Memory64, multithreading and the 128-bit atomics operations to combat the ABA problem

In native lock-free programming, a tricky issue is the ABA problem, which is caused by two or more atomic mutations by other threads, occurring between an observer thread checking on a shared state variable.

https://en.wikipedia.org/wiki/ABA_problem

The common elegant solution for this is to use auxiliary information, which typically ends up with a shared state block of "2x sizeof(ptr)" kind of a setup. Native compilation targets allow performing atomic operations on such 128-bit quantities, on 64-bit platforms.

When porting Unity to use WebAssembly's Memory64, we now find ourselves in a pickle with this scheme. Multithreading libraries that Unity uses, perform 128-bit atomic operations on shared memory (the usual: load/store/cas/exchange/add/and/or/xor).

We are looking at removing these 128-bit CAS support in the library code that assume its existence. Have there been considerations for adding 128-bit atomic operation support to multithreading+memory64 modes, or would that be something possible to consider?

No files or tests are named. Start by reviewing the WebAssembly threads and Memory64 requirements described here, especially the requested 128-bit load/store/CAS/exchange/add/and/or/xor operations; done means establishing whether these operations can be specified and supported in multithreading+Memory64 modes, or documenting the constraints that rule them out.