experiment with how to pick the right SIMD implementation

[folkertdev]

we currently use the stdlib macros in many places. I believe this is reasonably fast for us (we don't do it in tight loops), but still those macros have overhead. We might benefit from a different approach. I came across

https://crates.io/crates/multiversion

which apparently has some tricks up its sleeve.

[gdonval]

Wouldn't std::Simd be a great way to fully generalize SIMD implementations? Then multiversion can be used as intended (to automatically compile multiple runtime version and do the dispatch).

[bjorn3]

std::simd::Simd isn't stable yet, so zlib-rs can't use it yet.

[folkertdev]

And even if it were, std::simd::Simd is supposed to be cross-platform, and it's implied that it generates decent code on all platforms it supports. We do have some very target-specific SIMD instructions that I don't think the standard library ever will nor should support.

What we would like to see is native multi-versioning https://rust-lang.github.io/rust-project-goals/2025h1/simd-multiversioning.html. We're making do with const generics at the moment.