heap/GCIncomingRefCounted.h

   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
  32 namespace JSC {
  33
  34 class 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
  42 template<typename T>
  43 class GCIncomingRefCounted : public DeferrableRefCounted<T> {
  44 public:
  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
  88 private:
  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