As defined in
Intel® Itanium ™ Processor-specific Application Binary Interface, section 3.3.5, the return pointer register (rp) shall
contain a valid return address, such that if the application
program returns from the main entry routine, the implementation
shall cause the application to exit normally, using the returned
value as the exit status. Further, the unwind information for this
"bottom of stack" routine in the implementation shall
provide a mechanism for recognizing the bottom of the stack during
a stack unwind.