From 39820ca657f2a6dd758973a72133a3556599c3d9 Mon Sep 17 00:00:00 2001
From: Brian Sullivan <briansul@microsoft.com>
Date: Fri, 20 Sep 2019 12:05:01 -0700
Subject: [PATCH 1/3] New fix - only disable GT_CLS_VAR's

---
 src/jit/optimizer.cpp | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/jit/optimizer.cpp b/src/jit/optimizer.cpp
index b63c454d6cea..7072bbfdaef0 100644
--- a/src/jit/optimizer.cpp
+++ b/src/jit/optimizer.cpp
@@ -6795,8 +6795,21 @@ void Compiler::optHoistLoopBlocks(unsigned loopNum, ArrayStack<BasicBlock*>* blo
 
         bool IsTreeVNInvariant(GenTree* tree)
         {
-            return m_compiler->optVNIsLoopInvariant(tree->gtVNPair.GetLiberal(), m_loopNum,
-                                                    &m_hoistContext->m_curLoopVnInvariantCache);
+            ValueNum vn = tree->gtVNPair.GetLiberal();
+
+            if (m_compiler->vnStore->IsVNConstant(vn))
+            {
+                // It is unsafe to allow a GT_CLS_VAR that has been assigned a constant.
+                // The logic in optVNIsLoopInvariant woudl consider it to be loop-invariant, even
+                // if the assignment of the constant to the GT_CLS_VAR was inside the loop.
+                //
+                if (tree->OperIs(GT_CLS_VAR))
+                {
+                    return false;
+                }
+            }
+
+            return m_compiler->optVNIsLoopInvariant(vn, m_loopNum, &m_hoistContext->m_curLoopVnInvariantCache);
         }
 
     public:

From e511f560fe73734c1aed2798546e05ac9d66ec44 Mon Sep 17 00:00:00 2001
From: Brian Sullivan <briansul@microsoft.com>
Date: Fri, 20 Sep 2019 14:05:30 -0700
Subject: [PATCH 2/3] Fix typo

---
 src/jit/optimizer.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/jit/optimizer.cpp b/src/jit/optimizer.cpp
index 7072bbfdaef0..f1b75a29f5ac 100644
--- a/src/jit/optimizer.cpp
+++ b/src/jit/optimizer.cpp
@@ -6800,7 +6800,7 @@ void Compiler::optHoistLoopBlocks(unsigned loopNum, ArrayStack<BasicBlock*>* blo
             if (m_compiler->vnStore->IsVNConstant(vn))
             {
                 // It is unsafe to allow a GT_CLS_VAR that has been assigned a constant.
-                // The logic in optVNIsLoopInvariant woudl consider it to be loop-invariant, even
+                // The logic in optVNIsLoopInvariant would consider it to be loop-invariant, even
                 // if the assignment of the constant to the GT_CLS_VAR was inside the loop.
                 //
                 if (tree->OperIs(GT_CLS_VAR))

From 37fed7f7be4ccb049f87984c7388e9a0ee420baf Mon Sep 17 00:00:00 2001
From: Brian Sullivan <briansul@microsoft.com>
Date: Sat, 21 Sep 2019 13:57:25 -0700
Subject: [PATCH 3/3] Added back test:  JIT\Regression\JitBlue\GitHub_26417

---
 .../JitBlue/GitHub_26417/GitHub_26417.cs      | 47 +++++++++++++++++++
 .../JitBlue/GitHub_26417/GitHub_26417.csproj  | 12 +++++
 2 files changed, 59 insertions(+)
 create mode 100644 tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.cs
 create mode 100644 tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.csproj

diff --git a/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.cs b/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.cs
new file mode 100644
index 000000000000..6871962398e4
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.cs
@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Runtime.CompilerServices;
+
+class GitHub_26417
+{
+    static int   _a;
+
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    static void MyWriteLine(int v)
+    {
+        Console.WriteLine(v);
+        if (v == 0)
+        {
+            throw new Exception();
+        }
+    }
+
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    static void Test()
+    {
+        _a = 1;
+        
+        while (_a == 1)
+        {
+            MyWriteLine(_a);
+            _a = 0;
+        }
+    }
+
+    static int Main()
+    {
+        int result = 100;
+        try {
+            Test();
+        }
+        catch (Exception)
+        {
+            Console.WriteLine("FAILED");
+            result = -1;
+        }
+        return result;
+    }
+}
diff --git a/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.csproj b/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.csproj
new file mode 100644
index 000000000000..f3e1cbd44b40
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/GitHub_26417/GitHub_26417.csproj
@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>