JIT: Restore EH-region check in if-conversion flow validation

src/coreclr/jit/ifconversion.cpp

@@ -130,9 +130,22 @@ bool OptIfConversionDsc::IfConvertCheckFlow()
         return false;
     }
 
+    // The Then/Else blocks will be removed by if-conversion, so they must be in the same
+    // EH region as m_startBlock. Otherwise they may be the start of a try/handler region
+    // (and thus marked BBF_DONT_REMOVE), or removing them could leave dangling EH state.
+    if (!BasicBlock::sameEHRegion(falseBb, m_startBlock))
+    {
+        return false;
+    }
+
     m_doElseConversion = trueBb->GetUniquePred(m_compiler) != nullptr;
     m_finalBlock       = m_doElseConversion ? trueBb->GetUniqueSucc() : trueBb;
 
+    if (m_doElseConversion && !BasicBlock::sameEHRegion(trueBb, m_startBlock))
+    {
+        return false;
+    }
+
     // m_finalBlock is only allowed to be null if both return.
     // E.g: Then block exits by throwing an exception => we bail here.
     if (m_finalBlock == nullptr && (!falseBb->KindIs(BBJ_RETURN) || !trueBb->KindIs(BBJ_RETURN)))

src/tests/JIT/Regression/JitBlue/Runtime_127446/Runtime_127446.cs

@@ -0,0 +1,53 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Runtime_127446;
+
+using System;
+using Xunit;
+
+public class Runtime_127446
+{
+    // Regression test: the JIT's if-conversion phase used to assert when the
+    // "Then" block was the start of an EH region (and therefore marked
+    // BBF_DONT_REMOVE). Compilation should succeed without hitting the assert.
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        try
+        {
+            M0();
+        }
+        catch (NullReferenceException)
+        {
+        }
+        catch (DivideByZeroException)
+        {
+        }
+    }
+
+    private static void M0()
+    {
+        int var1 = default(int);
+        bool[,] var4 = default(bool[,]);
+        if (var4[0, 0])
+        {
+            try
+            {
+                var1 = 0;
+            }
+            catch (System.Exception)
+            {
+                try
+                {
+                    var4[0, 0] = var4[0, 0];
+                }
+                catch (System.Exception)
+                {
+                }
+            }
+        }
+
+        var1 = (0 / var1);
+    }
+}

src/tests/JIT/Regression/Regression_o_3.csproj

@@ -150,6 +150,7 @@
     <Compile Include="JitBlue\Runtime_97625\Runtime_97625.cs" />
     <Compile Include="JitBlue\Runtime_98068\Runtime_98068.cs" />
     <Compile Include="JitBlue\Runtime_99391\Runtime_99391.cs" />
+    <Compile Include="JitBlue\Runtime_127446\Runtime_127446.cs" />
     <Compile Include="JitBlue\WPF_3226\CSharpRepro\WPF_3226.cs" />
   </ItemGroup>
   <Import Project="$(TestSourceDir)MergedTestRunner.targets" />