[libcxx-commits] [libcxx] [libc++] atomic timed wait (PR #172214)

[via libcxx-commits]

================
@@ -117,11 +128,20 @@ static void __platform_wake_by_address(void const* __ptr, bool __notify_one) {
  */
 
 template <std::size_t _Size>
-static void __platform_wait_on_address(void const* __ptr, void const* __val) {
+static void __platform_wait_on_address(void const* __ptr, void const* __val, uint64_t __timeout_ns) {
   static_assert(_Size == 8, "Can only wait on 8 bytes value");
-  char buffer[_Size];
-  std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
-  _umtx_op(const_cast<void*>(__ptr), UMTX_OP_WAIT, *reinterpret_cast<__cxx_contention_t*>(&buffer), nullptr, nullptr);
+  if (__timeout_ns == 0) {
+    char buffer[_Size];
+    std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
+    _umtx_op(const_cast<void*>(__ptr), UMTX_OP_WAIT, *reinterpret_cast<__cxx_contention_t*>(&buffer), nullptr, nullptr);
+  } else {
+    // TODO the doc says it supports timeout but does not say how to use it
+    // https://man.freebsd.org/cgi/man.cgi?query=_umtx_op
----------------
huixie90 wrote:

Claude wrote the following code for me

```
    _umtx_time ut;
    ut._timeout.tv_sec  = __timeout_ns / 1'000'000'000;
    ut._timeout.tv_nsec = __timeout_ns % 1'000'000'000;
    ut._flags           = 0;               // Relative time (not absolute)
    ut._clockid         = CLOCK_MONOTONIC; // Use monotonic clock

    _umtx_op(const_cast<void*>(__ptr),
             UMTX_OP_WAIT,
             *reinterpret_cast<__cxx_contention_t*>(&buffer),
             reinterpret_cast<void*>(sizeof(ut)), // Pass size as uaddr
             &ut);                                // Pass _umtx_time structure as uaddr2
```

And I wrote a test

```
int main(int, char**) {
  std::counting_semaphore<> s(0);
  (void)s.try_acquire_for(std::chrono::seconds(300));

  return 0;
}

```

and the FreeBSD CI seems to do the right thing

```
302.25s: llvm-libc++-shared.cfg.in :: std/thread/thread.semaphore/lost_wakeup.timed2.pass.cpp
```

https://github.com/llvm/llvm-project/pull/172214