/src/mozilla-central/js/src/gc/WeakMap.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/. */

#ifndef gc_WeakMap_h
#define gc_WeakMap_h

#include "mozilla/LinkedList.h"

#include "gc/Barrier.h"
#include "gc/DeletePolicy.h"
#include "gc/Zone.h"
#include "js/HashTable.h"

namespace JS {
class Zone;
} // namespace JS

namespace js {

class GCMarker;
class WeakMapBase;
struct WeakMapTracer;

namespace gc {
struct WeakMarkable;
} // namespace gc

// A subclass template of js::HashMap whose keys and values may be garbage-collected. When
// a key is collected, the table entry disappears, dropping its reference to the value.
//
// More precisely:
//
//     A WeakMap entry is live if and only if both the WeakMap and the entry's key
//     are live. An entry holds a strong reference to its value.
//
// You must call this table's 'trace' method when its owning object is reached
// by the garbage collection tracer. Once a table is known to be live, the
// implementation takes care of the special weak marking (ie, marking through
// the implicit edges stored in the map) and of removing (sweeping) table
// entries when collection is complete.

typedef HashSet<WeakMapBase*, DefaultHasher<WeakMapBase*>, SystemAllocPolicy> WeakMapSet;

// Common base class for all WeakMap specializations, used for calling
// subclasses' GC-related methods.
class WeakMapBase : public mozilla::LinkedListElement<WeakMapBase>
{
    friend class js::GCMarker;

  public:
    WeakMapBase(JSObject* memOf, JS::Zone* zone);
    virtual ~WeakMapBase();

    JS::Zone* zone() const { return zone_; }

    // Garbage collector entry points.

    // Unmark all weak maps in a zone.
    static void unmarkZone(JS::Zone* zone);

    // Mark all the weakmaps in a zone.
    static void traceZone(JS::Zone* zone, JSTracer* tracer);

    // Check all weak maps in a zone that have been marked as live in this garbage
    // collection, and mark the values of all entries that have become strong references
    // to them. Return true if we marked any new values, indicating that we need to make
    // another pass. In other words, mark my marked maps' marked members' mid-collection.
    static bool markZoneIteratively(JS::Zone* zone, GCMarker* marker);

    // Add zone edges for weakmaps with key delegates in a different zone.
    static bool findInterZoneEdges(JS::Zone* zone);

    // Sweep the weak maps in a zone, removing dead weak maps and removing
    // entries of live weak maps whose keys are dead.
    static void sweepZone(JS::Zone* zone);

    // Trace all delayed weak map bindings. Used by the cycle collector.
    static void traceAllMappings(WeakMapTracer* tracer);

    // Save information about which weak maps are marked for a zone.
    static bool saveZoneMarkedWeakMaps(JS::Zone* zone, WeakMapSet& markedWeakMaps);

    // Restore information about which weak maps are marked for many zones.
    static void restoreMarkedWeakMaps(WeakMapSet& markedWeakMaps);

  protected:
    // Instance member functions called by the above. Instantiations of WeakMap override
    // these with definitions appropriate for their Key and Value types.
    virtual void trace(JSTracer* tracer) = 0;
    virtual bool findZoneEdges() = 0;
    virtual void sweep() = 0;
    virtual void traceMappings(WeakMapTracer* tracer) = 0;
    virtual void clearAndCompact() = 0;

    // Any weakmap key types that want to participate in the non-iterative
    // ephemeron marking must override this method.
    virtual void markEntry(GCMarker* marker, gc::Cell* markedCell, JS::GCCellPtr l) = 0;

    virtual bool markIteratively(GCMarker* marker) = 0;

  protected:
    // Object that this weak map is part of, if any.
    GCPtrObject memberOf;

    // Zone containing this weak map.
    JS::Zone* zone_;

    // Whether this object has been traced during garbage collection.
    bool marked;
};

template <class Key, class Value>
class WeakMap : public HashMap<Key, Value, MovableCellHasher<Key>, ZoneAllocPolicy>,
                public WeakMapBase
{
  public:
    typedef HashMap<Key, Value, MovableCellHasher<Key>, ZoneAllocPolicy> Base;
    typedef typename Base::Enum Enum;
    typedef typename Base::Lookup Lookup;
    typedef typename Base::Entry Entry;
    typedef typename Base::Range Range;
    typedef typename Base::Ptr Ptr;
    typedef typename Base::AddPtr AddPtr;

    explicit WeakMap(JSContext* cx, JSObject* memOf = nullptr);

    // Overwritten to add a read barrier to prevent an incorrectly gray value
    // from escaping the weak map. See the UnmarkGrayTracer::onChild comment in
    // gc/Marking.cpp.
    Ptr lookup(const Lookup& l) const {
        Ptr p = Base::lookup(l);
        if (p) {
            exposeGCThingToActiveJS(p->value());
        }
        return p;
    }

    AddPtr lookupForAdd(const Lookup& l) {
        AddPtr p = Base::lookupForAdd(l);
        if (p) {
            exposeGCThingToActiveJS(p->value());
        }
        return p;
    }

    // Resolve ambiguity with LinkedListElement<>::remove.
    using Base::remove;

    void markEntry(GCMarker* marker, gc::Cell* markedCell, JS::GCCellPtr origKey) override;

    void trace(JSTracer* trc) override;

  protected:
    static void addWeakEntry(GCMarker* marker, JS::GCCellPtr key,
                             const gc::WeakMarkable& markable);

    bool markIteratively(GCMarker* marker) override;

    JSObject* getDelegate(JSObject* key) const;
    JSObject* getDelegate(JSScript* script) const;
    JSObject* getDelegate(LazyScript* script) const;

  private:
    void exposeGCThingToActiveJS(const JS::Value& v) const { JS::ExposeValueToActiveJS(v); }
    void exposeGCThingToActiveJS(JSObject* obj) const { JS::ExposeObjectToActiveJS(obj); }

    bool keyNeedsMark(JSObject* key) const;
    bool keyNeedsMark(JSScript* script) const;
    bool keyNeedsMark(LazyScript* script) const;

    bool findZoneEdges() override {
        // This is overridden by ObjectValueMap.
        return true;
    }

    void sweep() override;

    void clearAndCompact() override {
        Base::clear();
        Base::compact();
    }

    /* memberOf can be nullptr, which means that the map is not part of a JSObject. */
    void traceMappings(WeakMapTracer* tracer) override;

  protected:
#if DEBUG
    void assertEntriesNotAboutToBeFinalized();
#endif
};


class ObjectValueMap : public WeakMap<HeapPtr<JSObject*>, HeapPtr<Value>>
{
  public:
    ObjectValueMap(JSContext* cx, JSObject* obj)
      : WeakMap(cx, obj)
    {}

    bool findZoneEdges() override;
};


// Generic weak map for mapping objects to other objects.
class ObjectWeakMap
{
    ObjectValueMap map;

  public:
    explicit ObjectWeakMap(JSContext* cx);

    JS::Zone* zone() const { return map.zone(); }

    JSObject* lookup(const JSObject* obj);
    bool add(JSContext* cx, JSObject* obj, JSObject* target);
    void clear();

    void trace(JSTracer* trc);
    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);
    size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
        return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
    }

#ifdef JSGC_HASH_TABLE_CHECKS
    void checkAfterMovingGC();
#endif
};

} /* namespace js */

namespace JS {

template <>
struct DeletePolicy<js::ObjectValueMap> : public js::GCManagedDeletePolicy<js::ObjectValueMap>
{};

} /* namespace JS */

#endif /* gc_WeakMap_h */

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Created: 2018-09-25 14:53