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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef vm_GetterSetter_h
#define vm_GetterSetter_h
#include "gc/Barrier.h" // js::GCPtr<JSObject*>
#include "gc/Cell.h" // js::gc::TenuredCellWithGCPointer
#include "js/TypeDecls.h" // JS::HandleObject
#include "js/UbiNode.h" // JS::ubi::TracerConcrete
namespace js {
// [SMDOC] Getter/Setter Properties
// Getter/setter properties are implemented similar to plain data properties:
// the shape contains the property's key, attributes, and slot number, but the
// getter/setter objects are stored separately as part of the object.
// To simplify the NativeObject and Shape code, a single slot is allocated for
// each getter/setter property (again similar to data properties). This slot
// contains a PrivateGCThingValue pointing to a js::GetterSetter instance.
// js::GetterSetter
// ================
// js::GetterSetter is an immutable type that stores the getter/setter objects.
// Because accessor properties can be defined with only a getter or only a
// setter, a GetterSetter's objects can be nullptr.
// JIT/IC Guards
// =============
// An object's shape implies a certain property is an accessor, but it does not
// imply the identity of the getter/setter objects. This means IC code needs to
// guard on the slot value (the GetterSetter*) when optimizing a call to a
// particular getter/setter function.
// See EmitGuardGetterSetterSlot in jit/CacheIR.cpp.
// HadGetterSetterChange Optimization
// ==================================
// Some getters and setters defined on the prototype chain are very hot, for
// example the 'length' getter for typed arrays. To avoid the GetterSetter guard
// in the common case, when attaching a stub for a known 'holder' object, we
// use the HadGetterSetterChange object flag.
// When this flag is not set, the object is guaranteed to get a different shape
// when an accessor property is either deleted or mutated, because when that
// happens the HadGetterSetterChange will be set which triggers a shape change.
// This means CacheIR does not have to guard on the GetterSetter slot for
// accessors on the prototype chain until the first time an accessor property is
// mutated or deleted.
class GetterSetter : public gc::TenuredCellWithGCPointer<JSObject> {
friend class gc::CellAllocator;
// Getter object, stored in the cell header.
JSObject* getter() const { return headerPtr(); }
GCPtr<JSObject*> setter_;
#ifndef JS_64BIT
// Ensure size >= MinCellSize on 32-bit platforms.
uint64_t padding_ = 0;
GetterSetter(HandleObject getter, HandleObject setter);
static GetterSetter* create(JSContext* cx, HandleObject getter,
HandleObject setter);
JSObject* setter() const { return setter_; }
static const JS::TraceKind TraceKind = JS::TraceKind::GetterSetter;
void traceChildren(JSTracer* trc);
void finalize(JS::GCContext* gcx) {
// Nothing to do.
} // namespace js
// JS::ubi::Nodes can point to GetterSetters; they're js::gc::Cell instances
// with no associated compartment.
namespace JS {
namespace ubi {
template <>
class Concrete<js::GetterSetter> : TracerConcrete<js::GetterSetter> {
explicit Concrete(js::GetterSetter* ptr)
: TracerConcrete<js::GetterSetter>(ptr) {}
static void construct(void* storage, js::GetterSetter* ptr) {
new (storage) Concrete(ptr);
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
} // namespace ubi
} // namespace JS
#endif // vm_GetterSetter_h