Question

I understand that in C-like languages I can perform a boolean not with the not operator: (C)

int myBool = TRUE;
myBool = !myBool;

But my question is: How is this implemented behind the scenes? My guess is by using jumps, but those could be inefficient if used excessively: (Intel x86 syntax)

; assume eax holds boolean
  test eax, eax
  jnz short boolTrue
  inc eax ; eax was 0, now 1
  jmp short after
boolTrue: ; eax non-zero
  xor eax, eax ; eax now 0
after:

As shown, it requires 5 instructions with at least one jump and one bit-wise and (test). There has to be an easier way to do this because I’ve seen code bases that do “double-nots” (if (!!fileHandle)) for some weird reason.

So (as said above): How do compilers do boolean !s on x86?

Answer 1

You can do it with SETZ (set (if) zero (flag is set)) which require no branches.