c4 VS qbe-rs

The C4 compiler [https://github.com/rswier/c4] is a self-hosting compiler for a subset of the C programming language that produces executable x86 code. You can understand and audit this code in a couple of hours (its 528 lines).

It could be an interesting exercise to bootstrap up from something like this to a working linux environment based solely on source code compilation : no binary inputs. Of course a full linux environment has way too much source code for one person or team to audit, but at least it rules out RoTT style binary compiler contamination.

I was surprised to see nodes still have two pointers ("references") given that you now know that that the first pointer will always point exactly to the next node. I've see https://github.com/rswier/c4 use that. Granted it doesn't make for the most readable code, but it's even smaller and faster.

VMs with simple instruction sets is a fun topic. Some years ago I got inspired by the amazing rswier/c4 compiler by Robert Swierczek and explored the smallest instruction set I could get away with to create VMs that could run non-trivial workloads.

C4 comes to mind (C in 4 functions), https://github.com/rswier/c4.

have you considered adding a backend for LLVM? perhaps a bit heavyweight, but it could be a good way to get C/C++, fortran, rust, etc. if that's something you'd like!

I use Alpine Linux (no GNU bloat btw), dwm (Sucks less!), and I edit all my C (no bloat language) through busybox ed and compile my programs with (c4)[https://github.com/rswier/c4]

Keep in mind you can create a "usable" C compiler by yourself, and is doable in surprisingly low amount of code. Try https://github.com/rswier/c4/blob/master/c4.c

> 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.

How big is the whole backend? I've heard that it is small but I wanted to compare it to QBE which is around 8 KLoC and it is quite interesting too.