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diff --git a/js/public/GCPolicyAPI.h b/js/public/GCPolicyAPI.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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/. */
+
+// GC Policy Mechanism
+
+// A GCPolicy controls how the GC interacts with both direct pointers to GC
+// things (e.g. JSObject* or JSString*), tagged and/or optional pointers to GC
+// things (e.g.  Value or jsid), and C++ container types (e.g.
+// JSPropertyDescriptor or GCHashMap).
+//
+// The GCPolicy provides at a minimum:
+//
+//   static T initial()
+//       - Construct and return an empty T.
+//
+//   static void trace(JSTracer, T* tp, const char* name)
+//       - Trace the edge |*tp|, calling the edge |name|. Containers like
+//         GCHashMap and GCHashSet use this method to trace their children.
+//
+//   static bool needsSweep(T* tp)
+//       - Return true if |*tp| is about to be finalized. Otherwise, update the
+//         edge for moving GC, and return false. Containers like GCHashMap and
+//         GCHashSet use this method to decide when to remove an entry: if this
+//         function returns true on a key/value/member/etc, its entry is dropped
+//         from the container. Specializing this method is the standard way to
+//         get custom weak behavior from a container type.
+//
+// The default GCPolicy<T> assumes that T has a default constructor and |trace|
+// and |needsSweep| methods, and forwards to them. GCPolicy has appropriate
+// specializations for pointers to GC things and pointer-like types like
+// JS::Heap<T> and mozilla::UniquePtr<T>.
+//
+// There are some stock structs your specializations can inherit from.
+// IgnoreGCPolicy<T> does nothing. StructGCPolicy<T> forwards the methods to the
+// referent type T.
+
+#ifndef GCPolicyAPI_h
+#define GCPolicyAPI_h
+
+#include "mozilla/UniquePtr.h"
+
+#include "js/TraceKind.h"
+#include "js/TracingAPI.h"
+
+// Expand the given macro D for each public GC pointer.
+#define FOR_EACH_PUBLIC_GC_POINTER_TYPE(D) \
+    D(JS::Symbol*) \
+    D(JSAtom*) \
+    D(JSFunction*) \
+    D(JSObject*) \
+    D(JSScript*) \
+    D(JSString*)
+
+// Expand the given macro D for each public tagged GC pointer type.
+#define FOR_EACH_PUBLIC_TAGGED_GC_POINTER_TYPE(D) \
+    D(JS::Value) \
+    D(jsid)
+
+#define FOR_EACH_PUBLIC_AGGREGATE_GC_POINTER_TYPE(D) \
+    D(JSPropertyDescriptor)
+
+class JSAtom;
+class JSFunction;
+class JSObject;
+class JSScript;
+class JSString;
+namespace JS {
+class Symbol;
+}
+
+namespace JS {
+
+// Defines a policy for container types with non-GC, i.e. C storage. This
+// policy dispatches to the underlying struct for GC interactions.
+template <typename T>
+struct StructGCPolicy
+{
+    static T initial() {
+        return T();
+    }
+
+    static void trace(JSTracer* trc, T* tp, const char* name) {
+        tp->trace(trc);
+    }
+
+    static void sweep(T* tp) {
+        return tp->sweep();
+    }
+
+    static bool needsSweep(T* tp) {
+        return tp->needsSweep();
+    }
+};
+
+// The default GC policy attempts to defer to methods on the underlying type.
+// Most C++ structures that contain a default constructor, a trace function and
+// a sweep function will work out of the box with Rooted, Handle, GCVector,
+// and GCHash{Set,Map}.
+template <typename T> struct GCPolicy : public StructGCPolicy<T> {};
+
+// This policy ignores any GC interaction, e.g. for non-GC types.
+template <typename T>
+struct IgnoreGCPolicy {
+    static T initial() { return T(); }
+    static void trace(JSTracer* trc, T* t, const char* name) {}
+    static bool needsSweep(T* v) { return false; }
+};
+template <> struct GCPolicy<uint32_t> : public IgnoreGCPolicy<uint32_t> {};
+template <> struct GCPolicy<uint64_t> : public IgnoreGCPolicy<uint64_t> {};
+
+template <typename T>
+struct GCPointerPolicy
+{
+    static T initial() { return nullptr; }
+    static void trace(JSTracer* trc, T* vp, const char* name) {
+        if (*vp)
+            js::UnsafeTraceManuallyBarrieredEdge(trc, vp, name);
+    }
+    static bool needsSweep(T* vp) {
+        if (*vp)
+            return js::gc::IsAboutToBeFinalizedUnbarriered(vp);
+        return false;
+    }
+};
+template <> struct GCPolicy<JS::Symbol*> : public GCPointerPolicy<JS::Symbol*> {};
+template <> struct GCPolicy<JSAtom*> : public GCPointerPolicy<JSAtom*> {};
+template <> struct GCPolicy<JSFunction*> : public GCPointerPolicy<JSFunction*> {};
+template <> struct GCPolicy<JSObject*> : public GCPointerPolicy<JSObject*> {};
+template <> struct GCPolicy<JSScript*> : public GCPointerPolicy<JSScript*> {};
+template <> struct GCPolicy<JSString*> : public GCPointerPolicy<JSString*> {};
+
+template <typename T>
+struct GCPolicy<JS::Heap<T>>
+{
+    static void trace(JSTracer* trc, JS::Heap<T>* thingp, const char* name) {
+        TraceEdge(trc, thingp, name);
+    }
+    static bool needsSweep(JS::Heap<T>* thingp) {
+        return js::gc::EdgeNeedsSweep(thingp);
+    }
+};
+
+// GCPolicy<UniquePtr<T>> forwards the contained pointer to GCPolicy<T>.
+template <typename T, typename D>
+struct GCPolicy<mozilla::UniquePtr<T, D>>
+{
+    static mozilla::UniquePtr<T,D> initial() { return mozilla::UniquePtr<T,D>(); }
+    static void trace(JSTracer* trc, mozilla::UniquePtr<T,D>* tp, const char* name) {
+        if (tp->get())
+            GCPolicy<T>::trace(trc, tp->get(), name);
+    }
+    static bool needsSweep(mozilla::UniquePtr<T,D>* tp) {
+        if (tp->get())
+            return GCPolicy<T>::needsSweep(tp->get());
+        return false;
+    }
+};
+
+} // namespace JS
+
+#endif // GCPolicyAPI_h