durin VS qbe-rs

So I think I may be one of the few people in the world who has actually implemented a GC using LLVM's statepoint infrastructure. It's poorly documented and there are some gotchas, but I'd say it's definitely usable, and it works with basically any collector design, including moving collectors (I'm using Immix) and has no runtime bookkeeping overhead and allows LLVM to optimize the code without worrying about GC, which is nice. It's actually gotten a bit better with LLVM 13, too. If you're curious what a LLVM-based GC looks like, mine is in this folder. Of course, if you just want some sort of GC, you can also just link it with Boehm which is quite easy and has pretty good performance - this is what e.g. Crystal does, although they're talking about switching.

I also fixed lots of bugs in the GC and backend, so it should be a lot more stable now.

Recently, I've been working on adding garbage collection to Pika. I've successfully written an Immix-based garbage collector that works with the LLVM GC support infrastructure, and I'm currently working on integrating the GC with Pika, or really Durin, the dependently-typed intermediate representation that Pika compiles to. Because types are passed around at runtime, objects of unknown type and size can be stored unboxed in polymorphic data structures; but that makes keeping track of type information for heap allocations somewhat harder, because type information needs to be allocated and constructed at runtime in some cases. It's an interesting design problem, because you want constructing type information to be fast; but the GC will run much more often, so maximizing tracing speed by avoiding e.g. indirection in type information is important; and you also want to construct as much type information as possible at compile time and embed it as constants.

Another problem with LLVM I’ve heard about is that it’s intermediate language or API or something is a moving, informally-specified target. People who know LLVM internals might weigh in on that claim. If true, it’s actually easier to target C or a subset of Rust just because it’s static and well-understood.

Two projects sought to mitigate these issues by going in different directions. One was a compiler backend that aimed to be easy to learn with well-specified IL. The other aimed to formalize LLVM’s IL.

http://c9x.me/compile/

https://github.com/AliveToolkit/alive2

There have also been typed, assembly languages to support verification from groups like FLINT. One can also compile language-specific analysis with a certified to LLVM IL compiler. Integrating pieces from different languages can have risks. That (IIRC) is being mitigated by people doing secure, abstract compilation.

> I think it uses a different backend than LLVM

harec uses https://c9x.me/compile/

Have you considered QBE?

I really hope other new projects (like QBE) can really grow and become widely used

I really liked targetting QBE (https://c9x.me/compile/) as an IR, as it gave me lots of back-end optimisations for free 😊.

There is: QBE.

You might be underestimating the accuracy of the CPU models LLVM uses.

For x86, the same data the code generator uses drives llvm-mca[1], which given a loop body can tell you the throughput, latency, and microarchitectural bottlenecks (decoding, ports, dependencies, store forwarding, etc.)—if not always precisely, then still not worse then IACA, the tool written at Intel by people who presumably knew how the CPUs work, unlike LLVM contributors and the rest of us who can only guess and measure. This separately for Haswell, Sandy Bridge, Skylake, etc.; not “x86”.

Now, is this the best model you can get? Not exactly[2], but it’s close enough to not matter. Do we often need machine code to be optimized to that level of detail? Perhaps not[3], and with that in mind you can shave at least a factor of ten off LLVM’s considerable bulk at the cost of 20—30% of performance[4,5]. But if you do want those as well, it seems that the complexity of LLVM is a fair price, or has the right order of magnitude at least.

(Frontend not included, C++ frontend required to bootstrap sold separately, at a similar markup compared to a C-only frontend with somewhat worse ergonomics.)

[1] https://llvm.org/docs/CommandGuide/llvm-mca.html

[2] https://www.uops.info/

[3] https://briancallahan.net/blog/20211010.html

[4] https://c9x.me/compile/

[5] https://drewdevault.com/talks/qbe.html

You can try buildling you own backend like llvm. A good example or starting point is probably QBE since it is extremely small but very functional.