Add NEON encode and check

[Lynnesbian]

Add implementations for hex_encode and hex_check using ARM's NEON (aka AdvSIMD) SIMD instruction set. These implementations are based on the existing SSE4.2 ones - they're more or less direct translations.

These implementations are only active on aarch64 targets and not 32-bit ARM targets (armv7 etc), because NEON intrinsics on 32-bit ARM are unstable.

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Unfortunately, checking for NEON support at runtime is a difficult problem to solve. My current implementation is less than ideal:

https://github.com/Lynnesbian/faster-hex/blob/859221bbcfd2256047b5bf6d334f30beb906ee3f/src/lib.rs#L159-L171

I've found a variety of differing ways to get this information on Aarch64 platforms:

• Linux: The HWCAP interface (getauxval()), or reading /proc/cpuinfo
• Android: Google's cpu_features library
• Windows: Call IsProcessorFeaturePresent with PF_ARM_NEON_INSTRUCTIONS_AVAILABLE
• BSDs:
  • FreeBSD: elf_aux_info()
  • NetBSD: sysctlbyname with machdep.neon_present
  • OpenBSD: sysctl with CTL_MACHDEP and CPU_ID_AA64PFR0
  • It's worth noting that /proc/cpuinfo, if enabled by the given BSD, will also work

There's no nice, cross-platform, no-std method to do this, like there is with x86's cpuid. And worse - many of these methods only work for Aarch64, and not 32-bit ARM platforms.

I decided against including all of these methods in the vectorization_support function. They'd necessitate bringing in multiple new dependencies, and would make testing much more complicated.

[Lynnesbian]

Unfortunately, the only Aarch64 device I have access to for benchmarking with is my phone (a Samsung Galaxy A73). My Raspberry Pi 3B seems to have died since I last used it years ago 😢

Here are the relevant benchmark results from running cargo bench under Termux (which is, of course, far from an ideal benchmarking environment):

Bench	Result
bench_faster_hex_encode	91.559 ns
bench_faster_hex_encode_fallback	140.00 ns

You can view the full output from cargo bench here.

[eval-exec]

Thank you, is it possible to run the benchmark on the CI workflow?

[Lynnesbian]

Sure, how would I do that? Would I need to add benchmarks to the rust.yml workflow file?

[eval-exec]

Sure, how would I do that? Would I need to add benchmarks to the rust.yml workflow file?

Sure.

[Lynnesbian]

Unfortunately it seems like GitHub doesn't have any Aarch64 runners available at this time, but they're aiming for them to be available by the end of the year.

This means there's currently no way to run the CI on an Aarch64 runner, unless you want to set up self-hosted runners.