No, you "only" have to write a backend. Which is still a lot of work, but much less work than another compiler. There are multiple ways it can be designed, but one idea that I like the most right now is to create an API for LLVM, and then implement the same API ourselves with a fast backend. That way we reuse everything in current LFortran, including lowering to LLVM, we just have to refactor it to use our own API instead.