chromium/v8/src/objects/transitions.h

// 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.

#ifndef V8_OBJECTS_TRANSITIONS_H_
#define V8_OBJECTS_TRANSITIONS_H_

#include <optional>

#include "src/common/checks.h"
#include "src/execution/isolate.h"
#include "src/objects/descriptor-array.h"
#include "src/objects/elements-kind.h"
#include "src/objects/map.h"
#include "src/objects/maybe-object.h"
#include "src/objects/name.h"
#include "src/objects/objects.h"

// Has to be the last include (doesn't have include guards):
#include "src/objects/object-macros.h"

namespace v8::internal {

// Find all transitions with given name and calls the callback.
ForEachTransitionCallback;

// Descriptor for the contents of special side-step transition arrays.
// Side-step transitions are accessed through the TransitionsAccessor which
// enforces adherence to this format. The entries are either weak, Empty, or
// Unreachable.
struct SideStepTransition { … };

std::ostream& operator<<(std::ostream& os, SideStepTransition::Kind sidestep);

// TransitionsAccessor is a helper class to encapsulate access to the various
// ways a Map can store transitions to other maps in its respective field at
// Map::kTransitionsOrPrototypeInfo.
// It caches state information internally, which becomes stale when a Map's
// transitions storage changes or when a GC cycle clears dead transitions;
// so while a TransitionsAccessor instance can be used for several read-only
// operations in a row (provided no GC happens between them), it must be
// discarded and recreated after "Insert" and "UpdateHandler" operations.
//
// Internal details: a Map's field either holds an in-place weak reference to a
// transition target, or a StoreIC handler for a transitioning store (which in
// turn points to its target map), or a TransitionArray for several target maps
// and/or handlers as well as prototype and ElementsKind transitions.  Property
// details (and in case of inline target storage, the key) are retrieved from
// the target map's descriptor array.  Stored transitions are weak in the GC
// sense: both single transitions stored inline and TransitionArray fields are
// cleared when the map they refer to is not otherwise reachable.
class V8_EXPORT_PRIVATE TransitionsAccessor { … };

// TransitionArrays are fixed arrays used to hold map transitions for property,
// constant, and element changes.
// The TransitionArray class exposes a very low-level interface. Most clients
// should use TransitionsAccessors.
// TransitionArrays have the following format:
// [0] Tagged<Smi>(0) or WeakFixedArray of prototype transitions (strong ref)
// [1] Tagged<Smi>(0) or WeakFixedArray of side-step transitions (strong ref)
// [2] Number of transitions (can be zero after trimming)
// [3] First transition key (strong ref)
// [4] First transition target (weak ref)
// ...
// [4 + number of transitions * kTransitionSize]: start of slack
// TODO(olivf): The slots for prototype transitions and side-steps could be
// shared.
class TransitionArray : public WeakFixedArray { … };

}  // namespace v8::internal

#include "src/objects/object-macros-undef.h"

#endif  // V8_OBJECTS_TRANSITIONS_H_