`AtomicDoubleUsize`?

[Ddystopia]

Hello, some architectures have instructions to atomically load 2 words, like cmpxchg16b on x86_64. I don't think AtomicU128 is an answer, because there might be 32-bit architectures that allow 64 bit loads. That's why we have AtomicUnsize. Can we then have something like AtomicDoubleUsize or something similar?

[SkiFire13]

You could define it yourself:

#[cfg(target_pointer_width = "16")]
pub type AtomicDoubleUsize = std::sync::atomic::AtomicU32;

#[cfg(target_pointer_width = "32")]
pub type AtomicDoubleUsize = std::sync::atomic::AtomicU64;

#[cfg(target_pointer_width = "64")]
pub type AtomicDoubleUsize = std::sync::atomic::AtomicU128;

[HjVT]

But this assumes that native register size is == pointer size == there is support for atomic operations with wish double of that

[eggyal]

Under strict provenance, there are issues with using (atomic) integer types to hold pointers—which makes even having an AtomicDoubleUsize type or similar of limited use in such cases.

I raised this very concern in the unsafe code guidelines issue tracker a couple of days ago, and there has been some interesting discussion there since.

Packing pointers into double-word width atomics

[farnz]

That sounds like we should have both AtomicDoubleUsize and AtomicDoublePtr, with the same relationship as AtomicUsize and AtomicPtr.

[kornel]

There's also #[cfg(target_has_atomic="128")] which could be used to provide a fallback like Mutex.