The Chapel runtime atomics interface is modeled after the
C11 standard. See: http://en.cppreference.com/w/c/atomic

Currently we have a cstdlib, an intrinsics, and a locks
implementation. The cstdlib implementation is basically a wrapper for
C11 atomics. The intrinsics implementation uses compiler atomic
intrinsics to implement atomics and the locks version uses Chapel
locks. The cstdlib implementation is expected to have the best
performance in the long run, depending on the compiler's native
handling of C11 atomics, and the intrinsic version has better
performance than locks, but neither the standard atomics nor the
intrinsics are available on all systems.  (For gcc up to at least
6.2.0, though, there is a performance bug that inhibits more
optimization than necessary with standard atomics.)

We require the C11 standard atomic functions for the types:
   atomic_uint_least8_t
   atomic_uint_least16_t
   atomic_uint_least32_t
   atomic_uint_least64_t
   atomic_uintptr_t
   atomic_int_least8_t
   atomic_int_least16_t
   atomic_int_least32_t
   atomic_int_least64_t
   atomic_bool
and we require that the namespace has room for us to provide
type-specific routines that have the type appended to the names of the
equivalent generic functions, so that (for example),
atomic_fetch_add((uint_least64_t*) ptr, 100 ) will be written
atomic_fetch_add_uint_least64_t( (uint_least64_t*) ptr, 100 )

In addition, we assume that there is a function e.g.
void atomic_destroy_uint_least64_t(volatile A* obj, C value);
so that we have an opportunity to free memory occupied by a lock.


Here is what we expect from the C11 standard:


/////////////////
Memory Orderings:
/////////////////

   typedef enum {
     memory_order_relaxed,
     memory_order_consume,
     memory_order_acquire,
     memory_order_release,
     memory_order_acq_rel,
     memory_order_seq_cst
   } memory_order;

   // release, acq_rel, seq_cst store performs a release on affected memory.
   // acquire, acq_rel, seq_cst load operation performs acquire on affected memory.
   // consume a load operation performs a consume on affected memory
   // seq_cst is 'sequentially consistent'
   // At present our implementations are really the same regardless of the memory order


//////////////
Memory Fences:
//////////////

   // generic memory order-dependent fence synchronization primitive
   void atomic_thread_fence(memory_order order);

   // fence between a thread and a signal handler executed in the same thread
   void atomic_signal_fence(memory_order order);


////////////////////
For Atomic Integers:
////////////////////

   // the standard specifies many types, but Chapel uses only:
   atomic_uint_least8_t
   atomic_uint_least16_t
   atomic_uint_least32_t
   atomic_uint_least64_t
   atomic_uintptr_t
   atomic_int_least8_t
   atomic_int_least16_t
   atomic_int_least32_t
   atomic_int_least64_t
   atomic_bool

   // All the following functions are generic - the Chapel runtime will assume
   // that we have implementations with e.g. _uint_least64_t appended to the function
   // name. The non _explicit versions use memory_order_seq_cst.

   // is it implemented lock-free?
   bool atomic_is_lock_free(const volatile A* object);

   // non-atomically initialize an atomic variable (the name is a little confusing.)
   void atomic_init(volatile A* obj, C value);

   // gives us a chance to free any mechanisms used to guarantee atomicity (E.G locks)
   // Note that this function is not part of the C11 standard
   void atomic_destroy(volatile A* object);

   // stores a value in an atomic object
   C atomic_store(volatile A* object, C desired);
   C atomic_store_explicit(volatile A* object, C desired, memory_order order);

   // reads a value from an atomic object
   C atomic_load(volatile A* object);
   C atomic_load_explicit(volatile A* object, memory_order order);

   // swaps a value with the value of an atomic object
   C atomic_exchange(volatile A* object, C desired);
   C atomic_exchange_explicit(volatile A* object, C desired, memory_order order);

   // strong compare-and-swap.
   bool atomic_compare_exchange_strong(volatile A* object, C* expected, C desired);
   bool atomic_compare_exchange_strong_explicit(volatile A* object, C* expected, C desired, memory_order success, memory_order failure);

   // weak compare-and-swap can fail for no real reason.
   bool atomic_compare_exchange_weak(volatile A* object, C* expected, C desired);
   bool atomic_compare_exchange_weak_explicit(volatile A* object, C* expected, C desired, memory_order success, memory_order failure);

   // atomic fetch and op, for op in add, sub, or, and:
   // (atomic xor only supported on some platforms)
   // for non-bool integers
   C atomic_fetch_op(volatile A *object, M operand);
   C atomic_fetch_op_explicit(volatile A *object, M operand, memory_order order);


/////////////////
For Atomic Reals:
/////////////////

   // Chapel also has atomic support for reals (_real32 and _real64).
   // The main difference is that the fetch_op operations are not part
   // of the C standard, and those that do not make sense for reals
   // are not part of the interface.

   // non-atomically initialize an atomic variable (the name is a little confusing.)
   void atomic_init(volatile A* object, C value);

   // gives us a chance to free any mechanisms used to guarantee atomicity (E.G locks)
   // Note that this function is not part of the C11 standard
   void atomic_destroy(volatile A* object);

   // stores a value in an atomic object
   C atomic_store(volatile A* object, C desired);
   C atomic_store_explicit(volatile A* object, C desired, memory_order order);

   // reads a value from an atomic object
   C atomic_load(volatile A* object);
   C atomic_load_explicit(volatile A* object, memory_order order);

   // swaps a value with the value of an atomic object
   C atomic_exchange(volatile A* object, C desired);
   C atomic_exchange_explicit(volatile A* object, C desired, memory_order order);

   // strong compare-and-swap.
   bool atomic_compare_exchange_strong(volatile A* object, C* expected, C desired);
   bool atomic_compare_exchange_strong_explicit(volatile A* object, C* expected, C desired, memory_order success, memory_order failure);

   // weak compare-and-swap can fail for no real reason.
   bool atomic_compare_exchange_weak(volatile A* object, C* expected, C desired);
   bool atomic_compare_exchange_weak_explicit(volatile A* object, C* expected, C desired, memory_order success, memory_order failure);

   // atomic fetch and op, for op in add and sub
   C atomic_fetch_op(volatile A *object, M operand);
   C atomic_fetch_op_explicit(volatile A *object, M operand, memory_order order);

