Return scalar MulMod results and benchmark native reducers

EvenPerfectBitScanner.Benchmarks/GpuUInt128MulModBenchmarks.cs

@@ -0,0 +1,102 @@
+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 GpuUInt128MulModBenchmarks
+{
+    private static readonly MulModInput[] Inputs = new MulModInput[]
+    {
+        new(
+            new GpuUInt128(0UL, 5UL),
+            new GpuUInt128(0UL, 7UL),
+            new GpuUInt128(0UL, 97UL),
+            "TinyOperands"),
+        new(
+            new GpuUInt128(0UL, ulong.MaxValue - 3UL),
+            new GpuUInt128(0UL, ulong.MaxValue - 7UL),
+            new GpuUInt128(0UL, ulong.MaxValue - 11UL),
+            "LowWordHeavy"),
+        new(
+            new GpuUInt128(0x1234_5678_9ABC_DEF0UL, 0x0FED_CBA9_8765_4321UL),
+            new GpuUInt128(0x0ACE_BDF0_1357_9BDFUL, 0x0246_8ACE_ECA8_6421UL),
+            new GpuUInt128(0x1FFF_FFFF_FFFF_FFFFUL, 0xFFFF_FFFF_FFFF_FFF7UL),
+            "HighWordModulus"),
+        new(
+            new GpuUInt128(0x8000_0000_0000_0000UL, 0x0000_0000_0000_0001UL),
+            new GpuUInt128(0x7FFF_FFFF_FFFF_FFFFUL, 0x8000_0000_0000_0000UL),
+            new GpuUInt128(0x7FFF_FFFF_FFFF_FFFFUL, 0xFFFF_FFFF_FFFF_FFF1UL),
+            "MixedMagnitude"),
+    };
+
+    [ParamsSource(nameof(GetInputs))]
+    public MulModInput Input { get; set; }
+
+    public static IEnumerable<MulModInput> GetInputs() => Inputs;
+
+    [Benchmark(Baseline = true)]
+    public GpuUInt128 InPlaceMulMod()
+    {
+        GpuUInt128 value = Input.Left;
+        value.MulMod(Input.Right, Input.Modulus);
+        return value;
+    }
+
+    [Benchmark]
+    public GpuUInt128 AllocatePerIteration()
+    {
+        return MulModInline(Input.Left, Input.Right, Input.Modulus);
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static GpuUInt128 MulModInline(GpuUInt128 left, GpuUInt128 right, GpuUInt128 modulus)
+    {
+        GpuUInt128 a = left;
+        GpuUInt128 b = right;
+        GpuUInt128 result = new();
+
+        while (!b.IsZero)
+        {
+            if ((b.Low & 1UL) != 0UL)
+            {
+                result.Add(a);
+                if (result.CompareTo(modulus) >= 0)
+                {
+                    result.Sub(modulus);
+                }
+            }
+
+            a <<= 1;
+            if (a.CompareTo(modulus) >= 0)
+            {
+                a.Sub(modulus);
+            }
+
+            if (b.High == 0UL)
+            {
+                b = new GpuUInt128(b.Low >> 1);
+            }
+            else
+            {
+                ulong low = (b.Low >> 1) | (b.High << 63);
+                ulong high = b.High >> 1;
+                b = new GpuUInt128(high, low);
+            }
+        }
+
+        return result;
+    }
+
+    public readonly record struct MulModInput(GpuUInt128 Left, GpuUInt128 Right, GpuUInt128 Modulus, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}