Return scalar MulMod results and benchmark native reducers

EvenPerfectBitScanner.Benchmarks/GpuUInt128Mul64Benchmarks.cs

@@ -0,0 +1,57 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core.Gpu;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[SimpleJob(RuntimeMoniker.Net80)]
+[MemoryDiagnoser]
+public class GpuUInt128Mul64Benchmarks
+{
+    private static readonly Mul64Input[] Inputs = new Mul64Input[]
+    {
+        new(new GpuUInt128(0UL, 1UL), new GpuUInt128(0UL, 1UL), "TinyOperands"),
+        new(new GpuUInt128(0UL, ulong.MaxValue), new GpuUInt128(0UL, ulong.MaxValue), "LowWordMax"),
+        new(new GpuUInt128(1UL, 0UL), new GpuUInt128(0UL, ulong.MaxValue), "HighByLow"),
+        new(new GpuUInt128(ulong.MaxValue, ulong.MaxValue), new GpuUInt128(ulong.MaxValue, ulong.MaxValue), "AllBitsSet"),
+    };
+
+    [ParamsSource(nameof(GetInputs))]
+    public Mul64Input Input { get; set; }
+
+    public static IEnumerable<Mul64Input> GetInputs() => Inputs;
+
+    [Benchmark(Baseline = true)]
+    public GpuUInt128 HighProductMaterialisedInLocal()
+    {
+        GpuUInt128 value = Input.Multiplicand;
+        value.Mul64(Input.Multiplier);
+        return value;
+    }
+
+    [Benchmark]
+    public GpuUInt128 HighProductInline()
+    {
+        GpuUInt128 value = Input.Multiplicand;
+        Mul64Inline(ref value, Input.Multiplier);
+        return value;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static void Mul64Inline(ref GpuUInt128 left, GpuUInt128 right)
+    {
+        ulong operand = left.Low;
+        left.Low = operand * right.Low;
+        left.High = operand * right.High + GpuUInt128.MulHigh(operand, right.Low);
+    }
+
+    public readonly record struct Mul64Input(GpuUInt128 Multiplicand, GpuUInt128 Multiplier, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}