s_mp_mod_2d
void s_mp_mod_2d(mp_int *mp, mp_digit d); /* modulo 2^d in place */
s_mp_mod_2d(r, d);
s_mp_mod_2d(&val, k);
s_mp_mod_2d(&t1, k);
s_mp_mod_2d(&tmp2, k);
s_mp_mod_2d(rem, (mp_digit)ix);
s_mp_mod_2d(x, DIGIT_BIT * (USED(m) + 1));
s_mp_mod_2d(&q, DIGIT_BIT * (USED(m) + 1));