RFR(M): 8156915: introduce MethodHandle factory for array length

[Vladimir Ivanov]

>
> Thanks Vladimir! This is how I understood it, but I did not see the place in code where the actual switch between those "modes" is done, as I have seen they are compiled-up-front.

The decision is made in LambdaForm.prepare() and 
LF.checkInvocationCounter().

MethodHandleStatics.COMPILE_THRESHOLD controls when the switch happens.

>
> Theoretically at some point, the LFI code could be removed from JDK, right?
Yes, it's optional now. But LFI comprises tiny fraction of j.l.i code 
and it's invaluable when debugging LambdaForm issues. So, I expect it to 
stay.

Also, LambdaForm instances are still much more lightweight than classes 
they are compiled to. So, if we face a situation when too many 
LambdaForms are created, it's better so switch back to LFI than flooding 
the system with classes.

Best regards,
Vladimir Ivanov