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Fix for Random failures in System.Numerics.Tests.modpowTest.ModPowAxiom test #74112

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merged 2 commits into from
Aug 18, 2022
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Fix for Random failures in System.Numerics.Tests.modpowTest.ModPowAxiom test #74112

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e189f53
Fixed #70330
sakno Aug 17, 2022
ce0fb71
Removed pessimistic buffer cleanup
sakno Aug 17, 2022
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14 changes: 11 additions & 3 deletions src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -217,7 +217,11 @@ public static void Pow(ReadOnlySpan<uint> value, uint power,
Span<uint> valueCopy = (size <= StackAllocThreshold ?
stackalloc uint[StackAllocThreshold]
: valueCopyFromPool = ArrayPool<uint>.Shared.Rent(size)).Slice(0, size);
valueCopy.Clear();

// smallish optimization here:
// subsequent operations will copy the elements to the beginning of the buffer,
// no need to clear everything
valueCopy.Slice(value.Length).Clear();

if (value.Length > modulus.Length)
{
Expand Down Expand Up @@ -262,7 +266,11 @@ public static void Pow(ReadOnlySpan<uint> value, ReadOnlySpan<uint> power,
Span<uint> valueCopy = (size <= StackAllocThreshold ?
stackalloc uint[StackAllocThreshold]
: valueCopyFromPool = ArrayPool<uint>.Shared.Rent(size)).Slice(0, size);
valueCopy.Clear();

// smallish optimization here:
// subsequent operations will copy the elements to the beginning of the buffer,
// no need to clear everything
valueCopy.Slice(value.Length).Clear();

if (value.Length > modulus.Length)
{
Expand Down Expand Up @@ -464,7 +472,7 @@ private static Span<uint> PowCore(Span<uint> value, int valueLength,
power >>= 1;
}

return result.Slice(0, resultLength);
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@dakersnar dakersnar Aug 18, 2022

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Was this slice basically unnecessary in all cases, and causing problems in this edge case?

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@sakno sakno Aug 18, 2022

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Yes, correct. It doesn't exist for overload of PowCore with exponent of arbitrary length.

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@tannergooding tannergooding Aug 25, 2022

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Removing this slice introduced a regression for the case of:

value: 174475926811173371108797680334274448482027679060635889276326576767485928691470272416521788770673419883076040463212449631887959313694345475337324466914914839368969988215395980439434099840700753576299320684517028939674493640310781015739837159625716992654178036326745036841619965661654305787656989089177185300627
exponent: 152266387011071825363527751452194927195391511201743678326712300944812006670853968954038119879530327815990523885596782666640476470053822766345969220504425389995206265094671158964311234812226260664473986637197447367726400093404731405598815953979171704175885394570388678952091206779039984338579147114424357147650
modulus: 15548277136038299103

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@tannergooding tannergooding Aug 25, 2022

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In this scenario, the length of result is greater than the length of bits and so the result.CopyTo(bits) operation fails as the destination is too short.

I expect the actual issue here is that resultLength isn't being correctly tracked somewhere.

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@tannergooding tannergooding Aug 25, 2022

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Its not that resultLength is incorrect, its that bits will have been mutated and may contain temporary data

return result;
}

private static Span<uint> PowCore(Span<uint> value, int valueLength,
Expand Down
12 changes: 12 additions & 0 deletions src/libraries/System.Runtime.Numerics/tests/BigInteger/modpow.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -274,6 +274,18 @@ public static void ModPowAxiom()
}
}

[Fact]
public static void RegressionIssue70330()
{
byte[] tempByteArray1 = { 226, 32 };
byte[] tempByteArray2 = { 113 };
byte[] tempByteArray3 = { 15, 8, 201, 158, 96, 200, 233, 243, 184, 0, 33, 203, 210, 80, 174, 198, 244, 177, 223, 221, 168, 243, 233, 133, 103, 252, 219, 195, 187, 227, 215, 54, 66, 248, 37, 186, 232, 45, 227, 147, 100, 14, 121, 244, 56, 89, 181, 120, 205, 4, 59, 48, 65, 239, 221, 28, 30, 68, 55, 99, 237, 38, 56, 213, 40, 234, 136, 218, 42, 244, 222, 198, 205 };
VerifyIdentityString(
Print(tempByteArray3) + Print(tempByteArray2) + Print(tempByteArray1) + "tModPow",
Print(tempByteArray3) + Print(tempByteArray2) + Print(tempByteArray1) + "bPow" + " bRemainder"
);
}

[Fact]
public static void ModPowBoundary()
{
Expand Down