> Atomics and volatile solve different problems, though.

Yep, that's why atomics are only an alternative to uses of volatile _for atomicity_. For the original use case of accessing hardware registers, volatile is still the correct choice.

It is indeed possible for C11 atomics to be optimized, although interestingly, the three major compilers do very little such optimization. This paper [1] lists some optimizations that are implemented in LLVM and some that aren't; it's from 2015 but from some quick testing it seems like not much has changed since.

[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n44...