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

base/synchronization/spin_lock.cc

-// 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 "base/synchronization/spin_lock.h"
-
-#if defined(OS_WIN)
-#include <windows.h>
-#elif defined(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 quantum. This is
-// basically a worst-case fallback, and if you're hitting it with any frequency
-// you really should be using a proper lock (such as |base::Lock|)rather than
-// these spinlocks.
-#if defined(OS_WIN)
-#define YIELD_PROCESSOR YieldProcessor()
-#define YIELD_THREAD SwitchToThread()
-#elif defined(COMPILER_GCC) || defined(__clang__)
-#if defined(ARCH_CPU_X86_64) || defined(ARCH_CPU_X86)
-#define YIELD_PROCESSOR __asm__ __volatile__("pause")
-#elif defined(ARCH_CPU_ARMEL) || defined(ARCH_CPU_ARM64)
-#define YIELD_PROCESSOR __asm__ __volatile__("yield")
-#elif defined(ARCH_CPU_MIPSEL)
-// 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 defined(ARCH_CPU_MIPS64EL) && __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 defined(OS_POSIX)
-#define YIELD_THREAD sched_yield()
-#else
-#warning "Thread yield not supported on this OS."
-#define YIELD_THREAD ((void)0)
-#endif
-#endif
-
-namespace base {
-namespace subtle {
-
-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 (!lock_.load(std::memory_order_relaxed) &&
-            LIKELY(!lock_.exchange(true, std::memory_order_acquire)))
-          return;
-      }
-
-      // Give the OS a chance to schedule something on this core.
-      YIELD_THREAD;
-    } while (lock_.load(std::memory_order_relaxed));
-  } while (UNLIKELY(lock_.exchange(true, std::memory_order_acquire)));
-}
-
-}  // namespace subtle
-}  // namespace base