CPSC 388 – Compiler Design and Construction

1. CPSC 388 – Compiler Design and Construction Parameter Passing

2. Terminology int method(int I, int J){} I and J are formal parameters int a,b; method(a,b); a and b are actual parameters

3. Terminology • r-value refers to the value of an expression. If x has the value of 2 and y has the value of 3 then expressionr-value x 2 y 3 x+y 5 x==y false • l-value refers to the location or address of an expression. x if it is global x if it is local x+y • All expressions have an r-value. Only expressions that can appear on the left side of an assignment have an l-value.

4. Parameter Passing Modes • Pass by Value • Pass by Reference • Pass by Value-Restore • Pass by Name Programmer Compiler Writer

5. Pass by Value • In java and C you only have pass by value • In pascal it is pass by value unless the formal declaration starts with “var”, in C++ it is by value unless the formal contains a “&” //Pascal procedure header Procedure f(x: integer; var y: integer) // C++ function header void f(int x; int & y);

6. Pass by Value (continued) • The calling method copies the r-value of the actual into the called method’s activation record. • Changes to a formal have no effect on the actual. void f( int[] A ) { A[0] = 10; // change an element of parameter A A = null; // change A itself (but not the actual) } void g() { int[] B = new int [3]; B[0] = 5; f(B); System.out.println(B[0]); } • You can change what is pointed to by formal!

7. What happens when the following code executes? class Person { int age; String name; } class Test { static void changePerson(Person P) { P.age = 10; P = new Person(); P.name = "Joe"; } public static void main(String[] args) { Person P = new Person(); P.age = 2; P.name = "Ann"; changePerson(P); System.out.println(P.age); System.out.println(P.name); } }

8. Reference Parameters • calling method copies the l-value of the actual into the called method's activation record • What if the actual has no l-value? e.g. f(x+y) • Compiler can give an error • Compiler can create a temporary to hold the value and pass the address of this temporary

9. Bad Compiler Design with Reference Parameters void mistake(int x) { // x is a reference parameter x = x+1; } void main() { int a; mistake(1); a = 1; print(a); } Output was 2! Created a temporary for the INT_LIT of 1. All uses of 1 used this temporary

10. Reference Parameters in C++ class Person { public: String name; int age; }; void birthday(Person per) { per.age++; } void main() { Person P; P.age = 0; birthday(P); print(P.age); } P is a Person, not a pointer to a Person The parameter is passed by value (no &) So a copy of P is created in birthday’s AR

11. Reasons for using reference parameters • When the job of the called method is to modify the parameter • When the called method will not modify the parameter, but the parameter is very large.

12. You Try It static void swap(int x, int y) { int tmp = x; x = y; y = tmp; } • Assume that A is an array of integers, and that j and k are (different) array indexes. Draw the AR to illustrate what happens for the call: swap(A[j], A[k]); • With pass by value • With pass by reference

13. Code Generator with Parameters • Assembly code generator creates different code for pass by value and pass by reference • In pass by value activation record holds value of variable. Access variable by offset from the frame pointer, FP. • In pass by reference activation record holds address of the value of the variable

14. Assembly Code Example void f(int a) { a = a - 5; } void main() { int x = 10; f(x); } passed by value _____________ lw t0,(FP) #load a's r-value into t0 sub t0,t0,5 #t0 = t0 – 5 sw t0,(FP) #store result into f's AR passed by reference ________________ lw t0,(FP) #load a's l-value into t0 lw t1,(t0) #load a's r-value into t1 sub t1,t1,5 #t1 = t1 - 5 sw t1,(t0) #store result into main's AR

15. Assembly Code Example • Calls generate different assembly code as well int x,y; x=y=3; f(x,y); • What assembly code would be generated to populate the AR for function f if the first param is by reference and the second is by value?

16. Pass by Value-Restore • Used in Fortran IV and Ada • Values are copied into method’s AR • At end of method call values are copied back to calling method • Pass by value-restore has same effect as pass by reference EXCEPT when there are aliases.

17. Aliasing • Two expressions that have the same l-value • Occurs by: • Pointer manipulation • Parameter is pass by reference and global • 2 parameters passed by reference using same expression

18. Aliases Via Pointers class Person { int age; String name; } Person p, q; p = new Person(); q = p; • p.name and q.name are aliases (same l-value, refer to same location in memory) • p and q are not aliases!

19. You Try It • Create aliases by passing a global variable as a parameter (pass by reference) • Create aliases by passing the same variable as different parameters (pass by reference)

20. Different Results with Aliasing int x = 1; // a global variable void f(int & a) { a = 2; // a and x are aliases x = 0; } main() { f(x); cout << x; } What is the difference if the parameter is pass by reference Or pass by value-result?

21. Different Results with Aliasing void f(int &a, &b) { a = 2; b = 4; } main() { int x; f(x, x); cout << x; } What is the difference if the parameter is pass by reference Or pass by value-result? The order values are copied back matters!

22. You Try It • Assume all parameters are pass by value-result. • Question 1: Give a high-level description of what the code generator must do for a method call. • Question 2: Give the specific code that would be generated for the call shown below, assuming that variables x and y are stored at offsets -8 and -12 in the calling method's AR. int x, y; f(x, y);

23. Pass by Name Parameters • Used by Algol • Think of it like this: • Every call statement is replaced by the body of the called method. • Each occurrence of a formal parameter in the called method is replaced with the corresponding argument -- the actual text of the argument, not its value.

24. Example Pass by Name Parameter void Init(int x, int y) { for (int k = 0; k < 10; k++) { y = 0; x++; } } main() { int j; int A[10]; j = 0; Init(j, A[j]); }

25. Comparison of Methods of Parameter Passing • Call by Value • Call by Reference • Call by Value Result • Call by Name Does not cause Aliasing Efficient for large objects Faster than dereferencing in call by ref. Faster for params that are not used