__NIFBITS
uint32_t ifs_bits[howmany(256, __NIFBITS)];
((p)->ifs_bits[(n)/__NIFBITS] |= (1U << ((n) % __NIFBITS)))
((p)->ifs_bits[(n)/__NIFBITS] &= ~(1U << ((n) % __NIFBITS)))
((p)->ifs_bits[(n)/__NIFBITS] & (1U << ((n) % __NIFBITS)))