Created attachment 482957 [details] [diff] [review] fix iterNext() really wants the iterator to be at sp[-1], rather than fiddle with that and its slow paths, some ops just rearrange the stack.
I summon Andreas. /be
Fiddling with the stack here shouldn't affect perf, it's just internal bookkeeping. There's a chance when taking the slow path, there may be more memory shuffling, but not enough to cause concern.