Re: [PATCH] [libstdc++] Refactor/cleanup of atomic wait implementation

[Jonathan Wakely <jwakely@redhat.com>]

On 19/04/21 12:23 -0700, Thomas Rodgers wrote:
>+#if __cpp_lib_atomic_wait
>+  struct __atomic_semaphore
>+  {
>+    static constexpr ptrdiff_t _S_max = __gnu_cxx::__int_traits<int>::__max;
>+    explicit __atomic_semaphore(__detail::__platform_wait_t __count) noexcept
>+      : _M_counter(__count)
>     {
>-      static_assert(std::is_integral_v<_Tp>);
>-      static_assert(__gnu_cxx::__int_traits<_Tp>::__max
>-		      <= __gnu_cxx::__int_traits<ptrdiff_t>::__max);
>-      static constexpr ptrdiff_t _S_max = __gnu_cxx::__int_traits<_Tp>::__max;
>+      __glibcxx_assert(__count >= 0 && __count <= _S_max);
>+    }
>
>-      explicit __atomic_semaphore(_Tp __count) noexcept
>-	: _M_counter(__count)
>+    __atomic_semaphore(const __atomic_semaphore&) = delete;
>+    __atomic_semaphore& operator=(const __atomic_semaphore&) = delete;
>+
>+    static _GLIBCXX_ALWAYS_INLINE bool
>+    _S_do_try_acquire(__detail::__platform_wait_t* __counter,
>+		      __detail::__platform_wait_t& __old) noexcept
>+    {
>+      if (__old == 0)
>+	return false;
>+
>+      return __atomic_impl::compare_exchange_strong(__counter,
>+						    __old, __old - 1,
>+						    memory_order::acquire,
>+						    memory_order::release);

This violates the compare_exchange precondition:

Preconditions: The failure argument is neither memory_order::release nor memory_order::acq_rel.


Should this be relaxed? I don't think a failed try_acquire has to
synchronize, does it?