JIT: Skip loop unrolling when the test is not a backedge

src/coreclr/jit/optimizer.cpp

@@ -1377,6 +1377,19 @@ bool Compiler::optTryUnrollLoop(FlowGraphNaturalLoop* loop, bool* changedIR)
         return false;
     }
 
+    // The loop test must be both an exit and a backedge.
+    // FlowGraphNaturalLoop::AnalyzeIteration ensures it is an exit but we must
+    // make sure it is a backedge so that we can legally redirect it to the
+    // next iteration. If it isn't a backedge then redirecting it would skip
+    // all code between the loop test and the backedge.
+    assert(loop->ContainsBlock(iterInfo.TestBlock->GetTrueTarget()) !=
+           loop->ContainsBlock(iterInfo.TestBlock->GetFalseTarget()));
+    if (!iterInfo.TestBlock->TrueTargetIs(loop->GetHeader()) && !iterInfo.TestBlock->FalseTargetIs(loop->GetHeader()))
+    {
+        JITDUMP("Failed to unroll loop " FMT_LP ": test block is not a backedge\n", loop->GetIndex());
+        return false;
+    }
+
     // Get the loop data:
     //  - initial constant
     //  - limit constant

src/tests/JIT/Regression/JitBlue/Runtime_97321/Runtime_97321.cs

@@ -0,0 +1,33 @@
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_97321
+{
+    [Fact]
+    public static int TestEntryPoint() => Foo(false);
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int Foo(bool b)
+    {
+        int sum = 1;
+        int i = 0;
+        goto Header;
+Top:;
+        sum += 33;
+
+Header:;
+        if (b)
+        {
+            goto Header;
+        }
+
+Bottom:
+        i++;
+        if (i < 4)
+        {
+            goto Top;
+        }
+
+        return sum;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_97321/Runtime_97321.csproj

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