[browser][MT] enable MT PLINQ (#94214)

* wip

* keep ActiveIssue #91579

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Author: pavelsavara

src/libraries/System.Linq.Parallel/src/System.Linq.Parallel.csproj

@@ -5,7 +5,12 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile>
   </PropertyGroup>
-
+  <PropertyGroup>
+    <TargetPlatformIdentifier>$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)'))</TargetPlatformIdentifier>
+    <FeatureWasmThreads Condition="'$(TargetPlatformIdentifier)' == 'browser' and '$(MonoWasmBuildVariant)' == 'multithread'">true</FeatureWasmThreads>
+    <DefineConstants Condition="'$(FeatureWasmThreads)' == 'true'" >$(DefineConstants);FEATURE_WASM_THREADS</DefineConstants>
+  </PropertyGroup>
+  <!-- Compiled Source Files -->
   <ItemGroup>
     <Compile Include="System\Linq\Parallel\Channels\AsynchronousChannel.cs" />
     <Compile Include="System\Linq\Parallel\Channels\SynchronousChannel.cs" />

src/libraries/System.Linq.Parallel/src/System/Linq/ParallelEnumerable.cs

@@ -64,8 +64,12 @@ public static class ParallelEnumerable
 
         // When running in single partition mode, PLINQ operations will occur on a single partition and will not
         // be executed in parallel, but will retain PLINQ semantics (exceptions wrapped as aggregates, etc).
+#if !FEATURE_WASM_THREADS
         [System.Runtime.Versioning.SupportedOSPlatformGuard("browser")]
         internal static bool SinglePartitionMode => OperatingSystem.IsBrowser();
+#else
+        internal static bool SinglePartitionMode => false;
+#endif
 
         //-----------------------------------------------------------------------------------
         // Converts any IEnumerable<TSource> into something that can be the target of parallel

src/libraries/System.Threading.Tasks.Parallel/src/System.Threading.Tasks.Parallel.csproj

@@ -5,7 +5,11 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile>
   </PropertyGroup>
-
+  <PropertyGroup>
+    <TargetPlatformIdentifier>$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)'))</TargetPlatformIdentifier>
+    <FeatureWasmThreads Condition="'$(TargetPlatformIdentifier)' == 'browser' and '$(MonoWasmBuildVariant)' == 'multithread'">true</FeatureWasmThreads>
+    <DefineConstants Condition="'$(FeatureWasmThreads)' == 'true'" >$(DefineConstants);FEATURE_WASM_THREADS</DefineConstants>
+  </PropertyGroup>
   <ItemGroup>
     <Compile Include="System\Threading\Tasks\Parallel.cs" />
     <Compile Include="System\Threading\Tasks\Parallel.ForEachAsync.cs" />

src/libraries/System.Threading.Tasks.Parallel/src/System/Threading/Tasks/Parallel.cs

@@ -238,8 +238,12 @@ public static void Invoke(ParallelOptions parallelOptions, params Action[] actio
             {
                 // If we've gotten this far, it's time to process the actions.
 
+#if !FEATURE_WASM_THREADS
                 // Web browsers need special treatment that is implemented in TaskReplicator
                 if (OperatingSystem.IsBrowser() ||
+#else
+                if (
+#endif
                     // This is more efficient for a large number of actions, or for enforcing MaxDegreeOfParallelism:
                     (actionsCopy.Length > SMALL_ACTIONCOUNT_LIMIT) ||
                     (parallelOptions.MaxDegreeOfParallelism != -1 && parallelOptions.MaxDegreeOfParallelism < actionsCopy.Length)

src/libraries/System.Threading.Tasks.Parallel/src/System/Threading/Tasks/TaskReplicator.cs

@@ -131,6 +131,7 @@ public static void Run<TState>(ReplicatableUserAction<TState> action, ParallelOp
         {
             // Browser hosts do not support synchronous Wait so we want to run the
             //  replicated task directly instead of going through Task infrastructure
+#if !FEATURE_WASM_THREADS
             if (OperatingSystem.IsBrowser())
             {
                 // Since we are running on a single thread, we don't want the action to time out
@@ -142,6 +143,7 @@ public static void Run<TState>(ReplicatableUserAction<TState> action, ParallelOp
                     throw new Exception("Replicated tasks cannot yield in this single-threaded browser environment");
             }
             else
+#endif
             {
                 int maxConcurrencyLevel = (options.EffectiveMaxConcurrencyLevel > 0) ? options.EffectiveMaxConcurrencyLevel : int.MaxValue;