#include <assert.h>
#include <atomic>
#include <thread>

std::atomic<bool> x, y;
std::atomic<int> z;

void
write_x()
{
  x.store(true, std::memory_order_release);
}

void
write_y()
{
  y.store(true, std::memory_order_release);
}

void
read_x_then_y()
{
  while (!x.load(std::memory_order_acquire))
    ;
  if (y.load(std::memory_order_acquire))
    ++z;
}

void
read_y_then_x()
{
  while (!y.load(std::memory_order_acquire))
    ;
  if (x.load(std::memory_order_acquire))
    ++z;
}

int
main()
{
  x = false;
  y = false;
  z = 0;
  std::thread a(write_x);
  std::thread b(write_y);
  std::thread c(read_x_then_y);
  std::thread d(read_y_then_x);
  a.join();
  b.join();
  c.join();
  d.join();
  assert(z.load() != 0);
}