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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
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