src/objects/transitions.cc

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

#include "src/objects/transitions.h"

#include "src/base/small-vector.h"
#include "src/objects/objects-inl.h"
#include "src/objects/transitions-inl.h"
#include "src/utils/utils.h"

namespace v8 {
namespace internal {

// static
Tagged<Map> TransitionsAccessor::GetSimpleTransition(Isolate* isolate,
                                                     Handle<Map> map) {
  Tagged<MaybeObject> raw_transitions =
      map->raw_transitions(isolate, kAcquireLoad);
  switch (GetEncoding(isolate, raw_transitions)) {
    case kWeakRef:
      return Map::cast(raw_transitions.GetHeapObjectAssumeWeak());
    default:
      return Tagged<Map>();
  }
}

bool TransitionsAccessor::HasSimpleTransitionTo(Tagged<Map> map) {
  switch (encoding()) {
    case kWeakRef:
      return raw_transitions_.GetHeapObjectAssumeWeak() == map;
    case kPrototypeInfo:
    case kUninitialized:
    case kMigrationTarget:
    case kFullTransitionArray:
      return false;
  }
  UNREACHABLE();
}

// static
void TransitionsAccessor::Insert(Isolate* isolate, Handle<Map> map,
                                 Handle<Name> name, Handle<Map> target,
                                 TransitionKindFlag flag) {
  Encoding encoding = GetEncoding(isolate, map);
  DCHECK_NE(kPrototypeInfo, encoding);
  target->SetBackPointer(*map);

  // If the map doesn't have any transitions at all yet, install the new one.
  if (encoding == kUninitialized || encoding == kMigrationTarget) {
    if (flag == SIMPLE_PROPERTY_TRANSITION) {
      ReplaceTransitions(isolate, map, MakeWeak(*target));
      return;
    }
    // If the flag requires a full TransitionArray, allocate one.
    Handle<TransitionArray> result =
        isolate->factory()->NewTransitionArray(1, 0);
    result->Set(0, *name, MakeWeak(*target));
    ReplaceTransitions(isolate, map, result);
    DCHECK_EQ(kFullTransitionArray, GetEncoding(isolate, *result));
    return;
  }

  if (encoding == kWeakRef) {
    Tagged<Map> simple_transition = GetSimpleTransition(isolate, map);
    DCHECK(!simple_transition.is_null());

    if (flag == SIMPLE_PROPERTY_TRANSITION) {
      Tagged<Name> key = GetSimpleTransitionKey(simple_transition);
      PropertyDetails old_details =
          simple_transition->GetLastDescriptorDetails(isolate);
      PropertyDetails new_details = GetTargetDetails(*name, *target);
      if (key->Equals(*name) && old_details.kind() == new_details.kind() &&
          old_details.attributes() == new_details.attributes()) {
        ReplaceTransitions(isolate, map, MakeWeak(*target));
        return;
      }
    }

    // Otherwise allocate a full TransitionArray with slack for a new entry.
    Handle<TransitionArray> result =
        isolate->factory()->NewTransitionArray(1, 1);

    // Reload `simple_transition`. Allocations might have caused it to be
    // cleared.
    simple_transition = GetSimpleTransition(isolate, map);
    if (simple_transition.is_null()) {
      result->Set(0, *name, MakeWeak(*target));
      ReplaceTransitions(isolate, map, result);
      DCHECK_EQ(kFullTransitionArray, GetEncoding(isolate, *result));
      return;
    }

    // Insert the original transition in index 0.
    result->Set(0, GetSimpleTransitionKey(simple_transition),
                MakeWeak(simple_transition));

    // Search for the correct index to insert the new transition.
    int insertion_index;
    int index;
    if (flag == SPECIAL_TRANSITION) {
      index =
          result->SearchSpecial(Symbol::cast(*name), false, &insertion_index);
    } else {
      PropertyDetails details = GetTargetDetails(*name, *target);
      index = result->Search(details.kind(), *name, details.attributes(),
                             &insertion_index);
    }
    DCHECK_EQ(index, kNotFound);
    USE(index);

    result->SetNumberOfTransitions(2);
    if (insertion_index == 0) {
      // If the new transition will be inserted in index 0, move the original
      // transition to index 1.
      result->Set(1, GetSimpleTransitionKey(simple_transition),
                  MakeWeak(simple_transition));
    }
    result->SetKey(insertion_index, *name);
    result->SetRawTarget(insertion_index, MakeWeak(*target));

    SLOW_DCHECK(result->IsSortedNoDuplicates());
    ReplaceTransitions(isolate, map, result);
    DCHECK_EQ(kFullTransitionArray, GetEncoding(isolate, *result));
    return;
  }

  // At this point, we know that the map has a full TransitionArray.
  DCHECK_EQ(kFullTransitionArray, encoding);

  int number_of_transitions = 0;
  int new_nof = 0;
  int insertion_index = kNotFound;
  const bool is_special_transition = flag == SPECIAL_TRANSITION;
  DCHECK_EQ(is_special_transition,
            IsSpecialTransition(ReadOnlyRoots(isolate), *name));
  PropertyDetails details = is_special_transition
                                ? PropertyDetails::Empty()
                                : GetTargetDetails(*name, *target);

  {
    DisallowGarbageCollection no_gc;
    Tagged<TransitionArray> array = GetTransitionArray(isolate, map);
    number_of_transitions = array->number_of_transitions();

    int index =
        is_special_transition
            ? array->SearchSpecial(Symbol::cast(*name), false, &insertion_index)
            : array->Search(details.kind(), *name, details.attributes(),
                            &insertion_index);
    // If an existing entry was found, overwrite it and return.
    if (index != kNotFound) {
      base::SharedMutexGuard<base::kExclusive> shared_mutex_guard(
          isolate->full_transition_array_access());
      array->SetRawTarget(index, MakeWeak(*target));
      return;
    }

    new_nof = number_of_transitions + 1;
    CHECK_LE(new_nof, kMaxNumberOfTransitions);
    DCHECK_GE(insertion_index, 0);
    DCHECK_LE(insertion_index, number_of_transitions);

    // If there is enough capacity, insert new entry into the existing array.
    if (new_nof <= array->Capacity()) {
      base::SharedMutexGuard<base::kExclusive> shared_mutex_guard(
          isolate->full_transition_array_access());
      array->SetNumberOfTransitions(new_nof);
      for (int i = number_of_transitions; i > insertion_index; --i) {
        array->SetKey(i, array->GetKey(i - 1));
        array->SetRawTarget(i, array->GetRawTarget(i - 1));
      }
      array->SetKey(insertion_index, *name);
      array->SetRawTarget(insertion_index, MakeWeak(*target));
      SLOW_DCHECK(array->IsSortedNoDuplicates());
      return;
    }
  }

  // We're gonna need a bigger TransitionArray.
  Handle<TransitionArray> result = isolate->factory()->NewTransitionArray(
      new_nof,
      Map::SlackForArraySize(number_of_transitions, kMaxNumberOfTransitions));

  // The map's transition array may have shrunk during the allocation above as
  // it was weakly traversed, though it is guaranteed not to disappear. Trim the
  // result copy if needed, and recompute variables.
  DisallowGarbageCollection no_gc;
  Tagged<TransitionArray> array = GetTransitionArray(isolate, map);
  if (array->number_of_transitions() != number_of_transitions) {
    DCHECK_LT(array->number_of_transitions(), number_of_transitions);

    int index =
        is_special_transition
            ? array->SearchSpecial(Symbol::cast(*name), false, &insertion_index)
            : array->Search(details.kind(), *name, details.attributes(),
                            &insertion_index);
    CHECK_EQ(index, kNotFound);
    USE(index);
    DCHECK_GE(insertion_index, 0);
    DCHECK_LE(insertion_index, number_of_transitions);

    number_of_transitions = array->number_of_transitions();
    new_nof = number_of_transitions + 1;
    result->SetNumberOfTransitions(new_nof);
  }

  if (array->HasPrototypeTransitions()) {
    result->SetPrototypeTransitions(array->GetPrototypeTransitions());
  }

  DCHECK_NE(kNotFound, insertion_index);
  for (int i = 0; i < insertion_index; ++i) {
    result->Set(i, array->GetKey(i), array->GetRawTarget(i));
  }
  result->Set(insertion_index, *name, MakeWeak(*target));
  for (int i = insertion_index; i < number_of_transitions; ++i) {
    result->Set(i + 1, array->GetKey(i), array->GetRawTarget(i));
  }

  SLOW_DCHECK(result->IsSortedNoDuplicates());
  ReplaceTransitions(isolate, map, result);
}

Tagged<Map> TransitionsAccessor::SearchTransition(
    Tagged<Name> name, PropertyKind kind, PropertyAttributes attributes) {
  DCHECK(IsUniqueName(name));
  switch (encoding()) {
    case kPrototypeInfo:
    case kUninitialized:
    case kMigrationTarget:
      return Tagged<Map>();
    case kWeakRef: {
      Tagged<Map> map = Map::cast(raw_transitions_.GetHeapObjectAssumeWeak());
      if (!IsMatchingMap(map, name, kind, attributes)) return Tagged<Map>();
      return map;
    }
    case kFullTransitionArray: {
      base::SharedMutexGuardIf<base::kShared> scope(
          isolate_->full_transition_array_access(), concurrent_access_);
      return transitions()->SearchAndGetTarget(kind, name, attributes);
    }
  }
  UNREACHABLE();
}

Tagged<Map> TransitionsAccessor::SearchSpecial(Tagged<Symbol> name) {
  if (encoding() != kFullTransitionArray) return Tagged<Map>();
  base::SharedMutexGuardIf<base::kShared> scope(
      isolate_->full_transition_array_access(), concurrent_access_);
  int transition = transitions()->SearchSpecial(name, concurrent_access_);
  if (transition == kNotFound) return Tagged<Map>();
  return transitions()->GetTarget(transition);
}

// static
bool TransitionsAccessor::IsSpecialTransition(ReadOnlyRoots roots,
                                              Tagged<Name> name) {
  if (!IsSymbol(name)) return false;
  return name == roots.nonextensible_symbol() ||
         name == roots.sealed_symbol() || name == roots.frozen_symbol() ||
         name == roots.elements_transition_symbol() ||
         name == roots.strict_function_transition_symbol();
}

MaybeHandle<Map> TransitionsAccessor::FindTransitionToField(
    Handle<String> name) {
  DCHECK(IsInternalizedString(*name));
  DisallowGarbageCollection no_gc;
  Tagged<Map> target = SearchTransition(*name, PropertyKind::kData, NONE);
  if (target.is_null()) return MaybeHandle<Map>();
#ifdef DEBUG
  PropertyDetails details = target->GetLastDescriptorDetails(isolate_);
  DCHECK_EQ(NONE, details.attributes());
  DCHECK_EQ(PropertyKind::kData, details.kind());
  DCHECK_EQ(PropertyLocation::kField, details.location());
#endif
  return Handle<Map>(target, isolate_);
}

void TransitionsAccessor::ForEachTransitionTo(
    Tagged<Name> name, const ForEachTransitionCallback& callback,
    DisallowGarbageCollection* no_gc) {
  DCHECK(IsUniqueName(name));
  switch (encoding()) {
    case kPrototypeInfo:
    case kUninitialized:
    case kMigrationTarget:
      return;
    case kWeakRef: {
      Tagged<Map> target =
          Map::cast(raw_transitions_.GetHeapObjectAssumeWeak());
      InternalIndex descriptor = target->LastAdded();
      Tagged<DescriptorArray> descriptors =
          target->instance_descriptors(kRelaxedLoad);
      Tagged<Name> key = descriptors->GetKey(descriptor);
      if (key == name) {
        callback(target);
      }
      return;
    }
    case kFullTransitionArray: {
      base::SharedMutexGuardIf<base::kShared> scope(
          isolate_->full_transition_array_access(), concurrent_access_);
      return transitions()->ForEachTransitionTo(name, callback);
    }
  }
  UNREACHABLE();
}

// static
bool TransitionsAccessor::CanHaveMoreTransitions(Isolate* isolate,
                                                 Handle<Map> map) {
  if (map->is_dictionary_map()) return false;
  Tagged<MaybeObject> raw_transitions =
      map->raw_transitions(isolate, kAcquireLoad);
  if (GetEncoding(isolate, raw_transitions) == kFullTransitionArray) {
    return GetTransitionArray(isolate, raw_transitions)
               ->number_of_transitions() < kMaxNumberOfTransitions;
  }
  return true;
}

// static
bool TransitionsAccessor::IsMatchingMap(Tagged<Map> target, Tagged<Name> name,
                                        PropertyKind kind,
                                        PropertyAttributes attributes) {
  InternalIndex descriptor = target->LastAdded();
  Tagged<DescriptorArray> descriptors =
      target->instance_descriptors(kRelaxedLoad);
  Tagged<Name> key = descriptors->GetKey(descriptor);
  if (key != name) return false;
  return descriptors->GetDetails(descriptor)
      .HasKindAndAttributes(kind, attributes);
}

// static
bool TransitionArray::CompactPrototypeTransitionArray(
    Isolate* isolate, Tagged<WeakFixedArray> array) {
  const int header = kProtoTransitionHeaderSize;
  int number_of_transitions = NumberOfPrototypeTransitions(array);
  if (number_of_transitions == 0) {
    // Empty array cannot be compacted.
    return false;
  }
  int new_number_of_transitions = 0;
  for (int i = 0; i < number_of_transitions; i++) {
    Tagged<MaybeObject> target = array->get(header + i);
    DCHECK(target.IsCleared() ||
           (target.IsWeak() && IsMap(target.GetHeapObject())));
    if (!target.IsCleared()) {
      if (new_number_of_transitions != i) {
        array->set(header + new_number_of_transitions, target);
      }
      new_number_of_transitions++;
    }
  }
  // Fill slots that became free with undefined value.
  Tagged<MaybeObject> undefined = *isolate->factory()->undefined_value();
  for (int i = new_number_of_transitions; i < number_of_transitions; i++) {
    array->set(header + i, undefined);
  }
  if (number_of_transitions != new_number_of_transitions) {
    SetNumberOfPrototypeTransitions(array, new_number_of_transitions);
  }
  return new_number_of_transitions < number_of_transitions;
}

// static
Handle<WeakFixedArray> TransitionArray::GrowPrototypeTransitionArray(
    Handle<WeakFixedArray> array, int new_capacity, Isolate* isolate) {
  // Grow array by factor 2 up to MaxCachedPrototypeTransitions.
  int capacity = array->length() - kProtoTransitionHeaderSize;
  new_capacity = std::min({kMaxCachedPrototypeTransitions, new_capacity});
  DCHECK_GT(new_capacity, capacity);
  int grow_by = new_capacity - capacity;
  Handle<WeakFixedArray> new_array =
      isolate->factory()->CopyWeakFixedArrayAndGrow(array, grow_by);
  if (capacity < 0) {
    // There was no prototype transitions array before, so the size
    // couldn't be copied. Initialize it explicitly.
    SetNumberOfPrototypeTransitions(*new_array, 0);
  }
  return new_array;
}

// static
void TransitionsAccessor::PutPrototypeTransition(Isolate* isolate,
                                                 Handle<Map> map,
                                                 Handle<Object> prototype,
                                                 Handle<Map> target_map) {
  DCHECK(IsMap(HeapObject::cast(*prototype)->map()));
  // Don't cache prototype transition if this map is either shared, or a map of
  // a prototype.
  if (map->is_prototype_map()) return;
  if (map->is_dictionary_map() || !v8_flags.cache_prototype_transitions) return;

  const int header = TransitionArray::kProtoTransitionHeaderSize;

  Handle<WeakFixedArray> cache(GetPrototypeTransitions(isolate, map), isolate);
  int capacity = cache->length() - header;
  int transitions = TransitionArray::NumberOfPrototypeTransitions(*cache) + 1;

  // We're not using a MutexGuard for {full_transition_array_access}, because
  // we'll need to release it before growing the transition array (if needed),
  // in order to avoid deadlock if a background thread is waiting for the shared
  // mutex outside of a safepoint. And after growing the array, we'll need to
  // re-lock it.
  base::SharedMutex* transition_array_mutex =
      isolate->full_transition_array_access();

  transition_array_mutex->LockExclusive();
  if (transitions > capacity) {
    // Grow the array if compacting it doesn't free space.
    if (!TransitionArray::CompactPrototypeTransitionArray(isolate, *cache)) {
      transition_array_mutex->UnlockExclusive();
      if (capacity == TransitionArray::kMaxCachedPrototypeTransitions) return;

      // GrowPrototypeTransitionArray can allocate, so it shouldn't hold the
      // exclusive lock on {full_transition_array_access} mutex, since
      // background threads could be waiting for the shared lock (outside of a
      // safe point). This is not an issue, because GrowPrototypeTransitionArray
      // doesn't actually modify in place the array, but instead return a new
      // array.
      transition_array_mutex->LockShared();
      cache = TransitionArray::GrowPrototypeTransitionArray(
          cache, 2 * transitions, isolate);
      transition_array_mutex->UnlockShared();

      transition_array_mutex->LockExclusive();
      SetPrototypeTransitions(isolate, map, cache);
    }
  }

  // Reload number of transitions as they might have been compacted.
  int last = TransitionArray::NumberOfPrototypeTransitions(*cache);
  int entry = header + last;

  cache->set(entry, MakeWeak(*target_map));
  TransitionArray::SetNumberOfPrototypeTransitions(*cache, last + 1);

  transition_array_mutex->UnlockExclusive();
}

// static
Handle<Map> TransitionsAccessor::GetPrototypeTransition(
    Isolate* isolate, Handle<Map> map, Handle<Object> prototype_handle) {
  DisallowGarbageCollection no_gc;
  Tagged<Object> prototype = *prototype_handle;
  Tagged<WeakFixedArray> cache = GetPrototypeTransitions(isolate, map);
  int length = TransitionArray::NumberOfPrototypeTransitions(cache);
  for (int i = 0; i < length; i++) {
    Tagged<MaybeObject> target =
        cache->get(TransitionArray::kProtoTransitionHeaderSize + i);
    DCHECK(target.IsWeakOrCleared());
    Tagged<HeapObject> heap_object;
    if (target.GetHeapObjectIfWeak(&heap_object)) {
      Tagged<Map> target_map = Map::cast(heap_object);
      if (target_map->prototype() == prototype) {
        return handle(target_map, isolate);
      }
    }
  }
  return Handle<Map>();
}

// static
Tagged<WeakFixedArray> TransitionsAccessor::GetPrototypeTransitions(
    Isolate* isolate, Handle<Map> map) {
  Tagged<MaybeObject> raw_transitions =
      map->raw_transitions(isolate, kAcquireLoad);
  if (GetEncoding(isolate, raw_transitions) != kFullTransitionArray) {
    return ReadOnlyRoots(isolate).empty_weak_fixed_array();
  }
  Tagged<TransitionArray> transition_array =
      GetTransitionArray(isolate, raw_transitions);
  if (!transition_array->HasPrototypeTransitions()) {
    return ReadOnlyRoots(isolate).empty_weak_fixed_array();
  }
  return transition_array->GetPrototypeTransitions();
}

// static
void TransitionArray::SetNumberOfPrototypeTransitions(
    Tagged<WeakFixedArray> proto_transitions, int value) {
  DCHECK_NE(proto_transitions->length(), 0);
  proto_transitions->set(kProtoTransitionNumberOfEntriesOffset,
                         Smi::FromInt(value));
}

int TransitionsAccessor::NumberOfTransitions() {
  switch (encoding()) {
    case kPrototypeInfo:
    case kUninitialized:
    case kMigrationTarget:
      return 0;
    case kWeakRef:
      return 1;
    case kFullTransitionArray:
      return transitions()->number_of_transitions();
  }
  UNREACHABLE();
}

// static
void TransitionsAccessor::SetMigrationTarget(Isolate* isolate, Handle<Map> map,
                                             Tagged<Map> migration_target) {
  // We only cache the migration target for maps with empty transitions for GC's
  // sake.
  if (GetEncoding(isolate, map) != kUninitialized) return;
  DCHECK(map->is_deprecated());
  map->set_raw_transitions(migration_target, kReleaseStore);
}

Tagged<Map> TransitionsAccessor::GetMigrationTarget() {
  if (encoding() == kMigrationTarget) {
    return Tagged<Map>::cast(map_->raw_transitions(kAcquireLoad));
  }
  return Tagged<Map>();
}

// static
void TransitionsAccessor::ReplaceTransitions(
    Isolate* isolate, Handle<Map> map, Tagged<MaybeObject> new_transitions) {
#if DEBUG
  if (GetEncoding(isolate, map) == kFullTransitionArray) {
    CheckNewTransitionsAreConsistent(
        isolate, map, new_transitions.GetHeapObjectAssumeStrong());
    DCHECK_NE(GetTransitionArray(isolate, map),
              new_transitions.GetHeapObjectAssumeStrong());
  }
#endif
  map->set_raw_transitions(new_transitions, kReleaseStore);
  USE(isolate);
}

// static
void TransitionsAccessor::ReplaceTransitions(
    Isolate* isolate, Handle<Map> map,
    Handle<TransitionArray> new_transitions) {
  ReplaceTransitions(isolate, map, *new_transitions);
}

// static
void TransitionsAccessor::SetPrototypeTransitions(
    Isolate* isolate, Handle<Map> map,
    Handle<WeakFixedArray> proto_transitions) {
  EnsureHasFullTransitionArray(isolate, map);
  GetTransitionArray(isolate, map->raw_transitions(isolate, kAcquireLoad))
      ->SetPrototypeTransitions(*proto_transitions);
}

// static
void TransitionsAccessor::EnsureHasFullTransitionArray(Isolate* isolate,
                                                       Handle<Map> map) {
  Encoding encoding =
      GetEncoding(isolate, map->raw_transitions(isolate, kAcquireLoad));
  if (encoding == kFullTransitionArray) return;
  int nof =
      (encoding == kUninitialized || encoding == kMigrationTarget) ? 0 : 1;
  Handle<TransitionArray> result = isolate->factory()->NewTransitionArray(nof);
  // Reload encoding after possible GC.
  encoding = GetEncoding(isolate, map->raw_transitions(isolate, kAcquireLoad));
  if (nof == 1) {
    if (encoding == kUninitialized) {
      // If allocation caused GC and cleared the target, trim the new array.
      result->SetNumberOfTransitions(0);
    } else {
      // Otherwise populate the new array.
      Tagged<Map> target = GetSimpleTransition(isolate, map);
      Tagged<Name> key = GetSimpleTransitionKey(target);
      result->Set(0, key, MakeWeak(target));
    }
  }
  ReplaceTransitions(isolate, map, result);
}

void TransitionsAccessor::TraverseTransitionTreeInternal(
    const TraverseCallback& callback, DisallowGarbageCollection* no_gc) {
  // Mostly arbitrary but more than enough to run the test suite in static
  // memory.
  static constexpr int kStaticStackSize = 16;
  base::SmallVector<Tagged<Map>, kStaticStackSize> stack;
  stack.emplace_back(map_);

  // Pre-order iterative depth-first-search.
  while (!stack.empty()) {
    Tagged<Map> current_map = stack.back();
    stack.pop_back();

    callback(current_map);

    Tagged<MaybeObject> raw_transitions =
        current_map->raw_transitions(isolate_, kAcquireLoad);
    Encoding encoding = GetEncoding(isolate_, raw_transitions);

    switch (encoding) {
      case kPrototypeInfo:
      case kUninitialized:
      case kMigrationTarget:
        break;
      case kWeakRef: {
        stack.emplace_back(
            Map::cast(raw_transitions.GetHeapObjectAssumeWeak()));
        break;
      }
      case kFullTransitionArray: {
        Tagged<TransitionArray> transitions =
            TransitionArray::cast(raw_transitions.GetHeapObjectAssumeStrong());
        if (transitions->HasPrototypeTransitions()) {
          Tagged<WeakFixedArray> proto_trans =
              transitions->GetPrototypeTransitions();
          int length =
              TransitionArray::NumberOfPrototypeTransitions(proto_trans);
          for (int i = 0; i < length; ++i) {
            int index = TransitionArray::kProtoTransitionHeaderSize + i;
            Tagged<MaybeObject> target = proto_trans->get(index);
            Tagged<HeapObject> heap_object;
            if (target.GetHeapObjectIfWeak(&heap_object)) {
              stack.emplace_back(Map::cast(heap_object));
            } else {
              DCHECK(target.IsCleared());
            }
          }
        }
        for (int i = 0; i < transitions->number_of_transitions(); ++i) {
          stack.emplace_back(transitions->GetTarget(i));
        }
        break;
      }
    }
  }
}

#ifdef DEBUG
// static
void TransitionsAccessor::CheckNewTransitionsAreConsistent(
    Isolate* isolate, Handle<Map> map, Tagged<Object> transitions) {
  // This function only handles full transition arrays.
  Tagged<TransitionArray> old_transitions = GetTransitionArray(isolate, map);
  DCHECK_EQ(kFullTransitionArray, GetEncoding(isolate, old_transitions));
  Tagged<TransitionArray> new_transitions = TransitionArray::cast(transitions);
  for (int i = 0; i < old_transitions->number_of_transitions(); i++) {
    Tagged<Map> target = old_transitions->GetTarget(i);
    if (target->instance_descriptors(isolate) ==
        map->instance_descriptors(isolate)) {
      Tagged<Name> key = old_transitions->GetKey(i);
      int new_target_index;
      if (IsSpecialTransition(ReadOnlyRoots(isolate), key)) {
        new_target_index = new_transitions->SearchSpecial(Symbol::cast(key));
      } else {
        PropertyDetails details = GetTargetDetails(key, target);
        new_target_index =
            new_transitions->Search(details.kind(), key, details.attributes());
      }
      DCHECK_NE(TransitionArray::kNotFound, new_target_index);
      DCHECK_EQ(target, new_transitions->GetTarget(new_target_index));
    }
  }
}
#endif

// Private non-static helper functions (operating on full transition arrays).

int TransitionArray::SearchDetails(int transition, PropertyKind kind,
                                   PropertyAttributes attributes,
                                   int* out_insertion_index) {
  int nof_transitions = number_of_transitions();
  DCHECK(transition < nof_transitions);
  Tagged<Name> key = GetKey(transition);
  for (; transition < nof_transitions && GetKey(transition) == key;
       transition++) {
    Tagged<Map> target = GetTarget(transition);
    PropertyDetails target_details =
        TransitionsAccessor::GetTargetDetails(key, target);

    int cmp = CompareDetails(kind, attributes, target_details.kind(),
                             target_details.attributes());
    if (cmp == 0) {
      return transition;
    } else if (cmp < 0) {
      break;
    }
  }
  if (out_insertion_index != nullptr) *out_insertion_index = transition;
  return kNotFound;
}

Tagged<Map> TransitionArray::SearchDetailsAndGetTarget(
    int transition, PropertyKind kind, PropertyAttributes attributes) {
  int nof_transitions = number_of_transitions();
  DCHECK(transition < nof_transitions);
  Tagged<Name> key = GetKey(transition);
  for (; transition < nof_transitions && GetKey(transition) == key;
       transition++) {
    Tagged<Map> target = GetTarget(transition);
    PropertyDetails target_details =
        TransitionsAccessor::GetTargetDetails(key, target);

    int cmp = CompareDetails(kind, attributes, target_details.kind(),
                             target_details.attributes());
    if (cmp == 0) {
      return target;
    } else if (cmp < 0) {
      break;
    }
  }
  return Tagged<Map>();
}

int TransitionArray::Search(PropertyKind kind, Tagged<Name> name,
                            PropertyAttributes attributes,
                            int* out_insertion_index) {
  int transition = SearchName(name, false, out_insertion_index);
  if (transition == kNotFound) return kNotFound;
  return SearchDetails(transition, kind, attributes, out_insertion_index);
}

Tagged<Map> TransitionArray::SearchAndGetTarget(PropertyKind kind,
                                                Tagged<Name> name,
                                                PropertyAttributes attributes) {
  int transition = SearchName(name);
  if (transition == kNotFound) {
    return Tagged<Map>();
  }
  return SearchDetailsAndGetTarget(transition, kind, attributes);
}

void TransitionArray::ForEachTransitionTo(
    Tagged<Name> name, const ForEachTransitionCallback& callback) {
  int transition = SearchName(name);
  if (transition == kNotFound) return;

  int nof_transitions = number_of_transitions();
  DCHECK(transition < nof_transitions);
  Tagged<Name> key = GetKey(transition);
  for (; transition < nof_transitions && GetKey(transition) == key;
       transition++) {
    Tagged<Map> target = GetTarget(transition);
    callback(target);
  }
}

void TransitionArray::Sort() {
  DisallowGarbageCollection no_gc;
  // In-place insertion sort.
  int length = number_of_transitions();
  ReadOnlyRoots roots = GetReadOnlyRoots();
  for (int i = 1; i < length; i++) {
    Tagged<Name> key = GetKey(i);
    Tagged<MaybeObject> target = GetRawTarget(i);
    PropertyKind kind = PropertyKind::kData;
    PropertyAttributes attributes = NONE;
    if (!TransitionsAccessor::IsSpecialTransition(roots, key)) {
      Tagged<Map> target_map = TransitionsAccessor::GetTargetFromRaw(target);
      PropertyDetails details =
          TransitionsAccessor::GetTargetDetails(key, target_map);
      kind = details.kind();
      attributes = details.attributes();
    }
    int j;
    for (j = i - 1; j >= 0; j--) {
      Tagged<Name> temp_key = GetKey(j);
      Tagged<MaybeObject> temp_target = GetRawTarget(j);
      PropertyKind temp_kind = PropertyKind::kData;
      PropertyAttributes temp_attributes = NONE;
      if (!TransitionsAccessor::IsSpecialTransition(roots, temp_key)) {
        Tagged<Map> temp_target_map =
            TransitionsAccessor::GetTargetFromRaw(temp_target);
        PropertyDetails details =
            TransitionsAccessor::GetTargetDetails(temp_key, temp_target_map);
        temp_kind = details.kind();
        temp_attributes = details.attributes();
      }
      int cmp =
          CompareKeys(temp_key, temp_key->hash(), temp_kind, temp_attributes,
                      key, key->hash(), kind, attributes);
      if (cmp > 0) {
        SetKey(j + 1, temp_key);
        SetRawTarget(j + 1, temp_target);
      } else {
        break;
      }
    }
    SetKey(j + 1, key);
    SetRawTarget(j + 1, target);
  }
  DCHECK(IsSortedNoDuplicates());
}

bool TransitionsAccessor::HasIntegrityLevelTransitionTo(
    Tagged<Map> to, Tagged<Symbol>* out_symbol,
    PropertyAttributes* out_integrity_level) {
  ReadOnlyRoots roots(isolate_);
  if (SearchSpecial(roots.frozen_symbol()) == to) {
    if (out_integrity_level) *out_integrity_level = FROZEN;
    if (out_symbol) *out_symbol = roots.frozen_symbol();
  } else if (SearchSpecial(roots.sealed_symbol()) == to) {
    if (out_integrity_level) *out_integrity_level = SEALED;
    if (out_symbol) *out_symbol = roots.sealed_symbol();
  } else if (SearchSpecial(roots.nonextensible_symbol()) == to) {
    if (out_integrity_level) *out_integrity_level = NONE;
    if (out_symbol) *out_symbol = roots.nonextensible_symbol();
  } else {
    return false;
  }
  return true;
}

}  // namespace internal
}  // namespace v8