Comment by josephg
6 months ago
> The compiler is also relatively slow. Would Rust have been worth working with on 30+ year old hardware?
As I understand it, a lot of the slowness of the rust compiler comes about from llvm. And how rust and llvm interoperate. Rustc creates and sends gigabytes of stuff to llvm - which passes all of that to its optimizer. If you skip all that work - for example by running cargo check - the compiler is an order of magnitude faster.
If rust were invented in the 90s, it wouldn’t have used llvm. Rust could still have been implemented, and we’d probably have a much faster compiler as a result. But it would have missed out on all the benefits of llvm too. It would have needed its own backend to be written - which would have been more work. And the compiler probably wouldn’t have been as good at low level optimisations. And it probably wouldn’t have out of the box support for so many target platforms. At least, not from day 1.
The obvious(?) question is why it sends gigabytes of stuff to llvm and if that can't be reduced somehow.
> The obvious(?) question is why it sends gigabytes of stuff to llvm
IIRC it's a combination of technical debt from earlier in Rust's life (it's easier to generate naive LLVM IR and let LLVM's optimizer do the heavy lifting of chewing through that) and how Rust implements generics via monomorphization
> and if that can't be reduced somehow.
I believe the technical debt bit can be (and is being!) reduced by implementing optimizations and better IR generation in rustc itself. As for the monomorphization strategy, I thought I remembered reading something about how Rust technically allows for generics to be implemented via non-monomorphization strategies like type erasure/dynamic dispatch, but I can't seem to find that post/article/whatever it was now so I'm not sure I'm not making it up. That being said, there are patterns to reduce the amount of code generated (e.g., generic facade that forwards to a non-generic implementation), but those need to be manually implemented at the moment and I don't think there's significant work towards automating that at the moment.
For the latter, the term you want to search for is polymorphization. It is about making the compiler do the work that those manual façades do, automatically.
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