Perl 6 and Parrot Essentials, Second Edition

Subroutines

Subroutines and methods are the basic building blocks of larger programs. At the heart of every subroutine call are two fundamental actions: it has to store the current location so it can come back to it, and it has to transfer control to the subroutine. The bsr opcode does both. It pushes the address of the next instruction onto the control stack, and then branches to a label that marks the subroutine:

  print "in main\n"
  bsr _sub
  print "and back\n"
  end
_sub:
  print "in sub\n"
  ret

At the end of the subroutine, the ret instruction pops a location back off the control stack and goes there, returning control to the caller. The jsr opcode pushes the current location onto the call stack and jumps to a subroutine. Just like the jump opcode, it takes an absolute address in an integer register, so the address has to be calculated first with the set_addr opcode:

  print "in main\n"
  set_addr I0, _sub
  jsr I0
  print "and back\n"
  end
_sub:
  print "in sub\n"
  ret

Calling Conventions

A bsr or jsr is fine for a simple subroutine call, but few subroutines are quite that simple. The biggest issues revolve around register usage. Parrot has 32 registers of each type, and the caller and the subroutine share the same set of registers. How does the subroutine keep from destroying the caller’s values? More importantly, who is responsible for saving and restoring registers? Where are arguments for the subroutine stored? Where are the subroutine’s return values stored? A number of different answers ...