Revert of Remove old/unused atomicops code

base/atomicops_internals_gcc.h

+// Copyright (c) 2009 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.
+
+// This file is an internal atomic implementation, include base/atomicops.h
+// instead. This file is for platforms that use GCC intrinsics rather than
+// platform-specific assembly code for atomic operations.
+
+#ifndef BASE_ATOMICOPS_INTERNALS_GCC_H_
+#define BASE_ATOMICOPS_INTERNALS_GCC_H_
+
+namespace base {
+namespace subtle {
+
+inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
+                                         Atomic32 old_value,
+                                         Atomic32 new_value) {
+  Atomic32 prev_value;
+  do {
+    if (__sync_bool_compare_and_swap(ptr, old_value, new_value))
+      return old_value;
+    prev_value = *ptr;
+  } while (prev_value == old_value);
+  return prev_value;
+}
+
+inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr,
+                                         Atomic32 new_value) {
+  Atomic32 old_value;
+  do {
+    old_value = *ptr;
+  } while (!__sync_bool_compare_and_swap(ptr, old_value, new_value));
+  return old_value;
+}
+
+inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
+                                          Atomic32 increment) {
+  return Barrier_AtomicIncrement(ptr, increment);
+}
+
+inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
+                                        Atomic32 increment) {
+  for (;;) {
+    // Atomic exchange the old value with an incremented one.
+    Atomic32 old_value = *ptr;
+    Atomic32 new_value = old_value + increment;
+    if (__sync_bool_compare_and_swap(ptr, old_value, new_value)) {
+      // The exchange took place as expected.
+      return new_value;
+    }
+    // Otherwise, *ptr changed mid-loop and we need to retry.
+  }
+}
+
+inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
+                                       Atomic32 old_value,
+                                       Atomic32 new_value) {
+  // Since NoBarrier_CompareAndSwap uses __sync_bool_compare_and_swap, which
+  // is a full memory barrier, none is needed here or below in Release.
+  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+}
+
+inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
+                                       Atomic32 old_value,
+                                       Atomic32 new_value) {
+  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+}
+
+inline void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value) {
+  *ptr = value;
+}
+
+inline void MemoryBarrier() {
+  __sync_synchronize();
+}
+
+inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
+  *ptr = value;
+  MemoryBarrier();
+}
+
+inline void Release_Store(volatile Atomic32* ptr, Atomic32 value) {
+  MemoryBarrier();
+  *ptr = value;
+}
+
+inline Atomic32 NoBarrier_Load(volatile const Atomic32* ptr) {
+  return *ptr;
+}
+
+inline Atomic32 Acquire_Load(volatile const Atomic32* ptr) {
+  Atomic32 value = *ptr;
+  MemoryBarrier();
+  return value;
+}
+
+inline Atomic32 Release_Load(volatile const Atomic32* ptr) {
+  MemoryBarrier();
+  return *ptr;
+}
+
+}  // namespace subtle
+}  // namespace base
+
+#endif  // BASE_ATOMICOPS_INTERNALS_GCC_H_
+