Comment by hobofan | Hacker News Reader

Comment by hobofan | Hacker News Reader

> But a distinction between JavaScript and Rust+WebAssembly emerges when we consider the effort required to attain inlining and monomorphization, or to avoid allocations.

I'm not sure that is true. Having worked/interacted with a lot of people working with Rust on different experience levels, most of them (that includes me) don't have a deep knowledge of what Rust concept maps to a specific concept with which performance implications. And if they do it's often only partial. I'd say that right now, only very few people that don't work on the Rust compiler have a broad knowledge in that area. Sure, it's much better to have to Result of the optimization expressed in code itself, but I'd say that the amount of knowledge and effort required to get to such a level of optimization is similar to optimizing Javascript.

I also found the hint to `#[inline]` suggestions, a bit disingenuous. In the end they are just _suggestions_, and your are just as much at the mercy of the Rust/LLVM optimizer to accept them, as you are with a Javascript JIT.

I'm a big fan of Rust, and I'm a big fan of Rust+Webassembly (working with it is the most fun I had programming in a long time!). Generally I think that Rust has one of the better performance optimzation stories, I just don't a gree with some of the sentiments in the post. There are also enough other reasons to love Rust+WebAssembly than just the peformance!

> I'd say that the amount of knowledge and effort required to get to such a level of optimization is similar to optimizing Javascript.

Really? After reading all of the articles in this series (the original about porting to Rust, the rebuttal about optimizing JavaScript, and this one)?

I'm more familiar with Rust than JavaScript, but I found that other than the algorithmic optimization, the rest of the JavaScript optimizations were very non-obvious in order to achieve an effect that is entirely natural in Rust.

There's no need to be careful to avoid particular types of function calls, since there are no dynamic variadic function calls in Rust. There's no need to resort to using the equivalent of `eval` for monomorphization, it's a natural feature of the language. There's no need to do manual memory management by encoding values into typed arrays of integers; you can simply allocate vectors of structs, borrow references, and so on.

These are all things that had significant cost in JS, and needed to be worked around via some intensive profiling and knowledge of how JIT engines work, but just doing things naturally in Rust leads to a pretty much zero-cost solution.

It's true that if you want to really get into the nitty-gritty of micro-optimization in Rust, you need to learn some more and do things like profiling and inspecting the generated code to see what optimizations the compiler was able to apply or not.

But the Rust rewrite of source maps did none of that; they just rewrote it in fairly idiomatic Rust, and achieved a similar speedup to what a JIT engineer armed with a profiler, a deep knowledge of what affects how well a JIT works, and a willingness to do manual memory management in typed arrays was able to do.

> Having worked/interacted with a lot of people working with Rust on different experience levels, most of them (that includes me) don't have a deep knowledge of what Rust concept maps to a specific concept with which performance implications

What parts of Rust do you feel like you don't understand the performance implications of? I feel like the mental model for performance is relatively similar to C or C++, with relatively few things that would surprise you if you're familiar with modern C++ (in fact, a lot fewer performance surprises than modern C++ offers, in addition to the extra safety).

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