[libcxx-commits] [libcxx] [libc++] Fix FreeBSD atomic timed wait system call (PR #180400)

[Jessica Clarke via libcxx-commits]

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@@ -130,22 +130,20 @@ 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, uint64_t __timeout_ns) {
   static_assert(_Size == 8, "Can only wait on 8 bytes value");
-  char buffer[_Size];
+  alignas(__cxx_contention_t) char buffer[_Size];
   std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
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jrtc27 wrote:

So my understanding of the C++ object model is admittedly weaker than C's aliasing model, but I believe that, although you can access any object as if it were characters/bytes to see its representation, you cannot in general take an array of characters and reinterpret it as something else, without first creating such an object with a placement new? At which point why would this not just memcpy to a __cxx_contention_t directly? That's how std::bit_cast can be implemented. This seems to be a repeated pattern among other platforms, too.

To be honest I'm also not sure why we're using __cxx_contention_t in here. I'd expect __platform_wait_on_address to deal with the supported native type(s), with a fallback for those that aren't but need to be, and it's up to the layers above to deal with the __cxx_contention_t. As far as __platform_wait_on_address is concerned there's no reason it couldn't support `_Size == 4`, such an operation exists in FreeBSD, it's just that the upper layers dealing with __cxx_contention_t don't have a use for such an operation. But that's a bigger issue.

https://github.com/llvm/llvm-project/pull/180400