[faisal_ksa]

What is the benefit of having multiple compilers for programming languages? Is there a scenario where a GCC compiled rust program would do something that an LLVM one can't do?

Doesn't this cause fragmentation in the rust ecosystem?

P.S.:I understand that people can work on any project they want. And I don't have the right to tell them not to. I'm just curious about the technical reasons for having multiple compilers.

[zetaposter]

1. GCC has more backends than LLVM. 2. Competition is good in general. 3. I expect this will trigger inconsistencies between GCC and rustc; because Rust doesn't really have a specification. Which will force both parties to discuss and solve them.

[tomjakubowski]

Being on gcc, a long-lived platform, also helps ensure the survival of the language even if development of the current compiler (or LLVM) dies or withers.

[duckerude]

Does it? GCC's Java frontend died and is no longer shipped, they need maintainers like any other compiler.

[genewitch]

GCJ is no more? It was being used within recent memory for things, i thought.

I am out of the loop though, so if this is true, that's interesting and a bit weird.

[pjmlp]

Since 2009 actually.

Most contributors eventually moved into OpenJDK after it became available.

GCC folks left it around for a couple of years, because GCJ unit tests exercised parts of the compiler no one else did.

Eventually they decided it wasn't worth that maintenance cost to keep it around only for that purpose.

[apple4ever]

GCC support of Objective-C is very very poor.

[pjmlp]

It is at the level NeXT was forced to contribute back to upstream.

[astrange]

Reimplementing ObjC 2.0 would be very hard. You have to be precisely bug-compatible with Clang to implement ARC correctly.

[israrkhan]

and to back up your point... There should be at least 2 implementations for anything to be a spec/standard.

[faisal_ksa]

Thank you so much. The specification point is very important

[netheril96]

> I expect this will trigger inconsistencies between GCC and rustc; because Rust doesn't really have a specification. Which will force both parties to discuss and solve them.

More likely that GCC has to follow all the bugs and quirks of rustc or no people will use GCC for Rust.

[nickitolas]

> 1. GCC has more backends than LLVM

Did you mean target platforms? If so, how is this not already addressed by the rustc gcc (Via libgccjit) backend?

[sicp-enjoyer]

One advantage is it forces the language to articulate standards instead of the implementation defining the feature set. Standards tend to give stability and longevity to the language, as well as making it possible to write new compilers and make it more portable.

[melony]

Ah, a Lisp user. Common Lisp is Exhibit A for standardization. Every Lisp user claims it is great because of either standardization of advanced features in the days when the Berlin Wall has barely fallen or the mere existence of macros. No real first-party improvement to the language in almost three decades after ANSI standardisation. Massive fragmentation in the compiler ecosystem, rarely do libraries work out of the box on non-SBCL tooling. Yes, I can definitely see the advantage of standardization now, very much so.

[sicp-enjoyer]

> No real first-party improvement to the language in almost three decades

That sounds like a benefit to me :)

However, I think that's due to the general lack of interest in Lisp. You can see the C++ community has a similar ANSI standard and updates it every few years.

> Massive fragmentation in the compiler ecosystem

I wouldn't call it massive. They are pretty consistent, up until things like POSIX and FFI APIs. Let's agree there is some fragmentation. Isn't this still a better situation than if nothing was guaranteed?

[chlorion]

Yup I have had the same exact experience with Common Lisp and every time somebody talks about standardization being so great I think back on this.

Scheme is suffering from the same issues. Scheme is standardized, but implementations end up being incompatible with each other in subtle ways and the level of fragmentation is very painful. Scheme does get some updates unlike CL I guess, but all of the implementations either don't implement the the modern standard, don't have useful extensions for real world programming or are simply immature and don't have enough people working on them to get them into a nice state. In practice it's very difficult to use Scheme for anything non-trivial because of these issues.

I would much rather have no standard at all, and a single high quality implementation that everyone targeted instead of the current mess for both CL and Scheme. Until we get a new dialect that solves these issues Lisp is going to be more or less dead and irrelevant.

[shakna]

> I would much rather have no standard at all, and a single high quality implementation that everyone targeted instead of the current mess for both CL and Scheme.

That would be Chez Scheme [0], maintained actively by Cisco, a company that you may have heard of - who also use the language extensively.

Racket is porting to using Chez, because it is the industry standard, it's performant, and rock solid.

The GNU alternative to Chez is Guile. Emacs can run with Guile, and Guix is built on it. It's got a fairly large community.

Outside of Chez and Guile, there are implementations and communities, but comparatively, they're tiny. Those two are the only big names you need. Like GCC and Clang for C. There are other C compilers. But you only need to know those two.

[0] https://www.scheme.com/

[sicp-enjoyer]

> Racket is porting to using Chez,

I think it already happened in 8.0

[chlorion]

Thanks. I'm going to install guile and tinker around with it! So far it looks pretty good.

[Tyr42]

Racket?

[ithkuil]

Why do libraries not work across compilers of the compilers are implementing a standard? Are they perhaps not really implementing a standard?

[steveklabnik]

Simply having a document called "a standard" doesn't mean that:

1. the standard covers everything you wished it would cover

2. every implementation implements the standard, with no bugs

3. the standard doesn't itself contain incoherent or contradictory things

Standards are a tool, not magic interoperability sauce.

[pjmlp]

Hence the existence of standard certification for compilers as business.

[tialaramex]

Like the Goldman Rep says in The Big Short, "If you offer us free money, we ARE going to take it..."

[FrankHB]

There are simply lacking of such strong requirements in language standards. C/C++ even have the specific "linkage" concept to abstract the binary details under the source form away. And you may know, many libraries are distributed by binaries.

The standards implying binary compatibility rules are about ABI (application binary interface), which usually depend on the ISA (instruction-set architecture) or the OS (if any) being used. You cannot have the unique one once there are multiple ISAs/OSes supported. Even when you only want to rely on some external "exchanging" representations not tied to specific ISAs, there are already plenty of candidates: CLI, JVM, WebAssembly... Plus there are more than one executable (and mostly, runtime loadable) image formats widely used (PE/COFF, ELF, Mach-O ...). You will not have the unique combination, and any attempts to ensure it "work across compilers" in that way will likely finally just add a new instance not fully compatible to existing ones, making it more fragile.

[nickitolas]

Standards are often incomplete, or full of "implementation specific" behaviour, since AIUI standards often end up catering to implementations, instead of the other way around (For example C/C++ standards, you can read a plethora of blog posts about the experience of people trying to contribute to them, and some of the hurdles are related to how strongly tied to existing implementations they are). That means you can often have 2 "standards compliant" compilers that are wildly different. Another reason is compiler extensions. Sometimes a compiler is "standards compliant", but also implements a superset of the standard (Sometimes by default, sometimes under a flag) which means code gets written for that "superset" instead of according to a "standard" (for example, the linux kernel and gcc extensions to C).

[jcranmer]

Typically, some library features contain things that require cooperation with a specific compiler to work correctly. Consider something like std::is_standard_layout in C++, or java.lang.Object in Java, or std::panic::catch_unwind in Rust.

[sicp-enjoyer]

They typically do, unless they are wrapping C libraries or making platform specific API calls.

[charcircuit]

That sounds a little circular. The benefit of alternate compilers is that it makes making alternate compilers easier.

For stability compilers already have a large incentive not to break old programs. For longevity I don't really see how a standard affects it that much. For being more portable you do not need an entirely knew compiler.

[trimir2022]

Being able to specify a language outside an implementation is extremely useful to prevent hidden logical inconsistencies between different parts of the language, and makes the language more robust.

It also allows people to design new backends (looking at CUDA LLVM backends)by finding out the right abstraction to support performance. For example, implementing a C or C++ compatible CUDA backend required the C++ committee to make changes to the memory model / consistency guarantees of C++ atomics. If C or C++ had only depended on compiler implementation for it, then there would have just been different implementations with different guarantees with no consistencies between them, and no single way to even define why they were different.

[pjmlp]

Actually it was the other way around, CUDA was fixed to follow C++11 memory model.

There are a couple of CppCon talks on the subject.

[trimir2022]

Partially correct, even now, SG1 in C++ fixes a lot of things in C++ specification to allow for CUDA like approaches.

[not2b]

The reason it isn't circular is that if there is one implementation, even with good documentation, there will inevitably be lots of corner cases where the implementation does something, but it isn't written down anywhere. Independent implementations will discover many of these issues and they get clarified as part of the standards process.

So you can't really produce a high quality standard with only one implementation. You'll miss important details.

[xiphias2]

Rust is ,,A language empowering everyone to build reliable and efficient software.'' (from the home page)

Reliability at the extremes (where it may be even life or death situation) requires the developer knowing what the program (s)he is writing exactly expresses in Rust.

[charcircuit]

>requires the developer knowing what the program (s)he is writing exactly expresses in Rust.

That just need documentation. You don't need a standard for that.

[shadowofneptune]

Right now Rust often limits what it does to what is supported by LLVM. An example is the become statement. This is a reserved keyword which will eventually act as a jump to a function without saving a return address, the current stack frame becomes a new one. This is tricky since things like deconstructors need to still work. It is only recently that LLVM supported this well, and Clang did it first. Separate implementations and having a standard or some other form of communicatiom between implementers can help with these delays.

EDIT: Clang's version is the attribute 'musttail,' if anyone is interested.

[msla]

There's also political/legal considerations: The modern GCC codebase is derived from the egcs codebase, which forked off the original GCC codebase because the developers of GCC at that time didn't want to prioritize faster development speed:

https://gcc.gnu.org/wiki/History

Thus, a new strain of development can attract people who want to do things differently, and reduce tensions all around.

On the legal front, it's unlikely, but sometimes there's legal problems with continuing to use a certain codebase.

[daptaq]

Doesn't GCC support more architectures than LLVM? Wasn't that the issue a while back with the Rust dependency that a cryptography module for Python introduced?

[ATsch]

Adding a GCC backend for Rust (rust-codegen-gcc) does this already. I do not personally see the point of writing another frontend in C++.

[pjmlp]

Dependency on an existing Rust compiler during GCC bootstrap process.

[ATsch]

You already need an existing C++ compiler to bootstrap GCC though I don't see how this is much different. Plus there is already mrustc as a C++ Rust implementatation specifically designed for bootstrapping.

[pjmlp]

It is one more dependency, not written in C++, and you don't see the difference?

[zokier]

One big benefit in this case is that bootstrapping gcc is somewhat easier than rustc, and presumably gcc-rust can then be used to compile rustc if needed.

[smolder]

In addition to what others have said, since there is a plan to introduce Rust kernel modules into the Linux kernel, being able to compile with GCC helps avoid dependence on another toolchain, which was something I've seen mentioned as a concern w.r.t. Rust in the kernel.

[steveklabnik]

To be clear, while it's a concern some people on the internet have expressed, it's not an actual problem for landing the current work to get Rust in as a framework for writing drivers.

[topspin]

> What is the benefit of having multiple compilers for programming languages?

I'll give you one, or two, depending on what you'd like to count. At some point in the conceivable future we'll be able to compile some meaningful Rust code base with both compilers and measure; a.) how long it takes to compile and b.) the performance of the compiled code.

Obviously that will induce what it always has: incentive to improve.

[thrown_22]

>What is the benefit of having multiple compilers for programming languages?

Rust will need a standard.

The main reason why I don't take it seriously is that code written 5 years ago will often not compile today. For a language that pretends to be a systems language that is a non-starter. If you can't guarantee a 40 year shelf life of your code then no one working on systems cares.

People working on systems in the wild don't have the brain power to learn a new tool chain every decade, let alone every year. They are solving real problems and not writing blog posts.

[steveklabnik]

> code written 5 years ago will often not compile today.

citation needed. Yes, there's a few programs that relied on unsound things for which this is true, but that's a relatively small part of the overall amount of code.

[umanwizard]

Doesn’t every implementation of a new function on a standard type possibly break existing code?

For example if I have a trait Foo with a function bar, and I impl Foo for HashMap, and then a new version of std comes out that has named something HashMap::bar, now every call to my_map.bar() is ambiguous

[steveklabnik]

In that specific case, the inherent impl is preferred, so there’s no ambiguity. however this can still cause a breaking change if the inherent impl has a different type signature than the trait.

And yes, there are tons of things that can subtly break code. That’s why the rust project runs the entire open source ecosystems’ tests as part of the testing process for the compiler. It’s not all of the code in existence, but it’s pretty good at flushing out if something is going to cause disruption or not.

In practice, the experience that the vast majority of users report to us is that they do not experience breakage when upgrading the compiler.

[umanwizard]

> In that specific case, the inherent impl is preferred, so there’s no ambiguity.

That is even worse. It means your code can silently start doing the wrong thing rather than erroring, if the inherent impl does something different from the trait.

> And yes, there are tons of things that can subtly break code.

This isn't some obscure bug in some deep edge case though, it's a completely normal and common way of using the language (implementing your own traits on foreign types) predictably leading to breakage in an obvious way. I am not sure why it should be called "subtle".

Anyway, given this issue, I think the meme that Rust is backwards-compatible is really oversold. It'd be more honest to frame it as "we hope releases are backwards-compatible, but we like adding new functions to the stdlib, so no promises" rather than marketing BC as a major selling point as is done now.

> In practice, the experience that the vast majority of users report to us is that they do not experience breakage when upgrading the compiler.

It's anecdotal for sure, but I'm personally aware of times when the exact situation I'm describing has happened and caused headaches for people.

[thrown_22]

>but that's a relatively small part of the overall amount of code.

Yes and?

Systems programming isn't front end JS work where breaking things doesn't matter. It's no surprise that Rust came out of the browser space. Only people who don't take their work seriously could ever think the above is a justification and not a red flag for never using it.

I welcome Rust becoming ossified in GCC so I can build 30 year old code without modification like I can in C. Until then, it's a toy for people with more time than responsibility.

[rightbyte]

We are still arguing whether to bump the C standard from 89 to 99 where I work. I kinda like the pacing with C.

Or as Wikipedia puts it: "C17 addresses defects in C11 without introducing new language features."

[pjmlp]

Hopefully you won't have K&R C by the time you adopt C23 then.

[marcodiego]

It instantly inherits support for all OS's and architectures GCC support plus a lot of optimizations and debug information.

[maxwell86]

Rust already has a GCC backend that can do all that.

This post is about a new front-end.

[samatman]

A language with one implementation can't really be said to have a specification.

It may have a very detailed accompanying technical documentation of what the implementation is supposed to do, this may be called a specification, but a specification deserves the name with a minimum of two implementations.

[FrankHB]

This is technically incorrect. A programming language can be designed with specification in mind, even with a formal one (e.g. SML). It is just true that the specification is not likely effectively verified before more than one real implementations landed, if it is not formally verified. (Anyway, verification by testing of existing implementations _is_ the fallback where people cannot afford the cost of formal methods.)

[tsimionescu]

It helps ensure that the language standard is relevant, and it can help separate the standard committee from the compiler developers, helping bring in more interest groups to the table (since they're not entirely beholden to a single compiler dev team).

[avgcorrection]

Having to nominal competitors makes everything good. Like Pepsi/Coke, Republicans/Democrats.

[davidatbu]

This is answered in various replies to other comments.