JIT: Tail-merge fix self-loop handling

src/coreclr/jit/fgopt.cpp

@@ -5298,11 +5298,6 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
 
                     fgUnlinkStmt(predBlock, stmt);
 
-                    if (predBlock->isEmpty())
-                    {
-                        tryRemoveAndFixFlow(predBlock, commSucc);
-                    }
-
                     // Add one of the matching stmts to block, and
                     // update its flags.
                     //
@@ -5312,6 +5307,11 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
                         commSucc->CopyFlags(predBlock, BBF_COPY_PROPAGATE);
                     }
 
+                    if (predBlock->isEmpty())
+                    {
+                        tryRemoveAndFixFlow(predBlock, commSucc);
+                    }
+
                     madeChanges = true;
                 }
 
@@ -5363,20 +5363,24 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
 
                     // From most to least preferable.
                     //
-                    if (isNoSplit && isFallThrough)
+                    if (predBlock == commSucc)
                     {
                         return 0;
                     }
-                    if (isNoSplit)
+                    if (isNoSplit && isFallThrough)
                     {
                         return 1;
                     }
-                    if (isFallThrough)
+                    if (isNoSplit)
                     {
                         return 2;
                     }
+                    if (isFallThrough)
+                    {
+                        return 3;
+                    }
 
-                    return 3;
+                    return 4;
                 };
 
                 unsigned const rank = getRank();

src/tests/JIT/Regression/JitBlue/Runtime_133131/Runtime_133131.cs

@@ -0,0 +1,61 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Runtime_133131;
+
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_133131
+{
+    private static int s_v;
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    private static void TailMergePickSelfLoopAsVictim(int x)
+    {
+        if (x == 1)
+        {
+            goto P;
+        }
+        if (x == 2)
+        {
+            s_v = 77;
+            goto L;
+        }
+        return;
+    P:
+        s_v = 1;
+        goto L;
+    L:
+        s_v = 1;
+        goto L;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    private static void TailMergeDontRemoveSelfLoop(int x)
+    {
+        if (x >= 0)
+        {
+            goto P;
+        }
+        return;
+    L:
+        s_v = 1;
+        goto L;
+    P:
+        s_v = 1;
+        goto L;
+    }
+
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        s_v = 0;
+        TailMergePickSelfLoopAsVictim(-1);
+        Assert.Equal(0, s_v);
+
+        s_v = 0;
+        TailMergeDontRemoveSelfLoop(-1);
+        Assert.Equal(0, s_v);
+    }
+}

src/tests/JIT/Regression/Regression_ro_2.csproj

@@ -124,6 +124,7 @@
     <Compile Include="JitBlue\Runtime_131137\Runtime_131137.cs" />
     <Compile Include="JitBlue\Runtime_131226\Runtime_131226.cs" />
     <Compile Include="JitBlue\Runtime_131459\Runtime_131459.cs" />
+    <Compile Include="JitBlue\Runtime_133131\Runtime_133131.cs" />
     <Compile Include="JitBlue\Runtime_10337\Runtime_10337.cs" />
     <Compile Include="JitBlue\Runtime_125160\Runtime_125160.cs" />
     <Compile Include="JitBlue\GitHub_131472\GitHub_131472.cs" />