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

[Martin Storsjö via libcxx-commits]

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@@ -101,6 +105,40 @@ 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 __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) {
+    // 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));
+    }
+  } else {
+    // 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));
+    }
----------------
mstorsjo wrote:

Even though it's evident once one looks a bit more on the fallback implementation, it might be nice for clarity, to have an `} else { // The fallback implementation of waking does nothing, as the fallback wait implementation just does polling` or something along those lines? Just to make it clear that the regular pattern would be `if (funcPtr) { funcPtr(); } else { fallback(); }` but the fallback is empty here.

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