JIT: don't dead store untracked OSR locals based on ref count

src/coreclr/jit/lclvars.cpp

@@ -339,6 +339,12 @@ void Compiler::lvaInitTypeRef()
             {
                 JITDUMP("-- V%02u is OSR exposed\n", lclNum);
                 varDsc->lvIsOSRExposedLocal = true;
+
+                // Ensure that ref counts for exposed OSR locals take into account
+                // that some of the refs might be in the Tier0 parts of the method
+                // that get trimmed away.
+                //
+                varDsc->lvImplicitlyReferenced = 1;
             }
         }
     }

src/tests/JIT/opt/OSR/Runtime_89666.cs

@@ -0,0 +1,50 @@
+// 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.Runtime.InteropServices;
+using Xunit;
+
+public class Runtime_89666
+{
+    [Fact]
+    public static int Test()
+    {
+        byte[] a1 = new byte[100_000];
+        byte[] a2 = new byte[100_000];
+
+        Problem(a1.Length, a1);
+        Problem(a2.Length, a2);
+
+        for (int i = 0; i < a1.Length; i++)
+        {
+            if (a1[i] != a2[i])
+            {
+                Console.WriteLine($"Found diff at {i}: {a1[i]} != {a2[i]}");
+                return -1;
+            }
+        }
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Problem(int n, byte[] a)
+    {
+        Random random = new Random(1);
+        int value = 0;
+        Span<byte> span = MemoryMarshal.AsBytes(MemoryMarshal.CreateSpan(ref value, 1));
+        for (int i = 0; i < n; i++)
+        {
+            // This write must be kept in the OSR method
+            value = random.Next();
+            Write(span, i, a);
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Write(Span<byte> s, int i, byte[] a)
+    {
+        a[i] = s[0];
+    }
+}

src/tests/JIT/opt/OSR/Runtime_89666.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <DebugType />
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredPGO" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_QuickJitForLoops" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_OnStackReplacement" Value="1" />
+  </ItemGroup>
+</Project>