[Bug middle-end/59448] Code generation doesn't respect C11 address-dependency

["algrant at acm dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448

--- Comment #4 from algrant at acm dot org ---
So using g++,

  #include <atomic>
  int f1(std::atomic<int> const *p, std::atomic<int> const *q)
  {
    int flag = p->load(std::memory_order_consume);
    return flag ? (q + flag - flag)->load(std::memory_order_relaxed) : 0;
  }

demonstrates the same lack of ordering.  You suggest that this might
be a problem with the atomic built-ins - and yes, if this had been a
load-acquire, it would be a problem with the built-in not introducing a
barrier or using a load-acquire instruction.  But for a load-consume on
this architecture, no barrier is necessary to separate the load-consume
from a load that is address-dependent on it.  The programmer wrote a
dependency but the compiler lost track of it.

It's not necessary to demonstrate failure - there's an architectural 
race condition here.  Even if it doesn't fail now there's no guarantee
it will never fail on future more aggressively reordering cores.