1 /*
   2  * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  */
  23 
  24 /*
  25  * @test
  26  * @bug 8161720
  27  * @modules java.base/jdk.internal.misc
  28  * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions -Xbatch -XX:-TieredCompilation UnsafeSmallOffsetBooleanAccessTest
  29  * @run main/othervm -Xbatch UnsafeSmallOffsetBooleanAccessTest
  30  */
  31 
  32 import java.util.Random;
  33 import jdk.internal.misc.Unsafe;
  34 
  35 public class UnsafeSmallOffsetBooleanAccessTest {
  36     static final Unsafe UNSAFE = Unsafe.getUnsafe();
  37     static final long F_OFFSET;
  38     static final Random random = new Random();
  39 
  40     static {
  41         try {
  42             F_OFFSET = UNSAFE.objectFieldOffset(T.class.getDeclaredField("f"));
  43             System.out.println("The offset is: " + F_OFFSET);
  44         } catch (Exception e) {
  45             throw new Error(e);
  46         }
  47     }
  48 
  49     static class T {
  50         boolean f;
  51     }
  52 
  53     // Always return false in a way that is not obvious to the compiler.
  54     public static boolean myRandom() {
  55         if (random.nextInt(101) > 134) {
  56             return true;
  57         } else {
  58             return false;
  59         }
  60     }
  61 
  62     public static boolean test(T t) {
  63         boolean result = false;
  64         for (int i = 0; i < 20000; i++) {
  65             boolean random = myRandom();
  66             // If myRandom() returns false, access t.f.
  67             //
  68             // If myRandom() returns true, access virtual address
  69             // F_OFFSET. That address is most likely not mapped,
  70             // therefore the access will most likely cause a
  71             // crash. We're not concerned about that, though, because
  72             // myRandom() always returns false. However, the C2
  73             // compiler avoids normalization of the value returned by
  74             // getBoolean in this case.
  75             result = UNSAFE.getBoolean(myRandom() ? null : t, F_OFFSET);
  76         }
  77         return result;
  78     }
  79 
  80     public static void main(String[] args) {
  81         T t = new T();
  82         UNSAFE.putBoolean(t, F_OFFSET, true);
  83         System.out.println("The result for t is: " + test(t));
  84     }
  85 }