js/src/vm/GetterSetter.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef vm_GetterSetter_h
   8 #define vm_GetterSetter_h
   9
  10 #include "gc/Barrier.h"  // js::GCPtr<JSObject*>
  11 #include "gc/Cell.h"     // js::gc::TenuredCellWithGCPointer
  12
  13 #include "js/TypeDecls.h"  // JS::HandleObject
  14 #include "js/UbiNode.h"    // JS::ubi::TracerConcrete
  15
  16 namespace js {
  17
  18 // [SMDOC] Getter/Setter Properties
  19 //
  20 // Getter/setter properties are implemented similar to plain data properties:
  21 // the shape contains the property's key, attributes, and slot number, but the
  22 // getter/setter objects are stored separately as part of the object.
  23 //
  24 // To simplify the NativeObject and Shape code, a single slot is allocated for
  25 // each getter/setter property (again similar to data properties). This slot
  26 // contains a PrivateGCThingValue pointing to a js::GetterSetter instance.
  27 //
  28 // js::GetterSetter
  29 // ================
  30 // js::GetterSetter is an immutable type that stores the getter/setter objects.
  31 // Because accessor properties can be defined with only a getter or only a
  32 // setter, a GetterSetter's objects can be nullptr.
  33 //
  34 // JIT/IC Guards
  35 // =============
  36 // An object's shape implies a certain property is an accessor, but it does not
  37 // imply the identity of the getter/setter objects. This means IC code needs to
  38 // guard on the slot value (the GetterSetter*) when optimizing a call to a
  39 // particular getter/setter function.
  40 //
  41 // See EmitGuardGetterSetterSlot in jit/CacheIR.cpp.
  42 //
  43 // HadGetterSetterChange Optimization
  44 // ==================================
  45 // Some getters and setters defined on the prototype chain are very hot, for
  46 // example the 'length' getter for typed arrays. To avoid the GetterSetter guard
  47 // in the common case, when attaching a stub for a known 'holder' object, we
  48 // use the HadGetterSetterChange object flag.
  49 //
  50 // When this flag is not set, the object is guaranteed to get a different shape
  51 // when an accessor property is either deleted or mutated, because when that
  52 // happens the HadGetterSetterChange will be set which triggers a shape change.
  53 //
  54 // This means CacheIR does not have to guard on the GetterSetter slot for
  55 // accessors on the prototype chain until the first time an accessor property is
  56 // mutated or deleted.
  57 class GetterSetter : public gc::TenuredCellWithGCPointer<JSObject> {
  58   friend class gc::CellAllocator;
  59
  60  public:
  61   // Getter object, stored in the cell header.
  62   JSObject* getter() const { return headerPtr(); }
  63
  64   GCPtr<JSObject*> setter_;
  65
  66 #ifndef JS_64BIT
  67   // Ensure size >= MinCellSize on 32-bit platforms.
  68   uint64_t padding_ = 0;
  69 #endif
  70
  71  private:
  72   GetterSetter(HandleObject getter, HandleObject setter);
  73
  74  public:
  75   static GetterSetter* create(JSContext* cx, HandleObject getter,
  76                               HandleObject setter);
  77
  78   JSObject* setter() const { return setter_; }
  79
  80   static const JS::TraceKind TraceKind = JS::TraceKind::GetterSetter;
  81
  82   void traceChildren(JSTracer* trc);
  83
  84   void finalize(JS::GCContext* gcx) {
  85     // Nothing to do.
  86   }
  87 };
  88
  89 }  // namespace js
  90
  91 // JS::ubi::Nodes can point to GetterSetters; they're js::gc::Cell instances
  92 // with no associated compartment.
  93 namespace JS {
  94 namespace ubi {
  95
  96 template <>
  97 class Concrete<js::GetterSetter> : TracerConcrete<js::GetterSetter> {
  98  protected:
  99   explicit Concrete(js::GetterSetter* ptr)
 100       : TracerConcrete<js::GetterSetter>(ptr) {}
 101
 102  public:
 103   static void construct(void* storage, js::GetterSetter* ptr) {
 104     new (storage) Concrete(ptr);
 105   }
 106
 107   Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
 108
 109   const char16_t* typeName() const override { return concreteTypeName; }
 110   static const char16_t concreteTypeName[];
 111 };
 112
 113 }  // namespace ubi
 114 }  // namespace JS
 115
 116 #endif  // vm_GetterSetter_h