Use portable-atomic on platforms without u64 atomics

[linkmauve]

This has been tested on powerpc-unknown-linux-gnu, which supports atomics only up to u32. Without this commit, building quad-rand would fail with:

error[E0432]: unresolved import `core::sync::atomic::AtomicU64`
 --> src/lib.rs:5:26
  |
5 | use core::sync::atomic::{AtomicU64, Ordering};
  |                          ^^^^^^^^^
  |                          |
  |                          no `AtomicU64` in `sync::atomic`
  |                          help: a similar name exists in the module: `AtomicU32`

[not-fl3]

To be honest, for quad-rand I would rather use two AtomicU32. portable-atomic is a really big crate and we use so little of it. Even with default-features = false cargo will always download all this code, and iirc it even downloads feature-gated crates.

[linkmauve]

Do you suggest lowering RandGenerator::state to an AtomicU32 and changing the algorithm, or using two of them and decomposing the operation into non-atomic operations?

I don’t directly use quad-rand, but it is a dependency of kira which I’m using in my game engine, so I don’t have much opinion on that matter.

Edit: My bad, I had misread your message. I’m not sure I know the maths to emulate u64 wrapping add and multiply using u32, and besides that would render the whole operation non-atomic. Maybe using a Mutex<u64> would be nicer?

[not-fl3]

I doubt it would be possible to keep it atomic if AtomicU64 is not supported?