__sync_leaf_prefix
if (likely(__sync_leaf_prefix(p, id) == p->prefix))
p->prefix = __sync_leaf_prefix(p, id);
GEM_BUG_ON(__sync_leaf_prefix(p, id) == p->prefix);
GEM_BUG_ON(p->prefix != __sync_leaf_prefix(p, id));
if (likely(p && __sync_leaf_prefix(p, id) == p->prefix)) {