Comment by BoppreH

You can also use math to avoid most of the jumps:

    int run_switches(char *input) {
      int res = 0;
      while (true) {
        char c = *input++;
        if (c == '\0') return res;
        // Here's the trick:
        res += (c == 's') - (c == 'p');
      }
    }

This gives a 3.7x speed compared to loop-1.c. The lower line count is also nice.