Oh, I thought he does this already. Why was there a prepare, when it doesnt prepare the arg decoding.

TBH, the complexity of this step grew over time, and the overhead snuck up on us. The prep step does useful work (eg. determine stack space requirements). It's just that we don't have to do it again.

Something I should have mentioned is that we could have avoided the new APIs if only there was space in the ffi_cif to stash a plan pointer. And I didn't want to break ABIs for this.