JIT: Fix invalid zero-init suppression for untracked variables (#91580)

optRemoveRedundantZeroInits has logic to remove unnecessary zero inits
if we can determine that the local will be zeroed in the prolog. In
addition, it also has logic to suppress the prolog zero init if there is
a dominating initialization already. The latter logic was trying to
reason about liveness for untracked locals, which does not make sense.

Fix #91576

src/coreclr/jit/optimizer.cpp

@@ -9044,9 +9044,9 @@ void Compiler::optRemoveRedundantZeroInits()
     CompAllocator   allocator(getAllocator(CMK_ZeroInit));
     LclVarRefCounts refCounts(allocator);
     BitVecTraits    bitVecTraits(lvaCount, this);
-    BitVec          zeroInitLocals = BitVecOps::MakeEmpty(&bitVecTraits);
-    bool            hasGCSafePoint = false;
-    bool            canThrow       = false;
+    BitVec          zeroInitLocals         = BitVecOps::MakeEmpty(&bitVecTraits);
+    bool            hasGCSafePoint         = false;
+    bool            hasImplicitControlFlow = false;
 
     assert(fgNodeThreading == NodeThreading::AllTrees);
 
@@ -9057,6 +9057,8 @@ void Compiler::optRemoveRedundantZeroInits()
         CompAllocator   allocator(getAllocator(CMK_ZeroInit));
         LclVarRefCounts defsInBlock(allocator);
         bool            removedTrackedDefs = false;
+        bool            hasEHSuccs         = block->HasPotentialEHSuccs(this);
+
         for (Statement* stmt = block->FirstNonPhiDef(); stmt != nullptr;)
         {
             Statement* next = stmt->GetNextStmt();
@@ -9067,10 +9069,7 @@ void Compiler::optRemoveRedundantZeroInits()
                     hasGCSafePoint = true;
                 }
 
-                if ((tree->gtFlags & GTF_EXCEPT) != 0)
-                {
-                    canThrow = true;
-                }
+                hasImplicitControlFlow |= hasEHSuccs && ((tree->gtFlags & GTF_EXCEPT) != 0);
 
                 switch (tree->gtOper)
                 {
@@ -9216,7 +9215,8 @@ void Compiler::optRemoveRedundantZeroInits()
                             }
                         }
 
-                        if (!removedExplicitZeroInit && isEntire && (!canThrow || !lclDsc->lvLiveInOutOfHndlr))
+                        if (!removedExplicitZeroInit && isEntire &&
+                            (!hasImplicitControlFlow || (lclDsc->lvTracked && !lclDsc->lvLiveInOutOfHndlr)))
                         {
                             // If compMethodRequiresPInvokeFrame() returns true, lower may later
                             // insert a call to CORINFO_HELP_INIT_PINVOKE_FRAME which is a gc-safe point.

src/tests/JIT/Regression/JitBlue/Runtime_91576/Runtime_91576.cs

@@ -0,0 +1,51 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.aa
+
+// Generated by Fuzzlyn v1.6 on 2023-09-03 15:59:01
+// Run on X64 Windows
+// Seed: 11520325105937570553
+// Reduced from 294.5 KiB to 0.7 KiB in 00:04:32
+// Debug: Outputs False
+// Release: Outputs True
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_91576
+{
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        Assert.Throws<NullReferenceException>(() =>
+        {
+            Run(new int[1]);
+            Run(null);
+        });
+
+        return s_result;
+    }
+
+    static int s_result;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Run(int[] l)
+    {
+        bool b = false;
+        try
+        {
+            int result = l[0];
+            b = true;
+        }
+        finally
+        {
+            Check(ref b);
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Check(ref bool b)
+    {
+        s_result = b ? 101 : 100;
+    }
+}
+

src/tests/JIT/Regression/JitBlue/Runtime_91576/Runtime_91576.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>