8357105: C2: compilation fails with "assert(false) failed: empty program detected during loop optimization"

Backport-of: a300c356555019a42c19bf0c16184f6dee4ad96e

Author: GoeLin

test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsAppendUncommonTrap.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. 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 8357105
+ * @summary Test stacked string concatenations where the toString result
+ *          of the first StringBuilder chain is wired into an uncommon trap
+ *          located in the second one.
+ * @run main/othervm compiler.stringopts.TestStackedConcatsAppendUncommonTrap
+ * @run main/othervm -XX:-TieredCompilation -Xbatch
+ *                   -XX:CompileOnly=compiler.stringopts.TestStackedConcatsAppendUncommonTrap::*
+ *                   compiler.stringopts.TestStackedConcatsAppendUncommonTrap
+ */
+
+package compiler.stringopts;
+
+public class TestStackedConcatsAppendUncommonTrap {
+
+    public static void main (String... args) {
+        for (int i = 0; i < 10000; i++) {
+            String s = f(" ");
+            if (!s.equals("    ")) {
+                throw new RuntimeException("wrong result.");
+            }
+        }
+    }
+
+    static String f(String c) {
+        String s = " ";
+        s = new StringBuilder().append(s).append(s).toString();
+        s = new StringBuilder().append(s).append(s == c ? s : "  ").toString();
+        return s;
+    }
+}