Subroutines – parameter passing

1. Subroutines – parameter passing • passing data to/from a subroutine can be done through the parameters and through the return value of a function subroutine • parameter passing methods include: call-by-value - input parameters (declared with "IN") in Ada - default method for parameters in C, C++, and Pascal - parameters of primitive type in Java call-by-result - output parameters (declared with "OUT") in Ada

2. Subroutines – parameter passing Parameter passing methods (cont’d): call-by-value-result - in/out parameters (declared with "IN OUT") in Ada call-by-reference - large array parameters in Ada - array parameters in C and C++ - reference parameters (declared with "&") in C++ - reference parameters (declared with "VAR") in Pascal - object parameters in Java - default method for parameters in Fortran

3. Subroutines – parameter passing Parameter passing (cont’d) call-by-value • copy values of actual parameters into memory locations of formal parameters before executing the body of the subroutine; do nothing on return main a = 1 a: 1 b = 2 b: 2 call subr(a,b) pass 1,2 via stack print a,b print 1,2 subr(x,y) copy 1,2 into x,y ^ x = x + 1 x: /1/ 2 | y = x + y y: /2/ 4 | return ---------------------'

4. Subroutines – parameter passing Parameter passing (cont’d) • call-by-result - do nothing prior to executing the body of the subroutine; copy the final values of the formal parameters into the memory locations of the actual parameters on return main a = 1 a: 1 b = 2 b: 2 call subr(a,b) pass nothing receive ?,? from subr into a,b a: /1/ ? b: /2/ ? print a,b print ?,? subr(x,y) copy ?,? into x,y ^ x = x + 1 x: /?/ ? | y = x + y y: /?/ ? | return pass ?,? back via stack -------'

5. Subroutines – parameter passing Parameter passing (cont’d) • call-by-value-result - perform copying of values both before and after executing the body of the subroutine main a = 1 a: 1 b = 2 b: 2 call subr(a,b) pass 1,2 via stack receive 2,4 from subr into a,b a: /1/ 2 b: /2/ 4 print a,b print 2,4 subr(x,y) copy 1,2 into x,y ^ x = x + 1 x: /1/ 2 | y = x + y y: /2/ 4 | return pass 2,4 back via stack --'

6. Subroutines – parameter passing Parameter passing (cont’d) • call-by-reference • pass the addresses of the actual parameters • copy these addresses into the memory locations of the formal parameters • on each reference to a formal parameter in the body of the subroutine, perform an indirect reference to the corresponding actual parameter; • i.e. the formal parameter is an alias of the actual parameter, thus both the formal and actual parameter "name" refer to the same object • changes made using the formal parameter are being executed on the object passed as the actual parameter

7. Subroutines – parameter passing Parameter passing (cont’d) • call-by-reference (cont’d) main a = 1 a: 1 a: /1/ 2 action in subr b = 2 b: 2 b: /2/ 4 action in subr call subr(a,b) pass &a,&b via stack print a,b print 2,4 subr(x,y) copy &a,&b into x,y ^ x = x + 1 x: &a thus a = a + 1 | y = x + y y: &b thus b = a + b | return ------------------------------------'

8. Subroutines – parameter passing consider Fortran's call-by-reference applied to a constant parameter - what should the following code print? SUBROUTINE ADDONE( I ) I = I + 1 RETURN END ... ADDONE( 2 ) WRITE(6,10) 2 10 FORMAT('CONSTANT 2 =',I1)

9. Subroutines – parameter passing (tradeoffs) optimizations can pass parameters in registers (not using stack) can sometimes not save/restore registers when executing a leaf routine