Oh I think I misundetstood you (or you me), and I didn't really articulate my question well. I'm well aware of how stackless coroutines are implemented.

My main concern was whenever you do that, you lose the ability to look at backtrace when something goes wrong. So some implementations keep a separate stack for each coroutine (like Go), and switch SP whenever context is switched. That way you don't have bunch of random function calls in your stack trace. The problem though is that these individual stack for each couroutine has to be pretty small in general (since you would spawn hundreds of thousands of these). Go solved this temporarily using fragmented stack, and later ditched that in favor of copying the whole stack over. And I thought, you were talking about optimizations around this whole "stackfull" coroutines thing so that I can have my backtrace.