Fold generic method bodies by default

[MichalStrehovsky]

Fixes #103951.

Same method with different genericness are not distinguishable in the stack traces. They are still distinguishable in the debugger, but it might be acceptable, especially since we have an opt out (undocumented for now).

Cc @dotnet/ilc-contrib

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[jkotas]

They are still distinguishable in the debugger, but it might be acceptable, especially since we have an opt out (undocumented for now).

Do you think it would be possible to rename the symbol for folded copy such that the symbol name does not include the instantiation that happened to win?

[MichalStrehovsky]

They are still distinguishable in the debugger, but it might be acceptable, especially since we have an opt out (undocumented for now).

Do you think it would be possible to rename the symbol for folded copy such that the symbol name does not include the instantiation that happened to win?

I think it would make things quite complicated in general. We don't have facilities to change our mind about how symbols are named. We'd also need to invent a scheme for new stable names.

[MichalStrehovsky]

Size statistics

Pull request #117411

Project	Size before	Size after	Difference
TodosApi-linux	25538640	24666192	-872448
TodosApi-windows	22906368	22239232	-667136
avalonia.app-linux	19995056	19499440	-495616
avalonia.app-windows	17649664	17309696	-339968
hello-linux	1369040	1364944	-4096
hello-minimal-linux	1086248	1086248	0
hello-minimal-windows	837120	837120	0
hello-windows	1092608	1088512	-4096
kestrel-minimal-linux	5205784	5205784	0
kestrel-minimal-windows	4750848	4750848	0
reflection-linux	1992752	1964080	-28672
reflection-windows	1695232	1673728	-21504
webapiaot-linux	9866600	9653608	-212992
webapiaot-windows	9032704	8862720	-169984
winrt-component-full-windows	5720064	5594112	-125952
winrt-component-minimal-windows	1701888	1701888	0

[Copilot]

Pull Request Overview

This PR replaces the boolean “fold identical method bodies” flag with a three-way MethodBodyFoldingMode enum (None, Generic, All), defaults generic folding by build integration, and wires the new mode through the compiler and MSBuild targets.

• Swapped UseMethodBodyFolding(bool) for UseMethodBodyFolding(MethodBodyFoldingMode) in the CLI, builder, and RyuJit compilation paths.
• Introduced MethodBodyFoldingMode enum and revamped ObjectDataInterner to only fold based on generics when requested.
• Updated MSBuild targets to compute a default folding mode and emit the new --methodbodyfolding:<mode> argument.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/coreclr/tools/aot/ILCompiler/Program.cs	Parse string option into new MethodBodyFoldingMode
src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs	Change CLI option type from bool to string and update description
src/coreclr/tools/aot/ILCompiler.RyuJit/Compiler/RyuJitCompilationBuilder.cs	Switch interner creation to use new enum
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectDataInterner.cs	Add _genericsOnly flag, CanFold, and comparer support
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/DependencyAnalysis/NodeFactory.cs	Use CanFold instead of IsNull
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/CompilationBuilder.Aot.cs	Change builder API and add enum definition
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets	Compute and pass <mode> to --methodbodyfolding

Comments suppressed due to low confidence (3)

src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs:103

• The description is missing a closing parenthesis; change to e.g. "Fold identical method bodies (one of: none, generic, all)".

            new("--methodbodyfolding") { Description = "Fold identical method bodies (one of: none, generic, all" };

src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs:102

• Add an AddValidator to restrict the option to the valid values (none, generic, all) and produce a clear error when parsing fails.

        public Option<string> MethodBodyFolding { get; } =

src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs:104

• No tests appear to validate the new MethodBodyFolding modes; please add unit tests for parsing and default behavior to ensure coverage.

        public Option<string[]> InitAssemblies { get; } =

[MichalStrehovsky]

@sbomer is this something you'd be comfortable reviewing?

[sbomer]

Did you consider making the defaults dependent on DebuggerSupport?

[MichalStrehovsky]

Did you consider making the defaults dependent on DebuggerSupport?

I didn't think about it but gave it a thought now - here's my thinking:

• DebuggerSupport disables debugger support. For example, it controls whether we leave the breadcrumbs for the SOS extension to work. The SOS extension doesn't work at all with DebuggerSupport=false. Debugging is much harder.
• Method folding somewhat messes up the callstack, but not to the extent that it would be very difficult to reason about. It's more to the tune of how inlining and tail calls mess up callstack.

So I'd lean towards not tying it to that.