[prozolic]

In MethodBodyScanner.HandleCall, create the ValueNodeList directly instead of building an intermediate List<T> and copying it. Apply the same change to MethodBodyScanner in src\tools\illink\src\linker\Linker.Dataflow to keep the two in sync.

Also specify the initial capacity when creating a List<T> in ILCompiler and ILCompiler.Compiler where the number of elements is known up front. This avoids reallocating the backing array as the list grows.
In single-method mode, fill the generic argument array directly instead of going through a List<T> and ToArray().

I ran the NativeAOT smoketests and ILCompiler.*.Tests/Mono.Linker.Tests using the following commands and confirmed that they all passed in local.

.\build.cmd clr.aot -rc Release -lc Release
.\src\tests\build.cmd nativeaot Release tree nativeaot /p:LibrariesConfiguration=Release
.\src\tests\run.cmd runnativeaottests Release
dotnet test src/coreclr/tools/aot/ILCompiler.Trimming.Tests/ILCompiler.Trimming.Tests.csproj -c Release
dotnet test src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/ILCompiler.TypeSystem.Tests.csproj -c Release
dotnet test src/coreclr/tools/aot/ILCompiler.DependencyAnalysisFramework.Tests/ILCompiler.DependencyAnalysisFramework.Tests.csproj -c Release

.\build.cmd -s tools.illink -c Debug
dotnet test src/tools/illink/test/Mono.Linker.Tests/Mono.Linker.Tests.csproj -c Debug

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[Copilot]

🟡 Changes recommended

Performance measurements are missing, and the removed call sites leave dead synchronized constructors.

2 open findings

• Add performance measurements for optimization · New
• Remove unused ValueNodeList constructors · New

What changed in this PR

Reduces allocations in NativeAOT and ILLink compiler paths.

Changes:

• Builds ValueNodeList directly in synchronized dataflow scanners.
• Preallocates lists with known capacities.
• Fills generic argument arrays directly.

File	Description
src/​tools/​illink/​src/​linker/​Linker.Dataflow/​MethodBodyScanner.cs	Removes an intermediate list allocation.
src/​coreclr/​tools/​Common/​Compiler/​Win32Resources/​ResourceData.cs	Preallocates resource-type storage.
src/​coreclr/​tools/​aot/​ILCompiler/​Program.cs	Preallocates lists and directly fills generic arguments.
src/​coreclr/​tools/​aot/​ILCompiler.Compiler/​Compiler/​TypePreinit.cs	Preallocates serialized field values.
src/​coreclr/​tools/​aot/​ILCompiler.Compiler/​Compiler/​MetadataManager.cs	Preallocates generic metadata arguments.
src/​coreclr/​tools/​aot/​ILCompiler.Compiler/​Compiler/​DependencyAnalysis/​NativeLayoutVertexNode.cs	Preallocates layout collections.
src/​coreclr/​tools/​aot/​ILCompiler.Compiler/​Compiler/​Dataflow/​MethodBodyScanner.cs	Removes an intermediate list allocation.

🧠 Review effort: Balanced

[MichalStrehovsky]

I only skimmed this, but I'm not sure if any of this is worth it, sorry. Just as an example, the TypePreinit change makes it look like we care about perf here because we want to correctly size things. But if we care about perf, this should be an array not a list. And we shouldn't process it as IEnumerable down the line. But the truth is we don't care about the perf here and go for simplicity.

Perf fixes should come with perf measurements. We care about wall clock time. Making source code lines longer and perturbing git blame history with no wall clock time wins for it doesn't move the needle.

[prozolic]

@MichalStrehovsky I measured the build times and memory allocation for helloworld and webapi, but the differences were so small that they were negligible, so as you said, it doesn't really mean much.
Closing. Thanks for the review.