bn_addw_addw
bn_addw_addw(a[0], b[0] & mask, carry, &carry, &r[0]);
bn_addw_addw(a[0], b[0], carry, &carry, &r[0]);
bn_addw_addw(a0, b0, carry, &carry, &r0);
bn_addw_addw(a1, b1, carry, &carry, &r1);
bn_addw_addw(a2, b2, carry, &carry, &r2);
bn_addw_addw(a3, b3, carry, &carry, &r3);
bn_addw_addw(v, a[i + n_len], carry, &carry, &a[i + n_len]);
bn_addw_addw(carry_a, carry_n, carry, &carry, &tp[n_len]);