JIT: fix bug where a gc struct is not zero initialized

src/coreclr/jit/liveness.cpp

@@ -1988,25 +1988,45 @@ void Compiler::fgComputeLifeLIR(VARSET_TP& life, BasicBlock* block, VARSET_VALAR
                         if (blockRange.TryGetUse(node, &addrUse) &&
                             (addrUse.User()->OperIs(GT_STOREIND, GT_STORE_BLK, GT_STORE_OBJ)))
                         {
-                            // Remove the store. DCE will iteratively clean up any ununsed operands.
                             GenTreeIndir* const store = addrUse.User()->AsIndir();
 
-                            JITDUMP("Removing dead indirect store:\n");
-                            DISPNODE(store);
+                            // If this is a zero init of an explicit zero init gc local
+                            // that has at least one other reference, we will keep the zero init.
+                            //
+                            const LclVarDsc& varDsc              = lvaTable[node->AsLclVarCommon()->GetLclNum()];
+                            const bool       isExplicitInitLocal = varDsc.lvHasExplicitInit;
+                            const bool       isReferencedLocal   = varDsc.lvRefCnt() > 1;
+                            const bool       isZeroInit          = store->OperIsInitBlkOp();
+                            const bool       isGCInit            = varDsc.HasGCPtr();
 
-                            assert(store->Addr() == node);
-                            blockRange.Delete(this, block, node);
-
-                            GenTree* data =
-                                store->OperIs(GT_STOREIND) ? store->AsStoreInd()->Data() : store->AsBlk()->Data();
-                            data->SetUnusedValue();
-
-                            if (data->isIndir())
+                            if (isExplicitInitLocal && isReferencedLocal && isZeroInit && isGCInit)
                             {
-                                Lowering::TransformUnusedIndirection(data->AsIndir(), this, block);
+                                // Yes, we'd better keep it around.
+                                //
+                                JITDUMP("Keeping dead indirect store -- explicit zero init of gc type\n");
+                                DISPNODE(store);
                             }
+                            else
+                            {
+                                // Remove the store. DCE will iteratively clean up any ununsed operands.
+                                //
+                                JITDUMP("Removing dead indirect store:\n");
+                                DISPNODE(store);
+
+                                assert(store->Addr() == node);
+                                blockRange.Delete(this, block, node);
 
-                            fgRemoveDeadStoreLIR(store, block);
+                                GenTree* data =
+                                    store->OperIs(GT_STOREIND) ? store->AsStoreInd()->Data() : store->AsBlk()->Data();
+                                data->SetUnusedValue();
+
+                                if (data->isIndir())
+                                {
+                                    Lowering::TransformUnusedIndirection(data->AsIndir(), this, block);
+                                }
+
+                                fgRemoveDeadStoreLIR(store, block);
+                            }
                         }
                     }
                 }

src/coreclr/jit/optimizer.cpp

@@ -9492,7 +9492,7 @@ void Compiler::optRemoveRedundantZeroInits()
                                 // the prolog and this explicit intialization. Therefore, it doesn't
                                 // require zero initialization in the prolog.
                                 lclDsc->lvHasExplicitInit = 1;
-                                JITDUMP("Marking " FMT_LP " as having an explicit init\n", lclNum);
+                                JITDUMP("Marking V%02u as having an explicit init\n", lclNum);
                             }
                         }
                         break;

src/tests/JIT/Regression/JitBlue/Runtime_65694/Runtime_65694.cs

@@ -0,0 +1,73 @@
+// 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.Collections.Generic;
+using System.Runtime.CompilerServices;
+
+public struct Key
+{
+    public int a;
+    public string s;
+}
+
+public struct Problem
+{
+    public int x;
+    public double d;
+    public string s0;
+    public int y;
+    public double e;
+    public string s1;
+}
+
+public class Runtime_65694
+{
+    public Dictionary<Key, Problem> _d;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public void D()
+    {
+        Problem p = new Problem { s0 = "hello", s1 = "world", x = 33 };
+        Key k = new Key() { a = 0, s = "a" };
+        Dictionary<Key, Problem> d = new Dictionary<Key, Problem>();
+        d[k] = p;
+
+        _d = d;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void F() 
+    {
+        GC.Collect();
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public int G(Key k, bool b)
+    {
+        Problem p = default;
+
+        F();
+
+        if (b)
+        {
+            if (_d?.TryGetValue(k, out p) == true && (p.x == 33))
+            {
+                return 22;
+            }
+        }
+
+        return 0;
+    }
+
+    public static int Main()
+    {
+        var r = new Runtime_65694();
+        r.D();
+        int result = 0;
+        Key k = new Key() { a = 0, s = "a" };
+        result += r.G(k, true);
+        return result + 78;
+    }
+}
+

src/tests/JIT/Regression/JitBlue/Runtime_65694/Runtime_65694.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>