8256385: C2: fatal error: modified node is not on IGVN._worklist

Reviewed-by: chagedorn, roland

src/hotspot/share/opto/loopnode.cpp

@@ -656,6 +656,7 @@ SafePointNode* PhaseIdealLoop::find_safepoint(Node* back_control, Node* x, Ideal
     MergeMemNode* mm = NULL;
     if (mem->is_MergeMem()) {
       mm = mem->clone()->as_MergeMem();
+      _igvn._worklist.push(mm);
       for (MergeMemStream mms(mem->as_MergeMem()); mms.next_non_empty(); ) {
         if (mms.alias_idx() != Compile::AliasIdxBot && loop != get_loop(ctrl_or_self(mms.memory()))) {
           mm->set_memory_at(mms.alias_idx(), mem->as_MergeMem()->base_memory());

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

@@ -0,0 +1,54 @@
+/*
+ * 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 8256385
+ * @requires vm.debug == true & vm.flavor == "server"
+ * @summary Test for dead nodes that are not added to the IGVN worklist for removal.
+ * @run main/othervm -Xbatch compiler.c2.TestDeadNodeDuringIGVN
+ */
+package compiler.c2;
+
+public class TestDeadNodeDuringIGVN {
+    static int res;
+
+    static void test(int len) {
+        int array[] = new int[len];
+        for (long l = 0; l < 10; l++) {
+            float e = 1;
+            do { } while (++e < 2);
+            res += l;
+        }
+    }
+
+    public static void main(String[] args) {
+        for (int i = 0; i < 40_000; ++i) {
+            res = 0;
+            test(1);
+            if (res != 45) {
+                throw new RuntimeException("Test failed: res = " + res);
+            }
+        }
+    }
+}