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Merged
merged 12 commits into from
Nov 5, 2025
Merged

Use constant indices when populating Lists and arrays in collection expressions #80999

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536a406
Initial plan
Copilot Nov 3, 2025
fcb6eb3
Use constant indices when populating span for List in collection expr…
Copilot Nov 3, 2025
c210cc9
Update src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewr…
CyrusNajmabadi Nov 3, 2025
738dcea
Update src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewr…
CyrusNajmabadi Nov 3, 2025
7aa7cab
Fix
CyrusNajmabadi Nov 3, 2025
877b12b
Add test for fixed elements before spread in collection expressions
Copilot Nov 4, 2025
bd54c95
Add WorkItem attribute to ListConstruction_FixedElementsBeforeSpread …
Copilot Nov 4, 2025
9ffa846
Apply constant indices optimization to arrays
Copilot Nov 4, 2025
f9b7579
Simplify code by using null state of indexTemp
Copilot Nov 4, 2025
b5ed80e
Improve documentation and inline condition check
Copilot Nov 4, 2025
38fdece
Add comments explaining that spreads always require runtime-tracked i…
Copilot Nov 5, 2025
2fab349
Add WorkItem attributes to all updated tests
Copilot Nov 5, 2025
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132 changes: 92 additions & 40 deletions src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -831,12 +831,19 @@ private BoundExpression CreateAndPopulateArray(BoundCollectionExpression node, A

RewriteCollectionExpressionElementsIntoTemporaries(elements, numberIncludingLastSpread, localsBuilder, sideEffects);

// int index = 0;
BoundLocal indexTemp = _factory.StoreToTemp(
_factory.Literal(0),
out assignmentToTemp);
localsBuilder.Add(indexTemp);
sideEffects.Add(assignmentToTemp);
// When there are no spread elements, we can use constant indices for better codegen.
// When there are spread elements, we need a mutable index variable for the CopyTo optimization.
BoundLocal? indexTemp = null;

if (numberIncludingLastSpread > 0)
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@RikkiGibson RikkiGibson Nov 4, 2025

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Ah. Looking at the code from above, I recall now, that this optimization won't ever kick in, until we extend it so that normal elements before the first spread element use constant indices. Preconditions earlier on this code path, make it so the condition on this line will always be true, as currently written.

Sorry for the runaround.

{
// int index = 0;
indexTemp = _factory.StoreToTemp(
_factory.Literal(0),
out assignmentToTemp);
localsBuilder.Add(indexTemp);
sideEffects.Add(assignmentToTemp);
}

// ElementType[] array = new ElementType[N + s1.Length + ...];
BoundLocal arrayTemp = _factory.StoreToTemp(
Expand All @@ -848,6 +855,7 @@ private BoundExpression CreateAndPopulateArray(BoundCollectionExpression node, A
localsBuilder.Add(arrayTemp);
sideEffects.Add(assignmentToTemp);

int currentElementIndex = 0;
AddCollectionExpressionElements(
elements,
arrayTemp,
Expand All @@ -857,33 +865,51 @@ private BoundExpression CreateAndPopulateArray(BoundCollectionExpression node, A
addElement: (ArrayBuilder<BoundExpression> expressions, BoundExpression arrayTemp, BoundExpression rewrittenValue, bool isLastElement) =>
{
Debug.Assert(arrayTemp.Type is ArrayTypeSymbol);
Debug.Assert(indexTemp.Type is { SpecialType: SpecialType.System_Int32 });

var expressionSyntax = rewrittenValue.Syntax;
var elementType = ((ArrayTypeSymbol)arrayTemp.Type).ElementType;

// array[index] = element;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
_factory.ArrayAccess(arrayTemp, indexTemp),
rewrittenValue,
isRef: false,
elementType));
if (!isLastElement)
if (indexTemp is null)
{
// index = index + 1;
// array[0] = element; array[1] = element; etc.
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
indexTemp,
_factory.Binary(BinaryOperatorKind.Addition, indexTemp.Type, indexTemp, _factory.Literal(1)),
_factory.ArrayAccess(arrayTemp, _factory.Literal(currentElementIndex)),
rewrittenValue,
isRef: false,
indexTemp.Type));
elementType));
currentElementIndex++;
}
else
{
Debug.Assert(indexTemp.Type is { SpecialType: SpecialType.System_Int32 });

// array[index] = element;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
_factory.ArrayAccess(arrayTemp, indexTemp),
rewrittenValue,
isRef: false,
elementType));
if (!isLastElement)
{
// index = index + 1;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
indexTemp,
_factory.Binary(BinaryOperatorKind.Addition, indexTemp.Type, indexTemp, _factory.Literal(1)),
isRef: false,
indexTemp.Type));
}
}
},
tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression arrayTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>
{
Debug.Assert(indexTemp is not null, "Should not have spread elements when using constant indices");

if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))
return false;

Expand Down Expand Up @@ -1177,13 +1203,21 @@ private BoundExpression CreateAndPopulateList(BoundCollectionExpression node, Ty
// Populate the span.
var spanGetItem = ((MethodSymbol)_factory.WellKnownMember(WellKnownMember.System_Span_T__get_Item)).AsMember((NamedTypeSymbol)spanTemp.Type);

// int index = 0;
BoundLocal indexTemp = _factory.StoreToTemp(
_factory.Literal(0),
out assignmentToTemp);
localsBuilder.Add(indexTemp);
sideEffects.Add(assignmentToTemp);
// When there are no spread elements, we can use constant indices for better codegen.
// When there are spread elements, we need a mutable index variable for the CopyTo optimization.
BoundLocal? indexTemp = null;
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@CyrusNajmabadi CyrusNajmabadi Nov 3, 2025

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review with whitespace off.


if (numberIncludingLastSpread > 0)
{
// int index = 0;
indexTemp = _factory.StoreToTemp(
_factory.Literal(0),
out assignmentToTemp);
localsBuilder.Add(indexTemp);
sideEffects.Add(assignmentToTemp);
}
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@CyrusNajmabadi CyrusNajmabadi Nov 4, 2025

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@copilot CreateAndPopulateArray contains very similar logic for arrays. can you update the PR to use constant indices for those as well when possible?

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Copilot AI Nov 4, 2025

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Applied constant indices optimization to arrays in commit 9ffa846. Arrays now use the same optimization as Lists when there are no spread elements.


int currentElementIndex = 0;
AddCollectionExpressionElements(
elements,
spanTemp,
Expand All @@ -1193,33 +1227,51 @@ private BoundExpression CreateAndPopulateList(BoundCollectionExpression node, Ty
addElement: (ArrayBuilder<BoundExpression> expressions, BoundExpression spanTemp, BoundExpression rewrittenValue, bool isLastElement) =>
{
Debug.Assert(spanTemp.Type is NamedTypeSymbol);
Debug.Assert(indexTemp.Type is { SpecialType: SpecialType.System_Int32 });

var expressionSyntax = rewrittenValue.Syntax;
var elementType = ((NamedTypeSymbol)spanTemp.Type).TypeArgumentsWithAnnotationsNoUseSiteDiagnostics[0].Type;

// span[index] = element;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
_factory.Call(spanTemp, spanGetItem, indexTemp),
rewrittenValue,
isRef: false,
elementType));
if (!isLastElement)
if (indexTemp is null)
{
// index = index + 1;
// span[0] = element; span[1] = element; etc.
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
indexTemp,
_factory.Binary(BinaryOperatorKind.Addition, indexTemp.Type, indexTemp, _factory.Literal(1)),
_factory.Call(spanTemp, spanGetItem, _factory.Literal(currentElementIndex)),
rewrittenValue,
isRef: false,
indexTemp.Type));
elementType));
currentElementIndex++;
}
else
{
Debug.Assert(indexTemp.Type is { SpecialType: SpecialType.System_Int32 });

// span[index] = element;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
_factory.Call(spanTemp, spanGetItem, indexTemp),
rewrittenValue,
isRef: false,
elementType));
if (!isLastElement)
{
// index = index + 1;
expressions.Add(
new BoundAssignmentOperator(
expressionSyntax,
indexTemp,
_factory.Binary(BinaryOperatorKind.Addition, indexTemp.Type, indexTemp, _factory.Literal(1)),
isRef: false,
indexTemp.Type));
}
}
},
tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression spanTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>
{
Debug.Assert(indexTemp is not null, "Should not have spread elements when using constant indices");

if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))
return false;

Expand Down
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