[pron]

The compiler can only make this transformation in special cases, in particular, when it doesn't break any of the JVM's semantics (also, not all recursion is self-recursion, i.e. a tail call to the subroutine you're in). You can't just discard a frame of a call in a tail position, because some security mechanisms require knowing the full call-stack (plus, developers might hate you when their stack traces start missing crucial frames). So we're talking about explicit tail calls, in places that can be checked for the safety of the optimization.