8238756: C2: assert(((n) == __null || !VerifyIterativeGVN || !((n)->i…

…s_dead()))) failed: can not use dead node

Fix -XX:+VerifyIterativeGVN due to a dead node and add some basic flag testing.

Reviewed-by: roland, neliasso

src/hotspot/share/opto/loopopts.cpp

@@ -898,6 +898,7 @@ void PhaseIdealLoop::try_move_store_after_loop(Node* n) {
             // Move the store out of the loop if the LCA of all
             // users (except for the phi) is outside the loop.
             Node* hook = new Node(1);
+            hook->init_req(0, n_ctrl); // Add an input to prevent hook from being dead
             _igvn.rehash_node_delayed(phi);
             int count = phi->replace_edge(n, hook);
             assert(count > 0, "inconsistent phi");

test/hotspot/jtreg/compiler/c2/TestVerifyIterativeGVN.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2020, 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 8238756
+ * @requires vm.debug == true & vm.flavor == "server"
+ * @summary Run with -Xcomp to test -XX:+VerifyIterativeGVN in debug builds.
+ *
+ * @run main/othervm/timeout=300 -Xbatch -Xcomp -XX:+VerifyIterativeGVN compiler.c2.TestVerifyIterativeGVN
+ */
+package compiler.c2;
+
+public class TestVerifyIterativeGVN {
+    public static void main(String[] args) {
+    }
+}

test/hotspot/jtreg/compiler/loopopts/TestMoveStoreAfterLoopVerifyIterativeGVN.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2020, 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 8238756
+ * @requires vm.debug == true & vm.flavor == "server"
+ * @summary Test which triggers assertion in PhaseIdealLoop::try_move_store_after_loop with -XX:+VerifyIterativeGVN due to dead hook.
+ *
+ * @run main/othervm -Xbatch -XX:+VerifyIterativeGVN compiler.loopopts.TestMoveStoreAfterLoopVerifyIterativeGVN
+ */
+
+package compiler.loopopts;
+
+public class TestMoveStoreAfterLoopVerifyIterativeGVN {
+
+    private static int[] iArr = new int[10];
+
+    static void test() {
+        for (int i = 0; i < 10; i++) {
+            for (int j = 0; j <= 10; j++) {
+                iArr[i] = j;
+            }
+        }
+    }
+
+    static public void main(String[] args) {
+        for (int i = 0; i < 1000; i++) {
+            test();
+        }
+    }
+}