8231988: Unexpected test result caused by C2 IdealLoopTree::do_remove_empty_loop

Summary: Duplicate cmp node in empty loop if it has other users
Reviewed-by: neliasso, thartmann
Contributed-by: wanghuang3@huawei.com, xietuo@huawei.com

src/share/vm/opto/loopTransform.cpp

 /*
- * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2019, 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
@@ -2231,6 +2231,13 @@ bool IdealLoopTree::policy_do_remove_empty_loop( PhaseIdealLoop *phase ) {
     // We also need to replace the original limit to collapse loop exit.
     Node* cmp = cl->loopexit()->cmp_node();
     assert(cl->limit() == cmp->in(2), "sanity");
+    // Duplicate cmp node if it has other users
+    if (cmp->outcnt() > 1) {
+      cmp = cmp->clone();
+      cmp = phase->_igvn.register_new_node_with_optimizer(cmp);
+      BoolNode *bol = cl->loopexit()->in(CountedLoopEndNode::TestValue)->as_Bool();
+      phase->_igvn.replace_input_of(bol, 1, cmp); // put bol on worklist
+    }
     phase->_igvn._worklist.push(cmp->in(2)); // put limit on worklist
     phase->_igvn.replace_input_of(cmp, 2, exact_limit); // put cmp on worklist
   }

test/compiler/loopopts/TestRemoveEmptyLoop.java

+/*
+ * Copyright (c) 2019, Huawei Technologies Co. Ltd. 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 8231988
+ * @summary Unexpected test result caused by C2 IdealLoopTree::do_remove_empty_loop
+ *
+ * @run main/othervm -XX:-TieredCompilation -XX:-BackgroundCompilation
+ *      TestRemoveEmptyLoop
+ */
+
+public class TestRemoveEmptyLoop {
+
+    public void test() {
+        int i = 34;
+        for (; i > 0; i -= 11);
+        if (i < 0) {
+            // do nothing
+        } else {
+            throw new RuntimeException("Test failed.");
+        }
+    }
+
+    public static void main(String[] args) {
+        TestRemoveEmptyLoop _instance = new TestRemoveEmptyLoop();
+        for (int i = 0; i < 50000; i++) {
+            _instance.test();
+        }
+        System.out.println("Test passed.");
+    }
+
+}