Issue #18481 has been updated by maximecb (Maxime Chevalier-Boisvert).


> TBH, with my Fedora/CS/RHEL maintainer hat on, this proposal scares me.

I completely understand. This is a departure from the Ruby tradition so to speak and as such, has to be considered carefully.

> I am not sure if it will stop there. Because you might start to require some additional libraries, not just Rust itself.

If it makes you feel any better, we are very disciplined programmers. As it is, YJIT has no external run-time dependencies besides what comes with the C compiler, and an optional disassembler which we built without by default. We're very careful about bloat and dependency creep. We don't want to build a fragile software product.

> It won't be today, tomorrow or maybe even years, but should YJIT be successful, it will eventually become default.

That's true, but in order to get to that point, YJIT has to succeed, and we feel that we are running in to real maintainability challenges with C99. JIT compilers have to maintain a lot of auxiliary data structures that are associated with generated code. Intermediate representations, control flow graphs, etc. All of these things have to be allocated, initialized and freed correctly as they're being transformed. We run into situations where it's very easy to forget calling a function at the right place, etc. We also have more pedestrian issues such as, we have an x86 assembler, and soon we'll have an arm64 assembler. A lot of the instructions have the same name. You can add prefixes to your C function names to avoid clashes, but soon you end up with very long function names. 

We want to implement things like lazily marking a page of code as writable/executable to reduce the number of system calls that we make. This is also more challenging to implement in C, because again, you need auxiliary data structures to track this sort of thing, which you're going to have to manually track memory allocation for.

> Last but not least, we would have to deal with new language (which can be seen as an opportunity, of course ;) )
I might be pessimistic, but it won't make life of package maintainer easier.

I understand. I know that there are challenges. We want to be accommodating. Would you be provide some specifics of how difficult it would be to compile rust code while packaging Ruby for Fedora/CS/RHEL? Is there an existing rustc package? Would you be able to use rustup during bundling?

> BTW should this be start of Ruby migration/rewrite in Rust, that would be different discussion ;-P

Probably not happening but one can dream ;)

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Feature #18481: Porting YJIT to Rust (request for feedback)
https://bugs.ruby-lang.org/issues/18481#change-95932

* Author: maximecb (Maxime Chevalier-Boisvert)
* Status: Open
* Priority: Normal
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TL;DR: The YJIT team wants to explore using Rust to help develop YJIT. The rest of CRuby will continue to build without Rust tools and building YJIT will remain optional.

We°«re currently exploring the possibility of porting YJIT to Rust and working on a small proof of concept that should be ready next month. The motivation behind this is that we are facing challenges in terms of code maintainability. As you know, JIT compilers can get very complex, and C99 doesn't offer many tools to manage this complexity. There are no classes and methods, limited type checking, and it's hard to fully separate code into modules, for instance.

We believe that having access to object oriented programming and a more expressive type system would help us manage growing complexity better and also improve the safety/robustness of YJIT. For instance we would like to add Windows support and a new backend to YJIT. That means we°«ll have two separate backends (x86, arm64) and we°«ll need to support two different calling conventions (Microsoft, SystemV), but currently, we have limited tools to build the abstractions needed, such as preprocessor macros and if-statements.

We°«ve discussed the idea of porting YJIT to Rust with some of the Ruby core developers (@ko1, @k0kubun, @mame), and it seems they would be open to merging something like this if it works well. I°«m opening this ticket so that everyone can have a chance to provide feedback and participate in the discussion. We realize that adding Rust code to the CRuby codebase could be challenging and that there are open questions.

We are planning to make it so that YJIT will only need the Rust compiler and `cargo` to build. Building YJIT would then require the Rust compiler to be installed, but CRuby could build without YJIT and without the Rust compiler. There would be no new dependencies for the compiled binary. Rust is supported on Mac, Windows, BSDs, and Linux, which covers all the platforms on which we plan to support YJIT. Since Rust is based on LLVM, it has good support for cross-compilation.

We would like to solicit input from Ruby distributors who create `.deb` and `.rpm` packages. We will likely remain conservative when updating Rust versions to make OS packaging easier. We believe that in the general, the resulting code should be easier to maintain because it will be better organized, but the YJIT team will help out with YJIT-related backports and will be available to help if needed.

Value proposition:
- Rust type systems will catch more bugs early, help prevent new bugs
- Easier to manage growing complexity of YJIT
- Easier to maintain codebase, fewer °»footguns°…
- Easier for newcomers because the compiler catches more bugs
- Better performance because we can implement more sophisticated optimizations
- Easier to add support for new platforms (which adds complexity)
- Rust has mature and easy-to-install tools such as source code formatter and editor plugins
- Rust as a programming language community has a great deal of enthusiasm behind it. This could translate to more enthusiasm for YJIT and for Ruby as well.

Integration:
- YJIT will only depend on the Rust language and the standard library, no other dependencies
- YJIT will be able to build without an internet connection
- Rust has good support for cross-compilation
- Rust is supported on all platforms on which we plan to support with YJIT (Mac, Linux, Windows)
- The compiled CRuby binary won°«t have any new dependencies on shared libraries
- CRuby will still be able to build without `rustc`, with YJIT disabled






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