8347040: C2: assert(!loop->_body.contains(in)) failed

src/hotspot/share/opto/loopnode.cpp

@@ -3183,10 +3183,10 @@ void OuterStripMinedLoopNode::transform_to_counted_loop(PhaseIterGVN* igvn, Phas
         if (iloop->get_loop(iloop->get_ctrl(in)) != outer_loop_ilt) {
           continue;
         }
-        assert(!loop->_body.contains(in), "");
-        loop->_body.push(in);
         wq.push(in);
       }
+      assert(!loop->_body.contains(n), "Shouldn't append node to body twice");
+      loop->_body.push(n);
     }
     iloop->set_loop(safepoint, loop);
     loop->_body.push(safepoint);

test/hotspot/jtreg/compiler/longcountedloops/TestAssertWhenOuterStripMinedLoopRemoved.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025, Red Hat, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8347040
+ * @summary C2: assert(!loop->_body.contains(in)) failed
+ * @run main/othervm -XX:-BackgroundCompilation TestAssertWhenOuterStripMinedLoopRemoved
+ */
+
+
+import static java.lang.foreign.ValueLayout.JAVA_LONG;
+
+import java.lang.foreign.Arena;
+import java.lang.foreign.MemorySegment;
+
+public class TestAssertWhenOuterStripMinedLoopRemoved {
+    static final int COUNT = 100000;
+    static final MemorySegment segment = Arena.global().allocate(JAVA_LONG, COUNT);
+
+    public static void main(String[] args) {
+        for (int i = 0; i < 10000; i++) {
+            test();
+        }
+    }
+
+    static void test() {
+        var i = 0;
+        var j = 0;
+        while (i < 100000) {
+            segment.setAtIndex(JAVA_LONG, i++, 0);
+            segment.setAtIndex(JAVA_LONG, j++, 0);
+        }
+    }
+}