I think I remember one called preserve_none. That might have been in a downstream fork though.

The calling convention you want for a coroutine switch is caller saves everything live, callee saves nothing. As that means spilling is done on the live set, to the stack in use before switching. Return after stack switch then restores them.

So if a new convention is proposed that solves both fast stackful coroutines and bytecode interpreter overhead, that seems reasonable to me.