Neither golang.org/ref/mem nor golang.org/pkg/sync/atomic say anything about what
guarantees are made by the atomic operations wrt the memory model. They should be as
weak as possible, of course, but right now they are non-existence, which is a little too
weak.

We might say, for example, that an atomic.Store writing a value to a memory location
happens before an atomic.Load that reads that value from the memory location. Is that
something we want to say? If not, what do we want to say?

What about Add?

What about CompareAndSwap?