Bug 888548 - Part 3: Add enum support to mozilla::Atomic<T>. r=froydnj

Due to a bug in GCC, the compareExchange function is not available with enum types.

mfbt/Atomics.h

@@ -983,6 +983,27 @@ class Atomic<T*, Order> : public detail::AtomicBaseIncDec<T*, Order>
     Atomic(Atomic<T*, Order>& aOther) MOZ_DELETE;
 };
 
+/**
+ * Atomic<T> implementation for enum types.
+ *
+ * The atomic store and load operations and the atomic swap method is provided.
+ */
+template<typename T, MemoryOrdering Order>
+class Atomic<T, Order, typename EnableIf<IsEnum<T>::value>::Type>
+  : public detail::AtomicBase<T, Order>
+{
+    typedef typename detail::AtomicBase<T, Order> Base;
+
+  public:
+    Atomic() : Base() {}
+    Atomic(T aInit) : Base(aInit) {}
+
+    using Base::operator=;
+
+  private:
+    Atomic(Atomic<T, Order>& aOther) MOZ_DELETE;
+};
+
 } // namespace mozilla
 
 #endif /* mozilla_Atomics_h */

mfbt/tests/TestAtomics.cpp

@@ -128,6 +128,43 @@ TestPointerWithOrdering()
   MOZ_ASSERT(atomic == array1 + 3, "CAS should have changed atomic's value.");
 }
 
+template<MemoryOrdering Order>
+static void
+TestEnumWithOrdering()
+{
+  enum EnumType {
+    EnumType_0 = 0,
+    EnumType_1 = 1,
+    EnumType_2 = 2,
+    EnumType_3 = 3
+  };
+
+  Atomic<EnumType, Order> atomic(EnumType_2);
+  MOZ_ASSERT(atomic == EnumType_2, "Atomic variable did not initialize");
+
+  // Test assignment
+  DebugOnly<EnumType> result;
+  result = (atomic = EnumType_3);
+  MOZ_ASSERT(atomic == EnumType_3, "Atomic assignment failed");
+  MOZ_ASSERT(result == EnumType_3, "Atomic assignment returned the wrong value");
+
+  // Test exchange.
+  atomic = EnumType_1;
+  result = atomic.exchange(EnumType_2);
+  MOZ_ASSERT(atomic == EnumType_2, "Atomic exchange did not work");
+  MOZ_ASSERT(result == EnumType_1, "Atomic exchange returned the wrong value");
+
+  // Test CAS.
+  atomic = EnumType_1;
+  DebugOnly<bool> boolResult = atomic.compareExchange(EnumType_0, EnumType_2);
+  MOZ_ASSERT(!boolResult, "CAS should have returned false.");
+  MOZ_ASSERT(atomic == EnumType_1, "CAS shouldn't have done anything.");
+
+  boolResult = atomic.compareExchange(EnumType_1, EnumType_3);
+  MOZ_ASSERT(boolResult, "CAS should have succeeded.");
+  MOZ_ASSERT(atomic == EnumType_3, "CAS should have changed atomic's value.");
+}
+
 template <typename T>
 static void
 TestType()
@@ -146,6 +183,14 @@ TestPointer()
   TestPointerWithOrdering<T, Relaxed>();
 }
 
+static void
+TestEnum()
+{
+  TestEnumWithOrdering<SequentiallyConsistent>();
+  TestEnumWithOrdering<ReleaseAcquire>();
+  TestEnumWithOrdering<Relaxed>();
+}
+
 int main()
 {
   TestType<uint32_t>();
@@ -156,4 +201,5 @@ int main()
   TestPointer<float>();
   TestPointer<uint16_t*>();
   TestPointer<uint32_t*>();
+  TestEnum();
 }