I wonder how capable current AIs are with the "silent defense" variant of Stratego [1]? The article states the high level of uncertainty presents a challenge. With silent defense the uncertainty is even higher.

[1] https://www.hasbro.com/common/instruct/Stratego.PDF "When an attack is made, the attacker is the only player who has to declare the number of his or her piece. The defender does not reveal the number of his or her piece, but resolves the attack by removing whatever piece has a lower number from the gameboard. Players keep their own captured pieces. Exception: when a Scout attacks, the defender must reveal the number of his or her piece.

As a kid, a friend of mine had "Electronic Stratego"[1], the biggest gameplay change was that you could carry out fights without revealing the strength of either piece to the other side. I found this made for a much more interesting game and we had quite a bit of fun playing it.

1. https://boardgamegeek.com/boardgame/3513/electronic-stratego (We generally banned the use of the 'probing' feature)

If contention is expected, would it be better to first perform a relaxed read before the exchange? For example:

  auto lock() noexcept -> void {
      auto backoff = 1;
      do {
          while (locked_.load(std::memory_order_relaxed)) {
              for (auto i = 0; i < backoff; ++i) _mm_pause();
              backoff = backoff < 64 ? backoff << 1 : 64;
          }
      } while (locked_.exchange(true, std::memory_order_acquire);
  }

If you're expecting heavy contention, and there's no risk of any of the threads being descheduled, then FIFO spinlocks are probably best.

In a FIFO threads register themselves into a linked list, and the thread calling unlock() directly wakes the next. It's possible to have e.g. 20 threads in this case all spinning on their own cache lines (their private node), rather than a shared one (the lock head).

This can be coherence protocol optimal.

A dumb test and set spinlock, or variant thereof, is going to degrade quickly as all the cores are spinning on the same cacheline causing a lot of coherence traffic between cores (transitions between shared, exclusive and modified states)