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

[via libcxx-commits]

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@@ -101,6 +105,37 @@ 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) {
+  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);
+  } else {
+    __libcpp_thread_poll_with_backoff(
+        [=]() -> bool { return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val); },
+        __libcpp_timed_backoff_policy());
+  }
+}
+
+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)>(
+        GetProcAddress(GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll"), "WakeByAddressSingle"));
+    if (pWakeByAddressSingle != nullptr) {
+      pWakeByAddressSingle(const_cast<__cxx_atomic_contention_t*>(__ptr));
+    }
+  } else {
+    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));
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Alcaro wrote:

The fallback impl's __libcpp_platform_wake_by_address is just completely empty, the waiter just polls with increasing delays.

I agree it _looks_ wrong, but I think it's correct in practice.

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