[NativeAOT] Investigate AOT size savings opportunities around async

[MichalStrehovsky]

.NET Native had size optimizations in the structuring of the BCL code to avoid unnecessary generic expansions in the async infrastructure.

We lost these when CoreLibs between the projects were unified. We restored some of it (example: dotnet/corert#6913), but there is likely more that we could do. We could study the old optimizations in the history of the CoreRT repo, or do a new investigation. Not sure how much async changed between then and now.

@eerhardt saw that in a sample 20 MB app using npgsql package, 3 MB of stuff come from the CompilerServices namespace and that feels still a bit excessive.

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Issue Details

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.NET Native had size optimizations in the structuring of the BCL code to avoid unnecessary generic expansions in the async infrastructure.

We lost these when CoreLibs between the projects were unified. We restored some of it (example: dotnet/corert#6913), but there is likely more that we could do. We could study the old optimizations in the history of the CoreRT repo, or do a new investigation. Not sure how much async changed between then and now.

@eerhardt saw that in a sample 20 MB app using npgsql package, 3 MB of stuff come from the CompilerServices namespace and that feels still a bit excessive.

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-System.Runtime.CompilerServices
Milestone:	8.0.0

[teo-tsirpanis]

Would an API like a RuntimeFeature.IsSizeOptimized property help? Then we could do this:

if (!RuntimeFeature.IsDynamicCodeCompiled && RuntimeFeature.IsSizeOptimized)
{
    // Use the common interface as generic parameter.
}
else
{
    // Use the struct as generic parameter.
}

[MichalStrehovsky]

Would an API like a RuntimeFeature.IsSizeOptimized property help? Then we could do this:

if (!RuntimeFeature.IsDynamicCodeCompiled && RuntimeFeature.IsSizeOptimized)
{
    // Use the common interface as generic parameter.
}
else
{
    // Use the struct as generic parameter.
}

I don't know if any of the optimizations .NET Native had meaningfully affected throughput and would have to be guarded. dotnet/corert#6913 is not something that would really need to be guarded. It helps everywhere (less jitting, less working set).

[eerhardt]

Here's the MstatDumper dump of the npgsql app, in case anyone is interested in the full output.

npgsql-aot-size.txt

And tweaking that code to just dump what is in the System.Runtime.CompilerServices namespace produces:

System.Runtime.CompilerServices.txt