[libcxx-commits] [libcxx] [libc++] Refactor the predicate taking variant of `__cxx_atomic_wait` (PR #80596)

[Louis Dionne via libcxx-commits]

Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>,
Jan =?utf-8?q?Kokemüller?= <jan.kokemueller at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/80596 at github.com>


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@@ -62,26 +100,20 @@ struct __libcpp_atomic_wait_backoff_impl {
   }
 };
 
-template <class _Atp, class _Poll, class _BackoffTest>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
-__cxx_atomic_wait(_Atp* __a, _Poll&& __poll, _BackoffTest&& __backoff_test) {
-  __libcpp_atomic_wait_backoff_impl<_Atp, __decay_t<_BackoffTest> > __backoff_fn = {__a, __backoff_test};
-  return std::__libcpp_thread_poll_with_backoff(__poll, __backoff_fn);
-}
-
-template <class _Poll>
-struct __libcpp_atomic_wait_poll_as_backoff_test {
-  _Poll __poll_;
-
-  _LIBCPP_AVAILABILITY_SYNC
-  _LIBCPP_HIDE_FROM_ABI bool operator()(__cxx_contention_t&) const { return __poll_(); }
-};
-
+// The semantics of this function are similar to `atomic`'s
+// `.wait(T old, std::memory_order order)`, but instead of having a hardcoded
+// predicate (is the loaded value unequal to `old`?), the predicate function is
+// specified as an argument. The loaded value is given as an in-out argument to
+// the predicate. If the predicate function returns `true`,
+// `_cxx_atomic_wait_fn` will return. If the predicate function returns
+// `false`, it must set the argument to its current understanding of the atomic
+// value. The predicate function must not return `false` spuriously.
 template <class _Atp, class _Poll>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __cxx_atomic_wait(_Atp* __a, _Poll&& __poll) {
-  __libcpp_atomic_wait_backoff_impl<_Atp, __libcpp_atomic_wait_poll_as_backoff_test<_Poll&> > __backoff_fn = {
-      __a, {__poll}};
-  return std::__libcpp_thread_poll_with_backoff(__poll, __backoff_fn);
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+__cxx_atomic_wait_fn(_Atp* __a, _Poll&& __poll, memory_order __order) {
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ldionne wrote:

We often use `_fn` to denote things that are implementation details or function objects in the library. IMO, keeping this function named `__cxx_atomic_wait` would be better.

https://github.com/llvm/llvm-project/pull/80596