Return scalar MulMod results and benchmark native reducers

EvenPerfectBitScanner.Benchmarks/GpuUInt128BinaryGcdBenchmarks.cs

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+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 GpuUInt128BinaryGcdBenchmarks
+{
+    private static readonly BinaryGcdInput[] Inputs = new BinaryGcdInput[]
+    {
+        new(new GpuUInt128(0UL, 48UL), new GpuUInt128(0UL, 18UL), "SmallOperands"),
+        new(
+            new GpuUInt128(0x0123_4567_89AB_CDEFUL, 0xFEDC_BA98_7654_3210UL),
+            new GpuUInt128(0x0FED_CBA9_8765_4321UL, 0x0123_4567_89AB_CDEFUL),
+            "HighEntropy"),
+        new(new GpuUInt128(0UL, ulong.MaxValue), new GpuUInt128(0UL, ulong.MaxValue - 1UL), "LowWordHeavy"),
+        new(new GpuUInt128(0x8000_0000_0000_0000UL, 0UL), new GpuUInt128(0x7FFF_FFFF_FFFF_FFFFUL, 0UL), "HighWordOnly"),
+    };
+
+    [ParamsSource(nameof(GetInputs))]
+    public BinaryGcdInput Input { get; set; }
+
+    public static IEnumerable<BinaryGcdInput> GetInputs() => Inputs;
+
+    [Benchmark(Baseline = true)]
+    public GpuUInt128 ReusedVariables()
+    {
+        return GpuUInt128.BinaryGcd(Input.Left, Input.Right);
+    }
+
+    [Benchmark]
+    public GpuUInt128 TemporaryStructPerIteration()
+    {
+        return BinaryGcdWithTemporary(Input.Left, Input.Right);
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static GpuUInt128 BinaryGcdWithTemporary(GpuUInt128 u, GpuUInt128 v)
+    {
+        if (u.IsZero)
+        {
+            return v;
+        }
+
+        if (v.IsZero)
+        {
+            return u;
+        }
+
+        int shift = GpuUInt128.TrailingZeroCount(new GpuUInt128(u.High | v.High, u.Low | v.Low));
+        int zu = GpuUInt128.TrailingZeroCount(u);
+        u >>= zu;
+
+        do
+        {
+            int zv = GpuUInt128.TrailingZeroCount(v);
+            v >>= zv;
+            if (u > v)
+            {
+                (u, v) = (v, u);
+            }
+
+            v -= u;
+        }
+        while (!v.IsZero);
+
+        return u << shift;
+    }
+
+    public readonly record struct BinaryGcdInput(GpuUInt128 Left, GpuUInt128 Right, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}