Code Generation/Example 1

The Original Code

Consider the following C function:

int main() {
  int n = 45, i = 0, j = 1, *seg = &i;
  while (n-- > 0) {
    print(*seg = i + j);
    seg = (seg == &i) ? &j : &i;
  }
  return 0;
}

Postfix Code

The Postfix code for the above function is as follows:

Postfix code

TEXT ALIGN GLOBAL "main",FUNC LABEL "main" ENTER 16 ; n@-4, i@-8, j@-12, seg@-16 ;-- initialization: INT 45 LOCA -4 ; n = 45 INT 0 LOCA -8 ; i = 0 INT 1 LOCA -12 ; j = 1 LOCAL -8 ; &i LOCA -16 ; seg = &i ;-- while cycle: LABEL "while" LOCAL -4 ; &n LDINT DUP32 ; n (because of n--) INT 1 SUB ; n-1 LOCAL -4 STINT ; n = n-1 INT 0 GT ; n > 0 (n before decrement) JZ "endwhile" LOCAL -8 ; &i LDINT ; i LOCAL -12 ; &j LDINT ; j ADD ; i+j DUP32 ; same value (for assignment) LOCAL -16 ; &seg LDINT ; seg STINT ; *seg = i+j CALL "print" ; we assume that "print" does not return any value TRASH 4 ; trash argument (int) ;-- compute trenary operator's value LOCAL -16 ; &seg LDINT ; seg LOCAL -8 ; &i EQ ; seg == &i JZ "false3" ; jump to "false" expression of trenary operator LOCAL -12 ; &j JMP "end3" ; jump to end of trenary operator LABEL "false3" LOCAL -8 ; &i LABEL "end3" DUP32 ; trenary operator's value (duplicated because of assignment) LOCAL -16 STINT ; seg = (seg == &i) ? &j : &i TRASH 4 ; trash assignment's value, since we used it as an instruction JMP "while" ; restart while cycle LABEL "endwhile" ;-- rest of function INT 0 STFVAL32 ; return value is 0 LEAVE RET