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 }