--- /dev/null	2018-11-26 11:49:18.255023023 +0100
+++ new/test/hotspot/jtreg/gc/shenandoah/TestShenandoahStrDedup.java	2018-11-26 21:28:02.513171821 +0100
@@ -0,0 +1,136 @@
+/*
+ * 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 TestShenandoahStringDedup.java
+ * @summary Test Shenandoah string deduplication implementation
+ * @key gc
+ * @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 TestShenandoahStrDedup
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=passive      -XX:-ShenandoahDegeneratedGC -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestShenandoahStrDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=aggressive   -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestShenandoahStrDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC                                         -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestShenandoahStrDedup
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=traversal    -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestShenandoahStrDedup
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=compact      -XX:+UseStringDeduplication -Xmx256M -Xlog:gc+stats TestShenandoahStrDedup
+ */
+
+import java.lang.reflect.*;
+import java.util.*;
+import sun.misc.*;
+
+public class TestShenandoahStrDedup {
+  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");
+    }
+  }
+}