[TuxSH]

Somewhat related: I find annoying that C++ doesn't have fetch_update and that Rust's fetch_update doesn't support LL/SC.

Rust fetch_update uses the lowest common denominator, CAS, regardless of platform: https://godbolt.org/z/ncssGnsfx (see the call __aarch64_cas8_acq_rel). In hot loops this can mean double-digit perf loss.

[gpderetta]

It is very hard to support LL/SC in generalized user code as the specific rules of what cause an LL lease to fail are generally non-portable (possibly not even within an architecture).

It could be implemented with a CAS fallback of course, but it seems a performance trap.

You could add the logic to the compiler to detect which specific code sequences are LL/SC safe, but at that point just providing built-ins for the most common operations is simpler.