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Apply hardware intrinsics to BitArray.*Shift #113299

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e29dedd
Add test caces for BitArray.*Shift
kzrnm Jul 2, 2025
a01656a
hardware intrinsic in BitArray.*Shift
kzrnm Jul 2, 2025
a482701
ClearHighExtraBits
kzrnm Jul 3, 2025
e96a2b8
Update logic of Int32
kzrnm Jul 3, 2025
7681ebc
Merge branch 'main' into BitArrayShift
jeffhandley Sep 1, 2025
1c77a92
Merge branch 'main' into BitArrayShift
tannergooding Oct 24, 2025
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33 changes: 31 additions & 2 deletions src/libraries/System.Collections/tests/BitArray/BitArray_OperatorsTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@

using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using Xunit;

namespace System.Collections.Tests
Expand Down Expand Up @@ -245,13 +246,33 @@ public static void Xor_With_Resize(BitArray left, BitArray right, int newLeftLen

public static IEnumerable<object[]> Shift_Data()
{
foreach (int size in new[] { 0, 1, BitsPerInt32 / 2, BitsPerInt32, BitsPerInt32 + 1, 2 * BitsPerInt32, 2 * BitsPerInt32 + 1 })
Random random = new Random(0);
foreach (int size in new[] {
0,
1,
BitsPerInt32 / 2,
BitsPerInt32,
BitsPerInt32 + 1,
2 * BitsPerInt32 - 1,
2 * BitsPerInt32 + 1,
1023,
1024,
1025,
})
{
foreach (int shift in new[] { 0, 1, size / 2, size - 1, size }.Where(s => s >= 0).Distinct())
foreach (int shift in new[] {
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
size / 3, size / 2, size / 2 + 1, size - 1, size,
}.Where(s => s >= 0).Distinct())
{
yield return new object[] { size, new int[] { /* deliberately empty */ }, shift };
yield return new object[] { size, Enumerable.Range(0, size), shift };

int[] nums = Enumerable.Range(0, size).ToArray();
random.Shuffle(nums);
yield return new object[] { size, nums.Take(size / 2), shift };

if (size > 1)
{
foreach (int position in new[] { 0, size / 2, size - 1 })
Expand Down Expand Up @@ -290,6 +311,14 @@ public static void LeftShift(int length, IEnumerable<int> set, int shift)

int index = 0;
Assert.All(ba.Cast<bool>(), bit => Assert.Equal(expected[index++], bit));

(int byteIndex, int bitOffeset) = Math.DivRem(length, BitsPerByte);
if (bitOffeset != 0)
{
Span<byte> bs = CollectionsMarshal.AsBytes(ba);
Assert.Equal(byteIndex + 1, bs.Length);
Assert.Equal(0, bs[byteIndex] >> bitOffeset);
Comment on lines +315 to +320
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Corrected spelling of 'bitOffeset' to 'bitOffset'.

Suggested change
(int byteIndex, int bitOffeset) = Math.DivRem(length, BitsPerByte);
if (bitOffeset != 0)
{
Span<byte> bs = CollectionsMarshal.AsBytes(ba);
Assert.Equal(byteIndex + 1, bs.Length);
Assert.Equal(0, bs[byteIndex] >> bitOffeset);
(int byteIndex, int bitOffset) = Math.DivRem(length, BitsPerByte);
if (bitOffset != 0)
{
Span<byte> bs = CollectionsMarshal.AsBytes(ba);
Assert.Equal(byteIndex + 1, bs.Length);
Assert.Equal(0, bs[byteIndex] >> bitOffset);

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}
}

private static bool[] GetBoolArray(int length, IEnumerable<int> set)
Expand Down
181 changes: 139 additions & 42 deletions src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -514,51 +514,94 @@ public BitArray RightShift(int count)
return this;
}

Span<int> intSpan = MemoryMarshal.Cast<byte, int>((Span<byte>)_array);

Span<byte> thisSpan = new Span<byte>(_array, 0, GetByteArrayLengthFromBitLength(_bitLength));
int toIndex = 0;
int ints = GetInt32ArrayLengthFromBitLength(_bitLength);

if (count < _bitLength)
{
// We can not use Math.DivRem without taking a dependency on System.Runtime.Extensions
(int fromIndex, int shiftCount) = Math.DivRem(count, 32);
int extraBits = (int)((uint)_bitLength % 32);
(int fromIndex, int shiftCount) = Math.DivRem(count, BitsPerByte);
if (shiftCount == 0)
{
// Cannot use `(1u << extraBits) - 1u` as the mask
// because for extraBits == 0, we need the mask to be 111...111, not 0.
// In that case, we are shifting a uint by 32, which could be considered undefined.
// The result of a shift operation is undefined ... if the right operand
// is greater than or equal to the width in bits of the promoted left operand,
// https://learn.microsoft.com/cpp/c-language/bitwise-shift-operators?view=vs-2017
// However, the compiler protects us from undefined behaviour by constraining the
// right operand to between 0 and width - 1 (inclusive), i.e. right_operand = (right_operand % width).
uint mask = uint.MaxValue >> (BitsPerInt32 - extraBits);
intSpan[ints - 1] &= ReverseIfBE((int)mask);

intSpan.Slice((int)fromIndex, ints - fromIndex).CopyTo(intSpan);
toIndex = ints - fromIndex;
thisSpan.Slice(fromIndex).CopyTo(thisSpan);
toIndex = thisSpan.Length - fromIndex;
}
else
{
int lastIndex = ints - 1;
if (Vector512.IsHardwareAccelerated)
{
toIndex = Apply<Vector512<byte>>(shiftCount, fromIndex, thisSpan);
}
else if (Vector256.IsHardwareAccelerated)
{
toIndex = Apply<Vector256<byte>>(shiftCount, fromIndex, thisSpan);
}
else if (Vector128.IsHardwareAccelerated)
{
toIndex = Apply<Vector128<byte>>(shiftCount, fromIndex, thisSpan);
}
fromIndex += toIndex;

int carryCount = BitsPerByte - shiftCount;

ref byte p = ref MemoryMarshal.GetReference(thisSpan);

const uint shiftUnit = 0x01010101u;
uint shiftMask = (shiftUnit << carryCount) - shiftUnit;
uint carryMask = ~shiftMask;

while (fromIndex < lastIndex)
while (fromIndex < thisSpan.Length - 4)
{
uint right = (uint)ReverseIfBE(intSpan[fromIndex]) >> shiftCount;
int left = ReverseIfBE(intSpan[++fromIndex]) << (BitsPerInt32 - shiftCount);
intSpan[toIndex++] = ReverseIfBE(left | (int)right);
uint lo = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)fromIndex)) >>> shiftCount) & shiftMask;
uint hi = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)(fromIndex + 1))) << carryCount) & carryMask;
uint result = hi | lo;
Unsafe.WriteUnaligned(ref Unsafe.AddByteOffset(ref p, toIndex), result);

fromIndex += 4;
toIndex += 4;
}

uint mask = uint.MaxValue >> (BitsPerInt32 - extraBits);
mask &= (uint)ReverseIfBE(intSpan[fromIndex]);
intSpan[toIndex++] = ReverseIfBE((int)(mask >> shiftCount));
while (fromIndex < thisSpan.Length)
{
int lo = thisSpan[fromIndex] >>> shiftCount;
int hi =
fromIndex + 1 < thisSpan.Length
? thisSpan[fromIndex + 1] << carryCount
: 0;

thisSpan[toIndex] = (byte)(hi | lo);

fromIndex++;
toIndex++;
}
}
}

intSpan.Slice(toIndex, ints - toIndex).Clear();
thisSpan.Slice(toIndex).Clear();
_version++;
return this;

[MethodImpl(MethodImplOptions.AggressiveInlining)]
static int Apply<TVector>(int shiftCount, int fromIndex, Span<byte> thisSpan)
where TVector : ISimdVector<TVector, byte>
{
ref byte p = ref MemoryMarshal.GetReference(thisSpan);
int carryCount = BitsPerByte - shiftCount;

int toIndex = 0;

while (fromIndex <= thisSpan.Length - (TVector.ElementCount + 1))
{
TVector lo = TVector.LoadUnsafe(ref p, (uint)fromIndex) >>> shiftCount;
TVector hi = TVector.LoadUnsafe(ref p, (uint)(fromIndex + 1)) << carryCount;
TVector result = lo | hi;
result.StoreUnsafe(ref p, (uint)toIndex);

fromIndex += TVector.ElementCount;
toIndex += TVector.ElementCount;
}

return toIndex;
}
}

/// <summary>
Expand All @@ -577,41 +620,95 @@ public BitArray LeftShift(int count)
return this;
}

Span<int> intSpan = MemoryMarshal.Cast<byte, int>((Span<byte>)_array);
Span<byte> thisSpan = new Span<byte>(_array, 0, GetByteArrayLengthFromBitLength(_bitLength));

int lengthToClear;
if (count < _bitLength)
{
int lastIndex = (int)((uint)(_bitLength - 1) / BitsPerInt32);

(lengthToClear, int shiftCount) = Math.DivRem(count, BitsPerInt32);
(lengthToClear, int shiftCount) = Math.DivRem(count, BitsPerByte);

if (shiftCount == 0)
{
intSpan.Slice(0, lastIndex + 1 - lengthToClear).CopyTo(intSpan.Slice(lengthToClear));
thisSpan.Slice(0, thisSpan.Length - lengthToClear).CopyTo(thisSpan.Slice(lengthToClear));
}
else
{
int fromindex = lastIndex - lengthToClear;
int toIndex = thisSpan.Length;
int fromIndex = toIndex - lengthToClear;

if (Vector512.IsHardwareAccelerated)
{
toIndex = Apply<Vector512<byte>>(shiftCount, fromIndex, thisSpan);
}
else if (Vector256.IsHardwareAccelerated)
{
toIndex = Apply<Vector256<byte>>(shiftCount, fromIndex, thisSpan);
}
else if (Vector128.IsHardwareAccelerated)
{
toIndex = Apply<Vector128<byte>>(shiftCount, fromIndex, thisSpan);
}
fromIndex = toIndex - lengthToClear;

int carryCount = BitsPerByte - shiftCount;

ref byte p = ref MemoryMarshal.GetReference(thisSpan);

const uint shiftUnit = 0x01010101u;
uint carryMask = (shiftUnit << shiftCount) - shiftUnit;
uint shiftMask = ~carryMask;

while (fromIndex >= 5)
{
uint lo = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)(fromIndex -= 4))) << shiftCount) & shiftMask;
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[nitpick] The decrement operation fromIndex -= 4 is embedded within the offset calculation, which reduces readability. Consider separating this into two statements: first decrement fromIndex, then perform the read operation.

Suggested change
uint lo = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)(fromIndex -= 4))) << shiftCount) & shiftMask;
fromIndex -= 4;
uint lo = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)fromIndex)) << shiftCount) & shiftMask;

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uint hi = (Unsafe.ReadUnaligned<uint>(ref Unsafe.AddByteOffset(ref p, (uint)(fromIndex - 1))) >>> carryCount) & carryMask;
uint result = hi | lo;
Unsafe.WriteUnaligned(ref Unsafe.AddByteOffset(ref p, toIndex -= 4), result);
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[nitpick] The decrement operation toIndex -= 4 is embedded within the offset calculation, which reduces readability. Consider separating this into two statements: first decrement toIndex, then perform the write operation.

Suggested change
Unsafe.WriteUnaligned(ref Unsafe.AddByteOffset(ref p, toIndex -= 4), result);
toIndex -= 4;
Unsafe.WriteUnaligned(ref Unsafe.AddByteOffset(ref p, toIndex), result);

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}

while (fromindex > 0)
while (--fromIndex >= 0)
{
int left = ReverseIfBE(intSpan[fromindex]) << shiftCount;
uint right = (uint)ReverseIfBE(intSpan[--fromindex]) >> (BitsPerInt32 - shiftCount);
intSpan[lastIndex] = ReverseIfBE(left | (int)right);
lastIndex--;
int hi = thisSpan[fromIndex] << shiftCount;
int lo =
fromIndex > 0
? thisSpan[fromIndex - 1] >>> carryCount
: 0;

thisSpan[--toIndex] = (byte)(hi | lo);
}
intSpan[lastIndex] = ReverseIfBE(ReverseIfBE(intSpan[fromindex]) << shiftCount);

Debug.Assert(toIndex == lengthToClear);
}
}
else
{
lengthToClear = GetInt32ArrayLengthFromBitLength(_bitLength); // Clear all
lengthToClear = thisSpan.Length; // Clear all
}

intSpan.Slice(0, lengthToClear).Clear();
thisSpan.Slice(0, lengthToClear).Clear();
ClearHighExtraBits();
_version++;
return this;

[MethodImpl(MethodImplOptions.AggressiveInlining)]
static int Apply<TVector>(int shiftCount, int fromIndex, Span<byte> thisSpan)
where TVector : ISimdVector<TVector, byte>
{
ref byte p = ref MemoryMarshal.GetReference(thisSpan);
int carryCount = BitsPerByte - shiftCount;

int toIndex = thisSpan.Length;

while (fromIndex >= TVector.ElementCount + 1)
{
TVector hi = TVector.LoadUnsafe(ref p, (nuint)(fromIndex -= TVector.ElementCount)) << shiftCount;
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[nitpick] The decrement operation fromIndex -= TVector.ElementCount is embedded within the offset calculation, which reduces readability. Consider separating this into two statements: first decrement fromIndex, then perform the load operation.

Suggested change
TVector hi = TVector.LoadUnsafe(ref p, (nuint)(fromIndex -= TVector.ElementCount)) << shiftCount;
fromIndex -= TVector.ElementCount;
TVector hi = TVector.LoadUnsafe(ref p, (nuint)fromIndex) << shiftCount;

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TVector lo = TVector.LoadUnsafe(ref p, (nuint)(fromIndex - 1)) >>> carryCount;
TVector result = hi | lo;
result.StoreUnsafe(ref p, (nuint)(toIndex -= TVector.ElementCount));
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[nitpick] The decrement operation toIndex -= TVector.ElementCount is embedded within the offset calculation, which reduces readability. Consider separating this into two statements: first decrement toIndex, then perform the store operation.

Suggested change
result.StoreUnsafe(ref p, (nuint)(toIndex -= TVector.ElementCount));
toIndex -= TVector.ElementCount;
result.StoreUnsafe(ref p, (nuint)toIndex);

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}

return toIndex;
}
}

/// <summary>
Expand Down
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