gistfile1.txt

  BbAa contains BBBBbbbbAAAAaaaa
  DdCc contains DDDDddddCCCCcccc
  
  we want:      DDDDCCCCBBBBAAAA
  we want:      ddddccccbbbbaaaa
  
  const auto process_pair = [](const __m256i BbAa, const __m256i DdCc) {
    const __m256i DbCa = _mm256_blend_epi32(BbAa, DdCc, 0b11001100);
    const __m256i dBcA = _mm256_blend_epi32(BbAa, DdCc, 0b00110011);
    const __m256i DCba = _mm256_permute4x64_epi64(DbCa, 0b11011000);
    const __m256i dcBA = _mm256_permute4x64_epi64(dBcA, 0b11011000);
    const __m256i DCBA = _mm256_blend_epi32(DCba, dcBA, 0b00001111);
    const __m256i dcba = _mm256_blend_epi32(DCba, dcBA, 0b11110000);
    return std::make_tuple(dcba, DCBA);
  };