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static inline __m128i
dec_reshuffle (const __m128i in)
{
// in, bits, upper case are most significant bits, lower case are least significant bits
// 00llllll 00kkkkLL 00jjKKKK 00JJJJJJ
// 00iiiiii 00hhhhII 00ggHHHH 00GGGGGG
// 00ffffff 00eeeeFF 00ddEEEE 00DDDDDD
// 00cccccc 00bbbbCC 00aaBBBB 00AAAAAA
const __m128i merge_ab_and_bc = _mm_maddubs_epi16(in, _mm_set1_epi32(0x01400140));
// 0000kkkk LLllllll 0000JJJJ JJjjKKKK
// 0000hhhh IIiiiiii 0000GGGG GGggHHHH
// 0000eeee FFffffff 0000DDDD DDddEEEE
// 0000bbbb CCcccccc 0000AAAA AAaaBBBB
const __m128i out = _mm_madd_epi16(merge_ab_and_bc, _mm_set1_epi32(0x00011000));
// 00000000 JJJJJJjj KKKKkkkk LLllllll
// 00000000 GGGGGGgg HHHHhhhh IIiiiiii
// 00000000 DDDDDDdd EEEEeeee FFffffff
// 00000000 AAAAAAaa BBBBbbbb CCcccccc
// Pack bytes together:
return _mm_shuffle_epi8(out, _mm_setr_epi8(
2, 1, 0,
6, 5, 4,
10, 9, 8,
14, 13, 12,
-1, -1, -1, -1));
// 00000000 00000000 00000000 00000000
// LLllllll KKKKkkkk JJJJJJjj IIiiiiii
// HHHHhhhh GGGGGGgg FFffffff EEEEeeee
// DDDDDDdd CCcccccc BBBBbbbb AAAAAAaa
}
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