JIT: rely on DFS for cycle entry detection in optRemoveRedundantZeroInits

src/coreclr/jit/optimizer.cpp

@@ -5918,10 +5918,35 @@ void Compiler::optRemoveRedundantZeroInits()
 
     assert(fgNodeThreading == NodeThreading::AllTrees);
 
-    for (BasicBlock* block = fgFirstBB; (block != nullptr) && !block->HasFlag(BBF_MARKED);
-         block             = block->GetUniqueSucc())
+    for (BasicBlock* block = fgFirstBB; block != nullptr; block = block->GetUniqueSucc())
     {
-        block->SetFlags(BBF_MARKED);
+        if (m_dfsTree->HasCycle())
+        {
+            // See if this block is a cycle entry
+            //
+            bool stop = false;
+            for (FlowEdge* predEdge = BlockPredsWithEH(block); predEdge != nullptr;
+                 predEdge           = predEdge->getNextPredEdge())
+            {
+                BasicBlock* const predBlock = predEdge->getSourceBlock();
+                if (m_dfsTree->IsAncestor(block, predBlock))
+                {
+                    JITDUMP(FMT_BB " is part of a cycle, stopping the block scan\n", block->bbNum);
+                    stop = true;
+                    break;
+                }
+            }
+
+            // If so, stop looking for redundant zero inits
+            //
+            if (stop)
+            {
+                break;
+            }
+        }
+
+        JITDUMP("Analyzing " FMT_BB "\n", block->bbNum);
+
         CompAllocator   allocator(getAllocator(CMK_ZeroInit));
         LclVarRefCounts defsInBlock(allocator);
         bool            removedTrackedDefs = false;
@@ -6038,7 +6063,7 @@ void Compiler::optRemoveRedundantZeroInits()
 
                         if (tree->Data()->IsIntegralConst(0))
                         {
-                            bool bbInALoop  = block->HasFlag(BBF_BACKWARD_JUMP);
+                            bool bbInALoop  = false;
                             bool bbIsReturn = block->KindIs(BBJ_RETURN);
 
                             if (!bbInALoop || bbIsReturn)
@@ -6116,12 +6141,6 @@ void Compiler::optRemoveRedundantZeroInits()
             }
         }
     }
-
-    for (BasicBlock* block = fgFirstBB; (block != nullptr) && block->HasFlag(BBF_MARKED);
-         block             = block->GetUniqueSucc())
-    {
-        block->RemoveFlags(BBF_MARKED);
-    }
 }
 
 //------------------------------------------------------------------------

src/tests/JIT/Regression/JitBlue/Runtime_103477/Runtime_103477.cs

@@ -0,0 +1,71 @@
+// 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.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_103477
+{
+    static int s_count;
+
+    [Fact]
+    public static int Test()
+    {
+        int result = -1;
+        try
+        {
+            Problem();
+            result = 100;
+        }
+        catch (Exception)
+        {
+        }
+        return result;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Problem()
+    {
+        string s = "12151";
+        int i = 0;
+
+        char? a = null;
+        int count = 0;
+        while (true)
+        {
+            string? res = get(s, ref i, ref a);
+            if (res != null)
+            {
+                Count(res);
+                a = null; // !!! this line is removed from the published version
+                continue;
+            }
+
+            if (i >= s.Length)
+                break;
+
+            a = '.';
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Count(string s)
+    {
+        s_count++;
+        if (s_count > 5) throw new Exception();
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static string? get(string s, ref int index, ref char? a)
+    {
+        if (index >= s.Length)
+            return null;
+
+        if (a == '.')
+            return ".";
+
+        a ??= s[index++];
+        return (a == '1') ? "1" : null;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_103477/Runtime_103477.csproj

@@ -0,0 +1,5 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>