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Document unreachable guards in GPU by-divisor pipeline

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Marek Lemańczyk committed 2025-10-13 19:56:47 +02:00
1 parent 65e010ded5
commit bccb2f43c9
7 files changed
+178 -122
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PerfectNumbers.Core/Gpu/MersenneNumberDivisorByDivisorGpuTester.cs
+57 -41
View File
@@ -87,11 +87,13 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
batchCapacity = _gpuBatchSize;
}
if (allowedMax < 3UL)
{
divisorsExhausted = true;
return true;
}
// The GPU path always receives an allowed maximum well above the minimal divisor, so the legacy
// guard stays commented out to document the invariant without branching.
// if (allowedMax < 3UL)
// {
// divisorsExhausted = true;
// return true;
// }
bool composite;
bool coveredRange;
@@ -222,7 +224,8 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
ArrayView1D<ulong, Stride1D.Dense> exponentView;
ArrayView1D<byte, Stride1D.Dense> hitsView;
cycleCapacity = Math.Max(1, cycleCapacity);
// Batch resources expose a positive cycle capacity, so skip the redundant clamp here.
// cycleCapacity = Math.Max(1, cycleCapacity);
int chunkCapacity = Math.Min(batchCapacity, cycleCapacity);
int maxThreadsPerGroup = (int)accelerator.MaxNumThreadsPerGroup;
@@ -231,20 +234,22 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
chunkCapacity = Math.Min(chunkCapacity, maxThreadsPerGroup);
}
if (chunkCapacity <= 0)
{
chunkCapacity = cycleCapacity;
}
// cycleCapacity already bounds chunkCapacity above zero, so the fallback would never execute.
// if (chunkCapacity <= 0)
// {
// chunkCapacity = cycleCapacity;
// }
ulong[] filteredDivisors = ArrayPool<ulong>.Shared.Rent(chunkCapacity);
ulong[] divisorGaps = ArrayPool<ulong>.Shared.Rent(chunkCapacity);
UInt128 twoP128 = (UInt128)prime << 1;
if (twoP128 == UInt128.Zero)
{
coveredRange = true;
return false;
}
// Primes above one yield a non-zero 2p value, so the legacy zero guard remains commented out.
// if (twoP128 == UInt128.Zero)
// {
// coveredRange = true;
// return false;
// }
UInt128 allowedMax128 = allowedMax;
UInt128 firstDivisor128 = twoP128 + UInt128.One;
@@ -255,11 +260,13 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
}
UInt128 maxK128 = (allowedMax128 - UInt128.One) / twoP128;
if (maxK128 == UInt128.Zero)
{
coveredRange = true;
return false;
}
// Once the first divisor falls within the allowed range, at least one multiplier k is admissible,
// so keep the defensive zero check commented out.
// if (maxK128 == UInt128.Zero)
// {
// coveredRange = true;
// return false;
// }
ulong maxK = maxK128 > ulong.MaxValue ? ulong.MaxValue : (ulong)maxK128;
ulong currentK = 1UL;
@@ -301,10 +308,12 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
chunkCount = (int)remainingK;
}
if (chunkCount <= 0)
{
break;
}
// chunkCapacity and the remaining multiplier range are strictly positive here, so the zero
// chunk fallback never triggers on production scans.
// if (chunkCount <= 0)
// {
// break;
// }
Span<ulong> candidateSpan = cycleCandidates.AsSpan(0, chunkCount);
Span<ulong> gapSpan = divisorGaps.AsSpan(0, chunkCount);
@@ -465,10 +474,12 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
Action<Index1D, ArrayView<ulong>, ArrayView<byte>, byte> deltaKernel,
Action<KernelConfig, Index1D, ArrayView<byte>, ArrayView<byte>, byte, byte> scanKernel)
{
if (count <= 0)
{
return;
}
// GPU batches always contain at least one divisor gap, so the empty-count guard remains commented
// to eliminate redundant branching.
// if (count <= 0)
// {
// return;
// }
deltaKernel(count, gapView, deltaView, modulus);
SharedMemoryConfig sharedMemoryConfig = SharedMemoryConfig.RequestDynamic<int>(count);
@@ -491,11 +502,13 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
Span<byte> destination,
Action<Index1D, ArrayView<byte>, ArrayView<byte>, ArrayView<byte>, ArrayView<byte>, byte, ArrayView<byte>> maskKernel)
{
if (count <= 0)
{
destination.Clear();
return;
}
// Candidate batches always provide at least one entry, so the zero-count guard stays commented out
// to avoid clearing spans unnecessarily.
// if (count <= 0)
// {
// destination.Clear();
// return;
// }
maskKernel(count, remainder10View, remainder8View, remainder5View, remainder3View, lastIsSevenFlag, maskView);
maskView.CopyToCPU(ref MemoryMarshal.GetReference(destination), count);
@@ -503,10 +516,12 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
private static byte FetchLastRemainder(ArrayView1D<byte, Stride1D.Dense> remainderView, int count)
{
if (count <= 0)
{
return 0;
}
// Kernels always process at least one remainder in this path, so the empty-span guard remains
// commented to document the invariant.
// if (count <= 0)
// {
// return 0;
// }
byte value = 0;
remainderView.SubView(count - 1, 1).CopyToCPU(ref value, 1);
@@ -519,10 +534,11 @@ public sealed class MersenneNumberDivisorByDivisorGpuTester : IMersenneNumberDiv
if (result >= modulus)
{
result -= modulus;
if (result >= modulus)
{
result %= modulus;
}
// The step deltas remain below the modulus, so a single subtraction suffices on this path.
// if (result >= modulus)
// {
// result %= modulus;
// }
}
return (byte)result;