The effort to support architectures is now minimal with AI assistance. I made a hobby architecture (based on Intel, but with some changes; I was making an "alternate history" as if a few decisions in the past had been different) and it was pretty much trivial to spit out support not just in gcc and llvm, but I also, for fun, made WATCOM backends and a few other things.
With that said, RISC-V is a nice baseline for designing another architecture. Start with RISC-V, and go from there.
The main difficulty in support isn't actually making the changes (which is going to be largely defining various tables and other boilerplate, unless your ISA is really weird and does something unusual), it's in doing all of the politics necessary to get those changes committed upstream.
Maintining this type of long time fork is the kind of mind numbing, tedious thing LLMs are actually exceptionally good at. I did some backend work on Lean4 compiler and then they switched out parts of the codegen, it took Codex less than an hour to recover the feature set I had implemented but on the new backend.
> and it was pretty much trivial to spit out support not just in gcc and llvm, but I also, for fun, made WATCOM backends and a few other things
No links? You may have dreamt about it instead.