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[libc++] fix atomic::wait memory order on platforms with weak ordering #144257

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[libc++] fix atomic::wait memory order on platforms with weak ordering #144257

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5 changes: 4 additions & 1 deletion libcxx/src/atomic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -153,6 +153,9 @@ static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __conte
__cxx_contention_t __old_value) {
__cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
// We sleep as long as the monitored value hasn't changed.
#if !defined(__x86_64__) && !defined(__aarch64__)
__cxx_atomic_thread_fence(memory_order_seq_cst);
#endif
__libcpp_platform_wait_on_address(__platform_state, __old_value);
__cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
}
Expand All @@ -163,7 +166,7 @@ static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __conte
static void __libcpp_atomic_notify(void const volatile* __location) {
auto const __entry = __libcpp_contention_state(__location);
// The value sequence laundering happens on the next line below.
__cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
__cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_seq_cst);
__libcpp_contention_notify(
&__entry->__contention_state,
&__entry->__platform_state,
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