dotnet · jkotas · Jul 22, 2025 · Jul 5, 2025 · Jul 7, 2025 · Jul 8, 2025
diff --git a/docs/workflow/building/coreclr/nativeaot.md b/docs/workflow/building/coreclr/nativeaot.md
@@ -76,6 +76,19 @@ You should now be able to publish the project for Wasm: `dotnet publish -r brows
 
 Once you build the repo, you can use the produced binaries in one of four ways specified below ("Using built binaries", "Building packages", "Convenience Visual Studio "repro" project", "Running tests").
 
+## Building for Multithreaded packages
+
+This is a work in progress and far from functional.  Currently there exists just enough infrastructure to build packages for multithreaded runtime and libs, but they are not functional in the sense that they support multithreaded programs yet.
+To build the browser multithreaded packages:
+```
+build clr.aot+libs+nativeaot.packages -c Debug -a wasm -os browser '/p:WasmEnableThreads=true'
+```
+To build the runtime tests for browser
+```
+src\tests\build nativeaot Debug wasm tree nativeaot browser /p:LibrariesConfiguration=debug /p:WasmEnableThreads=true
+```
+
+
 ### Using built binaries
 
 In this workflow, you have a project file that you want to `dotnet publish`, but you want to use your own build of the compiler/runtime/framework. You need to be using a daily build of the .NET SDK downloaded from the dotnet/sdk repo. It's typically enough to just download the daily build ZIP file, unpack it, and make sure the unpacked directory is the first thing in your PATH. Don't forget to add a NuGet.config as specified by the dotnet/sdk repo- you'll hit restore issues otherwise.

diff --git a/eng/native/configurecompiler.cmake b/eng/native/configurecompiler.cmake
@@ -148,6 +148,10 @@ elseif (CLR_CMAKE_HOST_UNIX)
     add_compile_options(-Wno-alloca)
     add_compile_options(-Wno-implicit-int-float-conversion)
   endif()
+
+  if (CMAKE_USE_PTHREADS AND CLR_CMAKE_HOST_BROWSER)
+    add_compile_options(-pthread)
+  endif(CMAKE_USE_PTHREADS AND CLR_CMAKE_HOST_BROWSER)
 endif(MSVC)
 
 if (CLR_CMAKE_ENABLE_SANITIZERS)

diff --git a/eng/pipelines/runtimelab.yml b/eng/pipelines/runtimelab.yml
@@ -71,6 +71,26 @@ extends:
                   parameters:
                     librariesConfiguration: Debug
 
+        #
+        # Build and test Wasm Debug multithreaded libraries and Debug runtime
+        #
+        - template: /eng/pipelines/common/platform-matrix.yml
+          parameters:
+            jobTemplate: /eng/pipelines/common/global-build-job.yml
+            helixQueuesTemplate: /eng/pipelines/coreclr/templates/helix-queues-setup.yml
+            buildConfig: debug
+            platforms:
+            - browser_wasm_win
+            jobParameters:
+              timeoutInMinutes: 300
+              buildArgs: -s clr.aot+libs -c debug -rc $(_BuildConfig) '/p:WasmEnableThreads=true'
+              nameSuffix: Multithreaded
+              postBuildSteps:
+                - template: /eng/pipelines/runtimelab/runtimelab-post-build-steps.yml
+                  parameters:
+                    librariesConfiguration: Debug
+                    wasmEnableThreadsArg: /p:WasmEnableThreads=true
+
         #
         # Build and test with Debug libraries and Checked runtime
         #

diff --git a/eng/pipelines/runtimelab/runtimelab-post-build-steps.yml b/eng/pipelines/runtimelab/runtimelab-post-build-steps.yml
@@ -8,6 +8,7 @@ parameters:
   runtimeVariant: ''
   isOfficialBuild: false
   librariesConfiguration: Debug
+  wasmEnableThreadsArg: ''
 
 steps:
   # For NativeAOT-LLVM, we have just built the Wasm-targeting native artifacts (the runtime and libraries).
@@ -17,7 +18,7 @@ steps:
       displayName: Build the ILC and RyuJit cross-compilers
 
     # Build target packages (note: target libs already built).
-    - script: $(Build.SourcesDirectory)/build$(scriptExt) nativeaot.packages -os ${{ parameters.osGroup }} -a wasm -c $(buildConfigUpper) $(_officialBuildParameter) -ci
+    - script: $(Build.SourcesDirectory)/build$(scriptExt) nativeaot.packages -os ${{ parameters.osGroup }} -a wasm -c $(buildConfigUpper) $(_officialBuildParameter) -ci $(wasmEnableThreadsArg)
       displayName: Build target packages
 
     # Build host packages.
@@ -26,12 +27,12 @@ steps:
 
   # Build coreclr native test output outside of official build
   - ${{ if ne(parameters.isOfficialBuild, true) }}:
-    - ${{ if and(eq(parameters.archType, 'wasm'), ne(parameters.nameSuffix, '')) }}:
+    - ${{ if and(eq(parameters.archType, 'wasm'), ne(parameters.nameSuffix, ''), eq(parameters.wasmEnableThreadsArg, '')) }}:
       - script: pwsh $(Build.SourcesDirectory)/eng/pipelines/runtimelab/set-ilc-emulation-environment.ps1 -Arch $(hostedTargetArch)
         displayName: Set up ILC emulation environment
 
     - ${{ if eq(parameters.archType, 'wasm') }}:
-      - script: $(Build.SourcesDirectory)/src/tests/build$(scriptExt) nativeaot $(buildConfigUpper) ${{ parameters.osGroup }} $(crossArg) $(_officialBuildParameter) ci tree nativeaot /p:LibrariesConfiguration=${{ parameters.librariesConfiguration }}
+      - script: $(Build.SourcesDirectory)/src/tests/build$(scriptExt) nativeaot $(buildConfigUpper) ${{ parameters.osGroup }} $(crossArg) $(_officialBuildParameter) ci tree nativeaot /p:LibrariesConfiguration=${{ parameters.librariesConfiguration }} ${{ parameters.wasmEnableThreadsArg }}
         displayName: Build runtime tests
     - ${{ else }}:
       - ${{ if eq(parameters.osGroup, 'windows') }}:
@@ -42,14 +43,15 @@ steps:
           displayName: Build runtime tests
 
     - ${{ if contains(parameters.platform, 'win') }}:
-      - script: $(Build.SourcesDirectory)/src/tests/run$(scriptExt) runnativeaottests $(buildConfigUpper) ${{ parameters.archType }} ${{ parameters.osGroup }}
-        displayName: Run runtime tests
+      - ${{ if eq(parameters.wasmEnableThreadsArg, '') }}:
+        - script: $(Build.SourcesDirectory)/src/tests/run$(scriptExt) runnativeaottests $(buildConfigUpper) ${{ parameters.archType }} ${{ parameters.osGroup }}
+          displayName: Run runtime tests
     - ${{ else }}:
       - script: $(Build.SourcesDirectory)/src/tests/run$(scriptExt) --runnativeaottests $(buildConfigUpper) ${{ parameters.archType }} ${{ parameters.osGroup }}
         displayName: Run runtime tests
 
     # Don't compile/run the libraries tests with emulated ILC to save CI time/resources.
-    - ${{ if and(eq(parameters.archType, 'wasm'), eq(parameters.nameSuffix, '')) }}:
+    - ${{ if and(eq(parameters.archType, 'wasm'), eq(parameters.nameSuffix, ''), eq(parameters.wasmEnableThreadsArg, '')) }}:
       - script: $(Build.SourcesDirectory)/build$(scriptExt) libs.tests -test -a ${{ parameters.archType }} -os ${{ parameters.osGroup }} -lc ${{ parameters.librariesConfiguration }} -rc $(buildConfigUpper) /p:TestNativeAot=true /p:RunSmokeTestsOnly=true
         displayName: Build and run WebAssembly libraries tests
 

diff --git a/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets b/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets
@@ -438,6 +438,7 @@ The .NET Foundation licenses this file to you under the MIT license.
       <CompileWasmArgs Condition="'$(NativeDebugSymbols)' == 'true'">$(CompileWasmArgs) -g3</CompileWasmArgs>
       <CompileWasmArgs Condition="'$(WasmEnableNonTrappingFloatToIntConversions)' == 'true'">$(CompileWasmArgs) -mnontrapping-fptoint</CompileWasmArgs>
       <CompileWasmArgs Condition="'$(IlcLlvmExceptionHandlingModel)' == 'wasm'">$(CompileWasmArgs) -fwasm-exceptions</CompileWasmArgs>
+      <CompileWasmArgs Condition="'$(WasmEnableThreads)' == 'true'">$(CompileWasmArgs) -pthread</CompileWasmArgs>
     </PropertyGroup>
 
     <PropertyGroup Condition="'$(_targetOS)' == 'browser'">
@@ -604,6 +605,8 @@ The .NET Foundation licenses this file to you under the MIT license.
       <CustomLinkerArg Include="-s MAXIMUM_MEMORY=$(EmccMaximumHeapSize)" Condition="'$(EmccMaximumHeapSize)' != ''" />
       <CustomLinkerArg Include="-s INITIAL_MEMORY=$(EmccInitialHeapSize)" Condition="'$(EmccInitialHeapSize)' != ''" />
       <CustomLinkerArg Include="-s ASSERTIONS=1" Condition="'$(EmccEnableAssertions)' == 'true'" />
+      <CustomLinkerArg Include="-pthread" Condition="'$(WasmEnableThreads)' == 'true'"/>
+      <CustomLinkerArg Include="-Wno-pthreads-mem-growth" Condition="'$(WasmEnableThreads)' == 'true'"/>
 
       <!-- See https://github.com/dotnet/runtimelab/issues/2357 for why we don't run wasm-opt (it would mangle our strack trace info). -->
       <CustomLinkerArg Condition="'$(Optimize)' == 'true'" Include="-O1" />

diff --git a/src/coreclr/nativeaot/CMakeLists.txt b/src/coreclr/nativeaot/CMakeLists.txt
@@ -32,6 +32,12 @@ if(CLR_CMAKE_TARGET_OS STREQUAL wasi)
   add_definitions(-DTARGET_UNIX)
 endif(CLR_CMAKE_TARGET_OS STREQUAL wasi)
 
+if((CLR_CMAKE_TARGET_OS STREQUAL wasi OR CLR_CMAKE_TARGET_OS STREQUAL emscripten) 
+  AND CMAKE_USE_PTHREADS)
+  add_definitions(-DFEATURE_WASM_MANAGED_THREADS)
+endif((CLR_CMAKE_TARGET_OS STREQUAL wasi OR CLR_CMAKE_TARGET_OS STREQUAL emscripten) 
+  AND CMAKE_USE_PTHREADS)
+
 if(CLR_CMAKE_TARGET_ANDROID)
     add_definitions(-DFEATURE_EMULATED_TLS)
 endif()

diff --git a/src/coreclr/nativeaot/Runtime/eventpipe/CMakeLists.txt b/src/coreclr/nativeaot/Runtime/eventpipe/CMakeLists.txt
@@ -2,6 +2,11 @@ project(Runtime)
 
 include(FindPython)
 
+# Workaround from https://github.com/dotnet/runtime/pull/117513
+if (NOT "$ENV{EMSDK_PYTHON}" STREQUAL "")
+  set(Python_EXECUTABLE "$ENV{EMSDK_PYTHON}")
+endif()
+
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 set(EVENT_MANIFEST ${VM_DIR}/ClrEtwAll.man)

diff --git a/src/coreclr/nativeaot/System.Private.CoreLib/src/System.Private.CoreLib.csproj b/src/coreclr/nativeaot/System.Private.CoreLib/src/System.Private.CoreLib.csproj
@@ -35,6 +35,9 @@
   <PropertyGroup Condition="'$(TargetsSingleThreadedWasm)' != 'true'">
     <FeaturePortableThreadPool>true</FeaturePortableThreadPool>
     <FeaturePortableTimer>true</FeaturePortableTimer>
+    <!-- LLVM-TODO: Remove FeatureWasmManagedThreads in favor of WasmEnableThreads. -->
+    <FeatureWasmManagedThreads Condition="'$(WasmEnableThreads)' == 'true'">true</FeatureWasmManagedThreads>
+    <DefineConstants Condition="'$(WasmEnableThreads)' == 'true'">$(DefineConstants);FEATURE_WASM_MANAGED_THREADS</DefineConstants>
   </PropertyGroup>
   <PropertyGroup>
     <FeatureHardwareIntrinsics>true</FeatureHardwareIntrinsics>
@@ -347,6 +350,8 @@
     <Compile Include="System\Diagnostics\StackFrame.NativeAot.Wasm.cs" />
     <Compile Include="System\Diagnostics\StackFrame.NativeAot.Wasi.cs" Condition="'$(TargetsWasi)' == 'true'" />
     <Compile Include="System\Diagnostics\StackFrame.NativeAot.Browser.cs" Condition="'$(TargetsBrowser)' == 'true'" />
+    <Compile Include="$(MonoBclSourcesRoot)\System\Threading\PortableThreadPool.Browser.Threads.Mono.cs" Condition="'$(TargetsBrowser)' == 'true' and '$(WasmEnableThreads)' == 'true'" />
+    <Compile Include="$(MonoBclSourcesRoot)\System\Threading\ThreadPool.Browser.Threads.Mono.cs" Condition="'$(TargetsBrowser)' == 'true' and '$(WasmEnableThreads)' == 'true'" />
   </ItemGroup>
 
   <!-- WASM-specific things. Keep in sync with Mono. -->

diff --git a/src/coreclr/nativeaot/System.Private.CoreLib/src/System/Threading/Monitor.NativeAot.cs b/src/coreclr/nativeaot/System.Private.CoreLib/src/System/Threading/Monitor.NativeAot.cs
@@ -143,7 +143,9 @@ public static bool IsEntered(object obj)
 
         #region Public Wait/Pulse methods
 
+#if !FEATURE_WASM_MANAGED_THREADS
         [UnsupportedOSPlatform("browser")]
+#endif
         public static bool Wait(object obj, int millisecondsTimeout)
         {
             return GetCondition(obj).Wait(millisecondsTimeout, obj);

diff --git a/src/coreclr/runtime.proj b/src/coreclr/runtime.proj
@@ -94,6 +94,10 @@
       <_CoreClrBuildArg Include="-cmakeargs &quot;-DCLR_CMAKE_ESRP_CLIENT=$(DotNetEsrpToolPath)&quot;" />
     </ItemGroup>
 
+    <ItemGroup Condition="'$(WasmEnableThreads)' == 'true'">
+      <_CoreClrBuildArg Include="-cmakeargs &quot;-DCMAKE_USE_PTHREADS=1&quot;" />
+    </ItemGroup>
+
     <PropertyGroup>
       <_CoreClrBuildScript Condition="$([MSBuild]::IsOsPlatform(Windows))">build-runtime.cmd</_CoreClrBuildScript>
       <_CoreClrBuildScript Condition="!$([MSBuild]::IsOsPlatform(Windows))">build-runtime.sh</_CoreClrBuildScript>

diff --git a/src/installer/pkg/projects/nativeaot-packages.proj b/src/installer/pkg/projects/nativeaot-packages.proj
@@ -4,7 +4,8 @@
   <ItemGroup>
     <Project Include="Microsoft.DotNet.ILCompiler.LLVM\Microsoft.DotNet.ILCompiler.LLVM.pkgproj" />
     <ProjectReference Include="@(Project)" />
-    <ProjectReference Include="@(Project)" AdditionalProperties="PackageTargetRuntime=$(OutputRID)" />
+    <ProjectReference Include="@(Project)" Condition="'$(WasmEnableThreads)' != 'true'" AdditionalProperties="PackageTargetRuntime=$(OutputRID)" />
+    <ProjectReference Include="@(Project)" Condition="'$(WasmEnableThreads)' == 'true'" AdditionalProperties="PackageTargetRuntime=multithread.$(OutputRID)" />
     <ProjectReference Condition="'$(BuildHostTools)' == 'true'" Include="@(Project)" AdditionalProperties="PackageTargetRuntime=$(NETCoreSdkRuntimeIdentifier);PackHostILCompiler=true" />
   </ItemGroup>
 

diff --git a/src/libraries/Common/src/Interop/Browser/Interop.Runtime.NativeAOT.cs b/src/libraries/Common/src/Interop/Browser/Interop.Runtime.NativeAOT.cs
@@ -28,6 +28,30 @@ internal static unsafe partial class Runtime
         [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_cancel_promise", StringMarshalling = StringMarshalling.Utf16)]
         public static unsafe partial void CancelPromise(IntPtr gcHandle);
 
+#if FEATURE_WASM_MANAGED_THREADS
+        // Required by JavaScript/JSFunctionBinding.cs
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_invoke_js_function_send")]
+        public static unsafe partial void InvokeJSFunctionSend(nint targetNativeTID, nint functionHandle, nint data);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_invoke_jsimport_MT")]
+        public static unsafe partial void InvokeJSImportSync(nint signature, nint args);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_invoke_jsimport_sync_send")]
+        public static unsafe partial void InvokeJSImportSyncSend(nint targetNativeTID, nint signature, nint args);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_invoke_jsimport_async_post")]
+        public static unsafe partial void InvokeJSImportAsyncPost(nint targetNativeTID, nint signature, nint args);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_resolve_or_reject_promise_post")]
+        public static unsafe partial void ResolveOrRejectPromisePost(nint targetNativeTID, nint data);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_install_js_worker_interop_wrapper")]
+        public static unsafe partial void InstallWebWorkerInterop(nint proxyContextGCHandle, void* beforeSyncJSImport, void* afterSyncJSImport, void* pumpHandler);
+        // Required by JavaScript/JSProxyContext.cs
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_release_cs_owned_object_post")]
+        internal static unsafe partial void ReleaseCSOwnedObjectPost(nint targetNativeTID, nint jsHandle);
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_uninstall_js_worker_interop")]
+        public static unsafe partial void UninstallWebWorkerInterop();
+        // Required by JavaScript/CancelablePromise.cs
+        [LibraryImport(JSLibrary, EntryPoint = "mono_wasm_cancel_promise_post")]
+        public static unsafe partial void CancelPromisePost(nint targetNativeTID, nint taskHolderGCHandle);
+#endif
+
         #region Not used by NativeAOT
         public static IntPtr RegisterGCRoot(void* start, int bytesSize, IntPtr name) => throw new NotImplementedException();
         public static void DeregisterGCRoot(IntPtr handle) => throw new NotImplementedException();

diff --git a/src/mono/browser/runtime/loader/polyfills.ts b/src/mono/browser/runtime/loader/polyfills.ts
@@ -22,7 +22,8 @@ export function verifyEnvironment () {
     mono_assert(ENVIRONMENT_IS_SHELL || typeof globalThis.URL === "function", "This browser/engine doesn't support URL API. Please use a modern version. See also https://aka.ms/dotnet-wasm-features");
     mono_assert(typeof globalThis.BigInt64Array === "function", "This browser/engine doesn't support BigInt64Array API. Please use a modern version. See also https://aka.ms/dotnet-wasm-features");
     if (WasmEnableThreads) {
-        mono_assert(!ENVIRONMENT_IS_SHELL && !ENVIRONMENT_IS_NODE, "This build of dotnet is multi-threaded, it doesn't support shell environments like V8 or NodeJS. See also https://aka.ms/dotnet-wasm-features");
+        // TODO-LLVM: Comment this check out for now so we can run at least some tests.
+        //mono_assert(!ENVIRONMENT_IS_SHELL && !ENVIRONMENT_IS_NODE, "This build of dotnet is multi-threaded, it doesn't support shell environments like V8 or NodeJS. See also https://aka.ms/dotnet-wasm-features");
         mono_assert(globalThis.SharedArrayBuffer !== undefined, "SharedArrayBuffer is not enabled on this page. Please use a modern browser and set Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy http headers. See also https://aka.ms/dotnet-wasm-features");
         mono_assert(typeof globalThis.EventTarget === "function", "This browser/engine doesn't support EventTarget API. Please use a modern version. See also https://aka.ms/dotnet-wasm-features");
     }

diff --git a/src/tests/Common/dirs.proj b/src/tests/Common/dirs.proj
@@ -31,6 +31,11 @@
       <DisabledProjects Include="$(TestRoot)readytorun\**\*.csproj" Condition="'$(TestBuildMode)' == 'nativeaot'" />
     </ItemGroup>
 
+      <!-- TODO-LLVM: Reinstate when more thread support is implemented. -->
+    <ItemGroup Condition="'$(WasmEnableThreads)' == 'true'">
+      <DisabledProjects Include="$(TestRoot)nativeaot\**\PackagingTests.csproj" />
+    </ItemGroup>
+
     <ItemGroup>
       <!-- These projects are built separately in an isolated way. -->
       <DisabledProjects Include="$(TestRoot)nativeaot\SmokeTests\HelloWasm\PackagingTests\*.csproj" />

diff --git a/src/tests/build.proj b/src/tests/build.proj
@@ -490,6 +490,7 @@
       <GroupBuildCmd Condition="'$(MonoForceInterpreter)' == 'true'">$(GroupBuildCmd) "/p:MonoForceInterpreter=true"</GroupBuildCmd>
       <GroupBuildCmd Condition="'$(RunAOTCompilation)' == 'true'">$(GroupBuildCmd) "/p:RunAOTCompilation=true"</GroupBuildCmd>
       <GroupBuildCmd Condition="'$(DevTeamProvisioning)' != ''">$(GroupBuildCmd) "/p:DevTeamProvisioning=$(DevTeamProvisioning)"</GroupBuildCmd>
+      <GroupBuildCmd Condition="'$(WasmEnableThreads)' != ''">$(GroupBuildCmd) "/p:WasmEnableThreads=$(WasmEnableThreads)"</GroupBuildCmd>
     </PropertyGroup>
 
     <Message Importance="High" Text="$(MsgPrefix)Building managed test group $(__TestGroupToBuild): $(GroupBuildCmd)" />