entry_compose
*pptr = entry_compose(entry_ptr(entry),
root = entry_compose(newnodes[h], tagmask);
KASSERT(entry_tagmask(entry_compose(p, 0)) == 0);
KASSERT(entry_tagmask(entry_compose(p, 0)) == 0);
*vpp = entry_compose(p, entry_tagmask(*vpp));
*pptr = entry_compose(n, newmask);