DotProduct operation using SIMD

MemoryRef.cs

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace VectorSearch.Memory;

public readonly ref struct MemoryRef<T>
{
    private readonly ref T _value;

    public MemoryRef(ref T value)
    {
        _value = ref value;
    }
    
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ref T GetElement(nint index) => ref Unsafe.Add(ref _value, index);
    
    public static MemoryRef<T> GetReference(ReadOnlySpan<T> span)
    {
        return new MemoryRef<T>(ref MemoryMarshal.GetReference(span));
    }
}

Scalar.cs

using VectorSearch.Memory;

namespace VectorSearch.Math;

public static class Scalar
{
    public static double DotProduct(ReadOnlySpan<double> vector1, ReadOnlySpan<double> vector2)
    {
        VectorHelper.GuardMismatchedSizes(vector1, vector2);

        var vector1Ref = MemoryRef<double>.GetReference(vector1);
        var vector2Ref = MemoryRef<double>.GetReference(vector2);

        var dotProduct = 0d;
        
        nint dimensionCount = vector1.Length;
        nint i = 0;
        for (; i < dimensionCount; i++)
        {
            ref var vector1Value = ref vector1Ref.GetElement(i);
            ref var vector2Value = ref vector2Ref.GetElement(i);

            dotProduct += vector1Value * vector2Value;
        }
        
        return dotProduct;
    }
}

SIMD.cs

using System.Numerics;
using System.Runtime.CompilerServices;
using VectorSearch.Memory;

namespace VectorSearch.Math;

public static class SIMD
{
    public static double DotProduct(ReadOnlySpan<double> vector1, ReadOnlySpan<double> vector2)
    {
        const nint operationsPerIteration = 2;
        
        VectorHelper.GuardMismatchedSizes(vector1, vector2);
        
        if (!Vector.IsHardwareAccelerated || !Vector<double>.IsSupported || vector1.Length < Vector<double>.Count)
            return Scalar.DotProduct(vector1, vector2);
        
        var vector1Ref = MemoryRef<double>.GetReference(vector1);
        var vector2Ref = MemoryRef<double>.GetReference(vector2);

        var dotProduct = 0d;
        
        nint dimensionCount = vector1.Length;
        nint i = 0;
        nint step = Vector<double>.Count;
        nint bound = dimensionCount & ~(step * operationsPerIteration - 1);
        for (; i < bound; i += step * operationsPerIteration)
        {
            var simdRegister1Vector1 = Unsafe.As<double, Vector<double>>(ref vector1Ref.GetElement(i));
            var simdRegister1Vector2 = Unsafe.As<double, Vector<double>>(ref vector2Ref.GetElement(i));
            
            var simdRegister2Vector1 = Unsafe.As<double, Vector<double>>(ref vector1Ref.GetElement(i + step));
            var simdRegister2Vector2 = Unsafe.As<double, Vector<double>>(ref vector2Ref.GetElement(i + step));
            
            dotProduct += Vector.Dot(simdRegister1Vector1, simdRegister1Vector2) +
                          Vector.Dot(simdRegister2Vector1, simdRegister2Vector2);
        }

        for (; i < dimensionCount; i++)
        {
            var simdRegister1Vector1 = Unsafe.As<double, Vector<double>>(ref vector1Ref.GetElement(i));
            var simdRegister1Vector2 = Unsafe.As<double, Vector<double>>(ref vector2Ref.GetElement(i));

            dotProduct += Vector.Dot(simdRegister1Vector1, simdRegister1Vector2);
        }

        return dotProduct;
    }
}