I noticed a bug in Guile 3.0.9’s aarch64 atomics handling, and found a couple of apparent solutions (1, 2), but one of them is weird enough for me t

Are aarch64 atomics really this sensitive? (eighty-twenty news)

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2024-04-22 10:30:03

I noticed a bug in Guile 3.0.9’s aarch64 atomics handling, and found a couple of apparent solutions (1, 2), but one of them is weird enough for me to write this post.

Long story short, the problem was with the equivalent of C’s atomic_exchange. Here’s the code that Guile’s JIT was generating:

which is fine, if you have an ARM v8.1 device to hand, but not if you don’t have a machine with Large System Extensions. So I tried, on a hunch, the second patch, which just changed the target of the cbnz, producing code like this:

Are aarch64 atomics really this sensitive? Is there only One True Instruction Sequence that should be used to implement atomic_exchange? Why does making this seemingly-insignificant change produce such a noticeable effect?