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[net11.0] Fully trim assemblies in R2R image when using SdkOnly link mode in Debug#24909

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[net11.0] Fully trim assemblies in R2R image when using SdkOnly link mode in Debug#24909
kotlarmilos wants to merge 2 commits intonet11.0from
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@kotlarmilos kotlarmilos commented Mar 17, 2026

When FilterReadyToRunAssemblies is active (Debug builds with CoreCLR R2R), only NuGet/framework assemblies enter the R2R composite image while user assemblies run interpreted for debugging.

Currently, with the default SdkOnly link mode, framework assemblies are only trimmed if they have IsTrimmable=true metadata. This leaves some assemblies untrimmed in the R2R image, increasing bundle size unnecessarily since these assemblies won't be used for debugging.

This change adds a _SetR2RAssemblyTrimMode target that, when _LinkMode == 'SdkOnly' and R2R filtering is active:

  • Forces TrimMode=link and IsTrimmable=true on all NuGet/framework assemblies (entering R2R image)
  • Forces TrimMode=copy on user assemblies (interpreted, kept intact for debugging)

If the user explicitly sets Full or None link mode, their choice is respected.

Results (MAUI template app, Debug, CoreCLR, iOS arm64):

Metric Before After Delta
Bundle Size 140.42 MB 112.66 MB -27.76 MB (-19.8%)
Total Startup 183.89 ms 172.25 ms -11.64 ms (-6.3%)

@kotlarmilos
[CoreCLR] Fully trim assemblies in R2R image when using SdkOnly link …
07630e8 
…mode

When FilterReadyToRunAssemblies is active (Debug builds with CoreCLR R2R),
only NuGet/framework assemblies enter the R2R composite image while user
assemblies run interpreted for debugging.

Currently, with the default SdkOnly link mode, framework assemblies are
only trimmed if they have IsTrimmable=true metadata. This leaves some
assemblies untrimmed in the R2R image, increasing bundle size.

Add _SetR2RAssemblyTrimMode target that forces TrimMode=link on all
NuGet/framework assemblies entering the R2R image and TrimMode=copy on
user assemblies when _LinkMode is SdkOnly.

Results (MAUI template, Debug, CoreCLR, iOS arm64):
- Bundle size: 140.42 MB -> 112.66 MB (-27.76 MB, -19.8%)
- Startup: 183.89 ms -> 172.25 ms (-11.64 ms, -6.3%)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@kotlarmilos kotlarmilos changed the title [CoreCLR] Fully trim assemblies in R2R image when using SdkOnly link mode [net11.0] Fully trim assemblies in R2R image when using SdkOnly link mode in Debug Mar 17, 2026
@kotlarmilos kotlarmilos self-assigned this Mar 17, 2026
@kotlarmilos kotlarmilos mentioned this pull request Mar 17, 2026
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@kotlarmilos kotlarmilos added this to the .NET 11 milestone Mar 17, 2026
@kotlarmilos kotlarmilos added iOS Issues affecting iOS CoreCLR Bugs related to our CoreCLR support labels Mar 17, 2026
@kotlarmilos kotlarmilos marked this pull request as ready for review March 17, 2026 12:34
Copilot AI reviewed Mar 17, 2026
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vs-mobiletools-engineering-service2 commented Mar 17, 2026

✅ [CI Build #b9a2ea1] Build passed (Build packages) ✅

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✅ [PR Build #b9a2ea1] Build passed (Detect API changes) ✅

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✅ [CI Build #b9a2ea1] Build passed (Build macOS tests) ✅

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🔥 [CI Build #b9a2ea1] Test results 🔥

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❌ Tests failed on VSTS: test results

0 tests crashed, 1 tests failed, 154 tests passed.

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rolfbjarne commented Mar 18, 2026

Currently, with the default SdkOnly link mode, framework assemblies are only trimmed if they have IsTrimmable=true metadata. This leaves some assemblies untrimmed in the R2R image, increasing bundle size unnecessarily since these assemblies won't be used for debugging.

If a framework assembly doesn't have IsTrimmable=true, then presumably it's not trimmable - so I'm not sure we should override that.

If a framework assembly is indeed trimmable, then it seems like the better solution would be to set IsTrimmable=true for those assemblies.

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rolfbjarne commented Mar 18, 2026

Also which assemblies are these?

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kotlarmilos commented Mar 18, 2026 •
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The assemblies in the R2R image that don't have IsTrimmable=true are:

  • Microsoft.Maui.Controls.dll
  • Microsoft.Maui.dll
  • Microsoft.Maui.Graphics.dll
  • Microsoft.Maui.Controls.Xaml.dll
  • System.Private.CoreLib.dll

I initially misunderstood SdkOnly. I thought it only trimmed SDK/framework assemblies, not all assemblies with IsTrimmable=true. Since the default IsTrimmable=false, I assume that many third-party nuget packs don't have it explicitly set, but might be wrong.

If a framework assembly is indeed trimmable, then it seems like the better solution would be to set IsTrimmable=true for those assemblies.

What do you have in mind here?

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rolfbjarne commented Mar 19, 2026

The assemblies in the R2R image that don't have IsTrimmable=true are:

  • Microsoft.Maui.Controls.dll
  • Microsoft.Maui.dll
  • Microsoft.Maui.Graphics.dll
  • Microsoft.Maui.Controls.Xaml.dll
  • System.Private.CoreLib.dll

Note that IsTrimmable can be set as an assembly attribute as well: https://github.com/dotnet/runtime/blob/main/docs/design/tools/illink/trimmed-assemblies.md#net-6

And looking at System.Private.CoreLib.dll, it has:

[assembly: System.Reflection.AssemblyMetadata ("IsTrimmable", "True")]

so it's strange to see SPC.dll in this list.

If a framework assembly is indeed trimmable, then it seems like the better solution would be to set IsTrimmable=true for those assemblies.

What do you have in mind here?

Set the same AssemblyMetadata attribute on the MAUI assemblies as well (if they're indeed trimmable).

This has a few additional advantages:

  • Apps behave the same between Debug and Release
  • Release also gets the same size gain

I initially misunderstood SdkOnly. I thought it only trimmed SDK/framework assemblies, not all assemblies with IsTrimmable=true. Since the default IsTrimmable=false, I assume that many third-party nuget packs don't have it explicitly set, but might be wrong.

That's a historical accident: "SdkOnly" is old, and before .NET that's exactly what it meant: trim the framework assemblies only. In .NET things are somewhat different (to start: what's a "framework assembly" exactly?), so we changed the meaning a little bit to be "all assemblies that declare themselves to be trimmable", which was both close enough to the truth (only BCL assemblies declared themselves to be trimmmable, at least at first), and it's also close in spirit (trim all assemblies that are known to trimmer safe).

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kotlarmilos commented Mar 19, 2026

Thanks. Closing this PR in favour of dotnet/maui#34573

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