map-updater.h | Explore in Territory

// Copyright 2017 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.

#ifndef V8_OBJECTS_MAP_UPDATER_H_
#define V8_OBJECTS_MAP_UPDATER_H_

#include <optional>

#include "src/common/globals.h"
#include "src/handles/handles.h"
#include "src/objects/elements-kind.h"
#include "src/objects/field-type.h"
#include "src/objects/map.h"
#include "src/objects/property-details.h"

namespace v8::internal {

// The |MapUpdater| class implements all sorts of map reconfigurations
// including changes of elements kind, property attributes, property kind,
// property location and field representations/type changes. It ensures that
// the reconfigured map and all the intermediate maps are properly integrated
// into the existing transition tree.
//
// To avoid high degrees over polymorphism, and to stabilize quickly, on every
// rewrite the new type is deduced by merging the current type with any
// potential new (partial) version of the type in the transition tree.
// To do this, on each rewrite:
// - Search the root of the transition tree using FindRootMap, remember
//   the integrity level (preventExtensions/seal/freeze) of transitions.
// - Find/create a |root_map| with the requested |new_elements_kind|.
// - Find |target_map|, the newest matching version of this map using the
//   "updated" |old_map|'s descriptor array (i.e. whose entry at |modify_index|
//   is considered to be of |new_kind| and having |new_attributes|) to walk
//   the transition tree. If there was an integrity level transition on the path
//   to the old map, use the descriptor array of the map preceding the first
//   integrity level transition (|integrity_source_map|), and try to replay
//   the integrity level transition afterwards.
// - Merge/generalize the "updated" descriptor array of the |old_map| and
//   descriptor array of the |target_map|.
// - Generalize the |modify_index| descriptor using |new_representation| and
//   |new_field_type|.
// - Walk the tree again starting from the root towards |target_map|. Stop at
//   |split_map|, the first map whose descriptor array does not match the merged
//   descriptor array.
// - If |target_map| == |split_map|, and there are no integrity level
//   transitions, |target_map| is in the expected state. Return it.
// - Otherwise, invalidate the outdated transition target from |target_map|, and
//   replace its transition tree with a new branch for the updated descriptors.
// - If the |old_map| had integrity level transition, create the new map for it.
class V8_EXPORT_PRIVATE MapUpdater { … };

}  // namespace v8::internal

#endif  // V8_OBJECTS_MAP_UPDATER_H_
chromium/v8/src/objects/map-updater.h
