Fix direct thread-static alignment fallback

src/coreclr/vm/threadstatics.cpp

@@ -751,8 +751,8 @@ void GetTLSIndexForThreadStatic(MethodTable* pMT, bool gcStatic, TLSIndex* pInde
                 {
                     g_directThreadLocalTLSBytesAvailable = newBytesAvailable - alignmentAdjust;
                     newTLSIndex = TLSIndex(TLSIndexType::DirectOnThreadLocalData, actualIndexOffset);
+                    usedDirectOnThreadLocalDataPath = true;
                 }
-                usedDirectOnThreadLocalDataPath = true;
             }
             if (usedDirectOnThreadLocalDataPath)
                 VolatileStore(&g_pMethodTablesForDirectThreadLocalData[IndexOffsetToDirectThreadLocalIndex(newTLSIndex.GetIndexOffset())], pMT);

src/tests/baseservices/threading/threadstatic/threadstatic09.cs

@@ -0,0 +1,72 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading;
+
+public static class ThreadStaticAlignmentFallback
+{
+    private const int Pass = 100;
+    private const int Fail = -1;
+    private const int PaddingCount = 12;
+
+    public static int Main()
+    {
+        int paddingCount = new Random().Next(PaddingCount);
+        Console.WriteLine($"Using {paddingCount} padding fields.");
+        return InitializePadding<object>(paddingCount) && LongStorage.Check() ? Pass : Fail;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static bool InitializePadding<T>(int count)
+    {
+        if (count == 0)
+        {
+            return true;
+        }
+
+        if (!Padding<T>.Check())
+        {
+            return false;
+        }
+
+        return InitializePadding<Padding<T>>(count - 1);
+    }
+
+    private sealed class Padding<T>
+    {
+        [ThreadStatic]
+        private static int s_value;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public static bool Check()
+        {
+            if (s_value != 0)
+            {
+                return false;
+            }
+
+            s_value = 42;
+            return s_value == 42;
+        }
+    }
+
+    private static class LongStorage
+    {
+        [ThreadStatic]
+        private static long s_value;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public static bool Check()
+        {
+            if (Interlocked.Increment(ref s_value) != 1)
+            {
+                return false;
+            }
+
+            s_value = 0x1122334455667788;
+            return s_value == 0x1122334455667788;
+        }
+    }
+}

src/tests/baseservices/threading/threadstatic/threadstatic09.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+    <BuildAsStandalone>true</BuildAsStandalone>
+    <ReferenceXUnitWrapperGenerator>false</ReferenceXUnitWrapperGenerator>
+    <NativeAotIncompatible>true</NativeAotIncompatible>
+    <CLRTestTargetUnsupported Condition="'$(RuntimeFlavor)' != 'coreclr'">true</CLRTestTargetUnsupported>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="threadstatic09.cs" />
+  </ItemGroup>
+</Project>