Sum simd performance

Program.cs

using System.Numerics;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;

var b = new Benchmarks();
b.Setup();
var result = b.Sum(b._input);
Console.WriteLine(result.Item1);
Console.WriteLine(result.Item2);
result = b.SumSimd2(b._input);
Console.WriteLine(result.Item1);
Console.WriteLine(result.Item2);

BenchmarkRunner.Run<Benchmarks>();

[HardwareCounters(
        HardwareCounter.BranchMispredictions,
        HardwareCounter.BranchInstructions)]
public class Benchmarks
{
    public byte[] _input = [];

    [GlobalSetup]
    public void Setup()
    {
        _input = Enumerable.Range(1, 1_000_000).Select(x => (byte)(x % 256)).ToArray();
        Random.Shared.Shuffle(_input);
    }

    [Benchmark]
    public void Sum() => Sum(_input);

    [Benchmark]
    public void SumSimd2() => SumSimd2(_input);

    public (uint, uint) Sum(byte[] input)
    {
        uint sumSmall = 0;
        uint sumTotal = 0;
        for (var i = 0; i < input.Length; i++)
        {
            if (input[i] < 128)
                sumSmall += input[i];
            sumTotal += input[i];
        }
        return (sumSmall, sumTotal);
    }

    public (uint, uint) SumSimd2(byte[] input)
    {
        Vector<uint> vSumSmall = Vector<uint>.Zero;
        Vector<uint> vSumTotal = Vector<uint>.Zero;
        Vector<ushort> vInterSmall = Vector<ushort>.Zero;
        Vector<ushort> vInterTotal = Vector<ushort>.Zero;
        byte b = 0;

        Vector<byte> vLimit = Vector.Create<byte>(128);
        int i = input.Length - Vector<byte>.Count;
        for (; i >= 0; i -= Vector<byte>.Count)
        {
            var vCurrent = Vector.LoadUnsafe(ref input[i]);
            var vMask = Vector.LessThan(vCurrent, vLimit);

            var vDiff = Vector.ConditionalSelect(vMask, vCurrent, Vector<byte>.Zero);
            Vector.Widen(vDiff, out var lower, out var upper);
            vInterSmall += lower + upper;
            Vector.Widen(vCurrent, out lower, out upper);
            vInterTotal += lower + upper;

            unchecked { b += 2; }
            if (b == 0)
            {
                Vector.Widen(vInterSmall, out var lower2, out var upper2);
                vSumSmall += lower2 + upper2;
                Vector.Widen(vInterTotal, out lower2, out upper2);
                vSumTotal += lower2 + upper2;
                vInterSmall = Vector<ushort>.Zero;
                vInterTotal = Vector<ushort>.Zero;
            }
        }

        Vector.Widen(vInterSmall, out var flower2, out var fupper2);
        vSumSmall += flower2 + fupper2;
        Vector.Widen(vInterTotal, out flower2, out fupper2);
        vSumTotal += flower2 + fupper2;
        var sumSmall = Vector.Sum(vSumSmall);
        var sumTotal = Vector.Sum(vSumTotal);

        for (i += Vector<byte>.Count - 1; i >= 0; i--)
        {
            if (input[i] < 128)
                sumSmall += input[i];
            sumTotal += input[i];
        }

        return (sumSmall, sumTotal);
    }
}

Sumsimd.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="BenchmarkDotNet" Version="0.14.0" />
    <PackageReference Include="BenchmarkDotNet.Diagnostics.Windows" Version="0.14.0" />
  </ItemGroup>

</Project>