Method __release_weak

system/lib/libcxx/src/memory.cpp:59–88

#endif // _LIBCPP_SHARED_PTR_DEFINE_LEGACY_INLINE_FUNCTIONS

void __shared_weak_count::__release_weak() noexcept {
  // NOTE: The acquire load here is an optimization of the very
  // common case where a shared pointer is being destructed while
  // having no other contended references.
  //
  // BENEFIT: We avoid expensive atomic stores like XADD and STREX
  // in a common case.  Those instructions are slow and do nasty
  // things to caches.
  //
  // IS THIS SAFE?  Yes.  During weak destruction, if we see that we
  // are the last reference, we know that no-one else is accessing
  // us. If someone were accessing us, then they would be doing so
  // while the last shared / weak_ptr was being destructed, and
  // that's undefined anyway.
  //
  // If we see anything other than a 0, then we have possible
  // contention, and need to use an atomicrmw primitive.
  // The same arguments don't apply for increment, where it is legal
  // (though inadvisable) to share shared_ptr references between
  // threads, and have them all get copied at once.  The argument
  // also doesn't apply for __release_shared, because an outstanding
  // weak_ptr::lock() could read / modify the shared count.
  if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Acquire) == 0) {
    // no need to do this store, because we are about
    // to destroy everything.
    //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release);
    __on_zero_shared_weak();
  } else if (__libcpp_atomic_refcount_decrement(__shared_weak_owners_) == -1)
    __on_zero_shared_weak();
}

__shared_weak_count* __shared_weak_count::lock() noexcept {
  long object_owners = __libcpp_atomic_load(&__shared_owners_);