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

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Merged
merged 12 commits into from
Nov 5, 2025
+396 −274
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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68 changes: 48 additions & 20 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 @@ -1177,13 +1177,22 @@ 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.
bool useConstantIndices = numberIncludingLastSpread == 0;
BoundLocal? indexTemp = null;
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@CyrusNajmabadi CyrusNajmabadi Nov 3, 2025

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review with whitespace off.


if (!useConstantIndices)
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@RikkiGibson RikkiGibson Nov 4, 2025 •
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Perhaps it would simplify things to use the null state of the indexTemp itself as indication of whether it should be used.

You could inline the value of useConstantIndices here, and in other places which are checking useConstantIndices, check indexTemp == null instead.

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@CyrusNajmabadi CyrusNajmabadi Nov 4, 2025

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@copilot I like this idea. Let's inline numberIncludingLastSpread != 0 into this if check again. Then document that if indexTemp is null it means we're using constant compile-time indices, otherwise we're using a runtime tracked one.

then, the checks below just check if indexTemp is null or not, removing the need for some of the asserts as well.

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Copilot AI Nov 4, 2025

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Simplified the code by using the null state of indexTemp in commit f9b7579. The useConstantIndices boolean has been removed, and the code now checks indexTemp is null instead.

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Copilot AI Nov 4, 2025

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Inlined the condition and improved documentation in commit b5ed80e. The code now uses numberIncludingLastSpread != 0 in the if check, with clear documentation that indexTemp's null state indicates constant compile-time vs runtime-tracked indices. Removed redundant Debug.Assert calls.

{
// 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 +1202,52 @@ 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 (useConstantIndices)
{
// 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 is { Type.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(!useConstantIndices, "Should not have spread elements when using constant indices");
Debug.Assert(indexTemp is not null);
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@CyrusNajmabadi CyrusNajmabadi Nov 5, 2025

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@copilot have a similar comment here.

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Copilot AI Nov 5, 2025

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The comment was already added in commit 38fdece at line 1270 (same location).


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

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