[libcxx-commits] [libcxx] [libc++] Improve performance of std::atomic_flag on Windows (PR #163524)

[via libcxx-commits]

================
@@ -101,6 +105,46 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
   _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), UMTX_OP_WAKE, __notify_one ? 1 : INT_MAX, nullptr, nullptr);
 }
 
+#elif defined(_WIN32)
+
+static void
+__libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
+  // WaitOnAddress was added in Windows 8 (build 9200)
+  static auto wait_on_address = reinterpret_cast<BOOL(WINAPI*)(volatile void*, PVOID, SIZE_T, DWORD)>(
+      GetProcAddress(GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll"), "WaitOnAddress"));
+  if (wait_on_address != nullptr) {
----------------
Andarwinux wrote:

If we can rely on llvm-lib, it's very simple, but if we have to be compatible with ar, it becomes very ugly.

```
if(LIBCXX_ENABLE_STATIC AND MINGW)
find_library(libsynchronization synchronization REQUIRED)
add_custom_command(
    TARGET cxx_static
    POST_BUILD
    COMMAND llvm-lib /out:$<TARGET_FILE:cxx_static> $<TARGET_FILE:cxx_static> ${libsynchronization}
)
endif()
```

https://github.com/llvm/llvm-project/pull/163524