phtree_compare
SPLAY_PROTOTYPE(phtree, pool_item_header, ph_node, phtree_compare);
SPLAY_GENERATE(phtree, pool_item_header, ph_node, phtree_compare);
if (ph != NULL && phtree_compare(&tmp, ph) >= 0) {
KASSERT(ph == NULL || phtree_compare(&tmp, ph) < 0);