Fearless SIMD v1.0

This is quite amazing. Fearless SIMD is what finally allowed my FFT crate, PhastFT, to run on stable Rust [0]. PhastFT previously depended on Rust’s std::simd (as well as multiversion), which still requires nightly [1].

One of the main contributors to v1.0 of fearless_simd started out by helping me evaluate portable SIMD crates for PhastFT. Once we landed on fearless_simd, he ported PhastFT from std::simd to fearless_simd. We did find that more functionality was needed than fearless_simd offered at the time. So, he started contributing significantly to fearless_simd. It’s really rewarding to see how working on a hobby open source project can help improve the Rust ecosystem.

[0] https://github.com/smu160/PhastFT

[1] https://doc.rust-lang.org/std/simd/index.html

I’m currently working on adding better SIMD support to Dart (https://github.com/dart-lang/sdk/issues/64170) and I have a question for the author or others here.

Does Rust or any other language support customizing the compiler so that interprocedural analyses can track custom subsets of, for example, doubles so that the compiler can choose the most efficient instruction sequence for example for min/max? If we know a double is never NaN then we can emit only one instruction on x86, but have to emit one more on arm64. If we know a double is never zero and never NaN, we can emit a single instruction on both.

This whole conversation between relaxed SIMD and deterministic SIMD seems to only exist because our compilers are not smart enough and/or their whole program analyses don’t support any plugin-like capabilities.

There are other examples where if we know a SIMD bitmask is canonical (all 1s per lane) then we can implement horizontal reductions more efficiently. This is very niche and I doubt that any language supports interprocedural analyses with such a rich domain, so it feels like a hole in the programming language space.

Sounds more like a book title than the name of a library, to be honest.

Have been really happy with Fearless simd, I used it to write [memchr-n][1], which searches for instances of an arbitrary set of bytes, with simd, which actually tends to [outperform][2] the rust memchr crate, even for sets of 1-3 bytes (what's supported by memchr)

[1]: https://github.com/Dr-Emann/memchr_n#performance [2]: https://github.com/BurntSushi/memchr/pull/241

Related. Others?

Towards fearless SIMD, 7 years later - https://news.ycombinator.com/item?id=43519823 - March 2025 (175 comments)

Towards fearless SIMD - https://news.ycombinator.com/item?id=18293209 - Oct 2018 (81 comments)

How does the crate’s SIMD performance compare to ISPC, which already has very easy dynamic dispatching?

Pretty happy this broke the 0.x curse!

I started an early Rust SIMD crate with similar aims (SIMDeez) and so I know how hard it is to do this well, so wanted to say congratulations to everyone who worked on Fearless SIMD. Tools like this are really nice for being able to leverage the gigantic performance modern CPUs make available without having to write intrinsics for each platform.

Congratulations.

Nice work. Looking forward to it.