Port fix for dotnet#1241 to 3.1

Author: CarolEidt

src/jit/morph.cpp

@@ -10186,6 +10186,7 @@ GenTree* Compiler::fgMorphCopyBlock(GenTree* tree)
         GenTreeLclVarCommon* lclVarTree        = nullptr;
         GenTreeLclVarCommon* srcLclVarTree     = nullptr;
         unsigned             destLclNum        = BAD_VAR_NUM;
+        unsigned             modifiedLclNum    = BAD_VAR_NUM;
         LclVarDsc*           destLclVar        = nullptr;
         FieldSeqNode*        destFldSeq        = nullptr;
         bool                 destDoFldAsg      = false;
@@ -10201,10 +10202,11 @@ GenTree* Compiler::fgMorphCopyBlock(GenTree* tree)
         {
             blockWidthIsConst = true;
             destOnStack       = true;
+            modifiedLclNum    = dest->AsLclVarCommon()->GetLclNum();
             if (dest->gtOper == GT_LCL_VAR)
             {
                 lclVarTree = dest->AsLclVarCommon();
-                destLclNum = lclVarTree->gtLclNum;
+                destLclNum = modifiedLclNum;
                 destLclVar = &lvaTable[destLclNum];
                 if (destLclVar->lvType == TYP_STRUCT)
                 {
@@ -10259,26 +10261,27 @@ GenTree* Compiler::fgMorphCopyBlock(GenTree* tree)
                 noway_assert(destAddr->TypeGet() == TYP_BYREF || destAddr->TypeGet() == TYP_I_IMPL);
                 if (destAddr->IsLocalAddrExpr(this, &lclVarTree, &destFldSeq))
                 {
-                    destOnStack = true;
-                    destLclNum  = lclVarTree->gtLclNum;
-                    destLclVar  = &lvaTable[destLclNum];
+                    destOnStack    = true;
+                    destLclNum     = lclVarTree->GetLclNum();
+                    modifiedLclNum = destLclNum;
+                    destLclVar     = &lvaTable[destLclNum];
                 }
             }
         }
 
-        if (destLclVar != nullptr)
-        {
 #if LOCAL_ASSERTION_PROP
-            // Kill everything about destLclNum (and its field locals)
-            if (optLocalAssertionProp)
+        // Kill everything about modifiedLclNum (and its field locals)
+        if ((modifiedLclNum != BAD_VAR_NUM) && optLocalAssertionProp)
+        {
+            if (optAssertionCount > 0)
             {
-                if (optAssertionCount > 0)
-                {
-                    fgKillDependentAssertions(destLclNum DEBUGARG(tree));
-                }
+                fgKillDependentAssertions(modifiedLclNum DEBUGARG(tree));
             }
+        }
 #endif // LOCAL_ASSERTION_PROP
 
+        if (destLclVar != nullptr)
+        {
             if (destLclVar->lvPromoted && blockWidthIsConst)
             {
                 noway_assert(varTypeIsStruct(destLclVar));

tests/src/JIT/Regression/JitBlue/Runtime_1241/Runtime_1241.cs

@@ -0,0 +1,95 @@
+// 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.Linq;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+
+namespace Runtime_1241
+{
+
+    public struct Vertex
+    {
+        public Vector3 Position;
+        public Vector2 TexCoords;
+
+        public Vertex(Vector3 pos, Vector2 tex)
+        {
+            Position = pos;
+            TexCoords = tex;
+        }
+    }
+
+    class Program
+    {
+        static int Main()
+        {
+            int returnVal = 100;
+
+            // This prints all zeros in the failure case.
+            Console.WriteLine("Replacing array element with new struct directly");
+            {
+                var bug = Bug.Create();
+                bug.MutateBroken();
+
+                Console.WriteLine(bug.Vertices[0].Position);
+                if ((bug.Vertices[0].Position.X != 1) || (bug.Vertices[0].Position.Y != 1) || (bug.Vertices[0].Position.Z != 1))
+                {
+                    returnVal = -1;
+                }
+            }
+
+            // Works
+            Console.WriteLine("Replacing array element with new struct, stored in a local variable first");
+            {
+                var bug = Bug.Create();
+                bug.MutateWorks();
+
+                Console.WriteLine(bug.Vertices[0].Position);
+                if ((bug.Vertices[0].Position.X != 1) || (bug.Vertices[0].Position.Y != 1) || (bug.Vertices[0].Position.Z != 1))
+                {
+                    returnVal = -1;
+                }
+            }
+
+            return returnVal;
+        }
+    }
+
+    public class Bug
+    {
+        public static Bug Create()
+        {
+            return new Bug
+            {
+                Vertices = Enumerable.Range(1, 100).Select(i => new Vertex(new Vector3(i), Vector2.One)).ToArray()
+            };
+        }
+
+        public Vertex[] Vertices { get; set; }
+
+        public void MutateBroken()
+        {
+            for (var i = 0; i < Vertices.Length; i++)
+            {
+                var vert = Vertices[i];
+
+                Vertices[i] = new Vertex(vert.Position, vert.TexCoords);
+            }
+        }
+
+        public void MutateWorks()
+        {
+            for (var i = 0; i < Vertices.Length; i++)
+            {
+                var vert = Vertices[i];
+
+                var newVert = new Vertex(vert.Position, vert.TexCoords);
+
+                Vertices[i] = newVert;
+            }
+        }
+    }
+}

tests/src/JIT/Regression/JitBlue/Runtime_1241/Runtime_1241.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType />
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>