Merge pull request #143 from mlemanczyk/codex/optimize-random-byte-generation-in-pollardrho

Improve Pollard-Rho random generation performance

PerfectNumbers.Core/DeterministicRandom.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Threading;
+
+namespace PerfectNumbers.Core;
+
+internal static class DeterministicRandom
+{
+    [ThreadStatic]
+    private static ulong s_threadState;
+
+    private static long s_seedCounter;
+
+    public static ulong NextUInt64()
+    {
+        ulong state = s_threadState;
+        if (state == 0UL)
+        {
+            state = InitializeThreadState();
+        }
+
+        state ^= state >> 12;
+        state ^= state << 25;
+        state ^= state >> 27;
+        s_threadState = state;
+        return state * 2685821657736338717UL;
+    }
+
+    public static UInt128 NextUInt128()
+    {
+        UInt128 high = (UInt128)NextUInt64();
+        UInt128 low = (UInt128)NextUInt64();
+        return (high << 64) | low;
+    }
+
+    private static ulong InitializeThreadState()
+    {
+        ulong seed = (ulong)Interlocked.Increment(ref s_seedCounter);
+        seed += 0x9E3779B97F4A7C15UL;
+        seed = (seed ^ (seed >> 30)) * 0xBF58476D1CE4E5B9UL;
+        seed = (seed ^ (seed >> 27)) * 0x94D049BB133111EBUL;
+        seed ^= seed >> 31;
+        if (seed == 0UL)
+        {
+            seed = 0x9E3779B97F4A7C15UL;
+        }
+
+        return seed;
+    }
+}

PerfectNumbers.Core/MersenneDivisorCycles.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Text;
-using System.Threading;
 using PerfectNumbers.Core.Gpu;
 using ILGPU;
 using ILGPU.Runtime;
@@ -16,11 +15,6 @@ public class MersenneDivisorCycles
     // Lightweight read-mostly cache for small divisors (<= 4,000,000). 0 => unknown
     private ulong[]? _smallCycles;
 
-    [ThreadStatic]
-    private static ulong _threadRandomState;
-
-    private static long _randomSeedCounter;
-
     public static MersenneDivisorCycles Shared { get; } = new MersenneDivisorCycles();
 
     public MersenneDivisorCycles()
@@ -535,8 +529,8 @@ public class MersenneDivisorCycles
 
         while (true)
         {
-            ulong c = (NextRandomUInt64() % (n - 1UL)) + 1UL;
-            ulong x = (NextRandomUInt64() % (n - 2UL)) + 2UL;
+            ulong c = (DeterministicRandom.NextUInt64() % (n - 1UL)) + 1UL;
+            ulong x = (DeterministicRandom.NextUInt64() % (n - 2UL)) + 2UL;
             ulong y = x;
             ulong d = 1UL;
 
@@ -595,36 +589,6 @@ public class MersenneDivisorCycles
         }
     }
 
-    private static ulong NextRandomUInt64()
-    {
-        ulong state = _threadRandomState;
-        if (state == 0UL)
-        {
-            state = InitializeThreadRandomState();
-        }
-
-        state ^= state >> 12;
-        state ^= state << 25;
-        state ^= state >> 27;
-        _threadRandomState = state;
-        return state * 2685821657736338717UL;
-    }
-
-    private static ulong InitializeThreadRandomState()
-    {
-        ulong seed = (ulong)Interlocked.Increment(ref _randomSeedCounter);
-        seed += 0x9E3779B97F4A7C15UL;
-        seed = (seed ^ (seed >> 30)) * 0xBF58476D1CE4E5B9UL;
-        seed = (seed ^ (seed >> 27)) * 0x94D049BB133111EBUL;
-        seed ^= seed >> 31;
-        if (seed == 0UL)
-        {
-            seed = 0x9E3779B97F4A7C15UL;
-        }
-
-        return seed;
-    }
-
     public readonly struct FactorCacheEntry
     {
         public FactorCacheEntry(ulong[] primes, byte[] exponents, int count)

PerfectNumbers.Core/MersennePrimeFactorTester.cs

@@ -371,10 +371,6 @@ public static class MersennePrimeFactorTester
             return 2UL;
         }
 
-        var rng = new Random(1234567); // TODO: Replace this RNG with the deterministic span-based
-                                       // sequence measured faster in PollardRhoBenchmarks so we avoid
-                                       // per-call Random allocations.
-
         ulong c, d, x, y, diff;
         while (true)
         {
@@ -383,8 +379,8 @@ public static class MersennePrimeFactorTester
                 return 0UL;
             }
 
-            c = (ulong)rng.Next(2, int.MaxValue);
-            x = (ulong)rng.Next(2, int.MaxValue);
+            c = (DeterministicRandom.NextUInt64() % (n - 1UL)) + 1UL;
+            x = (DeterministicRandom.NextUInt64() % (n - 2UL)) + 2UL;
             y = x;
             d = 1UL;
 
@@ -523,9 +519,6 @@ public static class MersennePrimeFactorTester
             return 2;
         }
 
-        var rng = new Random(1234567); // TODO: Replace with the deterministic UInt128 sequence from
-                                       // PollardRho128Benchmarks so we eliminate Random allocations.
-
         UInt128 c, d, x, y, diff;
         while (true)
         {
@@ -534,8 +527,8 @@ public static class MersennePrimeFactorTester
                 return 0;
             }
 
-            c = (UInt128)rng.NextInt64(2, long.MaxValue);
-            x = (UInt128)rng.NextInt64(2, long.MaxValue);
+            c = (DeterministicRandom.NextUInt128() % (n - 1)) + 1;
+            x = (DeterministicRandom.NextUInt128() % (n - 2)) + 2;
             y = x;
             d = 1;
 

PerfectNumbers.Core/PrimeOrderCalculator.cs

@@ -4,7 +4,6 @@ using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
-using System.Security.Cryptography;
 using PerfectNumbers.Core.Gpu;
 
 namespace PerfectNumbers.Core;
@@ -1165,7 +1164,11 @@ internal static partial class PrimeOrderCalculator
             return true;
         }
 
-        Span<byte> buffer = stackalloc byte[8];
+        ulong c;
+        ulong x;
+        ulong y;
+        ulong d;
+        ulong diff = 0UL;
 
         while (true)
         {
@@ -1174,12 +1177,10 @@ internal static partial class PrimeOrderCalculator
                 return false;
             }
 
-            RandomNumberGenerator.Fill(buffer);
-            ulong c = (BinaryPrimitives.ReadUInt64LittleEndian(buffer) % (n - 1UL)) + 1UL;
-            RandomNumberGenerator.Fill(buffer);
-            ulong x = (BinaryPrimitives.ReadUInt64LittleEndian(buffer) % (n - 2UL)) + 2UL;
-            ulong y = x;
-            ulong d = 1UL;
+            c = (DeterministicRandom.NextUInt64() % (n - 1UL)) + 1UL;
+            x = (DeterministicRandom.NextUInt64() % (n - 2UL)) + 2UL;
+            y = x;
+            d = 1UL;
 
             while (d == 1UL)
             {
@@ -1191,7 +1192,7 @@ internal static partial class PrimeOrderCalculator
                 x = AdvancePolynomial(x, c, n);
                 y = AdvancePolynomial(y, c, n);
                 y = AdvancePolynomial(y, c, n);
-                ulong diff = x > y ? x - y : y - x;
+                diff = x > y ? x - y : y - x;
                 d = BinaryGcd(diff, n);
             }
 
@@ -1396,23 +1397,25 @@ internal static partial class PrimeOrderCalculator
             return 2UL;
         }
 
-        Span<byte> buffer = stackalloc byte[8];
+        ulong c;
+        ulong x;
+        ulong y;
+        ulong d;
+        ulong diff = 0UL;
+
         while (true)
         {
-            RandomNumberGenerator.Fill(buffer);
-            ulong c = (BinaryPrimitives.ReadUInt64LittleEndian(buffer) % (n - 1UL)) + 1UL;
-
-            RandomNumberGenerator.Fill(buffer);
-            ulong x = (BinaryPrimitives.ReadUInt64LittleEndian(buffer) % (n - 2UL)) + 2UL;
-            ulong y = x;
-            ulong d = 1UL;
+            c = (DeterministicRandom.NextUInt64() % (n - 1UL)) + 1UL;
+            x = (DeterministicRandom.NextUInt64() % (n - 2UL)) + 2UL;
+            y = x;
+            d = 1UL;
 
             while (d == 1UL)
             {
                 x = AdvancePolynomial(x, c, n);
                 y = AdvancePolynomial(y, c, n);
                 y = AdvancePolynomial(y, c, n);
-                ulong diff = x > y ? x - y : y - x;
+                diff = x > y ? x - y : y - x;
                 d = BinaryGcd(diff, n);
             }