set_elements_kind(new_kind);
    }
    return new_map;
  }
  
  namespace {
  
  bool CanHoldValue(DescriptorArray descriptors, InternalIndex descriptor,
                    PropertyConstness constness, Object value) {
    PropertyDetails details = descriptors.GetDetails(descriptor);
    if (details.location() == PropertyLocation::kField) {
      if (details.kind() == PropertyKind::kData) {
        return IsGeneralizableTo(constness, details.constness()) &&
               value.FitsRepresentation(details.representation()) &&
               descriptors.GetFieldType(descriptor).NowContains(value);
      } else {
        DCHECK_EQ(PropertyKind::kAccessor, details.kind());
        return false;
      }
  
    } else {
      DCHECK_EQ(PropertyLocation::kDescriptor, details.location());
      DCHECK_EQ(PropertyConstness::kConst, details.constness());
      DCHECK_EQ(PropertyKind::kAccessor, details.kind());
      return false;
    }
    UNREACHABLE();
  }
  
  Handle<Map> UpdateDescriptorForValue(Isolate* isolate, Handle<Map> map,
                                       InternalIndex descriptor,
                                       PropertyConstness constness,
                                       Handle<Object> value) {
    if (CanHoldValue(map->instance_descriptors(isolate), descriptor, constness,
                     *value)) {
      return map;
    }
  
    PropertyAttributes attributes =
        map->instance_descriptors(isolate).GetDetails(descriptor).attributes();
    Representation representation = value->OptimalRepresentation(isolate);
    Handle<FieldType> type = value->OptimalType(isolate, representation);
  
    MapUpdater mu(isolate, map);
    return mu.ReconfigureToDataField(descriptor, attributes, constness,
                                     representation, type);
  }
  
  }  // namespace
  
  // static
  Handle<Map> Map::PrepareForDataProperty(Isolate* isolate, Handle<Map> map,
                                          InternalIndex descriptor,
                                          PropertyConstness constness,
                                          Handle<Object> value) {
    // Update to the newest map before storing the property.
    map = Update(isolate, map);
    // Dictionaries can store any property value.
    DCHECK(!map->is_dictionary_map());
    return UpdateDescriptorForValue(isolate, map, descriptor, constness, value);
  }
  
  Handle<Map> Map::TransitionToDataProperty(Isolate* isolate, Handle<Map> map,
                                            Handle<Name> name,
                                            Handle<Object> value,
                                            PropertyAttributes attributes,
                                            PropertyConstness constness,
                                            StoreOrigin store_origin) {
    RCS_SCOPE(isolate,
              map->IsDetached(isolate)
                  ? RuntimeCallCounterId::kPrototypeMap_TransitionToDataProperty
                  : RuntimeCallCounterId::kMap_TransitionToDataProperty);
  
    DCHECK(name->IsUniqueName());
    DCHECK(!map->is_dictionary_map());
  
    // Migrate to the newest map before storing the property.
    map = Update(isolate, map);
  
    MaybeHandle<Map> maybe_transition = TransitionsAccessor::SearchTransition(
        isolate, map, *name, PropertyKind::kData, attributes);
    Handle<Map> transition;
    if (maybe_transition.ToHandle(&transition)) {
      InternalIndex descriptor = transition->LastAdded();
  
      DCHECK_EQ(attributes, transition->instance_descriptors(isolate)
                                .GetDetails(descriptor)
                                .attributes());
  
      return UpdateDescriptorForValue(isolate, transition, descriptor, constness,
                                      value);
    }
  
    // Do not track transitions during bootstrapping.
    TransitionFlag flag =
        isolate->bootstrapper()->IsActive() ? OMIT_TRANSITION : INSERT_TRANSITION;
    MaybeHandle<Map> maybe_map;
    if (!map->TooManyFastProperties(store_origin)) {
      Representation representation = value->OptimalRepresentation(isolate);
      Handle<FieldType> type = value->OptimalType(isolate, representation);
      maybe_map = Map::CopyWithField(isolate, map, name, type, attributes,
                                     constness, representation, flag);
    }
  
    Handle<Map> result;
    if (!maybe_map.ToHandle(&result)) {
      const char* reason = "TooManyFastProperties";
  #if V8_TRACE_MAPS
      std::unique_ptr<base::ScopedVector<char>> buffer;
      if (FLAG_log_maps) {
        base::ScopedVector<char> name_buffer(100);
        name->NameShortPrint(name_buffer);
        buffer.reset(new base::ScopedVector<char>(128));
        SNPrintF(*buffer, "TooManyFastProperties %s", name_buffer.begin());
        reason = buffer->begin();
      }
  #endif
      Handle<Object> maybe_constructor(map->GetConstructor(), isolate);
      if (FLAG_feedback_normalization && map->new_target_is_base() &&
          maybe_constructor->IsJSFunction() &&
          !JSFunction::cast(*maybe_constructor).shared().native()) {
        Handle<JSFunction> constructor =
            Handle<JSFunction>::cast(maybe_constructor);
        DCHECK_NE(*constructor,
                  constructor->context().native_context().object_function());
        Handle<Map> initial_map(constructor->initial_map(), isolate);
        result = Map::Normalize(isolate, initial_map, CLEAR_INOBJECT_PROPERTIES,
                                reason);
        initial_map->DeprecateTransitionTree(isolate);
        Handle<HeapObject> prototype(result->prototype(), isolate);
        JSFunction::SetInitialMap(isolate, constructor, result, prototype);
  
        // Deoptimize all code that embeds the previous initial map.
        initial_map->dependent_code().DeoptimizeDependentCodeGroup(
            isolate, DependentCode::kInitialMapChangedGroup);
        if (!result->EquivalentToForNormalization(*map,
                                                  CLEAR_INOBJECT_PROPERTIES)) {
          result =
              Map::Normalize(isolate, map, CLEAR_INOBJECT_PROPERTIES, reason);
        }
      } else {
        result = Map::Normalize(isolate, map, CLEAR_INOBJECT_PROPERTIES, reason);
      }
    }
  
    return result;
  }
  
  Handle<Map> Map::TransitionToAccessorProperty(Isolate* isolate, Handle<Map> map,
                                                Handle<Name> name,
                                                InternalIndex descriptor,
                                                Handle<Object> getter,
                                                Handle<Object> setter,
                                                PropertyAttributes attributes) {
    RCS_SCOPE(
        isolate,
        map->IsDetached(isolate)
            ? RuntimeCallCounterId::kPrototypeMap_TransitionToAccessorProperty
            : RuntimeCallCounterId::kMap_TransitionToAccessorProperty);
  
    // At least one of the accessors needs to be a new value.
    DCHECK(!getter->IsNull(isolate) || !setter->IsNull(isolate));
    DCHECK(name->IsUniqueName());
  
    // Migrate to the newest map before transitioning to the new property.
    map = Update(isolate, map);
  
    // Dictionary maps can always have additional data properties.
    if (map->is_dictionary_map()) return map;
  
    PropertyNormalizationMode mode = map->is_prototype_map()
                                         ? KEEP_INOBJECT_PROPERTIES
                                         : CLEAR_INOBJECT_PROPERTIES;
  
    MaybeHandle<Map> maybe_transition = TransitionsAccessor::SearchTransition(
        isolate, map, *name, PropertyKind::kAccessor, attributes);
    Handle<Map> transition;
    if (maybe_transition.ToHandle(&transition)) {
      DescriptorArray descriptors = transition->instance_descriptors(isolate);
      InternalIndex last_descriptor = transition->LastAdded();
      DCHECK(descriptors.GetKey(last_descriptor).Equals(*name));
  
      DCHECK_EQ(PropertyKind::kAccessor,
                descriptors.GetDetails(last_descriptor).kind());
      DCHECK_EQ(attributes, descriptors.GetDetails(last_descriptor).attributes());
  
      Handle<Object> maybe_pair(descriptors.GetStrongValue(last_descriptor),
                                isolate);
      if (!maybe_pair->IsAccessorPair()) {
        return Map::Normalize(isolate, map, mode,
                              "TransitionToAccessorFromNonPair");
      }
  
      Handle<AccessorPair> pair = Handle<AccessorPair>::cast(maybe_pair);
      if (!pair->Equals(*getter, *setter)) {
        return Map::Normalize(isolate, map, mode,
                              "TransitionToDifferentAccessor");
      }
  
      return transition;
    }
  
    Handle<AccessorPair> pair;
    DescriptorArray old_descriptors = map->instance_descriptors(isolate);
    if (descriptor.is_found()) {
      if (descriptor != map->LastAdded()) {
        return Map::Normalize(isolate, map, mode, "AccessorsOverwritingNonLast");
      }
      PropertyDetails old_details = old_descriptors.GetDetails(descriptor);
      if (old_details.kind() != PropertyKind::kAccessor) {
        return Map::Normalize(isolate, map, mode,
                              "AccessorsOverwritingNonAccessors");
      }
  
      if (old_details.attributes() != attributes) {
        return Map::Normalize(isolate, map, mode, "AccessorsWithAttributes");
      }
  
      Handle<Object> maybe_pair(old_descriptors.GetStrongValue(descriptor),
                                isolate);
      if (!maybe_pair->IsAccessorPair()) {
        return Map::Normalize(isolate, map, mode, "AccessorsOverwritingNonPair");
      }
  
      Handle<AccessorPair> current_pair = Handle<AccessorPair>::cast(maybe_pair);
      if (current_pair->Equals(*getter, *setter)) return map;
  
      bool overwriting_accessor = false;
      if (!getter->IsNull(isolate) &&
          !current_pair->get(ACCESSOR_GETTER).IsNull(isolate) &&
          current_pair->get(ACCESSOR_GETTER) != *getter) {
        overwriting_accessor = true;
      }
      if (!setter->IsNull(isolate) &&
          !current_pair->get(ACCESSOR_SETTER).IsNull(isolate) &&
          current_pair->get(ACCESSOR_SETTER) != *setter) {
        overwriting_accessor = true;
      }
      if (overwriting_accessor) {
        return Map::Normalize(isolate, map, mode,
                              "AccessorsOverwritingAccessors");
      }
  
      pair = AccessorPair::Copy(isolate, Handle<AccessorPair>::cast(maybe_pair));
    } else if (map->NumberOfOwnDescriptors() >= kMaxNumberOfDescriptors ||
               map->TooManyFastProperties(StoreOrigin::kNamed)) {
      return Map::Normalize(isolate, map, CLEAR_INOBJECT_PROPERTIES,
                            "TooManyAccessors");
    } else {
      pair = isolate->factory()->NewAccessorPair();
    }
  
    pair->SetComponents(*getter, *setter);
  
    // Do not track transitions during bootstrapping.
    TransitionFlag flag =
        isolate->bootstrapper()->IsActive() ? OMIT_TRANSITION : INSERT_TRANSITION;
    Descriptor d = Descriptor::AccessorConstant(name, pair, attributes);
    return Map::CopyInsertDescriptor(isolate, map, &d, flag);
  }
  
  Handle<Map> Map::CopyAddDescriptor(Isolate* isolate, Handle<Map> map,
                                     Descriptor* descriptor,
                                     TransitionFlag flag) {
    Handle<DescriptorArray> descriptors(map->instance_descriptors(isolate),
                                        isolate);
  
    // Share descriptors only if map owns descriptors and it not an initial map.
    if (flag == INSERT_TRANSITION && map->owns_descriptors() &&
        !map->GetBackPointer().IsUndefined(isolate) &&
        TransitionsAccessor::CanHaveMoreTransitions(isolate, map)) {
      return ShareDescriptor(isolate, map, descriptors, descriptor);
    }
  
    int nof = map->NumberOfOwnDescriptors();
    Handle<DescriptorArray> new_descriptors =
        DescriptorArray::CopyUpTo(isolate, descriptors, nof, 1);
    new_descriptors->Append(descriptor);
  
    return CopyReplaceDescriptors(isolate, map, new_descriptors, flag,
                                  descriptor->GetKey(), "CopyAddDescriptor",
                                  SIMPLE_PROPERTY_TRANSITION);
  }
  
  Handle<Map> Map::CopyInsertDescriptor(Isolate* isolate, Handle<Map> map,
                                        Descriptor* descriptor,
                                        TransitionFlag flag) {
    Handle<DescriptorArray> old_descriptors(map->instance_descriptors(isolate),
                                            isolate);
  
    // We replace the key if it is already present.
    InternalIndex index =
        old_descriptors->SearchWithCache(isolate, *descriptor->GetKey(), *map);
    if (index.is_found()) {
      return CopyReplaceDescriptor(isolate, map, old_descriptors, descriptor,
                                   index, flag);
    }
    return CopyAddDescriptor(isolate, map, descriptor, flag);
  }
  
  Handle<Map> Map::CopyReplaceDescriptor(Isolate* isolate, Handle<Map> map,
                                         Handle<DescriptorArray> descriptors,
                                         Descriptor* descriptor,
                                         InternalIndex insertion_index,
                                         TransitionFlag flag) {
    Handle<Name> key = descriptor->GetKey();
    DCHECK_EQ(*key, descriptors->GetKey(insertion_index));
    // This function does not support replacing property fields as
    // that would break property field counters.
    DCHECK_NE(PropertyLocation::kField, descriptor->GetDetails().location());
    DCHECK_NE(PropertyLocation::kField,
              descriptors->GetDetails(insertion_index).location());
  
    Handle<DescriptorArray> new_descriptors = DescriptorArray::CopyUpTo(
        isolate, descriptors, map->NumberOfOwnDescriptors());
  
    new_descriptors->Replace(insertion_index, descriptor);
  
    SimpleTransitionFlag simple_flag =
        (insertion_index.as_int() == descriptors->number_of_descriptors() - 1)
            ? SIMPLE_PROPERTY_TRANSITION
            : PROPERTY_TRANSITION;
    return CopyReplaceDescriptors(isolate, map, new_descriptors, flag, key,
                                  "CopyReplaceDescriptor", simple_flag);
  }
  
  int Map::Hash() {
    // For performance reasons we only hash the 2 most variable fields of a map:
    // prototype and bit_field2.
  
    HeapObject prototype = this->prototype();
    int prototype_hash;
  
    if (prototype.IsNull()) {
      // No identity hash for null, so just pick a random number.
      prototype_hash = 1;
    } else {
      JSReceiver receiver = JSReceiver::cast(prototype);
      Isolate* isolate = GetIsolateFromWritableObject(receiver);
      prototype_hash = receiver.GetOrCreateIdentityHash(isolate).value();
    }
  
    return prototype_hash ^ bit_field2();
  }
  
  namespace {
  
  bool CheckEquivalent(const Map first, const Map second) {
    return first.GetConstructor() == second.GetConstructor() &&
           first.prototype() == second.prototype() &&
           first.instance_type() == second.instance_type() &&
           first.bit_field() == second.bit_field() &&
           first.is_extensible() == second.is_extensible() &&
           first.new_target_is_base() == second.new_target_is_base();
  }
  
  }  // namespace
  
  bool Map::EquivalentToForTransition(const Map other,
                                      ConcurrencyMode cmode) const {
    CHECK_EQ(GetConstructor(), other.GetConstructor());
    CHECK_EQ(instance_type(), other.instance_type());
  
    if (bit_field() != other.bit_field()) return false;
    if (new_target_is_base() != other.new_target_is_base()) return false;
    if (prototype() != other.prototype()) return false;
    if (InstanceTypeChecker::IsJSFunction(instance_type())) {
      // JSFunctions require more checks to ensure that sloppy function is
      // not equivalent to strict function.
      int nof =
          std::min(NumberOfOwnDescriptors(), other.NumberOfOwnDescriptors());
      DescriptorArray this_descriptors = cmode == ConcurrencyMode::kConcurrent
                                             ? instance_descriptors(kAcquireLoad)
                                             : instance_descriptors();
      DescriptorArray that_descriptors =
          cmode == ConcurrencyMode::kConcurrent
              ? other.instance_descriptors(kAcquireLoad)
              : other.instance_descriptors();
      return this_descriptors.IsEqualUpTo(that_descriptors, nof);
    }
    return true;
  }
  
  bool Map::EquivalentToForElementsKindTransition(const Map other,
                                                  ConcurrencyMode cmode) const {
    if (!EquivalentToForTransition(other, cmode)) return false;
  #ifdef DEBUG
    // Ensure that we don't try to generate elements kind transitions from maps
    // with fields that may be generalized in-place. This must already be handled
    // during addition of a new field.
    DescriptorArray descriptors = cmode == ConcurrencyMode::kConcurrent
                                      ? instance_descriptors(kAcquireLoad)
                                      : instance_descriptors();
    for (InternalIndex i : IterateOwnDescriptors()) {
      PropertyDetails details = descriptors.GetDetails(i);
      if (details.location() == PropertyLocation::kField) {
        DCHECK(IsMostGeneralFieldType(details.representation(),
                                      descriptors.GetFieldType(i)));
      }
    }
  #endif
    return true;
  }
  
  bool Map::EquivalentToForNormalization(const Map other,
                                         ElementsKind elements_kind,
                                         PropertyNormalizationMode mode) const {
    int properties =
        mode == CLEAR_INOBJECT_PROPERTIES ? 0 : other.GetInObjectProperties();
    // Make sure the elements_kind bits are in bit_field2.
    DCHECK_EQ(this->elements_kind(),
              Map::Bits2::ElementsKindBits::decode(bit_field2()));
    int adjusted_other_bit_field2 =
        Map::Bits2::ElementsKindBits::update(other.bit_field2(), elements_kind);
    return CheckEquivalent(*this, other) &&
           bit_field2() == adjusted_other_bit_field2 &&
           GetInObjectProperties() == properties &&
           JSObject::GetEmbedderFieldCount(*this) ==
               JSObject::GetEmbedderFieldCount(other);
  }
  
  int Map::ComputeMinObjectSlack(Isolate* isolate) {
    // Has to be an initial map.
    DCHECK(GetBackPointer().IsUndefined(isolate));
  
    int slack = UnusedPropertyFields();
    TransitionsAccessor transitions(isolate, *this);
    TransitionsAccessor::TraverseCallback callback = [&](Map map) {
      slack = std::min(slack, map.UnusedPropertyFields());
    };
    transitions.TraverseTransitionTree(callback);
    return slack;
  }
  
  void Map::SetInstanceDescriptors(Isolate* isolate, DescriptorArray descriptors,
                                   int number_of_own_descriptors) {
    set_instance_descriptors(descriptors, kReleaseStore);
    SetNumberOfOwnDescriptors(number_of_own_descriptors);
  #ifndef V8_DISABLE_WRITE_BARRIERS
    WriteBarrier::Marking(descriptors, number_of_own_descriptors);
  #endif
  }
  
  // static
  Handle<PrototypeInfo> Map::GetOrCreatePrototypeInfo(Handle<JSObject> prototype,
                                                      Isolate* isolate) {
    Object maybe_proto_info = prototype->map().prototype_info();
    if (maybe_proto_info.IsPrototypeInfo()) {
      return handle(PrototypeInfo::cast(maybe_proto_info), isolate);
    }
    Handle<PrototypeInfo> proto_info = isolate->factory()->NewPrototypeInfo();
    prototype->map().set_prototype_info(*proto_info, kReleaseStore);
    return proto_info;
  }
  
  // static
  Handle<PrototypeInfo> Map::GetOrCreatePrototypeInfo(Handle<Map> prototype_map,
                                                      Isolate* isolate) {
    Object maybe_proto_info = prototype_map->prototype_info();
    if (maybe_proto_info.IsPrototypeInfo()) {
      return handle(PrototypeInfo::cast(maybe_proto_info), isolate);
    }
    Handle<PrototypeInfo> proto_info = isolate->factory()->NewPrototypeInfo();
    prototype_map->set_prototype_info(*proto_info, kReleaseStore);
    return proto_info;
  }
  
  // static
  void Map::SetShouldBeFastPrototypeMap(Handle<Map> map, bool value,
                                        Isolate* isolate) {
    if (value == false && !map->prototype_info().IsPrototypeInfo()) {
      // "False" is the implicit default value, so there's nothing to do.
      return;
    }
    GetOrCreatePrototypeInfo(map, isolate)->set_should_be_fast_map(value);
  }
  
  // static
  Handle<Object> Map::GetOrCreatePrototypeChainValidityCell(Handle<Map> map,
                                                            Isolate* isolate) {
    Handle<Object> maybe_prototype;
    if (map->IsJSGlobalObjectMap()) {
      DCHECK(map->is_prototype_map());
      // Global object is prototype of a global proxy and therefore we can
      // use its validity cell for guarding global object's prototype change.
      maybe_prototype = isolate->global_object();
    } else {
      maybe_prototype =
          handle(map->GetPrototypeChainRootMap(isolate).prototype(), isolate);
    }
    if (!maybe_prototype->IsJSObject()) {
      return handle(Smi::FromInt(Map::kPrototypeChainValid), isolate);
    }
    Handle<JSObject> prototype = Handle<JSObject>::cast(maybe_prototype);
    // Ensure the prototype is registered with its own prototypes so its cell
    // will be invalidated when necessary.
    JSObject::LazyRegisterPrototypeUser(handle(prototype->map(), isolate),
                                        isolate);
  
    Object maybe_cell = prototype->map().prototype_validity_cell();
    // Return existing cell if it's still valid.
    if (maybe_cell.IsCell()) {
      Handle<Cell> cell(Cell::cast(maybe_cell), isolate);
      if (cell->value() == Smi::FromInt(Map::kPrototypeChainValid)) {
        return cell;
      }
    }
    // Otherwise create a new cell.
    Handle<Cell> cell = isolate->factory()->NewCell(
        handle(Smi::FromInt(Map::kPrototypeChainValid), isolate));
    prototype->map().set_prototype_validity_cell(*cell);
    return cell;
  }
  
  // static
  bool Map::IsPrototypeChainInvalidated(Map map) {
    DCHECK(map.is_prototype_map());
    Object maybe_cell = map.prototype_validity_cell();
    if (maybe_cell.IsCell()) {
      Cell cell = Cell::cast(maybe_cell);
      return cell.value() != Smi::FromInt(Map::kPrototypeChainValid);
    }
    return true;
  }
  
  // static
  void Map::SetPrototype(Isolate* isolate, Handle<Map> map,
                         Handle<HeapObject> prototype,
                         bool enable_prototype_setup_mode) {
    RCS_SCOPE(isolate, RuntimeCallCounterId::kMap_SetPrototype);
  
    if (prototype->IsJSObject()) {
      Handle<JSObject> prototype_jsobj = Handle<JSObject>::cast(prototype);
      JSObject::OptimizeAsPrototype(prototype_jsobj, enable_prototype_setup_mode);
    } else {
      DCHECK(prototype->IsNull(isolate) || prototype->IsJSProxy());
    }
  
    WriteBarrierMode wb_mode =
        prototype->IsNull(isolate) ? SKIP_WRITE_BARRIER : UPDATE_WRITE_BARRIER;
    map->set_prototype(*prototype, wb_mode);
  }
  
  void Map::StartInobjectSlackTracking() {
    DCHECK(!IsInobjectSlackTrackingInProgress());
    if (UnusedPropertyFields() == 0) return;
    set_construction_counter(Map::kSlackTrackingCounterStart);
  }
  
  Handle<Map> Map::TransitionToPrototype(Isolate* isolate, Handle<Map> map,
                                         Handle<HeapObject> prototype) {
    Handle<Map> new_map =
        TransitionsAccessor::GetPrototypeTransition(isolate, map, prototype);
    if (new_map.is_null()) {
      new_map = Copy(isolate, map, "TransitionToPrototype");
      TransitionsAccessor::PutPrototypeTransition(isolate, map, prototype,
                                                  new_map);
      Map::SetPrototype(isolate, new_map, prototype);
    }
    return new_map;
  }
  
  Handle<NormalizedMapCache> NormalizedMapCache::New(Isolate* isolate) {
    Handle<WeakFixedArray> array(
        isolate->factory()->NewWeakFixedArray(kEntries, AllocationType::kOld));
    return Handle<NormalizedMapCache>::cast(array);
  }
  
  MaybeHandle<Map> NormalizedMapCache::Get(Handle<Map> fast_map,
                                           ElementsKind elements_kind,
                                           PropertyNormalizationMode mode) {
    DisallowGarbageCollection no_gc;
    MaybeObject value = WeakFixedArray::Get(GetIndex(fast_map));
    HeapObject heap_object;
    if (!value->GetHeapObjectIfWeak(&heap_object)) {
      return MaybeHandle<Map>();
    }
  
    Map normalized_map = Map::cast(heap_object);
    if (!normalized_map.EquivalentToForNormalization(*fast_map, elements_kind,
                                                     mode)) {
      return MaybeHandle<Map>();
    }
    return handle(normalized_map, GetIsolate());
  }
  
  void NormalizedMapCache::Set(Handle<Map> fast_map, Handle<Map> normalized_map) {
    DisallowGarbageCollection no_gc;
    DCHECK(normalized_map->is_dictionary_map());
    WeakFixedArray::Set(GetIndex(fast_map),
                        HeapObjectReference::Weak(*normalized_map));
  }
  
  }  // namespace internal
  }  // namespace v8
  