Implement Atomic32 using C++11's std::atomic

Chromoting is planning to bump up the iOS deployment target to 10.0
before but is blocked by WebRTC because it uses deprecated OSAtomic*
APIs.

This CL replaces the platform specific Atomic32 implementation and uses
C++11's std::atomic instead.

Bug: webrtc:8413
Change-Id: Icbd29bb92eec60066589bbabf53e0d50df2f9b0d
Reviewed-on: https://webrtc-review.googlesource.com/12740
Commit-Queue: Yuwei Huang <yuweih@google.com>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20376}

system_wrappers/BUILD.gn

@@ -27,7 +27,7 @@ rtc_static_library("system_wrappers") {
     "include/sleep.h",
     "include/timestamp_extrapolator.h",
     "source/aligned_malloc.cc",
-    "source/atomic32_win.cc",
+    "source/atomic32.cc",
     "source/clock.cc",
     "source/cpu_features.cc",
     "source/cpu_info.cc",
@@ -76,13 +76,8 @@ rtc_static_library("system_wrappers") {
     libs += [ "rt" ]
   }
 
-  if (is_linux || is_android) {
-    sources += [ "source/atomic32_non_darwin_unix.cc" ]
-  }
-
   if (is_ios || is_mac) {
     defines += [ "WEBRTC_THREAD_RR" ]
-    sources += [ "source/atomic32_darwin.cc" ]
   }
 
   # TODO(jschuh): Bug 1348: fix this warning.

system_wrappers/include/atomic32.h

@@ -15,6 +15,8 @@
 #ifndef SYSTEM_WRAPPERS_INCLUDE_ATOMIC32_H_
 #define SYSTEM_WRAPPERS_INCLUDE_ATOMIC32_H_
 
+#include <atomic>
+
 #include <stddef.h>
 
 #include "common_types.h"  // NOLINT(build/include)
@@ -22,6 +24,9 @@
 
 namespace webrtc {
 
+// DEPRECATED: Please use std::atomic<int32_t> instead.
+// TODO(yuweih): Replace Atomic32 uses with std::atomic<int32_t> and remove this
+// class. (bugs.webrtc.org/8428)
 // 32 bit atomic variable.  Note that this class relies on the compiler to
 // align the 32 bit value correctly (on a 32 bit boundary), so as long as you're
 // not doing things like reinterpret_cast over some custom allocated memory
@@ -41,19 +46,12 @@ class Atomic32 {
   // Sets the value atomically to new_value if the value equals compare value.
   // The function returns true if the exchange happened.
   bool CompareExchange(int32_t new_value, int32_t compare_value);
-  int32_t Value() {
-    return *this += 0;
-  }
+  int32_t Value() const;
 
  private:
-  // Checks if |_value| is 32bit aligned.
-  inline bool Is32bitAligned() const {
-    return (reinterpret_cast<ptrdiff_t>(&value_) & 3) == 0;
-  }
-
   RTC_DISALLOW_COPY_AND_ASSIGN(Atomic32);
 
-  int32_t value_;
+  std::atomic<int32_t> value_;
 };
 
 }  // namespace webrtc

system_wrappers/source/atomic32.cc

@@ -1,5 +1,5 @@
 /*
- *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
@@ -11,39 +11,37 @@
 #include "system_wrappers/include/atomic32.h"
 
 #include <assert.h>
-#include <libkern/OSAtomic.h>
-#include <stdlib.h>
 
 #include "common_types.h"  // NOLINT(build/include)
 
 namespace webrtc {
 
-Atomic32::Atomic32(int32_t initial_value)
-    : value_(initial_value) {
-  assert(Is32bitAligned());
-}
+Atomic32::Atomic32(int32_t initial_value) : value_(initial_value) {}
 
-Atomic32::~Atomic32() {
-}
+Atomic32::~Atomic32() {}
 
 int32_t Atomic32::operator++() {
-  return OSAtomicIncrement32Barrier(&value_);
+  return ++value_;
 }
 
 int32_t Atomic32::operator--() {
-  return OSAtomicDecrement32Barrier(&value_);
+  return --value_;
 }
 
 int32_t Atomic32::operator+=(int32_t value) {
-  return OSAtomicAdd32Barrier(value, &value_);
+  return value_ += value;
 }
 
 int32_t Atomic32::operator-=(int32_t value) {
-  return OSAtomicAdd32Barrier(-value, &value_);
+  return value_ -= value;
 }
 
 bool Atomic32::CompareExchange(int32_t new_value, int32_t compare_value) {
-  return OSAtomicCompareAndSwap32Barrier(compare_value, new_value, &value_);
+  return value_.compare_exchange_strong(compare_value, new_value);
+}
+
+int32_t Atomic32::Value() const {
+  return value_.load();
 }
 
 }  // namespace webrtc

system_wrappers/source/atomic32_non_darwin_unix.cc

@@ -1,53 +0,0 @@
-/*
- *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#include "system_wrappers/include/atomic32.h"
-
-#include <assert.h>
-#include <inttypes.h>
-#include <malloc.h>
-
-#include "common_types.h"  // NOLINT(build/include)
-
-namespace webrtc {
-
-Atomic32::Atomic32(int32_t initial_value)
-    : value_(initial_value) {
-  assert(Is32bitAligned());
-}
-
-Atomic32::~Atomic32() {
-}
-
-int32_t Atomic32::operator++() {
-  return __sync_fetch_and_add(&value_, 1) + 1;
-}
-
-int32_t Atomic32::operator--() {
-  return __sync_fetch_and_sub(&value_, 1) - 1;
-}
-
-int32_t Atomic32::operator+=(int32_t value) {
-  int32_t return_value = __sync_fetch_and_add(&value_, value);
-  return_value += value;
-  return return_value;
-}
-
-int32_t Atomic32::operator-=(int32_t value) {
-  int32_t return_value = __sync_fetch_and_sub(&value_, value);
-  return_value -= value;
-  return return_value;
-}
-
-bool Atomic32::CompareExchange(int32_t new_value, int32_t compare_value) {
-  return __sync_bool_compare_and_swap(&value_, compare_value, new_value);
-}
-
-}  // namespace webrtc

system_wrappers/source/atomic32_win.cc

@@ -1,60 +0,0 @@
-/*
- *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#include "system_wrappers/include/atomic32.h"
-
-#include <assert.h>
-#include <windows.h>
-
-#include "common_types.h"  // NOLINT(build/include)
-
-namespace webrtc {
-
-Atomic32::Atomic32(int32_t initial_value)
-    : value_(initial_value) {
-  static_assert(sizeof(value_) == sizeof(LONG),
-                "counter variable is the expected size");
-  assert(Is32bitAligned());
-}
-
-Atomic32::~Atomic32() {
-}
-
-int32_t Atomic32::operator++() {
-  return static_cast<int32_t>(InterlockedIncrement(
-      reinterpret_cast<volatile LONG*>(&value_)));
-}
-
-int32_t Atomic32::operator--() {
-  return static_cast<int32_t>(InterlockedDecrement(
-      reinterpret_cast<volatile LONG*>(&value_)));
-}
-
-int32_t Atomic32::operator+=(int32_t value) {
-  return InterlockedExchangeAdd(reinterpret_cast<volatile LONG*>(&value_),
-                                value);
-}
-
-int32_t Atomic32::operator-=(int32_t value) {
-  return InterlockedExchangeAdd(reinterpret_cast<volatile LONG*>(&value_),
-                                -value);
-}
-
-bool Atomic32::CompareExchange(int32_t new_value, int32_t compare_value) {
-  const LONG old_value = InterlockedCompareExchange(
-      reinterpret_cast<volatile LONG*>(&value_),
-      new_value,
-      compare_value);
-
-  // If the old value and the compare value is the same an exchange happened.
-  return (old_value == compare_value);
-}
-
-}  // namespace webrtc