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Comment by littlestymaar

2 days ago

This kind of comment is funny because it reveals how uninformed people can be while having a strong opinion on a topic.

Yes grammar can impact how theoretically fast a compiler can be, and yes the type system ads more or less works depending on how it's designed, but none of these are what makes Rust compiler slow. Parsing and lexing are negligible fraction of compile time, and typing isn't particularly heavy in most cases (with the exception of niches crates who abuse the Turing completeness of the Trait system). You're not going to make big gains by changing these.

The massive gains are to be made later in the pipeline (or earlier, by having a way to avoid re-compiling pro macros and their dependencies before the actual compilation can even start).

6 comments

littlestymaar

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lsuresh  2 days ago

Hard agree. Practically all the bottlenecks we run into with Rust compilation have to do with the LLVM passes. The frontend doesn't even come close. (e.g. https://www.feldera.com/blog/cutting-down-rust-compile-times...)

  • edflsafoiewq  2 days ago

    The point was language design influences compiler performance. Rust is heavily designed around "zero-cost abstraction", ie. generate tons of IR and let the backend sort it out. Spending all the time in LLVM passes is what you would expect from this.

    • littlestymaar  2 days ago

      Had you read the linked blog post, you'd have seen that here this isn't so much an issue with LLVM having too much work, but rustc being currently unable to split the work into parallelizable chunks before sending it to LLVM, and as such it takes a very long time not because LLVM has too much things to do, but because it does it in a single-threaded fashion, leaving tons of performance on the table.

      > Rust is heavily designed around "zero-cost abstraction", ie. generate tons of IR and let the backend sort it out.

      Those two aren't equivalent: Rust is indeed designed around zero-cost abstraction, and it currently generates tons of IR for the backend to optimize, but it doesn't have to, it could run some optimizations in the front-end so it generates less IR. In fact there has been ongoing work to do exactly this to improve compiler performance. But this required rearchitecturing the compiler in depth (IIRC Rust's MIR has been introduced for that very reason).