Page:AITR-474.djvu/49

environments (we count two environments as identical if they involve the same variable bindings, regardless of the number of "frames" involved; that is, the environment is the same inside and outside a (LAMBDA () ...)). Assume a simple target machine with argument registers called reg1, reg2, etc.

main:   <code for a>            ;result in regl
        LOAD reg2,[name2]       ;[name2] is the closure for name2
        CALL-FUNCTION 2,[name1] ;call name1 with 2 arguments  

name1:  JUMP-IF-NIL reg1,name1a  
        RETURN                  ;return the value in reg1
name1a: CALL-FUNCTION 0,reg2    

name2:  <code for b>            ;result in reg1 
        LOAD reg2,[name4]       ;[name4] is the closure for name4 
        CALL-FUNCTION 2,[name3] ;call name3 with 2 arguments

name3:  JUMP-IF-NIL reg1,name3a 
        RETURN                  ;return the value in reg1 
name3a: CALL-FUNCTION 0,reg2    ;call function in reg2, 0 arguments 

name4:  <code for c>            ;result in reg1
        LOAD reg2,[name6]       ;[name6] is the closure for name6 
        CALL-FUNCTION 2,[name5] ;call name5 with 2 arguments 

name5:  JUMP-IF-NIL reg1,name5a 
        RETURN                  ;return the value in reg1
name5a: CALL-FUNCTION 0,reg2    ;call function in reg2, 0 arguments

name6:  LOAD reg1,'NIL          ;constant NIL in reg1
        RETURN

Now we make use of our knowledge that functions, and convert CALL-FUNCTION of certain variables always denote certain a known function to a simple GOTO. (We have actually done things backwards here; in practice this knowledge is used before generating any code. We have fudged over this issue here, but will return to it later. Our purpose here is merely to demonstrate the treatment of function calls as GOTOs.)

main:  <code for a>           ;result in reg1 L
       LOAD reg2,[name2]      ;[name2] is the closure for nameZ
       GOTO name1