ph_bitmap
ph->ph_bitmap + (idx / BITMAP_SIZE);
pool_item_bitmap_t *bitmap = ph->ph_bitmap + (idx / BITMAP_SIZE);
pool_item_bitmap_t *bitmap = ph->ph_bitmap;
pool_item_bitmap_t *bitmap = ph->ph_bitmap;
ph_bitmap[howmany(nelem, BITMAP_SIZE)]);
offsetof(struct pool_item_header, ph_bitmap[0]);