Luca Versari, one of the devs between both libraries, has a performance dashboard to compare performance between the two https://jxl-rs-perf.lucaversari.it/

jxl-rs started to outperform the C++ library 2 months ago.

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phire 4 hours ago | parent | next [-]

As much as I enjoy rust, there is no reason why a c++ library can’t be optimised to match the rust implementation.

It’s very rare that the actual performance benefits of rust implementations come from rust itself (though it does often push you to slightly better patterns). They usually come from the fact that rust implementations are usually a second (or 3rd, or 4th) iteration of the design, and the lessons learned help performance.

The other benefit of rust is that the stronger type system makes it easier to iterate and optimise without bugs creeping in. But once optimisations are implemented in rust, there isn’t that much pain to porting them back to c++, as long as someone cares enough to do so.

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magicalist 4 hours ago | parent | next [-]

> As much as I enjoy rust, there is no reason why a c++ library can’t be optimised to match the rust implementation.

I don't think anyone is claiming that. I took the GP's point as what was desired was a "safe, performant, compact, and compatible JPEG XL decoder in Rust" and "performant" (as defined as the speed of the c++ version) was passed two months ago.

> They usually come from the fact that rust implementations are usually a second (or 3rd, or 4th) iteration of the design, and the lessons learned help performance.

Sure, and looking at the recent commit histories, the team is focusing on getting the rust version ready for this milestone for obvious reasons, and is less concerned about immediate parity in the c++ codebase.

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bawolff 2 hours ago | parent | prev | next [-]

>As much as I enjoy rust, there is no reason why a c++ library can’t be optimised to match the rust implementation

but why would you bother? The rust library is the one that is being chosen to use. There is no point optimizing a library which is not going to be used.

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hn92726819 4 hours ago | parent | prev | next [-]

I don't understand why anyone would continue working on the c++ project once the rust one started beating it in performance

	▲	Snafuh 3 hours ago \| parent [-]
		jxl-rs (rust) is just a decoder while libjxl (C++) can also encode. There is a rust encoder in active developing by someone outside the core JPEG XL devs.

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pibaker 3 hours ago | parent | prev [-]

I suspect the real reason is safety. Rust isn't bulletproof but it's certainly much better than C++ when it comes to defending against memory corruption related attacks. And when you are building a decoder for untrusted data sent over the internet, this kind of thing matters a lot more.

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rfgplk 4 hours ago | parent | prev [-]

If he is one of the devs between both libraries, why is there a performance gap? Why not port the optimizations from one lib to the other? You can even create a pinned agent workflow that automatically translates optimizations between repos. Fairly trivial to implement actually.

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AlotOfReading 4 hours ago | parent | next [-]

Just because you can theoretically write equivalent code in both languages doesn't mean two idiomatic implementations in each language will be 1:1 with each other. I haven't looked at the code in question, but some examples of common differences:

A C++ program might do template metaprogramming at compile time and the same thing at runtime in Rust, or vice versa. The C++ version might use virtual functions that rust wouldn't use. The Rust version might simply give more optimization information to the backend. The C++ version might use fairly awful parts of the stdlib like iostreams or shared_ptr that rust simply implements better. Etc.

Or maybe they're just focused on the rust implementation as they should be.

	▲	josephg 2 hours ago \| parent [-]
		Yep. Also: The rust borrow checker makes it difficult to implement tree like structures with pointers like you would in C or C++. Safe rust trees are (imo) best written using vecs. Rust makes function arguments noalias. Rust adds runtime array bound checks. Rust and C++ do iteration quite differently. Rust encourages map/filter/reduce. I suspect this results in different assembly. Doing a “unity build” in rust is really easy (codegen-units=1). In C++, you need to make heavy modifications to your build system and sometimes your source too. But with some time you could probably port the optimisations across. If anyone has some spare tokens, Claude can be quite good at doing this sort of work. Show it both repositories and tell it to make the C++ code just as fast as rust.

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NewJazz 4 hours ago | parent | prev [-]

Because developers of security critical code don't flippantly merge LLM changes for marginal performance gain, and their time is incredibly valuable.