countdown
Return the number of times to invoke a compiled code object, after creation, before attempting to optimize it for the first time.
This number not only counts down by one every time the corresponding code is called, but it is also decreased by a larger amount every time the periodic timer goes off and polls the active interpreters to see what function they're currently running. These big decrements are arranged never to cross zero, allowing the next caller to do the optimization work.
Return
The number of invocations before initial optimization.