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

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

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 '

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 '

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 --'

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; o 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 o changes made using the formal parameter are being executed on the object passed as the actual parameter

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 '

Subroutines – parameter passing Consider the following code: int a = 5; /* global variable */ int subr( int b, int c ){ a = 4*a; b = b + 3; c = c + 2; return( a + b + c ); } void main(void){ int d = 1, e; e = subr( a, d ); } Show final values after calls to subr() for the variables listed below, by column, according to the specified parameter passing methods. (18 pts. total) b:call by value b:call by value-result b:call by reference c:call by value c:call by value-result c:call by reference a d e

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)

Subroutines – parameter passing (tradeoffs) optimizations can pass parameters in registers (not using stack) can sometimes not save/restore registers when executing a leaf routine advantagesdisadvantages Call-by- value loads and stores operate directly on formal parms copying overhead Call-by- reference no copying, allows subr to change the values of the actual parms indirect reference through addresses in formal parms to actual parms (i.e., you have to chase pointers)