1 files changed, 116 insertions, 0 deletions
diff --git a/js/src/vm/GetterSetter.h b/js/src/vm/GetterSetter.h
new file mode 100644
index 0000000000..d1e2fe4fc7
--- /dev/null
+++ b/js/src/vm/GetterSetter.h
@@ -0,0 +1,116 @@
+/* -*- 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 http://mozilla.org/MPL/2.0/. */
+
+#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;
+
+ public:
+  // 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;
+#endif
+
+ private:
+  GetterSetter(HandleObject getter, HandleObject setter);
+
+ public:
+  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> {
+ protected:
+  explicit Concrete(js::GetterSetter* ptr)
+      : TracerConcrete<js::GetterSetter>(ptr) {}
+
+ public:
+  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