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Comment by kzrdude | Hacker News Reader

Performance of the generated code is always the #1 priority in both Rust and llvm, performance improvements are rarely declined, and this prioritization needs to change radically, to get a much faster compiler.

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Prioritization is not that straightforward. Every open source project has different sub-groups with different goals. While we do care a ton about generated code, there is also movement by other folks to work on the compiler speed problem. It has been a lot of work to come this far, and it will be even more work to get to a better place, but that work is underway. That's not even to mention that since rustc is written in Rust, work to make generated code faster can also make the compiler faster.

I'm referring to the rust-analyzer work, which is effectively an outside-in re-write of the compiler, if you squint at it right.

At the end of the day, rustc is a very large codebase, and has already undergone major architectural changes, and will be undergoing even more. It is not fast to steer a large ship in a new direction, but that is what is happening.

See https://pbs.twimg.com/media/ENC1ls6XYAEdKkv?format=jpg&name=... for a time I ran https://github.com/erikbern/git-of-theseus on the rustc codebase. Thank goodness Rust is such a good refactoring language.

You are talking incremental improvements. Radical is needed not just using cranelift for debug builds, but something like that for all builds. Current llvm model does not work well for delivering a fast compiler (opt and non-debug)

You can deliver radical improvements in an incremental fashion. The move towards a query-based architecture, and the rust-analyzer work, are both fundamental, radical improvements.

If anything, the cranelift stuff is less radical. Doesn't require any architecture changes, "only" involves implementing the backend API.

then there's also ongoing work in rustc_codegen_cranelift, which [as of 2 weeks ago] can now compile rustc:

https://github.com/bjorn3/rustc_codegen_cranelift/issues/743...

Yeah to be clear, I am very excited about cranelift too!

Sure, that's seriously cool but I don't see how it changes the material balance so to speak... If you still want llvm to codegen the same items, we don't have any radical departure at all, just a different way to do the same thing, when we are release-compiling any particular crate, from scratch.

Oh sure, if you're talking purely about a final release build, then yes. But most of the time, compiles are not final release build compiles. People don't generally complain about final release build times. They complain about the build time as they're developing.

Final release build times are up there as build automation systems are often at capacity

For sure, and I don't mean to say that they don't matter. I have worked on codebases that only use release mode, for reasons, for example. I just hear less complaints about it, that's all I'm saying :)

Usually I've observed release is used by automated systems and debug locally. If you're not managing the automation or working closely with those people, you're less likely to see those complaints but they definitely exist.

I regularly need to run release builds while developing. Not as commonly as I need to do a debug build or a cargo check, but still often enough that the slowness is annoying.

What is wrong having 2 versions: slower at runtime but super fast compile for debug and the other way around for production?

Of course, that is the status quo for rustc (as it is for all production compilers I know of). The issues are that (1) debug builds are still slow enough to be annoying, and (2) they are sometimes _so much slower_ to execute that a debug build + test run is slower than a release build + run.

A lot of work is being done to address these issues. The big projects I'm aware of are a move to a query-based compiler (which should improve incremental build times) and the new cranelift backend, which should dramatically speed up debug build times.

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