qbe-rs VS asmjit

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.

I think QBE might be what you're looking for?

https://c9x.me/compile/

As far as I know, there is QBE, which is actually kinda underrated, and Cranelift, mainly designed for JIT compilation

Before that, I had spent a bit of time working with QBE, which is much simpler and really easy to write a frontend for. I switched to libgccjit though, because I got frustrated with a few of the things lacking from QBE (like the ability to easily keep track of where different variables live on the stack). I think for many hobby language projects, QBE would be a good option (my project was off the ground very fast using QBE, and I got pretty far before I ran into limitations I couldn't easily work around).

If one of your parameters is size/complexity of the backend and you prefer something smaller, have a look at qbe and cwerg

The alternatives are generally hidden inside of another compiler. The big exception seems to be qbe (https://c9x.me/compile/) however since the author appears to have written this code without peer review, it's not easy to read it's source code.

So my question is this: if a backend like QBE (~12k Loc) was added to Zig and Zig only had to compile Zig code (no C, etc) for that QBE backend -- about how many LoC would that Zig need to be?

GCC is written in C++ these days, so something like QBE(https://c9x.me/compile/) would be needed.

But an easy to create a JIT would be to use https://github.com/asmjit/asmjit, which is used in RPCS3.

You could go the way of https://asmjit.com (or forth) and make it your assembler DSL on top of the low-level call.