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diff --git a/Source/JavaScriptCore/heap/LargeAllocation.h b/Source/JavaScriptCore/heap/LargeAllocation.h
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+/*
+ * Copyright (C) 2016-2017 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. 
+ */
+
+#pragma once
+
+#include "MarkedBlock.h"
+#include "WeakSet.h"
+
+namespace JSC {
+
+class SlotVisitor;
+
+// WebKit has a good malloc that already knows what to do for large allocations. The GC shouldn't
+// have to think about such things. That's where LargeAllocation comes in. We will allocate large
+// objects directly using malloc, and put the LargeAllocation header just before them. We can detect
+// when a HeapCell* is a LargeAllocation because it will have the MarkedBlock::atomSize / 2 bit set.
+
+class LargeAllocation : public BasicRawSentinelNode<LargeAllocation> {
+public:
+    static LargeAllocation* tryCreate(Heap&, size_t, Subspace*);
+    
+    ~LargeAllocation();
+    
+    static LargeAllocation* fromCell(const void* cell)
+    {
+        return bitwise_cast<LargeAllocation*>(bitwise_cast<char*>(cell) - headerSize());
+    }
+    
+    HeapCell* cell() const
+    {
+        return bitwise_cast<HeapCell*>(bitwise_cast<char*>(this) + headerSize());
+    }
+    
+    static bool isLargeAllocation(HeapCell* cell)
+    {
+        return bitwise_cast<uintptr_t>(cell) & halfAlignment;
+    }
+    
+    void lastChanceToFinalize();
+    
+    Heap* heap() const { return m_weakSet.heap(); }
+    VM* vm() const { return m_weakSet.vm(); }
+    WeakSet& weakSet() { return m_weakSet; }
+    
+    void shrink();
+    
+    void visitWeakSet(SlotVisitor&);
+    void reapWeakSet();
+    
+    void clearNewlyAllocated() { m_isNewlyAllocated = false; }
+    void flip();
+    
+    bool isNewlyAllocated() const { return m_isNewlyAllocated; }
+    ALWAYS_INLINE bool isMarked() { return m_isMarked.load(std::memory_order_relaxed); }
+    ALWAYS_INLINE bool isMarked(HeapCell*) { return m_isMarked.load(std::memory_order_relaxed); }
+    ALWAYS_INLINE bool isMarkedConcurrently(HeapVersion, HeapCell*) { return m_isMarked.load(std::memory_order_relaxed); }
+    bool isLive() { return isMarked() || isNewlyAllocated(); }
+    
+    bool hasValidCell() const { return m_hasValidCell; }
+    
+    bool isEmpty();
+    
+    size_t cellSize() const { return m_cellSize; }
+    
+    bool aboveLowerBound(const void* rawPtr)
+    {
+        char* ptr = bitwise_cast<char*>(rawPtr);
+        char* begin = bitwise_cast<char*>(cell());
+        return ptr >= begin;
+    }
+    
+    bool belowUpperBound(const void* rawPtr)
+    {
+        char* ptr = bitwise_cast<char*>(rawPtr);
+        char* begin = bitwise_cast<char*>(cell());
+        char* end = begin + cellSize();
+        // We cannot #include IndexingHeader.h because reasons. The fact that IndexingHeader is 8
+        // bytes is wired deep into our engine, so this isn't so bad.
+        size_t sizeOfIndexingHeader = 8;
+        return ptr <= end + sizeOfIndexingHeader;
+    }
+    
+    bool contains(const void* rawPtr)
+    {
+        return aboveLowerBound(rawPtr) && belowUpperBound(rawPtr);
+    }
+    
+    const AllocatorAttributes& attributes() const { return m_attributes; }
+    
+    void aboutToMark(HeapVersion) { }
+    
+    ALWAYS_INLINE bool testAndSetMarked()
+    {
+        // This method is usually called when the object is already marked. This avoids us
+        // having to CAS in that case. It's profitable to reduce the total amount of CAS
+        // traffic.
+        if (isMarked())
+            return true;
+        return m_isMarked.compareExchangeStrong(false, true);
+    }
+    ALWAYS_INLINE bool testAndSetMarked(HeapCell*) { return testAndSetMarked(); }
+    void clearMarked() { m_isMarked.store(false); }
+    
+    void noteMarked() { }
+    
+#if ASSERT_DISABLED
+    void assertValidCell(VM&, HeapCell*) const { }
+#else
+    void assertValidCell(VM&, HeapCell*) const;
+#endif
+    
+    void sweep();
+    
+    void destroy();
+    
+    void dump(PrintStream&) const;
+    
+private:
+    LargeAllocation(Heap&, size_t, Subspace*);
+    
+    static const unsigned alignment = MarkedBlock::atomSize;
+    static const unsigned halfAlignment = alignment / 2;
+
+    static unsigned headerSize();
+    
+    size_t m_cellSize;
+    bool m_isNewlyAllocated;
+    bool m_hasValidCell;
+    Atomic<bool> m_isMarked;
+    AllocatorAttributes m_attributes;
+    Subspace* m_subspace;
+    WeakSet m_weakSet;
+};
+
+inline unsigned LargeAllocation::headerSize()
+{
+    return ((sizeof(LargeAllocation) + halfAlignment - 1) & ~(halfAlignment - 1)) | halfAlignment;
+}
+
+} // namespace JSC
+