Enable building a stubbed NativeAOT WASM CoreLib by jtschuster · Pull Request #134210 · dotnet/runtime

jtschuster · 2026-09-18T18:33:09Z

Building the repro project requires a number of helper methods in CoreLib, but stack unwinding, GC, and EH designs haven't been determined, so in the meantime we could just stub all of these out. This will enable ilc to at compile a WASM module (or at least moves one blocker), though it won't link or run yet.

Enable the managed NativeAOT CoreLib build for single-threaded browser WASM and provide shape-compatible stubs for runtime functionality that is not implemented yet. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 6cdcf8df-1e43-462e-b0a7-b4cea297b576

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 6cdcf8df-1e43-462e-b0a7-b4cea297b576

dotnet-policy-service · 2026-09-18T18:33:26Z

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

azure-pipelines · 2026-09-18T18:33:50Z

Azure Pipelines: Successfully started running 3 pipeline(s). 13 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

dotnet-policy-service · 2026-09-18T18:34:05Z

Tagging subscribers to this area: @agocke, @dotnet/ilc-contrib
See info in area-owners.md if you want to be subscribed.

Copilot

🟡 Changes recommended

The WASM exception helpers have an ABI mismatch and are missing the required RhpThrowExact export.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Adds browser-WASM NativeAOT CoreLib stubs and build configuration to unblock compilation while exception handling, stack unwinding, GC, and dynamic thunks remain unsupported.

Changes:

Adds WASM-specific source selection and platform definitions.
Stubs thunk allocation, stack tracing, frozen-object memory, and exception handling.
Removes unsupported native WASM imports and exports placeholder EH helpers.

File summaries

File	Summary
`src/coreclr/nativeaot/System.Private.CoreLib/src/System/Runtime/ThunkPool.NotSupported.cs`	Stubs dynamic thunk operations.
`src/coreclr/nativeaot/System.Private.CoreLib/src/System/Diagnostics/StackTrace.NativeAot.Browser.cs`	Disables browser-WASM stack traces.
`src/coreclr/nativeaot/System.Private.CoreLib/src/System.Private.CoreLib.csproj`	Selects WASM-specific implementations.
`src/coreclr/nativeaot/System.Private.CoreLib/src/Internal/Runtime/FrozenObjectHeapManager.Wasm.cs`	Stubs virtual memory operations.
`src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/RuntimeExports.cs`	Omits native stack-trace exports on WASM.
`src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/InternalCalls.cs`	Omits unsupported EH and stack-walking imports.
`src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/ExceptionHandling.wasm.cs`	Adds placeholder exception-handling exports.
`src/coreclr/nativeaot/Directory.Build.props`	Adds WASM platform definitions.

Review details

Files reviewed: 8/8 changed files
Comments generated: 2
Review effort level: Lite

jkotas · 2026-09-18T18:51:55Z

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// NativeAOT browser-WASM does not currently provide executable thunk allocation.


And never will...

jkotas · 2026-09-18T18:55:21Z

+    <DefineConstants>TARGET_32BIT;$(DefineConstants)</DefineConstants>
+    <!-- WASI support is outside the scope of the NativeAOT managed platform stubs. -->
+    <DefineConstants Condition="'$(TargetsBrowser)' == 'true'">TARGET_WASM;$(DefineConstants)</DefineConstants>


Suggested change

<DefineConstants>TARGET_32BIT;$(DefineConstants)</DefineConstants>



<DefineConstants Condition="'$(TargetsBrowser)' == 'true'">TARGET_WASM;$(DefineConstants)</DefineConstants>

<DefineConstants>TARGET_32BIT;TARGET_WASM;$(DefineConstants)</DefineConstants>

Is there a problem with doing the right thing?

jkotas · 2026-09-18T19:00:03Z

    <FeaturePortableThreadPool>true</FeaturePortableThreadPool>
-  </PropertyGroup>
-  <PropertyGroup>
    <FeaturePortableTimer>true</FeaturePortableTimer>


We should mirror regular CoreCLR CoreLib setup where possible. For example, this one should mirror:

runtime/src/coreclr/clr.featuredefines.props

Lines 25 to 27 in f881b48



<FeaturePortableTimer Condition="'$(WasmEnableThreads)' != 'true'">false</FeaturePortableTimer>

<FeaturePortableThreadPool Condition="'$(WasmEnableThreads)' != 'true'">false</FeaturePortableThreadPool>

jkotas · 2026-09-18T19:01:09Z

    <DefineConstants>SYSTEM_PRIVATE_CORELIB;$(DefineConstants)</DefineConstants>
    <SkipCommonResourcesIncludes>true</SkipCommonResourcesIncludes>
    <FeatureNativeAot>true</FeatureNativeAot>
+    <RuntimeFlavor>CoreCLR</RuntimeFlavor>


What is this for?

I had been using I had been using dotnet build src/coreclr/nativeaot/System.Private.CoreLib/src/System.Private.CoreLib.csproj , and Subsets.props sets the RuntimeFlavor to Mono when targeting browser if a clr subset is not selected. I can fix up clr.nativeaotlibs and use that instead.

… twice into one

Copilot

🟡 Changes recommended

Critical WASI build inconsistencies remain, and the NativeAOT support change belongs in the Arcade-managed source.

Get a fresh assessment by requesting another Copilot review.

Review details

Files reviewed: 10/11 changed files
Comments generated: 4
Review effort level: Lite

jkotas · 2026-09-19T04:13:37Z

+  <ItemGroup Condition="$(_subset.Contains('+clr.nativeaotlibs+')) and '$(TargetsBrowser)' == 'true' and '$(TargetsWasm)' == 'true'">
+    <!-- Test.CoreLib fails to build with the stubbed out nativeaot corelib -->
+    <ProjectToBuild Remove="$(CoreClrProjectRoot)nativeaot\Test.CoreLib\src\Test.CoreLib.csproj" />


We should drop "Browser" from all changes in this PR. I think all changes would be correct for WASI too.

jkotas · 2026-09-19T04:10:26Z

    <_NativeAotSupportedOS Condition="
-      '$(TargetOS)' != 'browser' and
      '$(TargetOS)' != 'haiku' and


Valid feedback

+        // Browser-WASM omits the native EH enumerator and StackFrameIterator implementation.
+#if !TARGET_WASM


+  <ItemGroup Condition="'$(_TargetsBrowserWasm)' == 'true'">
+    <Compile Include="System\Diagnostics\StackTrace.NativeAot.Browser.cs" />
+  </ItemGroup>


jkotas · 2026-09-19T04:15:20Z

    </Compile>
  </ItemGroup>
-  <ItemGroup Condition="'$(InPlaceRuntime)' == 'true'">
+  <ItemGroup Condition="'$(InPlaceRuntime)' == 'true' and '$(TargetOS)' == 'browser' and '$(TargetArchitecture)' == 'wasm'">


Test.CoreLib is excluded for now - this should not be needed.

jkotas · 2026-09-19T04:17:05Z

+        }
+
+        [RuntimeExport("RhpThrowEx")]
+        [MethodImpl(MethodImplOptions.NoInlining)]


Why NoInlining?

jtschuster added 4 commits September 18, 2026 10:59

remove extra stuff

9cfab06

Tighten browser WASM CoreLib stubs

f87741c

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 6cdcf8df-1e43-462e-b0a7-b4cea297b576

Simplify browser WASM CoreLib configuration

b71f149

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 6cdcf8df-1e43-462e-b0a7-b4cea297b576

jtschuster added this to the 12.0.0 milestone Sep 18, 2026

jtschuster requested a review from agocke September 18, 2026 18:33

jtschuster self-assigned this Sep 18, 2026

Copilot AI lite review requested due to automatic review settings September 18, 2026 18:33

jtschuster added the arch-wasm WebAssembly architecture label Sep 18, 2026

jtschuster requested a review from MichalStrehovsky as a code owner September 18, 2026 18:33

jtschuster added the area-NativeAOT-coreclr label Sep 18, 2026

github-project-automation Bot added this to AppModel Sep 18, 2026

Copilot started reviewing on behalf of jtschuster September 18, 2026 18:33 View session

Copilot AI reviewed Sep 18, 2026

View reviewed changes

Comment thread src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/ExceptionHandling.wasm.cs Outdated

Comment thread src/coreclr/nativeaot/Runtime.Base/src/System/Runtime/ExceptionHandling.wasm.cs Outdated

jkotas reviewed Sep 18, 2026

View reviewed changes

jtschuster added 3 commits September 18, 2026 14:39

Enable clr.nativeaotlibs subset for browser-wasm

9d47d3b

Fix RhpThrow* helpers and remove unnecessary comment

da4f8bb

Match clr.featuredefines.props and coalesce setting the same property…

0159f79

… twice into one

Copilot AI review requested due to automatic review settings September 18, 2026 21:40

Copilot started reviewing on behalf of jtschuster September 18, 2026 21:40 View session

Copilot AI reviewed Sep 18, 2026

View reviewed changes

jkotas reviewed Sep 19, 2026

View reviewed changes

	<!-- single thread -->
	<FeaturePortableTimer Condition="'$(WasmEnableThreads)' != 'true'">false</FeaturePortableTimer>
	<FeaturePortableThreadPool Condition="'$(WasmEnableThreads)' != 'true'">false</FeaturePortableThreadPool>

		// Browser-WASM omits the native EH enumerator and StackFrameIterator implementation.
		#if !TARGET_WASM

Conversation

jtschuster commented Sep 18, 2026

Uh oh!

dotnet-policy-service Bot commented Sep 18, 2026

Uh oh!

azure-pipelines Bot commented Sep 18, 2026

Uh oh!

dotnet-policy-service Bot commented Sep 18, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

🟡 Changes recommended

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

🟡 Changes recommended

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants