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Reduce allocations in CSharpSyntaxNode.GetStructure #80562

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merged 3 commits into from
Oct 6, 2025
Merged

Reduce allocations in CSharpSyntaxNode.GetStructure #80562

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fb1fcf6
*** WIP: Reduce allocations in CSharpSyntaxNode.GetStructure
ToddGrun Oct 4, 2025
981e9b2
Try out using a SmallDictionary instead of a List
ToddGrun Oct 5, 2025
0a04814
update comment
ToddGrun Oct 5, 2025
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53 changes: 27 additions & 26 deletions src/Compilers/CSharp/Portable/Syntax/InternalSyntax/CSharpSyntaxNode.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -250,8 +250,16 @@ public override bool IsTriviaWithEndOfLine()
}

// Use conditional weak table so we always return same identity for structured trivia
private static readonly ConditionalWeakTable<SyntaxNode, Dictionary<CodeAnalysis.SyntaxTrivia, WeakReference<SyntaxNode>>> s_structuresTable
= new ConditionalWeakTable<SyntaxNode, Dictionary<CodeAnalysis.SyntaxTrivia, WeakReference<SyntaxNode>>>();
//
// As there are commonly few structured trivia per parent, use a SmallDictionary for
// mapping from trivia to StructuredTriviaSyntax. Testing against roslyn, of parents
// containing structured trivia:
// 81.2% contain 1 structured trivia
// 96.5% contain 2 or fewer structured trivia
// 99.6% contain 4 or fewer structured trivia
// 100% contain 7 or fewer structured trivia
private static readonly ConditionalWeakTable<SyntaxNode, SmallDictionary<CodeAnalysis.SyntaxTrivia, SyntaxNode>> s_structuresTable
= new ConditionalWeakTable<SyntaxNode, SmallDictionary<CodeAnalysis.SyntaxTrivia, SyntaxNode>>();

/// <summary>
/// Gets the syntax node represented the structure of this trivia, if any. The HasStructure property can be used to
Expand All @@ -270,36 +278,29 @@ public override bool IsTriviaWithEndOfLine()
/// </remarks>
public override SyntaxNode GetStructure(Microsoft.CodeAnalysis.SyntaxTrivia trivia)
{
if (trivia.HasStructure)
if (!trivia.HasStructure)
{
var parent = trivia.Token.Parent;
if (parent != null)
{
SyntaxNode structure;
var structsInParent = s_structuresTable.GetOrCreateValue(parent);
lock (structsInParent)
{
if (!structsInParent.TryGetValue(trivia, out var weakStructure))
{
structure = CSharp.Syntax.StructuredTriviaSyntax.Create(trivia);
structsInParent.Add(trivia, new WeakReference<SyntaxNode>(structure));
}
else if (!weakStructure.TryGetTarget(out structure))
{
structure = CSharp.Syntax.StructuredTriviaSyntax.Create(trivia);
weakStructure.SetTarget(structure);
}
}
return null;
}

return structure;
}
else
var parent = trivia.Token.Parent;
if (parent is null)
{
return CSharp.Syntax.StructuredTriviaSyntax.Create(trivia);
}

SyntaxNode structure;
var structsInParent = s_structuresTable.GetOrCreateValue(parent);
lock (structsInParent)
{
if (!structsInParent.TryGetValue(trivia, out structure))
{
return CSharp.Syntax.StructuredTriviaSyntax.Create(trivia);
structure = CSharp.Syntax.StructuredTriviaSyntax.Create(trivia);
structsInParent.Add(trivia, structure);
}
}

return null;
return structure;
}
}
}
18 changes: 11 additions & 7 deletions src/Compilers/VisualBasic/Portable/Syntax/InternalSyntax/SyntaxNode.vb
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Expand Up @@ -203,7 +203,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Syntax.InternalSyntax
End Function

' Use conditional weak table so we always return same identity for structured trivia
Private Shared ReadOnly s_structuresTable As New ConditionalWeakTable(Of SyntaxNode, Dictionary(Of Microsoft.CodeAnalysis.SyntaxTrivia, WeakReference(Of SyntaxNode)))
'
' As there are commonly few structured trivia per parent, use a SmallDictionary for
' mapping from trivia to StructuredTriviaSyntax. Testing against roslyn, of parents
' containing structured trivia:
' 81.2% contain 1 structured trivia
' 96.5% contain 2 or fewer structured trivia
' 99.6% contain 4 or fewer structured trivia
' 100% contain 7 or fewer structured trivia
Private Shared ReadOnly s_structuresTable As New ConditionalWeakTable(Of SyntaxNode, SmallDictionary(Of Microsoft.CodeAnalysis.SyntaxTrivia, SyntaxNode))

Public Overrides Function GetStructure(trivia As Microsoft.CodeAnalysis.SyntaxTrivia) As SyntaxNode
If Not trivia.HasStructure Then
Expand All @@ -219,13 +227,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Syntax.InternalSyntax
Dim structsInParent = s_structuresTable.GetOrCreateValue(parent)

SyncLock structsInParent
Dim weakStructure As WeakReference(Of SyntaxNode) = Nothing
If Not structsInParent.TryGetValue(trivia, weakStructure) Then
If Not structsInParent.TryGetValue(trivia, [structure]) Then
[structure] = VisualBasic.Syntax.StructuredTriviaSyntax.Create(trivia)
structsInParent.Add(trivia, New WeakReference(Of SyntaxNode)([structure]))
ElseIf Not weakStructure.TryGetTarget([structure]) Then
[structure] = VisualBasic.Syntax.StructuredTriviaSyntax.Create(trivia)
weakStructure.SetTarget([structure])
structsInParent.Add(trivia, [structure])
End If
End SyncLock

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