Compiling Rust with GCC: an update

Posted Sep 13, 2022 14:04 UTC (Tue) by hunger (subscriber, #36242)
In reply to: Compiling Rust with GCC: an update by calumapplepie
Parent article: Compiling Rust with GCC: an update

> Part of the idea is to stabilize rust: right now, there are no bugs in rustc, because the language is defined as what rustc says is valid. There's no standard for how rust should behave; the documentation is informative, not normative.

While totally true: How will adding a gcc-based rust compiler change this?

The current process to improve Rust is to write an RFC and provide an implementation for rustc. This is then extensively tested, feedback on the implementation is collected, all available Rust projects will be built to see what breaks, etc. With this process gcc-based Rust will always have to catch up to the "real" rust and will never be a serious alternative! The only way to avoid this is to change the process and make that less code- and more paper-based. The process would need to produce a specifications for the compiler teams to implement later. This sounds like a huge step backwards to me! Just look at C++ to see how poorly that works: Compilers are always behind the specification and it is a huge pain for projects to agree on the exact features they can use in their code (without loosing too many users that need to stick with older compilers).

But in practice this will probably not be necessary: The gcc-based rust compiler plans to reuse parts of the original rust compiler as those are factored out and become available. E.g. Polonius, the library that should eventually contain the entire borrow checker. This will significantly reduce the costs to maintain the gcc-rust project and is thus obviously a good thing. So in the long-run all possible programs that work with rust code will converge towards a shared frontend, incl. rustc, rust-analyzer, a stand-alone gcc-based rust compiler and more. Many of the Rust features will be in this shared front-end code eventually, at which point we are back at having an implementation defined language. This also re-introduces the trusting-trust issues you bring up: If the shared front-end code produces malicious high-level code, then all non-malicious backend-implementations will faithfully produce malicious binaries from their inputs.

to post comments

Compiling Rust with GCC: an update

Posted Sep 13, 2022 17:39 UTC (Tue) by atnot (guest, #124910) [Link] (4 responses)

My question is: if all this comes to pass as written, then what exactly was the purpose of gccrs over rust-codegen-gcc and mrustc?

It won't be a fully independent implementation that can be used to find ambiguities, as nebulous as that idea was. Folks who insist that Rust must have multiple compilers because that's what C has have seemingly moved the goalposts elsewhere already anyway. It doesn't help bootstrapping because it won't be pure C++ like mrustc. The people who have a weirdly selective worry about dependence on permissively licensed software won't be happy either. Who exactly is left then? GCC developers? People who's mouth froths at the sight of a Code of Conduct? People who can install a modern gcc version and all of the other rust development tools, but not rustc? Are there really enough of those?

We've seen this play out a few times before, even: People demanded gcc frontends for D and Go, which are mostly abandoned and nobody uses, because why would you. I don't see how this vision of gccrs would be any different.

Compiling Rust with GCC: an update

Posted Sep 13, 2022 17:53 UTC (Tue) by rahulsundaram (subscriber, #21946) [Link] (2 responses)

> People demanded gcc frontends for D and Go, which are mostly abandoned and nobody uses, because why would you

I have seen this claim made in several places but this isn't what happened atleast for Go. It wasn't based on any demand from anyone. Go team themselves decided to do it.

https://go.dev/blog/gccgo-in-gcc-471

"The Go language has always been defined by a spec, not an implementation. The Go team has written two different compilers that implement that spec: gc and gccgo. Having two different implementations helps ensure that the spec is complete and correct: when the compilers disagree, we fix the spec, and change one or both compilers accordingly. Gc is the original compiler, and the go tool uses it by default. Gccgo is a different implementation with a different focus, and in this post we’ll take a closer look at it."

Compiling Rust with GCC: an update

Posted Sep 13, 2022 19:16 UTC (Tue) by atnot (guest, #124910) [Link] (1 responses)

I did indeed not know that, thanks for pointing that out! I'm not sure if it really makes things better though: A frontend for a relatively simple language with the full support of a team that earnestly believed in the specification with multiple implementations approach is today mostly irrelevant, has a single active contributor and appears to be at least half a year behind an already frozen spec. I don't hate the idea of multiple implementations, but it really gives reason to temper your expectations of what gccrs will deliver.

Compiling Rust with GCC: an update

Posted Sep 13, 2022 23:09 UTC (Tue) by rahulsundaram (subscriber, #21946) [Link]

> I did indeed not know that, thanks for pointing that out! I'm not sure if it really makes things better though

It may not be better but it is more accurate. Even if one were to stick to the same conclusion they had originally, it's helpful to validate the data points. The common narrative appears to be that GCC support for various languages including Rust are being added based on unreasonable demands for some unknown but definitely odd reasons and they will inevitably fail or splinter the language and the spec based approach is an old relic of the past that must be inherently doomed one way or the other. It doesn't leave room much room for acknowledging that multiple languages do have several successfully used implementations or even a mild curiosity of why things like gccrs is even funded by commercial organizations in the first place.

> I don't hate the idea of multiple implementations, but it really gives reason to temper your expectations of what gccrs will deliver.

This is something readily acknowledged in https://github.com/Rust-GCC/gccrs/wiki/Frequently-Asked-Q... but this is some extend mitigated by funding and possibility of code sharing via things like polonius.

Compiling Rust with GCC: an update

Posted Sep 23, 2022 14:42 UTC (Fri) by njs (subscriber, #40338) [Link]

Given that gccrs is funded by the grsecurity folks, and their big product is "the kernel, but compiled with proprietary gcc plugins", I always assumed the motivation for gccrs is that they want to get rust into the gcc pipeline early enough that it's before their plugins.