Expert Checksum with AVX2

crc32_expert_avx2.cs

public static unsafe uint ChecksumExpertAvx2(ReadOnlySpan<byte> arr)
{
        ref byte refSpan = ref MemoryMarshal.GetReference<byte>(arr);

        var z = 0;
        uint sum = 0;

        var vectorSum = Avx2.Xor(Vector256<byte>.Zero, Vector256<byte>.Zero).AsUInt32();
        var mask = Vector256.Create((byte)3, 2, 1, 0, 7, 6, 5, 4, 11, 10, 9, 8, 15, 14, 13, 12, (byte)3, 2, 1, 0, 7, 6, 5, 4, 11, 10, 9, 8, 15, 14, 13, 12);

        int limit = arr.Length - 128;
        while (z <= limit)
        {
            var v1 = Vector256.LoadUnsafe(ref Unsafe.Add(ref refSpan, z));
            var v2 = Vector256.LoadUnsafe(ref Unsafe.Add(ref refSpan, z + 32));
            var v3 = Vector256.LoadUnsafe(ref Unsafe.Add(ref refSpan, z + 64));
            var v4 = Vector256.LoadUnsafe(ref Unsafe.Add(ref refSpan, z + 96));

            var s1 = Avx2.Shuffle(v1, mask).AsUInt32();
            var s2 = Avx2.Shuffle(v2, mask).AsUInt32();
            var s3 = Avx2.Shuffle(v3, mask).AsUInt32();
            var s4 = Avx2.Shuffle(v4, mask).AsUInt32();

            var s5 = Avx2.Add(Avx2.Add(s1, s2), Avx2.Add(s3, s4));

            vectorSum = Avx2.Add(vectorSum, s5);

            z += 128;
        }

        ...
}