Designing a Lock-free, Wait-free Hash Map(shlomisteinberg.com)

  std::atomic<unsigned long> sum;
  void do_work(size_t N, unsigned long* a) {
    for (size_t i = 0; i < N; ++i) sum += a[i];
  }

  unsigned long sum(0); std::mutex M;
  void do_work(size_t N, unsigned long* a) {
    unsigned long s = 0;
    for (size_t i = 0; i < N; ++i) s += a[i];
    std::lock_guard<std::mutex> L(M); sum += s;
  }

  std::atomic<unsigned long> sum;
  void do_work(size_t N, unsigned long* a) {
    unsigned long s = 0;
    for (size_t i = 0; i < N; ++i) s += a[i];
    sum += s;
  }

    old_val <span style="color: #333333">=</span> ;