--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/gc/shenandoah/TestStringDedup.java	Mon Dec 10 15:47:44 2018 +0100
@@ -0,0 +1,145 @@
+/*
+ * Copyright (c) 2017, 2018, Red Hat, Inc. All rights reserved.
+ *
+ * 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 TestStringDedup
+ * @summary Test Shenandoah string deduplication implementation
+ * @key gc
+ * @requires vm.gc.Shenandoah
+ * @library /test/lib
+ * @modules java.base/jdk.internal.misc:open
+ * @modules java.base/java.lang:open
+ *          java.management
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=passive      -XX:+ShenandoahDegeneratedGC -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=passive      -XX:-ShenandoahDegeneratedGC -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=aggressive   -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC                                         -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=traversal    -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=compact      -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestStringDedup
+ */
+
+import java.lang.reflect.*;
+import java.util.*;
+
+import sun.misc.*;
+
+public class TestStringDedup {
+    private static Field valueField;
+    private static Unsafe unsafe;
+
+    private static final int UniqueStrings = 20;
+
+    static {
+        try {
+            Field field = Unsafe.class.getDeclaredField("theUnsafe");
+            field.setAccessible(true);
+            unsafe = (Unsafe) field.get(null);
+
+            valueField = String.class.getDeclaredField("value");
+            valueField.setAccessible(true);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    private static Object getValue(String string) {
+        try {
+            return valueField.get(string);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static class StringAndId {
+        private String str;
+        private int id;
+
+        public StringAndId(String str, int id) {
+            this.str = str;
+            this.id = id;
+        }
+
+        public String str() {
+            return str;
+        }
+
+        public int id() {
+            return id;
+        }
+    }
+
+    private static void generateStrings(ArrayList<StringAndId> strs, int unique_strs) {
+        Random rn = new Random();
+        for (int u = 0; u < unique_strs; u++) {
+            int n = rn.nextInt() % 10;
+            n = Math.max(n, 2);
+            for (int index = 0; index < n; index++) {
+                strs.add(new StringAndId("Unique String " + u, u));
+            }
+        }
+    }
+
+    private static int verifyDedepString(ArrayList<StringAndId> strs) {
+        HashMap<Object, StringAndId> seen = new HashMap<>();
+        int total = 0;
+        int dedup = 0;
+
+        for (StringAndId item : strs) {
+            total++;
+            StringAndId existing_item = seen.get(getValue(item.str()));
+            if (existing_item == null) {
+                seen.put(getValue(item.str()), item);
+            } else {
+                if (item.id() != existing_item.id() ||
+                        !item.str().equals(existing_item.str())) {
+                    System.out.println("StringDedup error:");
+                    System.out.println("String: " + item.str() + " != " + existing_item.str());
+                    throw new RuntimeException("StringDedup Test failed");
+                } else {
+                    dedup++;
+                }
+            }
+        }
+        System.out.println("Dedup: " + dedup + "/" + total + " unique: " + (total - dedup));
+        return (total - dedup);
+    }
+
+    public static void main(String[] args) {
+        ArrayList<StringAndId> astrs = new ArrayList<>();
+        generateStrings(astrs, UniqueStrings);
+        System.gc();
+        System.gc();
+        System.gc();
+        System.gc();
+        System.gc();
+
+        if (verifyDedepString(astrs) != UniqueStrings) {
+            // Can not guarantee all strings are deduplicated, there can
+            // still have pending items in queues.
+            System.out.println("Not all strings are deduplicated");
+        }
+    }
+}