Add hint::spin_loop() into Atomic*::*update()

[im-0]

Atomic*::update() and Atomic*::try_update() are implemented using a CAS loop and thus spin loop hint seems appropriate here.

Question: are there any relevant benchmarks to run to check that this patch does not regress anything? For example, I see that try_update() is used to implement RwLock. Are there any benchmarks for RwLock?

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[joboet]

Thanks for the pull request!

I'd argue against this: the CAS will only fail if the value is concurrently being changed but hint::spin_loop() is intended for cases where the value is repeatedly loaded to check whether some condition has been fulfilled, i.e. the thread is busy-waiting.

That's also why e.g. compare_exchange on targets that only have LL/SC instructions (like ARM) doesn't use the yield instruction, even though the value may be repeatedly loaded.