[NativeAOT] Metadata not trimmed for nested Interface types when compiling TerraFx.Interop.Windows

[jkotas]

Same repro as #66593 (comment)

Actual result: Nested Interface types like TerraFX.Interop.Windows.IEnumWbemClassObject+Interface types are present in the final binary

Expected result: TerraFX.Interop.Windows.IEnumWbemClassObject+Interface types are not present in the final binary (IL linker is able to trim these)

[jkotas]

I have tried to debug this using dgml log by adding --fulllog --dgmllog c:\repro\log.dgml to the command line. The generation of dgmllog is crashing with: An item with the same key has already been added. Key: (ILCompiler.DependencyAnalysis.PrecomputedDictionaryLayoutNode, ILCompiler.DependencyAnalysis.NativeLayoutTemplateTypeLayoutVertexNode)

[jkotas]

@MichalStrehovsky Does this ring any bells?

[agocke]

@jkotas Fixed by #66593?

[jkotas]

Fixed by #66593?

No. This issue is what's left after fixing #66593.

[MichalStrehovsky]

We don't trim interface lists. I played with it in the past (MichalStrehovsky/runtimelab@2a702bc), but it's a bit more involved (the above commit was enough to get a size estimate and I remember the size saving being interesting - more than 1% - but not interesting enough to spend time on it at that point).

It will require changing most of the occurrences of .RuntimeInterfaces in the dependency analysis within the compiler to use some kind of indirection. It will likely be something similar to how we track virtual method use.

I'll put this into Future since this is a nice to have size optimization.

[MichalStrehovsky]

I had a quick look - the only reason why we're generating the interfaces is because we're generating the struct that implements them. And the only reason why we generate the struct is because we called some instance methods on it and it became a potential target of reflection in the "IL Linker compat mode" we run by default (where whatever is called is considered reflection-accessible at runtime).

In the strict mode <IlcTrimMetadata>true</IlcTrimMetadata> we don't generate any of that. The repro in the top-post is 20% smaller with that setting.

I wish we just enabled the strict mode by default...