To be pedantic, C has no 8- or 16-bit addition operators, since everything sub-int is scaled up to int to do arithmetic. Therefore, the `x += 120;` line never overflows, since it is actually `x = (int8_t)((int)x + 120);`, and the possible range of `(int)x + 120` is comfortably within the range of expressible ints, while the conversion to int8_t is defined to wrap around when oversized. So there compiler can't optimize out do_important_thing in your example.