From b41e0e4210a8c02af76c9cb4660c9103f0218dfd Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Thu, 28 May 2026 06:30:01 -0700
Subject: [PATCH] JIT: Fix bug in GenTree::Equals (#128654)

Make sure we properly distinguish virtual stub calls to avoid incorrect
head/tail merges.

Fixes #128631.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
(cherry picked from commit dfd91b8a920820495f07b3529f350b46d9c2caef)
---
 src/coreclr/jit/gentree.cpp                   |  6 +++
 .../JitBlue/Runtime_128631/Runtime_128631.cs  | 43 +++++++++++++++++++
 .../Runtime_128631/Runtime_128631.csproj      |  9 ++++
 3 files changed, 58 insertions(+)
 create mode 100644 src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.cs
 create mode 100644 src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.csproj

diff --git a/src/coreclr/jit/gentree.cpp b/src/coreclr/jit/gentree.cpp
index 2ae5c17f1132f7..7f6a1a3b9ce2d4 100644
--- a/src/coreclr/jit/gentree.cpp
+++ b/src/coreclr/jit/gentree.cpp
@@ -2485,6 +2485,12 @@ bool GenTreeCall::Equals(GenTreeCall* c1, GenTreeCall* c2)
         {
             return false;
         }
+
+        // For virtual stub calls the stub addresses must agree.
+        if (c1->IsVirtualStub() && (c1->gtStubCallStubAddr != c2->gtStubCallStubAddr))
+        {
+            return false;
+        }
     }
     else
     {
diff --git a/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.cs b/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.cs
new file mode 100644
index 00000000000000..736bcb56988ea1
--- /dev/null
+++ b/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.cs
@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Two shared-generic interface call sites that canonicalize to __Canon must
+// not be merged by head/tail merge: each call has a distinct virtual stub
+// dispatch cell, so the calls are not interchangeable.
+
+namespace Runtime_128631;
+
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public interface IFoo<T> { object Get(); }
+
+public class Impl : IFoo<string>, IFoo<byte[]>
+{
+    [MethodImpl(MethodImplOptions.NoInlining)] object IFoo<string>.Get() => "STRING_VALUE";
+    [MethodImpl(MethodImplOptions.NoInlining)] object IFoo<byte[]>.Get() => new byte[] { 1, 2, 3 };
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public object Dispatch(string tag)
+    {
+        if (tag == "bytes")  return ((IFoo<byte[]>)this).Get();
+        if (tag == "string") return ((IFoo<string>)this).Get();
+        return null;
+    }
+}
+
+public class Runtime_128631
+{
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        var d = new Impl();
+        for (int i = 0; i < 200; i++)
+        {
+            d.Dispatch("bytes");
+            d.Dispatch("string");
+        }
+        object r = d.Dispatch("string");
+        return r is string ? 100 : -1;
+    }
+}
diff --git a/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.csproj b/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.csproj
new file mode 100644
index 00000000000000..501217e4d86892
--- /dev/null
+++ b/src/tests/JIT/Regression/JitBlue/Runtime_128631/Runtime_128631.csproj
@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>