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diff --git a/Source/JavaScriptCore/heap/GCIncomingRefCounted.h b/Source/JavaScriptCore/heap/GCIncomingRefCounted.h
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+/*
+ * Copyright (C) 2013 Apple 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:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. 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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 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 GCIncomingRefCounted_h
+#define GCIncomingRefCounted_h
+
+#include <wtf/DeferrableRefCounted.h>
+#include <wtf/Vector.h>
+
+namespace JSC {
+
+class JSCell;
+
+// A C-heap-allocated object that may have additional reference counts
+// due to incoming references from the heap, which are tracked in
+// reverse: the object knows its incoming references. Such objects also
+// have the invariant that they don't have references back into the GC
+// heap.
+
+template<typename T>
+class GCIncomingRefCounted : public DeferrableRefCounted<T> {
+public:
+    GCIncomingRefCounted()
+        : m_encodedPointer(0)
+    {
+    }
+    
+    ~GCIncomingRefCounted()
+    {
+        if (hasVectorOfCells())
+            delete vectorOfCells();
+    }
+    
+    size_t numberOfIncomingReferences() const
+    {
+        if (!hasAnyIncoming())
+            return 0;
+        if (hasSingleton())
+            return 1;
+        return vectorOfCells()->size();
+    }
+    
+    JSCell* incomingReferenceAt(size_t index) const
+    {
+        ASSERT(hasAnyIncoming());
+        if (hasSingleton()) {
+            ASSERT(!index);
+            return singleton();
+        }
+        return vectorOfCells()->at(index);
+    }
+    
+    // It's generally not a good idea to call this directly, since if this
+    // returns true, you're supposed to add this object to the GC's list.
+    // Call GCIncomingRefCountedSet::addReference() instead.
+    bool addIncomingReference(JSCell*);
+    
+    // A filter function returns true if we wish to keep the incoming
+    // reference, and false if we don't. This may delete the object,
+    // and if it does so, this returns true. In general, you don't want
+    // to use this with a filter function that can return false unless
+    // you're also walking the GC's list.
+    template<typename FilterFunctionType>
+    bool filterIncomingReferences(FilterFunctionType&);
+    
+private:
+    static uintptr_t singletonFlag() { return 1; }
+    
+    bool hasVectorOfCells() const { return !(m_encodedPointer & singletonFlag()); }
+    bool hasAnyIncoming() const { return !!m_encodedPointer; }
+    bool hasSingleton() const { return hasAnyIncoming() && !hasVectorOfCells(); }
+    
+    JSCell* singleton() const
+    {
+        ASSERT(hasSingleton());
+        return bitwise_cast<JSCell*>(m_encodedPointer & ~singletonFlag());
+    }
+    
+    Vector<JSCell*>* vectorOfCells() const
+    {
+        ASSERT(hasVectorOfCells());
+        return bitwise_cast<Vector<JSCell*>*>(m_encodedPointer);
+    }
+    
+    // Singleton flag is set: this is a JSCell*.
+    // Singleton flag not set: this is a pointer to a vector of cells.
+    uintptr_t m_encodedPointer;
+};
+
+} // namespace JSC
+
+#endif // GCIncomingRefCounted_h
+