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+/*
+ * Copyright (C) 2015-2016 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. ``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 WTF_WordLock_h
+#define WTF_WordLock_h
+
+#include <wtf/Atomics.h>
+#include <wtf/Compiler.h>
+#include <wtf/Locker.h>
+#include <wtf/Noncopyable.h>
+
+namespace TestWebKitAPI {
+struct LockInspector;
+};
+
+namespace WTF {
+
+// A WordLock is a fully adaptive mutex that uses sizeof(void*) storage. It has a fast path that is
+// similar to a spinlock, and a slow path that is similar to std::mutex. In most cases, you should use
+// Lock instead. WordLock sits lower in the stack and is used to implement Lock, so Lock is the main
+// client of WordLock.
+
+// NOTE: This is also a great lock to use if you are very low in the stack. For example,
+// PrintStream uses this so that ParkingLot and Lock can use PrintStream. This means that if you
+// try to use dataLog to debug this code, you will have a bad time.
+
+struct WordLockBase {
+    void lock()
+    {
+        if (LIKELY(m_word.compareExchangeWeak(0, isLockedBit, std::memory_order_acquire))) {
+            // WordLock acquired!
+            return;
+        }
+
+        lockSlow();
+    }
+
+    void unlock()
+    {
+        if (LIKELY(m_word.compareExchangeWeak(isLockedBit, 0, std::memory_order_release))) {
+            // WordLock released, and nobody was waiting!
+            return;
+        }
+
+        unlockSlow();
+    }
+
+    bool isHeld() const
+    {
+        return m_word.load(std::memory_order_acquire) & isLockedBit;
+    }
+
+    bool isLocked() const
+    {
+        return isHeld();
+    }
+
+protected:
+    friend struct TestWebKitAPI::LockInspector;
+    
+    static const uintptr_t isLockedBit = 1;
+    static const uintptr_t isQueueLockedBit = 2;
+    static const uintptr_t queueHeadMask = 3;
+
+    WTF_EXPORT_PRIVATE void lockSlow();
+    WTF_EXPORT_PRIVATE void unlockSlow();
+
+    // Method used for testing only.
+    bool isFullyReset() const
+    {
+        return !m_word.load();
+    }
+
+    Atomic<uintptr_t> m_word;
+};
+
+class WordLock : public WordLockBase {
+    WTF_MAKE_NONCOPYABLE(WordLock);
+public:
+    WordLock()
+    {
+        m_word.store(0, std::memory_order_relaxed);
+    }
+};
+
+typedef WordLockBase StaticWordLock;
+typedef Locker<WordLockBase> WordLockHolder;
+
+} // namespace WTF
+
+using WTF::WordLock;
+using WTF::WordLockHolder;
+using WTF::StaticWordLock;
+
+#endif // WTF_WordLock_h
+