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Comment by fulafel | Hacker News Reader

It seems that in https://perf.rust-lang.org/dashboard.html the improvements have essentially plateaued, I wonder how they decide how much work to invest there.

(Although as the language complexity continues to grow maybe that question is missing the opposite force of compiler slowdowns from added work?)

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For some insight into what more can be done, see

- https://internals.rust-lang.org/t/slow-incremental-compilati...

- https://internals.rust-lang.org/t/feedback-request-performan... (there is more context to that being published today in a blog post)

There are several more ideas that are in even earlier stages of discussion.

The log-scale plot hasn't plateaued yet, so there are still rooms for improvement. (Remind that rustc was initially quite slow at the very beginning.)

I wonder if these graphs capture the new multi-threaded frontend that's on nightly.

genuine question: how much can the frontend really impact compile times at the end of the day? I would guess most of the time spent compiling is on the backend, but IANA compiler engineer

One obvious example of this would be C++, where a smarter frontend that doesn't do textual inclusion of a million lines would significantly improve compile times.

that makes sense. are Rust's macros handled in the frontend? if so, I could see how multithreading could have a dramatic impact

But that's low-hanging fruit "optimization", no? Once you get around it, and this has been forever, the bottleneck is in the backend generation. So if Rust has already solved this, the area where they can improve most is the backend generation?

Most C++ builds I have worked with, and all of them being MMLoC codebases, were actually bottlenecked by the high memory pressure. In part due to heavy templates and symbol visibility.

I think the history of C++ implementations shows that it's not low hanging fruit, it's a huge effort to implement and the payoffs aren't game changing.

Modules? Yeah, I don't expect them to be a game changer either. Exactly because of the reasons I mentioned.

> How much can the frontend really impact compile times?

Your question is addressed in this blog post - Faster compilation with the parallel front-end in nightly (https://blog.rust-lang.org/2023/11/09/parallel-rustc.html). One the example shown there:

- Serial took 10.2 seconds in the frontend and 6.2 seconds in the backend.

- Parallel tool 5.9 seconds (-42%) and 5.3 seconds (-14.5%).

So to answer your question, parallelising the frontend can have a substantial impact.

You are right that frontend is only a subset of the total compilation time - backend and linking time matter too. Happily, that's something that's being worked on too!

There is an alternate backend called cranelift (https://bjorn3.github.io/2024/11/14/progress-report-nov-2024...) that compiles faster than LLVM, at the cost of producing slower binaries. That's a good trade-off for debug builds.

And there's work on changing the default linker to lld (https://blog.rust-lang.org/2024/05/17/enabling-rust-lld-on-l...).

We may see all 3 efforts land in the next year :)

These graphs only include stable releases, not nightly ones. Since the multi-threaded frontend hasn't landed on stable yet, we can't see it's effect on these graphs.

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