Fixed initialization of Tearable with libstdc++-v4.9.2

Summary: [Folly] Fixed initialization of `Tearable` with libstdc++-v4.9.2, which is missing definitions of `atomic_init`.

Reviewed By: davidtgoldblatt

Differential Revision: D6833004

fbshipit-source-id: 88d4cc91f90618a98df110bec5e434c35e750b16

folly/synchronization/Tearable.h

@@ -53,11 +53,7 @@ class Tearable {
       IsTriviallyCopyable<T>::value,
       "Tearable types must be trivially copyable.");
 
-  Tearable() {
-    for (std::size_t i = 0; i < kNumDataWords; ++i) {
-      std::atomic_init(&data_[i], RawWord{});
-    }
-  }
+  Tearable() = default;
 
   Tearable(const T& val) : Tearable() {
     store(val);
@@ -91,10 +87,15 @@ class Tearable {
     // trailing data word in write(), for instance).
     unsigned char data alignas(void*)[sizeof(void*)];
   };
+  // Because std::atomic_init is declared but undefined in libstdc++-v4.9.2:
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64658.
+  struct AtomicWord : std::atomic<RawWord> {
+    AtomicWord() noexcept : std::atomic<RawWord>{RawWord{}} {}
+  };
   const static std::size_t kNumDataWords =
       (sizeof(T) + sizeof(RawWord) - 1) / sizeof(RawWord);
 
-  std::atomic<RawWord> data_[kNumDataWords];
+  AtomicWord data_[kNumDataWords];
 };
 
 } // namespace folly

folly/synchronization/test/TearableTest.cpp

@@ -26,6 +26,8 @@ using namespace folly;
 namespace {
 
 struct Data {
+  using IsTriviallyCopyable = std::true_type;
+
   Data(unsigned char value) {
     setValue(value);
   }