Code Generation/Exercise 6

The Original Code

Consider the following C function:

int powmod(int base, int exp, int modulus) {
  int accum = 1, i = 0, basepow2 = base;
  while ((exp >> i) > 0) {
    if (((exp >> i) & 1) == 1) 
      accum = (accum * basepow2) % modulus;
    basepow2 = (basepow2 * basepow2) % modulus;
    i++;
  }
  return accum;
}

Postfix Code

The Postfix code for the above function is as follows:

Postfix code

TEXT ALIGN GLOBL powmod, FUNC LABEL powmod ENTER 12 INT 1 LOCAL -4 STINT INT 0 LOCAL -8 STINT LOCAL +8 LDINT LOCAL -12 STINT ALIGN LABEL while LOCAL +12 LDINT LOCAL -8 LDINT SHTRU INT 0 GT JZ while_end LOCAL +12 LDINT LOCAL -8 LDINT SHTRU INT 1 AND INT 1 EQ JZ if_end LOCAL -4 LDINT LOCAL -12 LDINT MUL LOCAL +16 LDINT MOD DUP32 LOCAL -4 STINT TRASH 4 ALIGN LABEL if_end LOCAL -12 LDINT LOCAL -12 LDINT MUL LOCAL +16 LDINT MOD DUP32 LOCAL -12 STINT TRASH 4 LOCAL -8 LDINT DUP32 INT 1 ADD LOCAL -8 STINT TRASH 4 JMP while LABEL while_end LOCAL -4 LDINT STFVAL32 LEAVE RET

Compiling and Running

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

pf2asm powmod.pf
yasm -felf powmod.asm