Use __kuser_cmpxchg64 for 64-bit atomics on pre-v6 ARM Linux/Android #82
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Ok, benchmarked on Graviton2 (Neoverse-N1)'s aarch32 mode. https://cirrus-ci.com/task/6137907514179584 |
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Currently, we are using fallback implementation for 64-bit atomics on pre-v6 ARM Linux/Android such as armv5te-unknown-linux-gnueabi and arm-linux-androideabi.
However, Linux kernel 3.1+ provides kernel user helpers for 64-bit atomics. This could be more efficient than a lock-based fallback implementation, because it calls native atomic instructions, depending on the actual CPU version.
This PR uses __kuser_cmpxchg64 on Linux kernel 3.1+, otherwise use fallback implementation as before.
Since Rust 1.64, the Linux kernel requirement for Rust when using std1 is 3.2+, so it should be possible to omit the dynamic kernel version check if the std feature is enabled on Rust 1.64+, but that has not yet been implemented.
Footnotes
https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html#affected-targets says "Targets which only use libcore and not libstd are unaffected." ↩