Thin-air-read bad case

thin-air.c

// Out of thin air writes are really scary.
// Example adapted from http://dl.acm.org/citation.cfm?id=2618134 to
// work under http://svr-pes20-cppmem.cl.cam.ac.uk/cppmem/index.html


// Can wind up with an = &b, bn = &a!
int main() {
  int *p1, *p2;
  atomic_int a, an, b, bn;
  a = &an;
  b = &bn;

  {{{ { p1 = a.load(memory_order_relaxed);
        atomic_store_explicit(*p1, &a, memory_order_relaxed); }
  ||| { p2 = b.load(memory_order_relaxed);
        atomic_store_explicit(*p2, &b, memory_order_relaxed); } }}};
  return 0;
}

// But if there are no atomics, everything is fine!

int main() {
  int *p1, *p2;
  int a, an, b, bn;
  a = &an;
  b = &bn;

  {{{ { p1 = a;
        *p1 = &a; }
  ||| { p2 = b;
        *p2 = &b; } }}};
  return 0;
}