cmp_in_x86.md

Basically, the cmp instruction is a subtraction that does not store the result, but only sets the flags. The flags are then used by the conditional jump instructions.

There are 6 flags the cmp instruction affects:

• ZF - Zero Flag
• SF - Sign Flag
• OF - Overflow Flag
• CF - Carry Flag
• PF - Parity Flag
• AF - Auxiliary Carry Flag

Here is the C equivalent of the cmp instruction:

static uint64_t count_bits(uint64_t x) {
  uint64_t res = 0;
  while (x) {
    res += x & 1;
    x >>= 1;
  }
  return res;
}

static void cmpq(core_t *core, uint64_t *src, uint64_t *dst) {
  uint64_t res = *dst - *src;
  core->cflag.CF = *dst < *src;
  core->cflag.ZF = *dst == *src;
  core->cflag.SF = res >> 63;
  core->cflag.OF = (*dst >> 63) != (res >> 63);
  core->cflag.PF = ~(count_bits(res & 0xff) & 1);
  core->cflag.AF = (*dst & 0xf) < (*src & 0xf);
}

Note that AT&T syntax is cmp src, dst, dst is the operand on the left side of subtraction, and src is the operand on the right side of subtraction.