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Use lazy reduction in the rescaling part of the keyswitching. #177

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May 27, 2020
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Use lazy reduction in the rescaling part of the keyswitching. #177

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38 changes: 29 additions & 9 deletions native/src/seal/evaluator.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2478,9 +2478,10 @@ namespace seal
inverse_ntt_negacyclic_harvey_lazy(t_last, key_ntt_tables[key_modulus_size - 1]);

// Add (p-1)/2 to change from flooring to rounding.
uint64_t half = key_modulus[key_modulus_size - 1].value() >> 1;
const uint64_t qk = key_modulus[key_modulus_size - 1].value();
const uint64_t qk_half = qk >> 1;
for_each_n(t_last, coeff_count, [&](auto J) {
*J = barrett_reduce_63(*J + half, key_modulus[key_modulus_size - 1]);
*J = barrett_reduce_63(*J + qk_half, key_modulus[key_modulus_size - 1]);
});

for_each_n(
Expand All @@ -2497,22 +2498,41 @@ namespace seal
CoeffIter t_ntt_iter(t_ntt.get());

// (ct mod 4qk) mod qi
modulo_poly_coeffs_63(t_last, coeff_count, *get<1>(J), t_ntt_iter);
uint64_t fix = barrett_reduce_63(half, *get<1>(J));
const uint64_t qi = get<1>(J)->value();
if (qk > qi) {
modulo_poly_coeffs_63(t_last, coeff_count, *get<1>(J), t_ntt_iter);
} else {
set_uint_uint(t_last, coeff_count, t_ntt_iter);
}

for_each_n(t_ntt_iter, coeff_count, [&](auto K) { *K = sub_uint_uint_mod(*K, fix, *get<1>(J)); });
// lazy substraction, results in [0, 2*qi).
const uint64_t fix = qi - barrett_reduce_63(qk_half, *get<1>(J));
for_each_n(t_ntt_iter, coeff_count, [&](auto K) { *K += fix; });

uint64_t Lqi; // some multiples of qi
if (scheme == scheme_type::CKKS)
{
ntt_negacyclic_harvey(t_ntt_iter, *get<2>(J));
ntt_negacyclic_harvey_lazy(t_ntt_iter, *get<2>(J));
#if SEAL_USER_MOD_BIT_COUNT_MAX > 60
Lqi = qi << 1;
// reduce from [0, 4qi) to [0, 2qi)
for_each_n(t_ntt_iter, coeff_count, [Lqi](auto K) { *K -= (Lqi & static_cast<uint64_t>(-static_cast<int64_t>(*K >= Lqi))); });
#else
// Since now SEAL use at most 60bit moduli, so 8*qi < 2^63.
// This ntt_negacyclic_harvey_lazy results in [0, 4*qi).
Lqi = qi << 2;
#endif
}
else if (scheme == scheme_type::BFV)
{
inverse_ntt_negacyclic_harvey(get<1>(get<0>(J)), *get<2>(J));
Lqi = qi << 1;
inverse_ntt_negacyclic_harvey_lazy(get<1>(get<0>(J)), *get<2>(J));
}

// ((ct mod qi) - (ct mod qk)) mod qi
sub_poly_poly_coeffmod(get<1>(get<0>(J)), t_ntt_iter, coeff_count, *get<1>(J), get<1>(get<0>(J)));
// ((ct mod qi) - (ct mod qk)) mod L * qi
for_each_n(IterTuple<CoeffIter, CoeffIter>(get<1>(get<0>(J)), t_ntt_iter), coeff_count,
[Lqi](auto IT) { *get<0>(IT) += Lqi - *get<1>(IT); });

// qk^(-1) * ((ct mod qi) - (ct mod qk)) mod qi
multiply_poly_scalar_coeffmod(
get<1>(get<0>(J)), coeff_count, *get<3>(J), *get<1>(J), get<1>(get<0>(J)));
Expand Down