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Fix incorrect result right shifting specific negative BigInteger by 32 bits #54115

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Merged
jeffhandley merged 1 commit into from
Jul 24, 2021
Merged

Fix incorrect result right shifting specific negative BigInteger by 32 bits #54115

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12 changes: 11 additions & 1 deletion src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1996,6 +1996,7 @@ public static explicit operator decimal(BigInteger value)
Span<uint> stackallocedXd = stackalloc uint[1];
Span<uint> xd = stackallocedXd;
bool negx = GetPartsForBitManipulation(ref value, ref xd);
bool trackSignBit = false;

if (negx)
{
Expand All @@ -2020,9 +2021,13 @@ public static explicit operator decimal(BigInteger value)
}

NumericsHelpers.DangerousMakeTwosComplement(xd); // Mutates xd
if (xd[^1] == 0)
{
trackSignBit = true;
}
}

int zl = xd.Length - digitShift;
int zl = xd.Length - digitShift + (trackSignBit ? 1: 0);
uint[]? zdArray = null;
Span<uint> zd = stackalloc uint[0];
if (zl > 0)
Expand Down Expand Up @@ -2057,6 +2062,11 @@ stackalloc uint[StackallocUInt32Limit].Slice(0, zl) :
if (negx)
{
NumericsHelpers.DangerousMakeTwosComplement(zd); // Mutates zd

if (trackSignBit)
{
zd[^1] = 1;
}
}

return new BigInteger(zd, zdArray, negx);
Expand Down
19 changes: 19 additions & 0 deletions src/libraries/System.Runtime.Numerics/tests/BigInteger/op_rightshift.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,15 @@ public static void RunRightShiftTests()
tempByteArray2 = new byte[] { (byte)32 };
VerifyRightShiftString(Print(tempByteArray2) + Print(tempByteArray1) + "b>>");
}

// RightShift Method - All One Uint Large BigIntegers - 32 bit Shift
for (int i = 0; i < s_samples; i++)
{
tempByteArray1 = GetRandomLengthAllOnesUIntByteArray(s_random);
tempByteArray2 = new byte[] { (byte)32 };
VerifyRightShiftString(Print(tempByteArray2) + Print(tempByteArray1) + "b>>");
}

// RightShift Method - Large BigIntegers - large - Shift
for (int i = 0; i < s_samples; i++)
{
Expand Down Expand Up @@ -211,6 +220,16 @@ private static byte[] GetRandomNegByteArray(Random random, int size)
return value;
}

private static byte[] GetRandomLengthAllOnesUIntByteArray(Random random)
{
int gap = random.Next(0, 128);
int byteLength = 4 + gap * 4 + 1;
byte[] array = new byte[byteLength];
array[0] = 1;
array[^1] = 0xFF;
return array;
}

private static string Print(byte[] bytes)
{
return MyBigIntImp.Print(bytes);
Expand Down