1/*
2 * Copyright (C) 2013 Apple Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 *    notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 *    notice, this list of conditions and the following disclaimer in the
11 *    documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
14 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
17 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
18 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
19 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
20 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
21 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
23 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#ifndef GCIncomingRefCounted_h
27#define GCIncomingRefCounted_h
28
29#include <wtf/DeferrableRefCounted.h>
30#include <wtf/Vector.h>
31
32namespace JSC {
33
34class JSCell;
35
36// A C-heap-allocated object that may have additional reference counts
37// due to incoming references from the heap, which are tracked in
38// reverse: the object knows its incoming references. Such objects also
39// have the invariant that they don't have references back into the GC
40// heap.
41
42template<typename T>
43class GCIncomingRefCounted : public DeferrableRefCounted<T> {
44public:
45    GCIncomingRefCounted()
46        : m_encodedPointer(0)
47    {
48    }
49
50    ~GCIncomingRefCounted()
51    {
52        if (hasVectorOfCells())
53            delete vectorOfCells();
54    }
55
56    size_t numberOfIncomingReferences() const
57    {
58        if (!hasAnyIncoming())
59            return 0;
60        if (hasSingleton())
61            return 1;
62        return vectorOfCells()->size();
63    }
64
65    JSCell* incomingReferenceAt(size_t index) const
66    {
67        ASSERT(hasAnyIncoming());
68        if (hasSingleton()) {
69            ASSERT(!index);
70            return singleton();
71        }
72        return vectorOfCells()->at(index);
73    }
74
75    // It's generally not a good idea to call this directly, since if this
76    // returns true, you're supposed to add this object to the GC's list.
77    // Call GCIncomingRefCountedSet::addReference() instead.
78    bool addIncomingReference(JSCell*);
79
80    // A filter function returns true if we wish to keep the incoming
81    // reference, and false if we don't. This may delete the object,
82    // and if it does so, this returns true. In general, you don't want
83    // to use this with a filter function that can return false unless
84    // you're also walking the GC's list.
85    template<typename FilterFunctionType>
86    bool filterIncomingReferences(FilterFunctionType&);
87
88private:
89    static uintptr_t singletonFlag() { return 1; }
90
91    bool hasVectorOfCells() const { return !(m_encodedPointer & singletonFlag()); }
92    bool hasAnyIncoming() const { return !!m_encodedPointer; }
93    bool hasSingleton() const { return hasAnyIncoming() && !hasVectorOfCells(); }
94
95    JSCell* singleton() const
96    {
97        ASSERT(hasSingleton());
98        return bitwise_cast<JSCell*>(m_encodedPointer & ~singletonFlag());
99    }
100
101    Vector<JSCell*>* vectorOfCells() const
102    {
103        ASSERT(hasVectorOfCells());
104        return bitwise_cast<Vector<JSCell*>*>(m_encodedPointer);
105    }
106
107    // Singleton flag is set: this is a JSCell*.
108    // Singleton flag not set: this is a pointer to a vector of cells.
109    uintptr_t m_encodedPointer;
110};
111
112} // namespace JSC
113
114#endif // GCIncomingRefCounted_h
115
116