Not everything in AVX2 is 256-bit

AVX2 has added support for 256-bit arguments for many operations on packed integers, although not all. Some instructions accept the 256-bit registers, but operates on 128-bit lanes rather whole register.

There are three major groups of instructions: packing (narrowing conversion), unpacking (interleave) and permutations; below is a full list of instructions (with intrinsics):

• valignr (_mm256_alignr_epi8)
• vpslldq (_mm256_bslli_epi128)
• vpsrldq (_mm256_bsrli_epi128)
• vmpsadbw (_mm256_mpsadbw_epu8)
• vpacksswb (_mm256_packs_epi16)
• vpackssdw (_mm256_packs_epi32)
• vpackuswb (_mm256_packus_epi16)
• vpackusdw (_mm256_packus_epi32)
• vperm2i128 (_mm256_permute2x128_si256)
• vpermq (_mm256_permute4x64_epi64)
• vpermpd (_mm256_permute4x64_pd)
• vpshufd (_mm256_shuffle_epi32)
• vpshufb (_mm256_shuffle_epi8)
• vpshufhw (_mm256_shufflehi_epi16)
• vpshuflw (_mm256_shufflelo_epi16)
• vpslldq (_mm256_slli_si256)
• vpsrldq (_mm256_srli_si256)
• vpunpckhwd (_mm256_unpackhi_epi16)
• vpunpckhdq (_mm256_unpackhi_epi32)
• vpunpckhqdq (_mm256_unpackhi_epi64)
• vpunpckhbw (_mm256_unpackhi_epi8)
• vpunpcklwd (_mm256_unpacklo_epi16)
• vpunpckldq (_mm256_unpacklo_epi32)
• vpunpcklqdq (_mm256_unpacklo_epi64)
• vpunpcklbw (_mm256_unpacklo_epi8)

For me the most surprising are packing instructions (vpack*) as they require additional shuffling (after or before the instruction) if we want to keep order of values. In some cases the order is crucial.

SSE: Generating mask where n leading (trailing) bytes are set

Informal specification:

__m128i mask_lower(const unsigned n) {
    
    assert(n < 16);
    switch (n) {
        case 0: return {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
        case 1: return {0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
        case 2: return {0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
        // ...
        case 14: return {0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00};
        case 15: return {0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
    }
}

__m128i mask_higher(const unsigned n) {
    
    assert(n < 16);
    return ~mask_lower(15 - n);
}

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