Function lut_dists_block32_4b

cpp/src/quantize/multi_codebook.hpp:272–314

template<int NBytes>
inline void lut_dists_block32_4b(const uint8_t* codes, const uint8_t* luts,
    uint8_t* dists_out, int64_t nblocks)
{
    static_assert(NBytes > 0, "Code length <= 0 is not valid");
    static const __m256i low_4bits_mask = _mm256_set1_epi8(0x0F);

    for (int64_t i = 0; i < nblocks; i++) {
        auto totals = _mm256_setzero_si256();
        for (uint8_t j = 0; j < NBytes; j++) {
            // auto x_col = load_si256i(codes);
            auto x_col = stream_load_si256i(codes);
            auto lut_low = load_si256i(luts);
            auto lut_high = load_si256i(luts + 32);
            // auto both_luts = load_si256i(luts);
            // auto lut_low = _mm256_permute2x128_si256(both_luts, both_luts, 0 + (0 << 4));
            // auto lut_high = _mm256_permute2x128_si256(both_luts, both_luts, 1 + (1 << 4));

            // compute distances via lookups; we have one table for the upper
            // 4 bits of each byte in x, and one for the lower 4 bits; the
            // shuffle instruction always looks at the lower 4 bits, so we
            // have to shift x to look at its upper 4 bits; also note that
            // we have to mask out the upper bit because the shuffle
            // instruction will zero the corresponding byte if this bit is set
            auto x_low = _mm256_and_si256(x_col, low_4bits_mask);
            auto x_high = _mm256_srli_epi16(x_col, 4);
            x_high = _mm256_and_si256(x_high, low_4bits_mask);

            auto dists_low = _mm256_shuffle_epi8(lut_low, x_low);
            auto dists_high = _mm256_shuffle_epi8(lut_high, x_high);

            totals = _mm256_adds_epu8(totals, dists_low);
            totals = _mm256_adds_epu8(totals, dists_high);

            codes += 32;
            luts += 64;
        }
        // _mm256_store_si256((__m256i*)dists_out, totals);
        _mm256_stream_si256((__m256i*)dists_out, totals); // "non-temporal memory hint"
        luts -= NBytes * 64;
        // luts -= 8 * 32;
        dists_out += 32;
    }
}

// for debugging; should have same behavior as above (vectorized and
// non-unpacking) func