Comment by benesch | Hacker News Reader

Comment by benesch | Hacker News Reader

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Part of what makes rustc's slow compile times so frustrating is that the slow compile times are mostly unrelated to all of the fancy compile-time analysis you listed. You can see this in action if you run `cargo check`. That will run all the special static analysis (typeck, borrowck, dropck, etc.) and complete in a very reasonable amount of time.

To a first approximation, rustc is slow because it generates terrible LLVM IR, and relies on a number of slow LLVM optimization passes to smooth that IR into something reasonable. As far as I can tell this is entirely a result of not having the necessary engineering resources to throw at the problem; Mozilla has a limited budget for Rust, and needs to spread that budget across not just the compiler toolchain, but the language and ecosystem.

Brian Anderson's been working on a blog post series that covers Rust's slow compile times in more detail, if you're curious: https://pingcap.com/blog/tag/Rust

Yeah, I realize that Rust has a lot of room for improvement. I'm just saying that compared to the alternative, you get a relatively inefficient compiler that, despite its inefficiency, still gives you a more efficient development process. You get instant feedback, which you can't get from a testkit, and you can eliminate massive amounts of testing.

If the focus is entirely on reducing compile times, it can very quickly lead down a path that limits runtime performance. There is so much potential for increased runtime performance in strongly typed languages, due to the guarantees made by the type system. It allows for analysis and reasoning that not just enables micro-optimizations but also macro-optimizations across entire codebases. Entire theses have been written describing optimizations that rely on immutability alone, and we haven't even touched so many other possibilities that rust's design enables (such as the ownership system).

I'm fairly familiar with the project to work on the LLVM-IR code emitter. If the focus is on reducing the complexity and cruft of the LLVM-IR emitted, that's a win all around (because you get faster code and faster compile times). I just hope that reducing compile times doesn't become the core objective, because that eliminates a lot of future potential.

Note that Rust doesn't really generate worse LLVM IR than clang does, given the same input. It's just that as a language, Rust needs MIR optimizations more than C and C++ do, given the increased abstraction (largely necessary for safety).

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