You potentially don't know who is "resuming", and so don't know what registers they will clobber. It would only be a "downside" if 1) your code uses a register 2) no other possible green threads do, and that isn't an invariant any compiler I know will promise, especially in the face of FFI calls.
If you're at the point where you want to do register allocation and spilling optimization across multitasking points, you're probably better off writing your own compiler instead of expecting a thread runtime to do it for you.
For comparison, normal threads have the same downside: the kernel saves and restores all registers (even more than what the example does), so you're not doing any worse than that.
You potentially don't know who is "resuming", and so don't know what registers they will clobber. It would only be a "downside" if 1) your code uses a register 2) no other possible green threads do, and that isn't an invariant any compiler I know will promise, especially in the face of FFI calls.
If you're at the point where you want to do register allocation and spilling optimization across multitasking points, you're probably better off writing your own compiler instead of expecting a thread runtime to do it for you.
For comparison, normal threads have the same downside: the kernel saves and restores all registers (even more than what the example does), so you're not doing any worse than that.