Revert of Move WTF::SpinLock to base::SpinLock.

third_party/WebKit/Source/wtf/SpinLock.cpp

+// Copyright 2015 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.
+
+#include "wtf/SpinLock.h"
+
+#include "wtf/Atomics.h"
+#include "wtf/CPU.h"
+#include "wtf/Compiler.h"
+
+#if OS(WIN)
+#include <windows.h>
+#elif OS(POSIX)
+#include <sched.h>
+#endif
+
+// The YIELD_PROCESSOR macro wraps an architecture specific-instruction that
+// informs the processor we're in a busy wait, so it can handle the branch more
+// intelligently and e.g. reduce power to our core or give more resources to the
+// other hyper-thread on this core. See the following for context:
+// https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
+//
+// The YIELD_THREAD macro tells the OS to relinquish our quanta. This is
+// basically a worst-case fallback, and if you're hitting it with any frequency
+// you really should be using proper lock rather than these spinlocks.
+#if OS(WIN)
+#define YIELD_PROCESSOR YieldProcessor()
+#define YIELD_THREAD SwitchToThread()
+#elif COMPILER(GCC) || COMPILER(CLANG)
+#if CPU(X86_64) || CPU(X86)
+#define YIELD_PROCESSOR __asm__ __volatile__("pause")
+#elif CPU(ARM) || CPU(ARM64)
+#define YIELD_PROCESSOR __asm__ __volatile__("yield")
+#elif CPU(MIPS)
+// The MIPS32 docs state that the PAUSE instruction is a no-op on older
+// architectures (first added in MIPS32r2). To avoid assembler errors when
+// targeting pre-r2, we must encode the instruction manually.
+#define YIELD_PROCESSOR __asm__ __volatile__(".word 0x00000140")
+#elif CPU(MIPS64) && __mips_isa_rev >= 2
+// Don't bother doing using .word here since r2 is the lowest supported mips64
+// that Chromium supports.
+#define YIELD_PROCESSOR __asm__ __volatile__("pause")
+#endif
+#endif
+
+#ifndef YIELD_PROCESSOR
+#warning "Processor yield not supported on this architecture."
+#define YIELD_PROCESSOR ((void)0)
+#endif
+
+#ifndef YIELD_THREAD
+#if OS(POSIX)
+#define YIELD_THREAD sched_yield()
+#else
+#warning "Thread yield not supported on this OS."
+#define YIELD_THREAD ((void)0)
+#endif
+#endif
+
+namespace WTF {
+
+void SpinLock::lockSlow() {
+  // The value of kYieldProcessorTries is cargo culted from TCMalloc, Windows
+  // critical section defaults, and various other recommendations.
+  // TODO(jschuh): Further tuning may be warranted.
+  static const int kYieldProcessorTries = 1000;
+  do {
+    do {
+      for (int count = 0; count < kYieldProcessorTries; ++count) {
+        // Let the Processor know we're spinning.
+        YIELD_PROCESSOR;
+        if (!m_lock.load(std::memory_order_relaxed) &&
+            LIKELY(!m_lock.exchange(true, std::memory_order_acquire)))
+          return;
+      }
+
+      // Give the OS a chance to schedule something on this core.
+      YIELD_THREAD;
+    } while (m_lock.load(std::memory_order_relaxed));
+  } while (UNLIKELY(m_lock.exchange(true, std::memory_order_acquire)));
+}
+
+}  // namespace WTF