root/Source/platform/heap/Visitor.h

INCLUDED FROM

/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
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 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef Visitor_h
#define Visitor_h

#include "platform/PlatformExport.h"
#include "platform/heap/ThreadState.h"
#include "wtf/Assertions.h"
#include "wtf/Deque.h"
#include "wtf/Forward.h"
#include "wtf/HashMap.h"
#include "wtf/HashSet.h"
#include "wtf/HashTraits.h"
#include "wtf/ListHashSet.h"
#include "wtf/OwnPtr.h"
#include "wtf/RefPtr.h"
#include "wtf/TypeTraits.h"

#ifndef NDEBUG
#define DEBUG_ONLY(x) x
#else
#define DEBUG_ONLY(x)
#endif

namespace WebCore {

class FinalizedHeapObjectHeader;
template<typename T> class GarbageCollectedFinalized;
class HeapObjectHeader;
template<typename T> class Member;
template<typename T> class WeakMember;
class Visitor;

template<bool needsTracing, bool isWeak, bool markWeakMembersStrongly, typename T, typename Traits> struct CollectionBackingTraceTrait;

// The TraceMethodDelegate is used to convert a trace method for type T to a TraceCallback.
// This allows us to pass a type's trace method as a parameter to the PersistentNode
// constructor. The PersistentNode constructor needs the specific trace method due an issue
// with the Windows compiler which instantiates even unused variables. This causes problems
// in header files where we have only forward declarations of classes.
template<typename T, void (T::*method)(Visitor*)>
struct TraceMethodDelegate {
    static void trampoline(Visitor* visitor, void* self) { (reinterpret_cast<T*>(self)->*method)(visitor); }
};

// GCInfo contains meta-data associated with objects allocated in the
// Blink heap. This meta-data consists of a function pointer used to
// trace the pointers in the object during garbage collection, an
// indication of whether or not the object needs a finalization
// callback, and a function pointer used to finalize the object when
// the garbage collector determines that the object is no longer
// reachable. There is a GCInfo struct for each class that directly
// inherits from GarbageCollected or GarbageCollectedFinalized.
struct GCInfo {
    bool hasFinalizer() const { return m_nonTrivialFinalizer; }
    TraceCallback m_trace;
    FinalizationCallback m_finalize;
    bool m_nonTrivialFinalizer;
};

// The FinalizerTraitImpl specifies how to finalize objects. Object
// that inherit from GarbageCollectedFinalized are finalized by
// calling their 'finalize' method which by default will call the
// destructor on the object.
template<typename T, bool isGarbageCollectedFinalized>
struct FinalizerTraitImpl;

template<typename T>
struct FinalizerTraitImpl<T, true> {
    static void finalize(void* obj) { static_cast<T*>(obj)->finalizeGarbageCollectedObject(); };
};

template<typename T>
struct FinalizerTraitImpl<T, false> {
    static void finalize(void* obj) { };
};

// The FinalizerTrait is used to determine if a type requires
// finalization and what finalization means.
//
// By default classes that inherit from GarbageCollectedFinalized need
// finalization and finalization means calling the 'finalize' method
// of the object. The FinalizerTrait can be specialized if the default
// behavior is not desired.
template<typename T>
struct FinalizerTrait {
    static const bool nonTrivialFinalizer = WTF::IsSubclassOfTemplate<T, GarbageCollectedFinalized>::value;
    static void finalize(void* obj) { FinalizerTraitImpl<T, nonTrivialFinalizer>::finalize(obj); }
};

// Trait to get the GCInfo structure for types that have their
// instances allocated in the Blink garbage-collected heap.
template<typename T> struct GCInfoTrait;

template<typename T> class GarbageCollected;
class GarbageCollectedMixin;
template<typename T, bool = WTF::IsSubclassOfTemplate<T, GarbageCollected>::value> class NeedsAdjustAndMark;

template<typename T>
class NeedsAdjustAndMark<T, true> {
public:
    static const bool value = false;
};

template<typename T>
class NeedsAdjustAndMark<T, false> {
public:
    static const bool value = WTF::IsSubclass<T, GarbageCollectedMixin>::value;
};

template<typename T, bool = NeedsAdjustAndMark<T>::value> class DefaultTraceTrait;

// The TraceTrait is used to specify how to mark an object pointer and
// how to trace all of the pointers in the object.
//
// By default, the 'trace' method implemented on an object itself is
// used to trace the pointers to other heap objects inside the object.
//
// However, the TraceTrait can be specialized to use a different
// implementation. A common case where a TraceTrait specialization is
// needed is when multiple inheritance leads to pointers that are not
// to the start of the object in the Blink garbage-collected heap. In
// that case the pointer has to be adjusted before marking.
template<typename T>
class TraceTrait {
public:
    // Default implementation of TraceTrait<T>::trace just statically
    // dispatches to the trace method of the class T.
    static void trace(Visitor* visitor, void* self)
    {
        static_cast<T*>(self)->trace(visitor);
    }

    static void mark(Visitor* visitor, const T* t)
    {
        DefaultTraceTrait<T>::mark(visitor, t);
    }

#ifndef NDEBUG
    static void checkGCInfo(Visitor* visitor, const T* t)
    {
        DefaultTraceTrait<T>::checkGCInfo(visitor, t);
    }
#endif
};

template<typename T> class TraceTrait<const T> : public TraceTrait<T> { };

template<typename Collection>
struct OffHeapCollectionTraceTrait;

template<typename T>
struct ObjectAliveTrait {
    static bool isAlive(Visitor*, T*);
};

// Visitor is used to traverse the Blink object graph. Used for the
// marking phase of the mark-sweep garbage collector.
//
// Pointers are marked and pushed on the marking stack by calling the
// |mark| method with the pointer as an argument.
//
// Pointers within objects are traced by calling the |trace| methods
// with the object as an argument. Tracing objects will mark all of the
// contained pointers and push them on the marking stack.
class PLATFORM_EXPORT Visitor {
public:
    virtual ~Visitor() { }

    // One-argument templated mark method. This uses the static type of
    // the argument to get the TraceTrait. By default, the mark method
    // of the TraceTrait just calls the virtual two-argument mark method on this
    // visitor, where the second argument is the static trace method of the trait.
    template<typename T>
    void mark(T* t)
    {
        if (!t)
            return;
#ifndef NDEBUG
        TraceTrait<T>::checkGCInfo(this, t);
#endif
        TraceTrait<T>::mark(this, t);
    }

    // Member version of the one-argument templated trace method.
    template<typename T>
    void trace(const Member<T>& t)
    {
        mark(t.get());
    }

    // Fallback method used only when we need to trace raw pointers of T.
    // This is the case when a member is a union where we do not support members.
    template<typename T>
    void trace(T* t)
    {
        mark(t);
    }

    // WeakMember version of the templated trace method. It doesn't keep
    // the traced thing alive, but will write null to the WeakMember later
    // if the pointed-to object is dead.
    template<typename T>
    void trace(const WeakMember<T>& t)
    {
        registerWeakCell(t.cell());
    }

    // Fallback trace method for part objects to allow individual
    // trace methods to trace through a part object with
    // visitor->trace(m_partObject).
    template<typename T>
    void trace(const T& t)
    {
        const_cast<T&>(t).trace(this);
    }

    // The following mark methods are for off-heap collections.
    template<typename T, size_t inlineCapacity>
    void trace(const Vector<T, inlineCapacity, WTF::DefaultAllocator>& vector)
    {
        OffHeapCollectionTraceTrait<Vector<T, inlineCapacity, WTF::DefaultAllocator> >::trace(this, vector);
    }

    template<typename T, typename U, typename V>
    void trace(const HashSet<T, U, V, WTF::DefaultAllocator>& hashSet)
    {
        OffHeapCollectionTraceTrait<HashSet<T, U, V, WTF::DefaultAllocator> >::trace(this, hashSet);
    }

    template<typename T, size_t inlineCapacity, typename U>
    void trace(const ListHashSet<T, inlineCapacity, U>& hashSet)
    {
        OffHeapCollectionTraceTrait<ListHashSet<T, inlineCapacity, U> >::trace(this, hashSet);
    }

    template<typename T, size_t N>
    void trace(const Deque<T, N>& deque)
    {
        OffHeapCollectionTraceTrait<Deque<T, N> >::trace(this, deque);
    }

    template<typename T, typename U, typename V, typename W, typename X>
    void trace(const HashMap<T, U, V, W, X, WTF::DefaultAllocator>& map)
    {
        OffHeapCollectionTraceTrait<HashMap<T, U, V, W, X, WTF::DefaultAllocator> >::trace(this, map);
    }

    // OwnPtrs that are traced are treated as part objects and the
    // trace method of the owned object is called.
    template<typename T>
    void trace(const OwnPtr<T>& t)
    {
        if (t)
            t->trace(this);
    }

    // This trace method is to trace a RefPtrWillBeMember when ENABLE(OILPAN)
    // is not enabled.
    // Remove this once we remove RefPtrWillBeMember.
    template<typename T>
    void trace(const RefPtr<T>&)
    {
#if ENABLE(OILPAN)
        // RefPtrs should never be traced.
        ASSERT_NOT_REACHED();
#endif
    }

#if !ENABLE(OILPAN)
    // Similarly, this trace method is to trace a RawPtrWillBeMember
    // when ENABLE(OILPAN) is not enabled.
    // Remove this once we remove RawPtrWillBeMember.
    template<typename T>
    void trace(const RawPtr<T>&)
    {
    }
#endif

    // This method marks an object and adds it to the set of objects
    // that should have their trace method called. Since not all
    // objects have vtables we have to have the callback as an
    // explicit argument, but we can use the templated one-argument
    // mark method above to automatically provide the callback
    // function.
    virtual void mark(const void*, TraceCallback) = 0;

    // Used to mark objects during conservative scanning.
    virtual void mark(HeapObjectHeader*, TraceCallback) = 0;
    virtual void mark(FinalizedHeapObjectHeader*, TraceCallback) = 0;

    // If the object calls this during the regular trace callback, then the
    // WeakPointerCallback argument may be called later, when the strong roots
    // have all been found. The WeakPointerCallback will normally use isAlive
    // to find out whether some pointers are pointing to dying objects. When
    // the WeakPointerCallback is done the object must have purged all pointers
    // to objects where isAlive returned false. In the weak callback it is not
    // allowed to touch other objects (except using isAlive) or to allocate on
    // the GC heap. Note that even removing things from HeapHashSet or
    // HeapHashMap can cause an allocation if the backing store resizes, but
    // these collections know to remove WeakMember elements safely.
    //
    // The weak pointer callbacks are run on the thread that owns the
    // object and other threads are not stopped during the
    // callbacks. Since isAlive is used in the callback to determine
    // if objects pointed to are alive it is crucial that the object
    // pointed to belong to the same thread as the object receiving
    // the weak callback. Since other threads have been resumed the
    // mark bits are not valid for objects from other threads.
    virtual void registerWeakMembers(const void* object, WeakPointerCallback callback) { registerWeakMembers(object, object, callback); }
    virtual void registerWeakMembers(const void*, const void*, WeakPointerCallback) = 0;

    template<typename T, void (T::*method)(Visitor*)>
    void registerWeakMembers(const T* obj)
    {
        registerWeakMembers(obj, &TraceMethodDelegate<T, method>::trampoline);
    }

    // For simple cases where you just want to zero out a cell when the thing
    // it is pointing at is garbage, you can use this. This will register a
    // callback for each cell that needs to be zeroed, so if you have a lot of
    // weak cells in your object you should still consider using
    // registerWeakMembers above.
    //
    // In contrast to registerWeakMembers, the weak cell callbacks are
    // run on the thread performing garbage collection. Therefore, all
    // threads are stopped during weak cell callbacks.
    template<typename T>
    void registerWeakCell(T** cell)
    {
        registerWeakCell(reinterpret_cast<void**>(cell), &handleWeakCell<T>);
    }

    virtual bool isMarked(const void*) = 0;

    template<typename T> inline bool isAlive(T* obj)
    {
        return ObjectAliveTrait<T>::isAlive(this, obj);
    }
    template<typename T> inline bool isAlive(const Member<T>& member)
    {
        return isAlive(member.get());
    }

#ifndef NDEBUG
    void checkGCInfo(const void*, const GCInfo*);
#endif

    // Macro to declare methods needed for each typed heap.
#define DECLARE_VISITOR_METHODS(Type)                                  \
    DEBUG_ONLY(void checkGCInfo(const Type*, const GCInfo*);)          \
    virtual void mark(const Type*, TraceCallback) = 0;                 \
    virtual bool isMarked(const Type*) = 0;

    FOR_EACH_TYPED_HEAP(DECLARE_VISITOR_METHODS)
#undef DECLARE_VISITOR_METHODS

protected:
    virtual void registerWeakCell(void**, WeakPointerCallback) = 0;

private:
    template<typename T>
    static void handleWeakCell(Visitor* self, void* obj)
    {
        T** cell = reinterpret_cast<T**>(obj);
        if (*cell && !self->isAlive(*cell))
            *cell = 0;
    }
};

template<typename T, typename HashFunctions, typename Traits>
struct OffHeapCollectionTraceTrait<WTF::HashSet<T, HashFunctions, Traits, WTF::DefaultAllocator> > {
    typedef WTF::HashSet<T, HashFunctions, Traits, WTF::DefaultAllocator> HashSet;

    static void trace(Visitor* visitor, const HashSet& set)
    {
        if (set.isEmpty())
            return;
        if (WTF::ShouldBeTraced<Traits>::value) {
            HashSet& iterSet = const_cast<HashSet&>(set);
            for (typename HashSet::iterator it = iterSet.begin(), end = iterSet.end(); it != end; ++it)
                CollectionBackingTraceTrait<WTF::ShouldBeTraced<Traits>::value, Traits::isWeak, false, T, Traits>::mark(visitor, *it);
        }
        COMPILE_ASSERT(!Traits::isWeak, WeakOffHeapCollectionsConsideredDangerous0);
    }
};

template<typename T, size_t inlineCapacity, typename HashFunctions>
struct OffHeapCollectionTraceTrait<WTF::ListHashSet<T, inlineCapacity, HashFunctions> > {
    typedef WTF::ListHashSet<T, inlineCapacity, HashFunctions> ListHashSet;

    static void trace(Visitor* visitor, const ListHashSet& set)
    {
        if (set.isEmpty())
            return;
        ListHashSet& iterSet = const_cast<ListHashSet&>(set);
        for (typename ListHashSet::iterator it = iterSet.begin(), end = iterSet.end(); it != end; ++it)
            visitor->trace(*it);
    }
};

template<typename Key, typename Value, typename HashFunctions, typename KeyTraits, typename ValueTraits>
struct OffHeapCollectionTraceTrait<WTF::HashMap<Key, Value, HashFunctions, KeyTraits, ValueTraits, WTF::DefaultAllocator> > {
    typedef WTF::HashMap<Key, Value, HashFunctions, KeyTraits, ValueTraits, WTF::DefaultAllocator> HashMap;

    static void trace(Visitor* visitor, const HashMap& map)
    {
        if (map.isEmpty())
            return;
        if (WTF::ShouldBeTraced<KeyTraits>::value || WTF::ShouldBeTraced<ValueTraits>::value) {
            HashMap& iterMap = const_cast<HashMap&>(map);
            for (typename HashMap::iterator it = iterMap.begin(), end = iterMap.end(); it != end; ++it) {
                CollectionBackingTraceTrait<WTF::ShouldBeTraced<KeyTraits>::value, KeyTraits::isWeak, false, Key, KeyTraits>::mark(visitor, it->key);
                CollectionBackingTraceTrait<WTF::ShouldBeTraced<ValueTraits>::value, ValueTraits::isWeak, false, Value, ValueTraits>::mark(visitor, it->value);
            }
        }
        COMPILE_ASSERT(!KeyTraits::isWeak, WeakOffHeapCollectionsConsideredDangerous1);
        COMPILE_ASSERT(!ValueTraits::isWeak, WeakOffHeapCollectionsConsideredDangerous2);
    }
};

// We trace vectors by using the trace trait on each element, which means you
// can have vectors of general objects (not just pointers to objects) that can
// be traced.
template<typename T, size_t N>
struct OffHeapCollectionTraceTrait<WTF::Vector<T, N, WTF::DefaultAllocator> > {
    typedef WTF::Vector<T, N, WTF::DefaultAllocator> Vector;

    static void trace(Visitor* visitor, const Vector& vector)
    {
        if (vector.isEmpty())
            return;
        for (typename Vector::const_iterator it = vector.begin(), end = vector.end(); it != end; ++it)
            TraceTrait<T>::trace(visitor, const_cast<T*>(it));
    }
};

template<typename T, size_t N>
struct OffHeapCollectionTraceTrait<WTF::Deque<T, N> > {
    typedef WTF::Deque<T, N> Deque;

    static void trace(Visitor* visitor, const Deque& deque)
    {
        if (deque.isEmpty())
            return;
        for (typename Deque::const_iterator it = deque.begin(), end = deque.end(); it != end; ++it)
            TraceTrait<T>::trace(visitor, const_cast<T*>(&(*it)));
    }
};

template<typename T, typename Traits = WTF::VectorTraits<T> >
class HeapVectorBacking;

template<typename Table>
class HeapHashTableBacking {
public:
    static void finalize(void* pointer);
};

template<typename T>
class DefaultTraceTrait<T, false> {
public:
    static void mark(Visitor* visitor, const T* t)
    {
        // Default mark method of the trait just calls the two-argument mark
        // method on the visitor. The second argument is the static trace method
        // of the trait, which by default calls the instance method
        // trace(Visitor*) on the object.
        visitor->mark(const_cast<T*>(t), &TraceTrait<T>::trace);
    }

#ifndef NDEBUG
    static void checkGCInfo(Visitor* visitor, const T* t)
    {
        visitor->checkGCInfo(const_cast<T*>(t), GCInfoTrait<T>::get());
    }
#endif
};

template<typename T>
class DefaultTraceTrait<T, true> {
public:
    static void mark(Visitor* visitor, const T* self)
    {
        self->adjustAndMark(visitor);
    }

#ifndef NDEBUG
    static void checkGCInfo(Visitor*, const T*) { }
#endif
};

template<typename T, bool = NeedsAdjustAndMark<T>::value> class DefaultObjectAliveTrait;

template<typename T>
class DefaultObjectAliveTrait<T, false> {
public:
    static bool isAlive(Visitor* visitor, T* obj)
    {
        return visitor->isMarked(obj);
    }
};

template<typename T>
class DefaultObjectAliveTrait<T, true> {
public:
    static bool isAlive(Visitor* visitor, T* obj)
    {
        return obj->isAlive(visitor);
    }
};

template<typename T> bool ObjectAliveTrait<T>::isAlive(Visitor* visitor, T* obj)
{
    return DefaultObjectAliveTrait<T>::isAlive(visitor, obj);
}

// The GarbageCollectedMixin interface and helper macro
// USING_GARBAGE_COLLECTED_MIXIN can be used to automatically define
// TraceTrait/ObjectAliveTrait on non-leftmost deriving classes
// which need to be garbage collected.
//
// Consider the following case:
// class B {};
// class A : public GarbageCollected, public B {};
//
// We can't correctly handle "Member<B> p = &a" as we can't compute addr of
// object header statically. This can be solved by using GarbageCollectedMixin:
// class B : public GarbageCollectedMixin {};
// class A : public GarbageCollected, public B {
//   USING_GARBAGE_COLLECTED_MIXIN(A)
// };
//
// With the helper, as long as we are using Member<B>, TypeTrait<B> will
// dispatch adjustAndMark dynamically to find collect addr of the object header.
// Note that this is only enabled for Member<B>. For Member<A> which we can
// compute the object header addr statically, this dynamic dispatch is not used.

class GarbageCollectedMixin {
public:
    virtual void adjustAndMark(Visitor*) const = 0;
    virtual bool isAlive(Visitor*) const = 0;
};

#define USING_GARBAGE_COLLECTED_MIXIN(TYPE) \
public: \
    virtual void adjustAndMark(Visitor* visitor) const OVERRIDE \
    { \
        typedef WTF::IsSubclassOfTemplate<TYPE, WebCore::GarbageCollected> IsSubclassOfGarbageCollected; \
        COMPILE_ASSERT(IsSubclassOfGarbageCollected::value, OnlyGarbageCollectedObjectsCanHaveGarbageCollectedMixins); \
        visitor->mark(this, &TraceTrait<TYPE>::trace);\
    } \
    virtual bool isAlive(Visitor* visitor) const OVERRIDE \
    { \
        return visitor->isAlive(this); \
    }

#if ENABLE(OILPAN)
#define WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(TYPE) USING_GARBAGE_COLLECTED_MIXIN(TYPE)
#else
#define WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(TYPE)
#endif

template<typename T>
struct GCInfoAtBase {
    static const GCInfo* get()
    {
        static const GCInfo gcInfo = {
            TraceTrait<T>::trace,
            FinalizerTrait<T>::finalize,
            FinalizerTrait<T>::nonTrivialFinalizer,
        };
        return &gcInfo;
    }
};

template<typename T> class GarbageCollected;
template<typename T, bool = WTF::IsSubclassOfTemplate<T, GarbageCollected>::value> struct GetGarbageCollectedBase;

template<typename T>
struct GetGarbageCollectedBase<T, true> {
    typedef typename T::GarbageCollectedBase type;
};

template<typename T>
struct GetGarbageCollectedBase<T, false> {
    typedef T type;
};

template<typename T>
struct GCInfoTrait {
    static const GCInfo* get()
    {
        return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
    }
};

}

#endif