One simple thing that LLM's doesn't have to do is use a calling convention. Compilers need to use them, at least to some extent because it's not known at compile time who will link against this function in the future. Humans kinda need them because it's a lot of mental load when everything is custom. But for a sufficiently smart llm, noticing a register doesn't need to be preserved because there is only two callers and neither of them care about it might be doable.