const int64_t start_us = NowMicros();
  int64_t iters = 0;
  while (true) {
    const int64_t iter_start_us = NowMicros();
    fn();
    const int64_t end_us = NowMicros();
    // Collect stats and decide whether to stop.
    stats->per_iter_us.push_back(end_us - iter_start_us);
    const int64_t total_us = end_us - start_us;
    ++iters;
    if ((max_us > 0 && total_us >= max_us) ||