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Eliminate GC allocations from CPU divisor scan hot path

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Marek Lemańczyk committed 2025-10-15 20:48:42 +02:00
1 parent b0ce7152f7
commit 763714307f
6 files changed
+174 -147
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PerfectNumbers.Core/Cpu/FactorCacheLease.cs
+32
View File
@@ -0,0 +1,32 @@
using System.Collections.Generic;
namespace PerfectNumbers.Core.Cpu;
internal struct FactorCacheLease
{
private Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? _cache;
public void EnsureInitialized(ref Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache)
{
Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? current = cache;
if (current is null)
{
current = ThreadStaticPools.RentMersenneFactorCacheDictionary();
cache = current;
}
_cache = current;
}
public void Dispose()
{
Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache = _cache;
if (cache is null)
{
return;
}
ThreadStaticPools.ReturnMersenneFactorCacheDictionary(cache);
_cache = null;
}
}
PerfectNumbers.Core/Cpu/MersenneCpuDivisorScanSession.cs
+82
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@@ -0,0 +1,82 @@
using System;
namespace PerfectNumbers.Core.Cpu;
internal sealed class MersenneCpuDivisorScanSession : IMersenneNumberDivisorByDivisorTester.IDivisorScanSession
{
private bool _disposed;
public void Reset()
{
_disposed = false;
}
public void CheckDivisor(
ulong divisor,
in MontgomeryDivisorData divisorData,
ulong divisorCycle,
in ReadOnlySpan<ulong> primes,
Span<byte> hits)
{
int length = primes.Length;
if (length == 0)
{
return;
}
MontgomeryDivisorData cachedData = divisorData;
if (cachedData.Modulus != divisor)
{
cachedData = MontgomeryDivisorDataCache.Get(divisor);
}
if (divisorCycle == 0UL)
{
divisorCycle = DivisorCycleCache.Shared.GetCycleLength(divisor);
if (divisorCycle == 0UL)
{
hits.Clear();
return;
}
}
// Keep these remainder steppers in place so future updates continue reusing the previously computed residues.
// They are critical for avoiding repeated full Montgomery exponentiation work when scanning divisors.
var exponentStepper = new ExponentRemainderStepper(cachedData);
if (!exponentStepper.IsValidModulus)
{
hits.Clear();
return;
}
var cycleStepper = new CycleRemainderStepper(divisorCycle);
ulong remainder = cycleStepper.Initialize(primes[0]);
hits[0] = remainder == 0UL
? (exponentStepper.ComputeNextIsUnity(primes[0]) ? (byte)1 : (byte)0)
: (byte)0;
for (int i = 1; i < length; i++)
{
remainder = cycleStepper.ComputeNext(primes[i]);
if (remainder != 0UL)
{
hits[i] = 0;
continue;
}
hits[i] = exponentStepper.ComputeNextIsUnity(primes[i]) ? (byte)1 : (byte)0;
}
}
public void Dispose()
{
if (_disposed)
{
return;
}
_disposed = true;
ThreadStaticPools.ReturnMersenneCpuDivisorSession(this);
}
}
PerfectNumbers.Core/Cpu/MersenneNumberDivisorByDivisorCpuTester.cs
+6 -121
View File
@@ -1,4 +1,5 @@
using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
namespace PerfectNumbers.Core.Cpu;
@@ -6,8 +7,6 @@ namespace PerfectNumbers.Core.Cpu;
public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDivisorByDivisorTester
{
private readonly object _sync = new();
[ThreadStatic]
private static DivisorScanSession? _threadLocalSession; // The nested session type stays private, so pool it here.
private ulong _divisorLimit;
private ulong _lastStatusDivisor;
@@ -44,6 +43,8 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
if (allowedMax < 3UL)
{
// EvenPerfectBitScanner routes primes below the small-divisor cutoff to the GPU path, so the CPU path still sees
// trivial candidates during targeted tests. Short-circuit here to keep those runs aligned with the production flow.
divisorsExhausted = true;
return true;
}
@@ -87,49 +88,13 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
public IMersenneNumberDivisorByDivisorTester.IDivisorScanSession CreateDivisorSession()
{
DivisorScanSession? session = _threadLocalSession;
MersenneCpuDivisorScanSession? session = ThreadStaticPools.RentMersenneCpuDivisorSession();
if (session is not null)
{
_threadLocalSession = null;
session.Reset();
return session;
}
return new DivisorScanSession(this);
}
private void ReturnSession(DivisorScanSession session)
{
_threadLocalSession = session;
}
private struct FactorCacheLease
{
private Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? _cache;
public void EnsureInitialized(ref Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache)
{
Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? current = cache;
if (current is null)
{
current = ThreadStaticPools.RentMersenneFactorCacheDictionary();
cache = current;
}
_cache = current;
}
public void Dispose()
{
Dictionary<ulong, MersenneDivisorCycles.FactorCacheEntry>? cache = _cache;
if (cache is null)
{
return;
}
ThreadStaticPools.ReturnMersenneFactorCacheDictionary(cache);
_cache = null;
}
return new MersenneCpuDivisorScanSession();
}
private bool CheckDivisors(
@@ -188,7 +153,7 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
byte remainder7 = (byte)(divisor % 7UL);
byte remainder11 = (byte)(divisor % 11UL);
// DivisorCycleCache cycleCache = DivisorCycleCache.Shared; // Preserve the reference for easy restoration if the fallback cache is re-enabled.
// DivisorCycleCache cycleCache = DivisorCycleCache.Shared; // Do not restore the fallback cache; divisors do not repeat on this path.
// Keep the divisibility filters aligned with the divisor-cycle generator so the
// CPU path never requests cycles that were skipped during cache creation.
@@ -320,85 +285,5 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
return Math.Min((1UL << (int)(prime - 1UL)) - 1UL, divisorLimit);
}
private sealed class DivisorScanSession : IMersenneNumberDivisorByDivisorTester.IDivisorScanSession
{
private readonly MersenneNumberDivisorByDivisorCpuTester _owner;
private bool _disposed;
internal DivisorScanSession(MersenneNumberDivisorByDivisorCpuTester owner)
{
_owner = owner;
}
internal void Reset()
{
_disposed = false;
}
public void CheckDivisor(ulong divisor, in MontgomeryDivisorData divisorData, ulong divisorCycle, in ReadOnlySpan<ulong> primes, Span<byte> hits)
{
int length = primes.Length;
if (length == 0)
{
return;
}
MontgomeryDivisorData cachedData = divisorData;
if (cachedData.Modulus != divisor)
{
cachedData = MontgomeryDivisorDataCache.Get(divisor);
}
if (divisorCycle == 0UL)
{
divisorCycle = DivisorCycleCache.Shared.GetCycleLength(divisor);
if (divisorCycle == 0UL)
{
hits.Clear();
return;
}
}
// Keep these remainder steppers in place so future updates continue reusing the previously computed residues.
// They are critical for avoiding repeated full Montgomery exponentiation work when scanning divisors.
var exponentStepper = new ExponentRemainderStepper(cachedData);
if (!exponentStepper.IsValidModulus)
{
hits.Clear();
return;
}
var cycleStepper = new CycleRemainderStepper(divisorCycle);
ulong remainder = cycleStepper.Initialize(primes[0]);
hits[0] = remainder == 0UL
? (exponentStepper.ComputeNextIsUnity(primes[0]) ? (byte)1 : (byte)0)
: (byte)0;
for (int i = 1; i < length; i++)
{
remainder = cycleStepper.ComputeNext(primes[i]);
if (remainder != 0UL)
{
hits[i] = 0;
continue;
}
hits[i] = exponentStepper.ComputeNextIsUnity(primes[i]) ? (byte)1 : (byte)0;
}
}
public void Dispose()
{
if (_disposed)
{
return;
}
_disposed = true;
_owner.ReturnSession(this);
}
}
}