See if unordered atomics are a feasible replacement for DisjointMut's potential unsoundness

[kkysen]

In a discussion of DisjointMut and its potential unsoundness (if there are any overlapping ranges in --release mode) on Reddit, u/afdbcreid suggested suggested Unordered atomics as a potential performant solution, as discussed on IRLO: unordered as a solution to “Bit-wise reasoning for atomic accesses”.

The only other potentially performant option we saw was to use Relaxed atomics. However, even Relaxed atomics are not optimized the same way in aggregate. An individual Relaxed load and store is the same instruction as a NotAtomic load and store, but, for example, 2 16-bit aligned AtomicU8 loads are not merged into a single AtomicU16 load. However, in LLVM's atomics model, there is a level between NotAtomic and Relaxed/Monotonic called Unordered. It is used by Java primarily and has even fewer guarantees compared to Relaxed/Monotonic, and as such, LLVM will do those same merging optimizations that we want. As such, it's possible it would result in good enough performance. However, Unordered is not in stable Rust, and is only an unstable intrinsic at the moment: good enough for some experimentation if we want to try it out, but far from stable. And fully trying it out, along with all of the necessary refactoring, and then evaluating performance would be significant work. Ideally, we'd try in the name of soundness, though time is a concern.