JIT: avoid cloning mid-entry loops with multiple non-loop entry preds

src/coreclr/jit/loopcloning.cpp

@@ -1724,6 +1724,25 @@ bool Compiler::optIsLoopClonable(unsigned loopInd)
         return false;
     }
 
+    // Reject cloning if this is a mid-entry loop and the entry has non-loop predecessors other than its head.
+    // This loop may be part of a larger looping construct that we didn't recognize.
+    //
+    // We should really fix this in optCanonicalizeLoop.
+    //
+    if (!loop.lpIsTopEntry())
+    {
+        for (BasicBlock* const entryPred : loop.lpEntry->PredBlocks())
+        {
+            if ((entryPred != loop.lpHead) && !loop.lpContains(entryPred))
+            {
+                JITDUMP("Loop cloning: rejecting loop " FMT_LP
+                        ". Is not top entry, and entry has multiple non-loop preds.\n",
+                        loopInd);
+                return false;
+            }
+        }
+    }
+
     // We've previously made a decision whether to have separate return epilogs, or branch to one.
     // There's a GCInfo limitation in the x86 case, so that there can be no more than SET_EPILOGCNT_MAX separate
     // epilogs.  Other architectures have a limit of 4 here for "historical reasons", but this should be revisited

src/tests/JIT/opt/Cloning/Runtime_70802.cs

@@ -0,0 +1,64 @@
+// 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 System.Threading;
+
+public class B 
+{
+    public virtual int V() => 33;
+}
+
+public class D : B
+{
+    public override int V() => 44;
+}
+
+class Runtime_70802
+{
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void G() {}
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static int F(B b, int n = 10, int m = 10)
+    {
+        int i = 0;
+        int j = 0;
+        int r = 0;
+        goto mid;
+        top:
+        G();
+        mid:
+        r += b.V();
+        i++;
+        if (i < n) goto top;
+        j++;
+        if (i < m) goto mid;
+        return r;     
+    }
+
+    public static int Main()
+    {
+        D d = new D();
+
+        for (int i = 0; i < 100; i++)
+        {
+            _ = F(d);
+            Thread.Sleep(15);
+        }
+
+        Thread.Sleep(50);
+
+        int r = 0;
+
+        for (int i = 0; i < 100; i++)
+        {
+            r += F(d);
+        }
+
+        Console.WriteLine($"result is {r} (expected 44000)");
+
+        return r / 440;
+    }
+}

src/tests/JIT/opt/Cloning/Runtime_70802.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+  <PropertyGroup>
+    <CLRTestBatchPreCommands><![CDATA[
+$(CLRTestBatchPreCommands)
+set COMPlus_TieredPGO=1
+]]></CLRTestBatchPreCommands>
+    <BashCLRTestPreCommands><![CDATA[
+$(BashCLRTestPreCommands)
+export COMPlus_TieredPGO=1
+]]></BashCLRTestPreCommands>
+  </PropertyGroup>
+</Project>