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Fix incorrect tracking of sign bit #59525

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Merged
pgovind merged 6 commits into from
Sep 28, 2021
Merged

Fix incorrect tracking of sign bit #59525

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b6c4a57
Fix incorrect tracking of sign bit
wzchua Sep 23, 2021
e83054f
Perform the 1 check earlier
wzchua Sep 23, 2021
7bbeb97
Add comment on checking special pattern
wzchua Sep 25, 2021
a14e984
Simplify fix
wzchua Sep 25, 2021
cd3c158
Update the test to target 32 bit right shift
wzchua Sep 25, 2021
5abd54a
Remove stray newline
wzchua Sep 28, 2021
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18 changes: 11 additions & 7 deletions 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 @@ -2021,10 +2021,14 @@ public static explicit operator decimal(BigInteger value)
}

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

// For a shift of N x 32 bit,
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@pgovind pgovind Sep 28, 2021

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Ah, helpful explanation.

// We check for a special case where its sign bit could be outside the uint array after 2's complement conversion.
// For example given [0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF], its 2's complement is [0x01, 0x00, 0x00]
// After a 32 bit right shift, it becomes [0x00, 0x00] which is [0x00, 0x00] when converted back.
// The expected result is [0x00, 0x00, 0xFFFFFFFF] (2's complement) or [0x00, 0x00, 0x01] when converted back
// If the 2's component's last element is a 0, we will track the sign externally
trackSignBit = smallShift == 0 && xd[^1] == 0;
}

int zl = xd.Length - digitShift + (trackSignBit ? 1: 0);
Expand Down Expand Up @@ -2061,12 +2065,12 @@ stackalloc uint[StackallocUInt32Limit].Slice(0, zl) :
}
if (negx)
{
NumericsHelpers.DangerousMakeTwosComplement(zd); // Mutates zd

// Set the tracked sign to the last element
if (trackSignBit)
{
zd[^1] = 1;
zd[^1] = 0xFFFFFFFF;
}
NumericsHelpers.DangerousMakeTwosComplement(zd); // Mutates zd
}

return new BigInteger(zd, zdArray, negx);
Expand Down
17 changes: 17 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 @@ -63,6 +63,14 @@ public static void RunRightShiftTests()
VerifyRightShiftString(Print(tempByteArray2) + Print(tempByteArray1) + "b>>");
}

// RightShift Method - Uint 0xffffffff 0x8000000 ... Large BigIntegers - 32 bit Shift
for (int i = 0; i < s_samples; i++)
{
tempByteArray1 = GetRandomLengthFirstUIntMaxSecondUIntMSBMaxArray(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 @@ -229,6 +237,15 @@ private static byte[] GetRandomLengthAllOnesUIntByteArray(Random random)
array[^1] = 0xFF;
return array;
}
private static byte[] GetRandomLengthFirstUIntMaxSecondUIntMSBMaxArray(Random random)
{
int gap = random.Next(0, 128);
int byteLength = 4 + gap * 4 + 1;
byte[] array = new byte[byteLength];
array[^6] = 0x80;
array[^1] = 0xFF;
return array;
}

private static string Print(byte[] bytes)
{
Expand Down