__sync_branch_prefix
if (__sync_branch_prefix(p, id) == p->prefix)
if (__sync_branch_prefix(p, id) != p->prefix)
if (__sync_branch_prefix(p, id) == p->prefix)
if (__sync_branch_prefix(p, id) != p->prefix) {
above = fls64(__sync_branch_prefix(p, id) ^ p->prefix);
next->prefix = __sync_branch_prefix(next, id);