Optimize GPU divisor heuristics pipeline

EvenPerfectBitScanner.Benchmarks/Pow2MontgomeryModWindowedKeepMontgomeryCpuBenchmarks.cs

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+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+/// <summary>
+/// Benchmarks CPU Montgomery exponentiation helpers for exponents at or above 138 million.
+/// Reuses the shared benchmark input cache so every method processes identical samples.
+/// </summary>
+[MemoryDiagnoser]
+[SimpleJob(RuntimeMoniker.Net80, launchCount: 1, warmupCount: 1, iterationCount: 5)]
+public class Pow2MontgomeryModWindowedKeepMontgomeryCpuBenchmarks
+{
+    private Pow2MontgomeryModWindowedBenchmarkInputs _inputs = null!;
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        _inputs = Pow2MontgomeryModWindowedBenchmarkInputsProvider.Instance;
+    }
+
+    /// <summary>
+    /// Baseline CPU windowed Montgomery exponentiation that converts the residue back to standard form.
+    /// </summary>
+    [Benchmark(Baseline = true)]
+    public ulong WindowedMontgomeryConvert()
+    {
+        ulong checksum = 0UL;
+        Pow2MontgomeryModWindowedBenchmarkCase[] cases = _inputs.Cases;
+
+        for (int i = 0; i < cases.Length; i++)
+        {
+            ref readonly Pow2MontgomeryModWindowedBenchmarkCase current = ref cases[i];
+            checksum ^= current.Exponent.Pow2MontgomeryModWindowedCpu(current.Divisor, keepMontgomery: false);
+        }
+
+        return checksum;
+    }
+
+    /// <summary>
+    /// CPU windowed Montgomery exponentiation that keeps the residue in the Montgomery domain.
+    /// </summary>
+    [Benchmark]
+    public ulong WindowedMontgomeryKeep()
+    {
+        ulong checksum = 0UL;
+        Pow2MontgomeryModWindowedBenchmarkCase[] cases = _inputs.Cases;
+
+        for (int i = 0; i < cases.Length; i++)
+        {
+            ref readonly Pow2MontgomeryModWindowedBenchmarkCase current = ref cases[i];
+            checksum ^= current.Exponent.Pow2MontgomeryModWindowedCpu(current.Divisor, keepMontgomery: true);
+        }
+
+        return checksum;
+    }
+
+    /// <summary>
+    /// Uses the known cycle length to fold the exponent before running the windowed Montgomery ladder.
+    /// </summary>
+    [Benchmark]
+    public ulong WindowedMontgomeryWithCycle()
+    {
+        ulong checksum = 0UL;
+        Pow2MontgomeryModWindowedBenchmarkCase[] cases = _inputs.Cases;
+
+        for (int i = 0; i < cases.Length; i++)
+        {
+            ref readonly Pow2MontgomeryModWindowedBenchmarkCase current = ref cases[i];
+            checksum ^= current.Exponent.Pow2MontgomeryModWithCycleCpu(current.CycleLength, current.Divisor);
+        }
+
+        return checksum;
+    }
+
+    /// <summary>
+    /// Runs the windowed Montgomery ladder on the reduced exponent that the divisor cycle exposes.
+    /// </summary>
+    [Benchmark]
+    public ulong WindowedMontgomeryFromCycleRemainder()
+    {
+        ulong checksum = 0UL;
+        Pow2MontgomeryModWindowedBenchmarkCase[] cases = _inputs.Cases;
+
+        for (int i = 0; i < cases.Length; i++)
+        {
+            ref readonly Pow2MontgomeryModWindowedBenchmarkCase current = ref cases[i];
+            checksum ^= current.ReducedExponent.Pow2MontgomeryModFromCycleRemainderCpu(current.Divisor);
+        }
+
+        return checksum;
+    }
+}