Code Generation/Exercise 7

The Original Code

Consider the following C function:

int *traverse(int vec[], int lim) {
  int *ptr = vec + lim - 1, ix = lim - 2;
  while (ix >= 0) {
    ptr = (vec[ix] > *ptr) ? vec + ix : ptr;
    ix--;
  }
  return ptr;
}

Postfix Code

The Postfix code for the above function is as follows:

Postfix code

TEXT ALIGN GLOBL traverse, FUNC LABEL traverse ENTER 8 ; ptr@-4 ix@-8 vec@+8 lim@+12 ; init prt (not assignment) LOCAL +8 LDINT ; vec LOCAL +12 LDINT ; lim INT 4 ; sizeof int MUL ADD ; vec + lim INT 1 INT 4 ; sizeof int MUL SUB ; vec + lim - 1 LOCAL -4 STINT ; ptr = vec + lim - 1 ; init ix (not assignment) LOCAL +12 LDINT ; lim INT 2 SUB LOCAL -8 STINT ; ix = lim -2 ; while start: test ALIGN LABEL while_test LOCAL -8 LDINT INT 0 GE ; ix >= 0 JZ while_end ; while block ; ?: operator LOCAL +8 LDINT ; vec LOCAL -8 LDINT ; ix INT 4 ; sizeof int MUL ADD ; vec + ix LDINT ; *(vec+ix) --> vec[ix] LOCAL -4 LDINT ; ptr LDINT ; *ptr GT JZ three_false_part ; ?: true part LOCAL +8 LDINT ; vec LOCAL -8 LDINT ; ix INT 4 ; sizeof int MUL ADD ; vec + ix JMP three_end ; ?: false part ALIGN LABEL three_false_part LOCAL -4 LDINT ; ptr ; ?: end ALIGN LABEL three_end DUP32 ; for assignment LOCAL -4 STINT ; ptr = ?: ... ; trash expression value (used as instruction) TRASH 4 ; decrement ix LOCAL -8 LDINT ; ix DUP32 INT 1 SUB LOCAL -8 STINT ; ix = ix - 1 TRASH 4 ; (ix-- used as instruction) ; end while block JMP while_test ; exit while cycle ALIGN LABEL while_end ; return ptr LOCAL -4 LDINT STFVAL32 LEAVE RET

Compiling and Running

To compile the Postfix code directly, pf2asm can be used:

pf2asm traverse.pf
yasm -felf traverse.asm