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There already seems to be a 6502 backend for LLVM specifically geared for c64:

https://github.com/c64scene-ar/llvm-6502

Which means you can just use the clang frontend and even write C++17 code which compiles down to 6502 ML.

I was unable to get any of the existing 3 llvm 6502 backends to produce reasonable code. Which is why I went this route. I meant to mention that in the video...

There's also https://github.com/puppeh/gcc-6502 and https://github.com/puppeh/gcc-6502-bits to get 6502 directly out of gcc.

I don't believe I found those, thank you.

By reasonable code do you mean performant, or just plain working?

If I had to guess, now knowing that there's a crushing stack size limitation for 6502 CPU, probably "just plain working."

6502 is notoriously difficult to target a C compiler to. Limited stack depth, only 3 registers, and I believe some other limitations to the ISA that make hand it hard to target C to it.

cc65 seems to be the only viable one around last I looked. And it doesn't support the 65816, only the 6502. WDC offers their own compiler, but it's not fully open/free.

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