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[libc++] Improve performance of std::atomic_flag on Windows #163524

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Merged
merged 8 commits into from
Nov 13, 2025
Merged

[libc++] Improve performance of std::atomic_flag on Windows #163524

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63ee5da
[libc++] Improve performance of std::atomic_flag on Windows
Oct 15, 2025
4515628
[libc++] Fixed naming, added comments.
Oct 15, 2025
49efee9
[libc++] More naming and comment changes as recommended
Oct 21, 2025
899aed7
[libc++] Corrected loading/unloading of API set
Oct 26, 2025
2586c83
[libc++] Removed assumption that NULL HMODULE value is safe to use
Oct 26, 2025
930b0c4
[libc++] Consolidated logic into helper function
Oct 26, 2025
27b395d
[libc++] Switched to assume API set is already loaded as per MS STL
Nov 2, 2025
b6df5a5
[libc++] Fixed build error and warning
Nov 4, 2025
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38 changes: 38 additions & 0 deletions libcxx/src/atomic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,10 @@
// OpenBSD has no indirect syscalls
# define _LIBCPP_FUTEX(...) futex(__VA_ARGS__)

#elif defined(_WIN32)

# include <windows.h>

#else // <- Add other operating systems here

// Baseline needs no new headers
Expand Down Expand Up @@ -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));
}
}
}

#else // <- Add other operating systems here

// Baseline is just a timed backoff
Expand Down
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