8219807: C2 crash in IfNode::up_one_dom(Node*, bool)

Guard against regions degraded to copies.

Reviewed-by: kvn

src/hotspot/share/opto/ifnode.cpp

@@ -633,7 +633,7 @@ Node* IfNode::up_one_dom(Node *curr, bool linear_only) {
     if( din4->is_Call() &&      // Handle a slow-path call on either arm
         (din4 = din4->in(0)) )
       din4 = din4->in(0);
-    if( din3 == din4 && din3->is_If() )
+    if (din3 != NULL && din3 == din4 && din3->is_If()) // Regions not degraded to a copy
       return din3;              // Skip around diamonds
   }
 

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

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 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
+ * 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 8219807
+ * @summary Test IfNode::up_one_dom() with dead regions.
+ * @compile -XDstringConcat=inline TestIfWithDeadRegion.java
+ * @run main/othervm -XX:CompileCommand=compileonly,compiler.c2.TestIfWithDeadRegion::test
+ *                   compiler.c2.TestIfWithDeadRegion
+ */
+
+package compiler.c2;
+
+import java.util.function.Supplier;
+
+public class TestIfWithDeadRegion {
+
+    static String msg;
+
+    static String getString(String s, int i) {
+        String current = s + String.valueOf(i);
+        System.nanoTime();
+        return current;
+    }
+
+    static void test(Supplier<String> supplier) {
+        msg = supplier.get();
+    }
+
+    public static void main(String[] args) {
+        for (int i = 0; i < 20_000; ++i) {
+            test(() -> getString("Test1", 42));
+            test(() -> getString("Test2", 42));
+        }
+    }
+}