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diff --git a/Source/JavaScriptCore/b3/B3FenceValue.h b/Source/JavaScriptCore/b3/B3FenceValue.h
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+/*
+ * Copyright (C) 2016 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.
+ */
+
+#pragma once
+
+#if ENABLE(B3_JIT)
+
+#include "B3HeapRange.h"
+#include "B3Value.h"
+
+namespace JSC { namespace B3 {
+
+class JS_EXPORT_PRIVATE FenceValue : public Value {
+public:
+    static bool accepts(Kind kind) { return kind == Fence; }
+    
+    ~FenceValue();
+    
+    // The read/write heaps are reflected in the effects() of this value. The compiler may change
+    // the lowering of a Fence based on the heaps. For example, if a fence does not write anything
+    // then it is understood to be a store-store fence. On x86, this may lead us to not emit any
+    // code, while on ARM we may emit a cheaper fence (dmb ishst instead of dmb ish). We will do
+    // the same optimization for load-load fences, which are expressed as a Fence that writes but
+    // does not read.
+    //
+    // This abstraction allows us to cover all of the fences on x86 and all of the standalone fences
+    // on ARM. X86 really just has one fence: mfence. This fence should be used to protect stores
+    // from being sunk below loads. WTF calls it the storeLoadFence. A classic example is the Steele
+    // barrier:
+    //
+    //     o.f = v  =>  o.f = v
+    //                  if (color(o) == black)
+    //                      log(o)
+    //
+    // We are trying to ensure that if the store to o.f occurs after the collector has started
+    // visiting o, then we will log o. Under sequential consistency, this would work. The collector
+    // would set color(o) to black just before it started visiting. But x86's illusion of sequential
+    // consistency is broken in exactly just this store->load ordering case. The store to o.f may
+    // get buffered, and it may occur some time after we have loaded and checked color(o). As well,
+    // the collector's store to set color(o) to black may get buffered and it may occur some time
+    // after the collector has finished visiting o. Therefore, we need mfences. In B3 we model this
+    // as a Fence that reads and writes some heaps. Setting writes to the empty set will cause B3 to
+    // not emit any barrier on x86.
+    //
+    // On ARM there are many more fences. The Fence instruction is meant to model just two of them:
+    // dmb ish and dmb ishst. You can emit a dmb ishst by using a Fence with an empty write heap.
+    // Otherwise, you will get a dmb ish.
+    // FIXME: Add fenced memory accesses. https://bugs.webkit.org/show_bug.cgi?id=162349
+    // FIXME: Add a Depend operation. https://bugs.webkit.org/show_bug.cgi?id=162350
+    HeapRange read { HeapRange::top() };
+    HeapRange write { HeapRange::top() };
+
+protected:
+    Value* cloneImpl() const override;
+
+private:
+    friend class Procedure;
+    
+    JS_EXPORT_PRIVATE FenceValue(Origin origin, HeapRange read, HeapRange write);
+    
+    JS_EXPORT_PRIVATE FenceValue(Origin origin);
+};
+
+} } // namespace JSC::B3
+
+#endif // ENABLE(B3_JIT)
+