I think we’re talking past each other. What I mean is that if you only need a load fence, that’s a lot cheaper (free on x64 if memory serves, just a compiler ba

I think we’re talking past each other. What I mean is that if you only need a load fence, that’s a lot cheaper (free on x64 if memory serves, just a compiler barrier) than lock cmpxchg. A mutex uses lock cmpxchg, so you’re arguably worse off than just using a mutex (or a reader writer mutex, which has similar performance characteristics for seldom write workloads.) About C++ using the same instruction for both, that’s up to their implementation. There’s no requirement that the more relaxed atomics actually be more relaxed. Only that the stronger atomics don’t permit more relaxed behavior. There may also be other requirements in the spec, that I’m not aware of, that require using the strong compare exchange. It’s been years since I’ve done any substantial lock free programming, I may be remembering incorrectly.