JIT: small OSR locals must be normalize on load

src/coreclr/jit/compiler.h

@@ -1099,14 +1099,14 @@ class LclVarDsc
     {
         return varTypeIsSmall(TypeGet()) &&
                // lvIsStructField is treated the same as the aliased local, see fgDoNormalizeOnStore.
-               (lvIsParam || m_addrExposed || lvIsStructField);
+               (lvIsParam || m_addrExposed || lvIsStructField || lvIsOSRLocal);
     }
 
     bool lvNormalizeOnStore() const
     {
         return varTypeIsSmall(TypeGet()) &&
                // lvIsStructField is treated the same as the aliased local, see fgDoNormalizeOnStore.
-               !(lvIsParam || m_addrExposed || lvIsStructField);
+               !(lvIsParam || m_addrExposed || lvIsStructField || lvIsOSRLocal);
     }
 
     void incRefCnts(weight_t weight, Compiler* pComp, RefCountState state = RCS_NORMAL, bool propagate = true);

src/tests/JIT/opt/OSR/normalizeonload.cs

@@ -0,0 +1,52 @@
+// 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;
+
+// Ensure small OSR locals are marked as normalize on load
+
+class Runtime_83959
+{
+    static bool B(out byte b)
+    {
+        b = 1;
+        return true;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void F() {}
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    [SkipLocalsInit]
+    static void WithOSR(int n, out char c)
+    {
+        c = (char) 0;
+        B(out byte b);
+        for (int i = 0; i < n; i++)
+        {
+            F();
+        }
+        // This load of `b` must be a single byte
+        c = (char) b;
+        c += (char) 99;
+    }
+
+    // Ensure stack is filled with nonzero data
+    static void FillStack(int n)
+    {
+        Span<int> s = stackalloc int[n];
+        for (int i = 0; i < n; i++)
+        {
+            s[i] = -1;
+        }
+    }
+
+    public static int Main()
+    {
+        char c = (char) 0;
+        FillStack(100);
+        WithOSR(50000, out c);
+        return (int) c;
+    }
+}

src/tests/JIT/opt/OSR/normalizeonload.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <DebugType />
+    <Optimize>True</Optimize>
+    <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_QuickJitForLoops" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_OnStackReplacement" Value="1" />
+  </ItemGroup>
+</Project>