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diff --git a/Source/JavaScriptCore/runtime/WriteBarrier.h b/Source/JavaScriptCore/runtime/WriteBarrier.h
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+++ b/Source/JavaScriptCore/runtime/WriteBarrier.h
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+/*
+ * Copyright (C) 2011, 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. AND ITS 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 APPLE INC. OR ITS 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 WriteBarrier_h
+#define WriteBarrier_h
+
+#include "GCAssertions.h"
+#include "HandleTypes.h"
+#include "Heap.h"
+#include "SamplingCounter.h"
+
+namespace JSC {
+
+namespace DFG {
+class DesiredWriteBarrier;
+}
+
+class JSCell;
+class VM;
+class JSGlobalObject;
+
+template<class T> class WriteBarrierBase;
+template<> class WriteBarrierBase<JSValue>;
+
+JS_EXPORT_PRIVATE void slowValidateCell(JSCell*);
+JS_EXPORT_PRIVATE void slowValidateCell(JSGlobalObject*);
+    
+#if ENABLE(GC_VALIDATION)
+template<class T> inline void validateCell(T cell)
+{
+    ASSERT_GC_OBJECT_INHERITS(cell, std::remove_pointer<T>::type::info());
+}
+
+template<> inline void validateCell<JSCell*>(JSCell* cell)
+{
+    slowValidateCell(cell);
+}
+
+template<> inline void validateCell<JSGlobalObject*>(JSGlobalObject* globalObject)
+{
+    slowValidateCell(globalObject);
+}
+#else
+template<class T> inline void validateCell(T)
+{
+}
+#endif
+
+// We have a separate base class with no constructors for use in Unions.
+template <typename T> class WriteBarrierBase {
+public:
+    void set(VM&, const JSCell* owner, T* value);
+    
+    // This is meant to be used like operator=, but is called copyFrom instead, in
+    // order to kindly inform the C++ compiler that its advice is not appreciated.
+    void copyFrom(const WriteBarrierBase<T>& other)
+    {
+        m_cell = other.m_cell;
+    }
+
+    void setMayBeNull(VM&, const JSCell* owner, T* value);
+
+    // Should only be used by JSCell during early initialisation
+    // when some basic types aren't yet completely instantiated
+    void setEarlyValue(VM&, const JSCell* owner, T* value);
+    
+    T* get() const
+    {
+        // Copy m_cell to a local to avoid multiple-read issues. (See <http://webkit.org/b/110854>)
+        JSCell* cell = m_cell;
+        if (cell)
+            validateCell(cell);
+        return reinterpret_cast<T*>(static_cast<void*>(cell));
+    }
+
+    T* operator*() const
+    {
+        ASSERT(m_cell);
+        validateCell<T>(static_cast<T*>(m_cell));
+        return static_cast<T*>(m_cell);
+    }
+
+    T* operator->() const
+    {
+        ASSERT(m_cell);
+        validateCell(static_cast<T*>(m_cell));
+        return static_cast<T*>(m_cell);
+    }
+
+    void clear() { m_cell = 0; }
+    
+    T** slot() { return reinterpret_cast<T**>(&m_cell); }
+    
+    explicit operator bool() const { return m_cell; }
+    
+    bool operator!() const { return !m_cell; }
+
+    void setWithoutWriteBarrier(T* value)
+    {
+#if ENABLE(WRITE_BARRIER_PROFILING)
+        WriteBarrierCounters::usesWithoutBarrierFromCpp.count();
+#endif
+        this->m_cell = reinterpret_cast<JSCell*>(value);
+    }
+
+    T* unvalidatedGet() const { return reinterpret_cast<T*>(static_cast<void*>(m_cell)); }
+
+private:
+    JSCell* m_cell;
+};
+
+template <> class WriteBarrierBase<Unknown> {
+public:
+    void set(VM&, const JSCell* owner, JSValue);
+    void setWithoutWriteBarrier(JSValue value)
+    {
+        m_value = JSValue::encode(value);
+    }
+
+    JSValue get() const
+    {
+        return JSValue::decode(m_value);
+    }
+    void clear() { m_value = JSValue::encode(JSValue()); }
+    void setUndefined() { m_value = JSValue::encode(jsUndefined()); }
+    void setStartingValue(JSValue value) { m_value = JSValue::encode(value); }
+    bool isNumber() const { return get().isNumber(); }
+    bool isObject() const { return get().isObject(); }
+    bool isNull() const { return get().isNull(); }
+    bool isGetterSetter() const { return get().isGetterSetter(); }
+    bool isCustomGetterSetter() const { return get().isCustomGetterSetter(); }
+    
+    JSValue* slot()
+    { 
+        union {
+            EncodedJSValue* v;
+            JSValue* slot;
+        } u;
+        u.v = &m_value;
+        return u.slot;
+    }
+    
+    int32_t* tagPointer() { return &bitwise_cast<EncodedValueDescriptor*>(&m_value)->asBits.tag; }
+    int32_t* payloadPointer() { return &bitwise_cast<EncodedValueDescriptor*>(&m_value)->asBits.payload; }
+    
+    explicit operator bool() const { return !!get(); }
+    bool operator!() const { return !get(); } 
+    
+private:
+    EncodedJSValue m_value;
+};
+
+template <typename T> class WriteBarrier : public WriteBarrierBase<T> {
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    WriteBarrier()
+    {
+        this->setWithoutWriteBarrier(0);
+    }
+
+    WriteBarrier(VM& vm, const JSCell* owner, T* value)
+    {
+        this->set(vm, owner, value);
+    }
+
+    WriteBarrier(DFG::DesiredWriteBarrier&, T* value)
+    {
+        ASSERT(isCompilationThread());
+        this->setWithoutWriteBarrier(value);
+    }
+
+    enum MayBeNullTag { MayBeNull };
+    WriteBarrier(VM& vm, const JSCell* owner, T* value, MayBeNullTag)
+    {
+        this->setMayBeNull(vm, owner, value);
+    }
+};
+
+enum UndefinedWriteBarrierTagType { UndefinedWriteBarrierTag };
+template <> class WriteBarrier<Unknown> : public WriteBarrierBase<Unknown> {
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    WriteBarrier()
+    {
+        this->setWithoutWriteBarrier(JSValue());
+    }
+    WriteBarrier(UndefinedWriteBarrierTagType)
+    {
+        this->setWithoutWriteBarrier(jsUndefined());
+    }
+
+    WriteBarrier(VM& vm, const JSCell* owner, JSValue value)
+    {
+        this->set(vm, owner, value);
+    }
+
+    WriteBarrier(DFG::DesiredWriteBarrier&, JSValue value)
+    {
+        ASSERT(isCompilationThread());
+        this->setWithoutWriteBarrier(value);
+    }
+};
+
+template <typename U, typename V> inline bool operator==(const WriteBarrierBase<U>& lhs, const WriteBarrierBase<V>& rhs)
+{
+    return lhs.get() == rhs.get();
+}
+
+} // namespace JSC
+
+#endif // WriteBarrier_h