Fair point, but I'd qualify that OOO is only observable for racy programs. In languages with shared memory concurrency, but safety, races are either statically disallowed (Rust), or their results are constrained by a (weak) memory model. Java, e.g. only makes rough guarantees that you don't see out-of-thin air values and that you can't violate the type system. Java also is a little stricter around volatile memory access (they are sequentially consistent, iirc). Also worth noting, some reorderings that you see are not due to hardware, but due to compiler optimizations. Compilers have to obey the memory model, too.

Generally, most programmers shouldn't need to be reasoning about OOO or memory order and be using fences. Rather, they should use libraries that are written by experts that use extra-lingual mechanisms (like compare-swap or fences) to guarantee a higher contract.

> Fair point, but I'd qualify that OOO is only observable for racy programs.

I mean, you're basically saying that it's only observable if you care to observe it.

Rust has atomic types that can observe races. Such "racy" code is useful because some concurrent code doesn't need the strictest guarantees (sequential consistency). Though, compared to C, Rust does not use the "Consume" memory ordering because so far it has turned out to be ill-defined.

AFAICT those can't observe OOO of CPUs, just nondeterminism from interleavings different threads. I.e. they will give you sequential consistency unless you have other, worse races.

Sequential consistency is absolutely optional, there are several other possible memory orderings that are far weaker. When programs involve multiple atomic locations concurrently, atomicity is only preserved at each separate location.