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How to speed up the Rust compiler in October 2022

75 points by itamarst 4y ago

moomin 4y ago
I learned quite recently that one of the limiting factors in the Rust compiler’s performance is the LLVM IR it produces being of poor quality. This means that LLVM itself takes much longer to produce code. Are there any initiatives to address this?
- emi2k01 4y ago
  Yes, I guess?: Cranelift, which is a LLVM-like library written in Rust. This could be used for faster debug builds in the future. There's a Rust backend using Cranelift [2]. If you mean addressing the poor quality of LLVM IR directly, then I don't know. [1]: https://github.com/bytecodealliance/wasmtime/blob/main/cranelift/README.md https://github.com/bytecodealliance/wasmtime/blob/main/crane... [2]: https://github.com/bjorn3/rustc_codegen_cranelift https://github.com/bjorn3/rustc_codegen_cranelift
- spoiler 4y ago
  I thought (maybe incorrectly) that most of the time is spent on linking in Rust codebase. Aside: out of curiosity, how did you find out about this fact/quirk about LLVM IR?
  - moomin 4y ago
    Amazingly, it's actually in the original rust-lang FAQ: https://prev.rust-lang.org/en-US/faq.html#why-is-rustc-slow https://prev.rust-lang.org/en-US/faq.html#why-is-rustc-slow
    - spoiler 4y ago
      Thank you!
- PoignardAzur 4y ago
  It's unclear. There were discussions on the rustc Zulip that recently pointed to LLVM being actually quite performant at canonicalizing poor-quality IR; so any time saved in the backend by producing better IR is offset by the additional time spent in the front-end to produce that IR. (I'm not a compiler developer, don't quote me on this)
AndyKelley 4y ago
This is inspiring work - thank you for sharing. That timer bot is a game changer.
mkj 4y ago
I guess the memory reduction from data structures might help rust-analyzer too? That would be pretty useful since it's always running in the background.
- PoignardAzur 4y ago
  AFAIR rust-analyzer is a completely separate implementation with its own data structures, so I don't think that will help much.
jokoon 4y ago
Aren't there a lot of sorts of things that are just impossible or difficult to do in rust, like GUI, or need to be entirely rewritten in rust? Rust can be interfaced with C, but I don't see if you can really benefit from rust safety since it can interact with unsafe languages. It's still better than not, but if the problem is safety, mixing languages might not be easy.

75 points by itamarst 4y ago

I learned quite recently that one of the limiting factors in the Rust compiler’s performance is the LLVM IR it produces being of poor quality. This means that LLVM itself takes much longer to produce code. Are there any initiatives to address this?

emi2k01 4y ago
Yes, I guess?: Cranelift, which is a LLVM-like library written in Rust. This could be used for faster debug builds in the future. There's a Rust backend using Cranelift [2]. If you mean addressing the poor quality of LLVM IR directly, then I don't know. [1]: https://github.com/bytecodealliance/wasmtime/blob/main/cranelift/README.md https://github.com/bytecodealliance/wasmtime/blob/main/crane... [2]: https://github.com/bjorn3/rustc_codegen_cranelift https://github.com/bjorn3/rustc_codegen_cranelift
spoiler 4y ago
I thought (maybe incorrectly) that most of the time is spent on linking in Rust codebase. Aside: out of curiosity, how did you find out about this fact/quirk about LLVM IR?
- moomin 4y ago
  Amazingly, it's actually in the original rust-lang FAQ: https://prev.rust-lang.org/en-US/faq.html#why-is-rustc-slow https://prev.rust-lang.org/en-US/faq.html#why-is-rustc-slow
  - spoiler 4y ago
    Thank you!
PoignardAzur 4y ago
It's unclear. There were discussions on the rustc Zulip that recently pointed to LLVM being actually quite performant at canonicalizing poor-quality IR; so any time saved in the backend by producing better IR is offset by the additional time spent in the front-end to produce that IR. (I'm not a compiler developer, don't quote me on this)

AndyKelley 4y ago

This is inspiring work - thank you for sharing. That timer bot is a game changer.

mkj 4y ago

I guess the memory reduction from data structures might help rust-analyzer too? That would be pretty useful since it's always running in the background.

PoignardAzur 4y ago
AFAIR rust-analyzer is a completely separate implementation with its own data structures, so I don't think that will help much.

jokoon 4y ago

Aren't there a lot of sorts of things that are just impossible or difficult to do in rust, like GUI, or need to be entirely rewritten in rust? Rust can be interfaced with C, but I don't see if you can really benefit from rust safety since it can interact with unsafe languages. It's still better than not, but if the problem is safety, mixing languages might not be easy.