Reland of "Create rtc::AtomicInt POD struct."

webrtc/base/atomicops.h

Index: webrtc/base/atomicops.h
diff --git a/webrtc/base/atomicops.h b/webrtc/base/atomicops.h
index a863566a670aad0b3d4904f0040c3c20ad845461..b21bd999f304d2f9cd9742fceebd80b3722e7ce3 100644
--- a/webrtc/base/atomicops.h
+++ b/webrtc/base/atomicops.h
@@ -21,6 +21,7 @@
 #endif  // defined(WEBRTC_WIN)
 
 namespace rtc {
+
 class AtomicOps {
  public:
 #if defined(WEBRTC_WIN)
@@ -61,8 +62,50 @@ class AtomicOps {
 #endif
 };
 
+// POD struct version of AtomicOps, prevents accidental non-atomic operator
+// usage (such as ++, -- or =). Functions are static, so that the AtomicInt::
+// prefix must be present in the code, clearly labeling the operations as
+// atomic.
+// Do not copy-initialize, since that performs non-atomic reads of value_. The
+// copy constructor needs to be present for brace initialization.
+struct AtomicInt {
+  AtomicInt() = delete;
+  // TODO(pbos): When MSVC allows brace initialization (or we move to
+  // std::atomic), remove copy constructor (or have it implicitly removed by
+  // std::atomic).
+  void operator=(const AtomicInt&) = delete;
+
+  // value_ is public to permit brace initialization. Should not be accessed
+  // directly.
+  volatile int value_;
+
+  // Atomically increments |i|, returns the resulting incremented value.
+  static int Increment(AtomicInt* i) {
+    return AtomicOps::Increment(&i->value_);
+  }
 
+  // Atomically decrements |i|, returns the resulting decremented value.
+  static int Decrement(AtomicInt* i) {
+    return AtomicOps::Decrement(&i->value_);
+  }
 
+  // Atomically loads |i|.
+  static int AcquireLoad(const AtomicInt* i) {
+    return AtomicOps::AcquireLoad(&i->value_);
+  }
+
+  // Atomically stores |value| in |i|.
+  static void ReleaseStore(AtomicInt* i, int value) {
+    AtomicOps::ReleaseStore(&i->value_, value);
+  }
+
+  // Attempts to compare-and-swaps |old_value| for |new_value| in |i| , returns
+  // |i|'s initial value. If equal to |old_value|, then the CAS succeeded,
+  // otherwise no operation is performed.
+  static int CompareAndSwap(AtomicInt* i, int old_value, int new_value) {
+    return AtomicOps::CompareAndSwap(&i->value_, old_value, new_value);
+  }
+};
 }
 
 #endif  // WEBRTC_BASE_ATOMICOPS_H_