why latter is fast ? (reject called from main loop continuously)

gistfile1.c

void
reject(UCHARP beg, UCHARP buf, UCHARP end, ENVP env){
  if (buf >= end) {
    if (env->print_count) {
      if (env->filename) {
        printf("%s:", env->filename);  
      }
      printf("%d\n", env->count);
    }
    return;
  }
  beg = buf;
  return s0(beg, buf, end, env);
}

/* remove if-statement ver. */

void
reject(UCHARP beg, UCHARP buf, UCHARP end, ENVP env){
  if (buf >= end) {
    return;
  }
  beg = buf;
  return s0(beg, buf, end, env);
}