[libc++] Fixed naming, added comments.

Author: Roger Sanders

libcxx/src/atomic.cpp

@@ -109,10 +109,11 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
 
 static void
 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
-  static auto pWaitOnAddress = reinterpret_cast<BOOL(WINAPI*)(volatile void*, PVOID, SIZE_T, DWORD)>(
+  // WaitOnAddress was added in Windows 8 (build 9200)
+  static auto __pWaitOnAddress = reinterpret_cast<BOOL(WINAPI*)(volatile void*, PVOID, SIZE_T, DWORD)>(
       GetProcAddress(GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll"), "WaitOnAddress"));
-  if (pWaitOnAddress != nullptr) {
-    pWaitOnAddress(const_cast<__cxx_atomic_contention_t*>(__ptr), &__val, sizeof(__val), INFINITE);
+  if (__pWaitOnAddress != nullptr) {
+    __pWaitOnAddress(const_cast<__cxx_atomic_contention_t*>(__ptr), &__val, sizeof(__val), INFINITE);
   } else {
     __libcpp_thread_poll_with_backoff(
         [=]() -> bool { return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val); },
@@ -122,16 +123,18 @@ __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __pt
 
 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) {
   if (__notify_one) {
-    static auto pWakeByAddressSingle = reinterpret_cast<void(WINAPI*)(PVOID)>(
+    // WakeByAddressSingle was added in Windows 8 (build 9200)
+    static auto __pWakeByAddressSingle = reinterpret_cast<void(WINAPI*)(PVOID)>(
         GetProcAddress(GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll"), "WakeByAddressSingle"));
-    if (pWakeByAddressSingle != nullptr) {
-      pWakeByAddressSingle(const_cast<__cxx_atomic_contention_t*>(__ptr));
+    if (__pWakeByAddressSingle != nullptr) {
+      __pWakeByAddressSingle(const_cast<__cxx_atomic_contention_t*>(__ptr));
     }
   } else {
-    static auto pWakeByAddressAll = reinterpret_cast<void(WINAPI*)(PVOID)>(
+    // WakeByAddressAll was added in Windows 8 (build 9200)
+    static auto __pWakeByAddressAll = reinterpret_cast<void(WINAPI*)(PVOID)>(
         GetProcAddress(GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll"), "WakeByAddressAll"));
-    if (pWakeByAddressAll != nullptr) {
-      pWakeByAddressAll(const_cast<__cxx_atomic_contention_t*>(__ptr));
+    if (__pWakeByAddressAll != nullptr) {
+      __pWakeByAddressAll(const_cast<__cxx_atomic_contention_t*>(__ptr));
     }
   }
 }