CSC448: Code Generation: Run-Time Environment IV [5/14]

Suppose that the language is extended with procedures that can declare local variables:

exp ::= <ID>
      | <INT>
      | exp "+" exp
      | exp "-" exp
      | exp "<=" exp
      | exp "=" exp

stat ::= <ID> ":=" exp
       | "label" <ID>
       | "if" exp "goto" <ID>
       | "print" exp
       | "var" <ID>
       | "invoke" <ID> "(" args ")"

proc ::= "proc" <ID> "(" params ")" { stat* }

params ::= ( <ID> ( "," <ID> )* )?
args   ::= ( exp ( "," exp )* )?

If the language does not permit recursive calls, how can we organize the run-time storage?

Arguments and local variables for a procedure are grouped together into an activation record or frame.

The position of local variables is known at compile-time.

How can we record information about the calling procedures? Note that the number of calling procedures is bounded by the number of procedures.

A sample program:

proc upto (x) {
  var y;
  y := 0;
  goto l2;
  label l1;
  print (y);
  y := y + 1;
  label l2;
  if (y <= x) goto l1;
}

proc f () {
  var z;
  z := 3;
  goto l4;
  label l3;
  print (z);
  z := z - 1;
  label l4;
  if (0 <= z) goto l3;
}