Java sidesteps this because we generate concrete machine instructions and have control over the low-level platform-specific representation. In other words, because our backend is lower level, not higher level. But yeah, if you don't have access to the low level representation -- like Rust and Kotlin -- you don't really have a choice except to wait for your backend to give you that access. Sometimes waiting is a good approach, though. I'm not even sure you need control over the stack representation; it's sufficient to know what it is, which you could maybe do even on top of LLVM (but I don't know LLVM well enough to say for sure).
Yeah, exactly. I believe you're correct that you just need to know what the stack looks like, not really change it much if at all. Any tweaks you may need to make are probably already available for things like split stacks or green threads.
The problem is that LLVM doesn't even let you see the stack layout until very late, and then only as debug info. You could probably hack something on top but it would probably wind up looking a lot like C++20, with a lot of extra allocation, or like Rust, with all the stack frame layout optimization duplicated into the frontend.
