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CSHARP-5666: Remove GetBitArray allocations in BsonClassMapSerializer.DeserializeClass #1767

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CSHARP-5666: Remove GetBitArray allocations in BsonClassMapSerializer.DeserializeClass #1767

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a7ff59a
CSHARP-5666: Remove GetBitArray allocations in BsonClassMapSerializer…
BorisDog Sep 1, 2025
ad91607
- Fix error message
BorisDog Sep 1, 2025
918ecac
- PR comments
BorisDog Sep 9, 2025
a913d58
- PR comments
BorisDog Sep 10, 2025
cc3bf13
- PR comments
BorisDog Sep 14, 2025
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166 changes: 91 additions & 75 deletions src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,9 +14,11 @@
*/

using System;
using System.Buffers;
using System.Collections.Generic;
using System.ComponentModel;
using System.Reflection;
using System.Runtime.CompilerServices;
using MongoDB.Bson.IO;
using MongoDB.Bson.Serialization.Conventions;
using MongoDB.Bson.Serialization.Serializers;
Expand Down Expand Up @@ -82,7 +84,7 @@ public override TClass Deserialize(BsonDeserializationContext context, BsonDeser
{
var bsonReader = context.Reader;

if (bsonReader.GetCurrentBsonType() == Bson.BsonType.Null)
if (bsonReader.GetCurrentBsonType() == BsonType.Null)
{
bsonReader.ReadNull();
return default(TClass);
Expand Down Expand Up @@ -149,7 +151,9 @@ public TClass DeserializeClass(BsonDeserializationContext context)
var discriminatorConvention = _classMap.GetDiscriminatorConvention();
var allMemberMaps = _classMap.AllMemberMaps;
var extraElementsMemberMapIndex = _classMap.ExtraElementsMemberMapIndex;
var memberMapBitArray = FastMemberMapHelper.GetBitArray(allMemberMaps.Count);

var (bitArrayLength, useStackAlloc) = FastMemberMapHelper.GetBitArrayLength(_classMap.AllMemberMaps.Count);
using var bitArray = useStackAlloc ? FastMemberMapHelper.GetBitArray(stackalloc uint[bitArrayLength]) : FastMemberMapHelper.GetBitArray(bitArrayLength);

bsonReader.ReadStartDocument();
var elementTrie = _classMap.ElementTrie;
Expand Down Expand Up @@ -193,7 +197,8 @@ public TClass DeserializeClass(BsonDeserializationContext context)
DeserializeExtraElementValue(context, values, elementName, memberMap);
}
}
memberMapBitArray[memberMapIndex >> 5] |= 1U << (memberMapIndex & 31);

bitArray.SetMemberIndex(memberMapIndex);
}
else
{
Expand Down Expand Up @@ -221,7 +226,7 @@ public TClass DeserializeClass(BsonDeserializationContext context)
{
DeserializeExtraElementValue(context, values, elementName, extraElementsMemberMap);
}
memberMapBitArray[extraElementsMemberMapIndex >> 5] |= 1U << (extraElementsMemberMapIndex & 31);
bitArray.SetMemberIndex(extraElementsMemberMapIndex);
}
else if (_classMap.IgnoreExtraElements)
{
Expand All @@ -239,51 +244,38 @@ public TClass DeserializeClass(BsonDeserializationContext context)
bsonReader.ReadEndDocument();

// check any members left over that we didn't have elements for (in blocks of 32 elements at a time)
for (var bitArrayIndex = 0; bitArrayIndex < memberMapBitArray.Length; ++bitArrayIndex)
var bitArraySpan = bitArray.Span;
for (var bitArrayIndex = 0; bitArrayIndex < bitArraySpan.Length; bitArrayIndex++)
{
var memberMapIndex = bitArrayIndex << 5;
var memberMapBlock = ~memberMapBitArray[bitArrayIndex]; // notice that bits are flipped so 1's are now the missing elements
var memberMapBlock = ~bitArraySpan[bitArrayIndex]; // notice that bits are flipped so 1's are now the missing elements

// work through this memberMapBlock of 32 elements
while (true)
for (; memberMapBlock != 0 && memberMapIndex < allMemberMaps.Count; memberMapIndex++, memberMapBlock >>= 1)
{
// examine missing elements (memberMapBlock is shifted right as we work through the block)
for (; (memberMapBlock & 1) != 0; ++memberMapIndex, memberMapBlock >>= 1)
{
var memberMap = allMemberMaps[memberMapIndex];
if (memberMap.IsReadOnly)
{
continue;
}

if (memberMap.IsRequired)
{
var fieldOrProperty = (memberMap.MemberInfo is FieldInfo) ? "field" : "property";
var message = string.Format(
"Required element '{0}' for {1} '{2}' of class {3} is missing.",
memberMap.ElementName, fieldOrProperty, memberMap.MemberName, _classMap.ClassType.FullName);
throw new FormatException(message);
}
if ((memberMapBlock & 1) == 0)
continue;

if (document != null)
{
memberMap.ApplyDefaultValue(document);
}
else if (memberMap.IsDefaultValueSpecified && !memberMap.IsReadOnly)
{
values[memberMap.ElementName] = memberMap.DefaultValue;
}
var memberMap = allMemberMaps[memberMapIndex];
if (memberMap.IsReadOnly)
{
continue;
}

if (memberMapBlock == 0)
if (memberMap.IsRequired)
{
break;
var fieldOrProperty = (memberMap.MemberInfo is FieldInfo) ? "field" : "property";
throw new FormatException($"Required element '{memberMap.ElementName}' for {fieldOrProperty} '{memberMap.MemberName}' of class {_classMap.ClassType.FullName} is missing.");
}

// skip ahead to the next missing element
var leastSignificantBit = FastMemberMapHelper.GetLeastSignificantBit(memberMapBlock);
memberMapIndex += leastSignificantBit;
memberMapBlock >>= leastSignificantBit;
if (document != null)
{
memberMap.ApplyDefaultValue(document);
}
else if (memberMap.IsDefaultValueSpecified && !memberMap.IsReadOnly)
{
values[memberMap.ElementName] = memberMap.DefaultValue;
}
}
}

Expand Down Expand Up @@ -335,13 +327,11 @@ public bool GetDocumentId(
idGenerator = idMemberMap.IdGenerator;
return true;
}
else
{
id = null;
idNominalType = null;
idGenerator = null;
return false;
}

id = null;
idNominalType = null;
idGenerator = null;
return false;
}

/// <summary>
Expand Down Expand Up @@ -694,47 +684,73 @@ private bool ShouldSerializeDiscriminator(Type nominalType)

// nested classes
// helper class that implements member map bit array helper functions
private static class FastMemberMapHelper
internal static class FastMemberMapHelper
{
public static uint[] GetBitArray(int memberCount)
internal ref struct BitArray()
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Maybe we should choose a different name so there is no confusion with System.Collections.BitArray?
Not sure it's completely necessary.

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@BorisDog BorisDog Sep 9, 2025 •
edited
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I think it's minor but still a valid point.
Changed to MemebersBitArray. Not sure about this naming though. Please suggest better alternatives if there are any.

{
var bitArrayOffset = memberCount & 31;
var bitArrayLength = memberCount >> 5;
if (bitArrayOffset == 0)
{
return new uint[bitArrayLength];
}
var bitArray = new uint[bitArrayLength + 1];
bitArray[bitArrayLength] = ~0U << bitArrayOffset; // set unused bits to 1
return bitArray;
}
private readonly ArrayPool<uint> _arrayPool;
private readonly Span<uint> _bitArray;
private readonly uint[] _rentedBuffer;
private bool _isDisposed = false;

// see http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightBinSearch
// also returns 31 if no bits are set; caller must check this case
public static int GetLeastSignificantBit(uint bitBlock)
{
var leastSignificantBit = 1;
if ((bitBlock & 65535) == 0)
public BitArray(Span<uint> bitArray) : this()
{
bitBlock >>= 16;
leastSignificantBit |= 16;
_arrayPool = null;
_bitArray = bitArray;
_rentedBuffer = null;
}
if ((bitBlock & 255) == 0)

public BitArray(int spanLength, uint[] rentedBuffer, ArrayPool<uint> arrayPool) : this()
{
bitBlock >>= 8;
leastSignificantBit |= 8;
_arrayPool = arrayPool;
_bitArray = rentedBuffer.AsSpan(0, spanLength);
_rentedBuffer = rentedBuffer;
}
if ((bitBlock & 15) == 0)

public Span<uint> Span => _bitArray;
public ArrayPool<uint> ArrayPool => _arrayPool;

public void SetMemberIndex(int memberMapIndex) =>
_bitArray[memberMapIndex >> 5] |= 1U << (memberMapIndex & 31);

public void Dispose()
{
bitBlock >>= 4;
leastSignificantBit |= 4;
if (_isDisposed)
return;

if (_rentedBuffer != null)
{
_arrayPool.Return(_rentedBuffer);
}
_isDisposed = true;
}
if ((bitBlock & 3) == 0)
}

public static (int BitArrayLength, bool UseStackAlloc) GetBitArrayLength(int membersCount)
{
var length = (membersCount + 31) >> 5;
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Rename length to lengthInUInts?

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Done.

return (length, length <= 8); // Use stackalloc for up to 256 members
}
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Some suggested renamings for clarity:

public static (int LengthInUInts, bool UseStackAlloc) GetLengthInUInts(int membersCoun
{
    var lengthInUInts = (membersCount + 31) >> 5;
    return (lengthInUInts, lengthInUInts <= 8); // Use stackalloc for up to 256 members
}

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Done.


public static BitArray GetBitArray(Span<uint> span) =>
new(ResetSpan(span));

public static BitArray GetBitArray(int length)
{
var rentedBuffer = ArrayPool<uint>.Shared.Rent(length);
ResetSpan(rentedBuffer);

return new(length, rentedBuffer, ArrayPool<uint>.Shared);
}

private static Span<uint> ResetSpan(Span<uint> span)
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Can we use Span.Clear instead?

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Thanks! Moved clearing to BitArray ctor for simplicity.

{
for (var i = 0; i < span.Length; i++)
{
bitBlock >>= 2;
leastSignificantBit |= 2;
span[i] = 0;
}
return leastSignificantBit - (int)(bitBlock & 1);

return span;
}
}
}
Expand Down
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