Go slices (and strings) know their length: http://research.swtch.com/godata http://research.swtch.com/godata So it's not a computation, just a struct member acc

Go slices (and strings) know their length: http://research.swtch.com/godata http://research.swtch.com/godata So it's not a computation, just a struct member access. And len() is a builtin, not a real function call.