Move WTF::SpinLock to base::SpinLock.

base/synchronization/spin_lock.h

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_SYNCHRONIZATION_SPIN_LOCK_H
+#define BASE_SYNCHRONIZATION_SPIN_LOCK_H
+
+#include <atomic>
+#include <memory>
+#include <mutex>
+
+#include "base/base_export.h"
+#include "base/compiler_specific.h"
+
+// Spinlock is a simple spinlock class based on the standard CPU primitive of
+// atomic increment and decrement of an int at a given memory address. These are
+// intended only for very short duration locks and assume a system with multiple
+// cores. For any potentially longer wait you should use a real lock, such as
+// |base::Lock|.
+//
+// |SpinLock|s MUST be globals. Using them as (e.g.) struct/class members will
+// result in an uninitialized lock, which is dangerously incorrect.
+
+namespace base {
+namespace subtle {
+
+class SpinLock {
+ public:
+  using Guard = std::lock_guard<SpinLock>;
+
+  ALWAYS_INLINE void lock() {
+    static_assert(sizeof(lock_) == sizeof(int),
+                  "int and lock_ are different sizes");
+    if (LIKELY(!lock_.exchange(true, std::memory_order_acquire)))
+      return;
+    LockSlow();
+  }
+
+  ALWAYS_INLINE void unlock() { lock_.store(false, std::memory_order_release); }
+
+ private:
+  // This is called if the initial attempt to acquire the lock fails. It's
+  // slower, but has a much better scheduling and power consumption behavior.
+  BASE_EXPORT void LockSlow();
+
+  std::atomic_int lock_;
+};
+
+}  // namespace subtle
+}  // namespace base
+
+#endif  // BASE_SYNCHRONIZATION_SPIN_LOCK_H