[browser][coreCLR] no COM, no swift, no x86 intrinsics by pavelsavara · Pull Request #125083 · dotnet/runtime

pavelsavara · 2026-03-02T23:23:43Z

Trim dead interop/intrinsics code from browser-WASM CoreCLR builds

Summary

This PR reduces the trimmed size of browser-WASM CoreCLR apps by removing ILLinker roots and native code for features that cannot run in the browser: COM interop, Dynamic Interface Castable, Swift interop, and hardware intrinsics metadata.

Motivation

When building trimmed browser-WASM apps with CoreCLR, several managed types survive trimming despite being unreachable at runtime. These types are kept alive by:

ILLink descriptor roots generated from corelib.h and rexcep.h — the descriptor generator processes DEFINE_CLASS/DEFINE_EXCEPTION entries and creates root entries that tell the linker to preserve types and methods.
Reverse P/Invoke stubs generated by PInvokeCollector.cs for [UnmanagedCallersOnly] methods on types that are unsupported on browser.
[CompExactlyDependsOn(typeof(...))] attribute metadata — this R2R-only attribute creates hard metadata type references (via typeof()) that the linker follows, keeping entire intrinsics type hierarchies alive.

Changes

1. Exclude COM/Interop exception types from WASM ILLink descriptors

Files: src/coreclr/vm/rexcep.h

Guard InvalidOleVariantTypeException, MarshalDirectiveException, COMException, ExternalException, and SEHException with #if !defined(TARGET_WASM) || !defined(FOR_ILLINK). This excludes them from the generated ILLink descriptor on WASM while keeping them available for the C++ compiler (which never defines FOR_ILLINK).

3. Exclude ComVariant from WASM ILLink descriptors

File: src/coreclr/vm/corelib.h

Guard COMVARIANT DEFINE_CLASS with #if !defined(TARGET_WASM) || !defined(FOR_ILLINK) to prevent rooting System.Runtime.InteropServices.Marshalling.ComVariant on WASM.

4. Exclude Swift interop types from WASM ILLink descriptors

File: src/coreclr/vm/corelib.h

Guard SwiftSelf, SwiftSelf<T>, SwiftError, and SwiftIndirectResult DEFINE_CLASS entries with #if !defined(TARGET_WASM) || !defined(FOR_ILLINK). Swift interop is not supported on WASM. The C++ code in callstubgenerator.cpp and stubgen.cpp still has access to CLASS__SWIFT_* constants during native compilation (since FOR_ILLINK is never defined for the C++ compiler).

5. Gate ComponentActivator ILLink root behind feature switch

File: src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml

The GetFunctionPointer method on ComponentActivator was unconditionally rooted. Now gated behind the System.Runtime.InteropServices.EnableConsumingManagedCodeFromNativeHosting feature switch, matching the other ComponentActivator methods in the same file.

6. Filter `[UnsupportedOSPlatform("browser")]` types from reverse P/Invoke stubs

Files: src/tasks/WasmAppBuilder/coreclr/PInvokeCollector.cs, src/coreclr/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.PlatformNotSupported.cs

Add IsUnsupportedOnBrowser() helper to PInvokeCollector.DoesMethodHaveCallbacks() — skips generating reverse P/Invoke callback stubs for methods on types annotated with [UnsupportedOSPlatform("browser")].
Add [UnsupportedOSPlatform("browser")] to ComActivator.PlatformNotSupported.cs so its [UnmanagedCallersOnly] methods (GetClassFactoryForTypeInternal, RegisterClassForTypeInternal, UnregisterClassForTypeInternal) are excluded from the generated stubs.

7. Strip `[CompExactlyDependsOn]` attribute on browser/WASI builds

File: src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/CompExactlyDependsOnAttribute.cs

Expand the existing [Conditional("unnecessary")] guard from #if MONO to #if MONO || TARGET_BROWSER || TARGET_WASI. This attribute is only needed for Ready2Run compilation, which browser and WASI targets do not use. The typeof(AdvSimd.Arm64) etc. arguments in 40+ methods across SPCL create hard metadata references that prevent the linker from trimming System.Runtime.Intrinsics.Arm (and X86) type hierarchies. Stripping the attribute at compile time removes these metadata references.

8. Regenerated callhelpers files

Files: src/coreclr/vm/wasm/callhelpers-pinvoke.cpp, src/coreclr/vm/wasm/callhelpers-reverse.cpp

Regenerated to reflect the ComActivator exclusion (3 stubs removed from reverse callbacks) and removal of System.Net.Primitives socket address P/Invokes that were removed upstream.

Design: The `FOR_ILLINK` guard pattern

The pattern #if !defined(TARGET_WASM) || !defined(FOR_ILLINK) is used for entries that must remain in native C++ code (for the enum/constant generator) but should be excluded from the ILLink root descriptor on WASM.

C++ compiler: FOR_ILLINK is never defined → condition is always true → entries are included
ILLink descriptor generator (CreateRuntimeRootILLinkDescriptorFile): passes DefineConstants=$(DefineConstants);FOR_ILLINK with a simple #if parser that treats unknown expressions (like defined() with &&/||) as false → entries are excluded on WASM
Non-WASM ILLink generation: TARGET_WASM is not defined → condition is true → entries are included

Testing

The build was verified with .\build.cmd -os browser -subset clr -c Debug /p:RuntimeFlavor=CoreCLR.

dotnet-policy-service · 2026-03-02T23:24:49Z

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Copilot

Pull request overview

This PR aims to reduce trimmed size for browser-WASM CoreCLR builds by removing/gating roots and native support code for features that aren’t supported in the browser (COM/interop-related pieces, DynamicInterfaceCastable, Swift interop, and intrinsics metadata rooting).

Changes:

Filters reverse P/Invoke stub generation for types marked [UnsupportedOSPlatform("browser")] and annotates ComActivator accordingly.
Gates ILLink rooting for ComponentActivator.GetFunctionPointer behind the existing native-hosting feature switch.
Adds TARGET_WASM guards to exclude DynamicInterfaceCastable native support on WASM and trims related ILLink roots / metadata retention paths.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/tasks/WasmAppBuilder/generate-coreclr-helpers.cmd	Updates helper generation script used for CoreCLR WASM callhelper regeneration.
src/tasks/WasmAppBuilder/coreclr/PInvokeCollector.cs	Skips generating reverse P/Invoke callback stubs for types unsupported on browser.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/CompExactlyDependsOnAttribute.cs	Removes `CompExactlyDependsOnAttribute` metadata for browser/WASI builds to improve trimming.
src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml	Gates `ComponentActivator.GetFunctionPointer` rooting behind the native-hosting feature switch.
src/coreclr/vm/wasm/callhelpers-reverse.cpp	Regenerated reverse thunk table reflecting removed callbacks.
src/coreclr/vm/wasm/callhelpers-pinvoke.cpp	Regenerated P/Invoke table reflecting upstream removals.
src/coreclr/vm/virtualcallstub.cpp	Removes DynamicInterfaceCastable resolver path from WASM builds.
src/coreclr/vm/rexcep.h	Attempts to exclude interop exception types from WASM ILLink descriptor generation.
src/coreclr/vm/methodtable.cpp	Removes IDynamicInterfaceCastable special method resolution on WASM.
src/coreclr/vm/jithelpers.cpp	Removes IDynamicInterfaceCastable cast path on WASM.
src/coreclr/vm/dynamicinterfacecastable.h	Wraps DynamicInterfaceCastable API surface out of WASM builds.
src/coreclr/vm/dynamicinterfacecastable.cpp	Wraps DynamicInterfaceCastable implementation out of WASM builds.
src/coreclr/vm/corelib.h	Attempts to exclude Swift interop and ComVariant from WASM ILLink descriptor generation; removes DynamicInterfaceCastableHelpers from WASM.
src/coreclr/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.PlatformNotSupported.cs	Marks `ComActivator` as unsupported on browser to suppress reverse stub generation.

src/tasks/WasmAppBuilder/generate-coreclr-helpers.cmd

src/coreclr/vm/corelib.h

src/coreclr/vm/rexcep.h

src/tasks/WasmAppBuilder/coreclr/PInvokeCollector.cs

src/coreclr/vm/corelib.h

Copilot

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

src/coreclr/vm/rexcep.h

src/coreclr/vm/corelib.h

maraf

The generator code looks good to me module the copilot caching comment

radekdoulik · 2026-03-18T13:39:06Z

.../System.Private.CoreLib/src/System/Runtime/CompilerServices/CompExactlyDependsOnAttribute.cs

-#if MONO
-    [Conditional("unnecessary")] // Mono doesn't use Ready2Run so we can remove this attribute to reduce size
+#if MONO || TARGET_BROWSER || TARGET_WASI
+    [Conditional("unnecessary")] // These platforms don't use Ready2Run so we can remove this attribute to reduce size


Please add WASM-TODO comment for future when we enable R2R

What is the WASM-TODO comment going to say? Add the attribute back?

I do not think we should be stripping it now if we will need to add it back soon.

radekdoulik · 2026-03-18T13:50:28Z

src/coreclr/vm/wasm/callhelpers-pinvoke.cpp

    int32_t SystemNative_FSync (void *);
    int32_t SystemNative_FTruncate (void *, int64_t);
    int32_t SystemNative_FUTimens (void *, void *);
+    int32_t SystemNative_FcntlGetIsNonBlocking (void *, void *);


I guess we need to regenerate the helpers.

radekdoulik

Beside the 2 comments it LGTM, thank you!

jkotas · 2026-03-18T15:52:28Z

src/coreclr/vm/corelib.h

 DEFINE_CLASS(CALLCONV_MEMBERFUNCTION,        CompilerServices,       CallConvMemberFunction)
 DEFINE_CLASS(CALLCONV_SWIFT,                 CompilerServices,       CallConvSwift)

+#if !defined(TARGET_WASM) || !defined(FOR_ILLINK)


Can this be ifdefed out for Apple instead of this? We support swift interop on Apple OSes only.

jkotas · 2026-03-18T15:53:34Z

src/coreclr/vm/corelib.h

 DEFINE_METHOD(HANDLE_MARSHALER,          THROW_SAFEHANDLE_FIELD_CHANGED, ThrowSafeHandleFieldChanged, SM_RetVoid)
 DEFINE_METHOD(HANDLE_MARSHALER,          THROW_CRITICALHANDLE_FIELD_CHANGED, ThrowCriticalHandleFieldChanged, SM_RetVoid)

+#if !defined(TARGET_WASM) || !defined(FOR_ILLINK)


Can this be ifdefed out for FEATURE_COMINTEROP instead of this?

jkotas · 2026-03-18T15:55:47Z

src/coreclr/vm/rexcep.h

 DEFINE_EXCEPTION(g_IONS,               IOException,                    false,  COR_E_IO, CTL_E_DEVICEIOERROR, STD_CTL_SCODE(31036), STD_CTL_SCODE(31037))

+#if !defined(TARGET_WASM) || !defined(FOR_ILLINK)
 DEFINE_EXCEPTION(g_InteropNS,          MarshalDirectiveException,      false,  COR_E_MARSHALDIRECTIVE)


I would expect that MarshalDirectiveException can be thrown even when we run on Wasm. What is the experience going to be on wasm when the IL linker trims it out? Is it going to be inscrutable crash?

jkotas · 2026-03-18T15:57:35Z

src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml

  </assembly>

-  <assembly fullname="System.Private.CoreLib">
+  <assembly fullname="System.Private.CoreLib" feature="System.Runtime.InteropServices.EnableConsumingManagedCodeFromNativeHosting" featurevalue="true">


Based on the comment, I think the whole point of this block is to keep ComponentActivator.GetFunctionPointer alive to get a reasonable experience when somebody tries to use this when EnableConsumingManagedCodeFromNativeHosting is disabled, and this change is going to break that.

jkotas · 2026-03-18T16:00:38Z

PR reduces the trimmed size of browser-WASM CoreCLR apps

Do you have any numbers for the size improvement?

pavelsavara added this to the 11.0.0 milestone Mar 2, 2026

pavelsavara self-assigned this Mar 2, 2026

Copilot AI review requested due to automatic review settings March 2, 2026 23:23

pavelsavara added arch-wasm WebAssembly architecture area-VM-coreclr size-reduction Issues impacting final app size primary for size sensitive workloads os-browser Browser variant of arch-wasm labels Mar 2, 2026

dotnet-policy-service bot added the linkable-framework Issues associated with delivering a linker friendly framework label Mar 2, 2026

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pavelsavara force-pushed the browser_trim_COM branch from c066985 to 77b2da7 Compare March 3, 2026 07:37

rebase

cb5be74

Copilot AI review requested due to automatic review settings March 13, 2026 15:29

pavelsavara force-pushed the browser_trim_COM branch from 77b2da7 to cb5be74 Compare March 13, 2026 15:29

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pavelsavara marked this pull request as ready for review March 18, 2026 08:53

pavelsavara requested a review from maraf as a code owner March 18, 2026 08:53

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Cache IsUnsupportedOnBrowser result per Type in PInvokeCollector #125722

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pavelsavara requested a review from radekdoulik March 18, 2026 11:13

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pavelsavara commented Mar 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Trim dead interop/intrinsics code from browser-WASM CoreCLR builds

Summary

Motivation

Changes

1. Exclude COM/Interop exception types from WASM ILLink descriptors

3. Exclude ComVariant from WASM ILLink descriptors

4. Exclude Swift interop types from WASM ILLink descriptors

5. Gate ComponentActivator ILLink root behind feature switch

6. Filter [UnsupportedOSPlatform("browser")] types from reverse P/Invoke stubs

7. Strip [CompExactlyDependsOn] attribute on browser/WASI builds

8. Regenerated callhelpers files

Design: The FOR_ILLINK guard pattern

Testing

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pavelsavara commented Mar 2, 2026 •

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6. Filter `[UnsupportedOSPlatform("browser")]` types from reverse P/Invoke stubs

7. Strip `[CompExactlyDependsOn]` attribute on browser/WASI builds

Design: The `FOR_ILLINK` guard pattern

jkotas Mar 18, 2026 •

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