[libc++] Removed assumption that NULL HMODULE value is safe to use

Author: Roger Sanders

libcxx/src/atomic.cpp

@@ -113,6 +113,7 @@ static HMODULE win32_get_win_core_synch_api_module() {
   // safely use std::unique_ptr here as a wrapper for the handle, with a destructor freeing the handle when this module
   // is unloaded.
   // https://learn.microsoft.com/en-us/windows/win32/winprog/windows-data-types
+  // https://devblogs.microsoft.com/oldnewthing/20180307-00/?p=98175
   static auto module_handle = std::unique_ptr<std::remove_pointer<HMODULE>::type, decltype(&FreeLibrary)>(
       LoadLibraryW(L"api-ms-win-core-synch-l1-2-0.dll"), &FreeLibrary);
   return module_handle.get();
@@ -122,7 +123,9 @@ 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(win32_get_win_core_synch_api_module(), "WaitOnAddress"));
+      (win32_get_win_core_synch_api_module() != NULL
+           ? GetProcAddress(win32_get_win_core_synch_api_module(), "WaitOnAddress")
+           : nullptr));
   if (wait_on_address != nullptr) {
     wait_on_address(const_cast<__cxx_atomic_contention_t*>(__ptr), &__val, sizeof(__val), INFINITE);
   } else {
@@ -136,7 +139,9 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
   if (__notify_one) {
     // WakeByAddressSingle was added in Windows 8 (build 9200)
     static auto wake_by_address_single = reinterpret_cast<void(WINAPI*)(PVOID)>(
-        GetProcAddress(win32_get_win_core_synch_api_module(), "WakeByAddressSingle"));
+        (win32_get_win_core_synch_api_module() != NULL
+             ? GetProcAddress(win32_get_win_core_synch_api_module(), "WakeByAddressSingle")
+             : nullptr));
     if (wake_by_address_single != nullptr) {
       wake_by_address_single(const_cast<__cxx_atomic_contention_t*>(__ptr));
     } else {
@@ -146,7 +151,9 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
   } else {
     // WakeByAddressAll was added in Windows 8 (build 9200)
     static auto wake_by_address_all = reinterpret_cast<void(WINAPI*)(PVOID)>(
-        GetProcAddress(win32_get_win_core_synch_api_module(), "WakeByAddressAll"));
+        (win32_get_win_core_synch_api_module() != NULL
+             ? GetProcAddress(win32_get_win_core_synch_api_module(), "WakeByAddressAll")
+             : nullptr));
     if (wake_by_address_all != nullptr) {
       wake_by_address_all(const_cast<__cxx_atomic_contention_t*>(__ptr));
     } else {