1 /*
2  * Copyright (c) 2013, 2018, Red Hat, Inc. All rights reserved.
3  *
4  * This code is free software; you can redistribute it and/or modify it
5  * under the terms of the GNU General Public License version 2 only, as
6  * published by the Free Software Foundation.
7  *
8  * This code is distributed in the hope that it will be useful, but WITHOUT
9  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
11  * version 2 for more details (a copy is included in the LICENSE file that
12  * accompanied this code).
13  *
14  * You should have received a copy of the GNU General Public License version
15  * 2 along with this work; if not, write to the Free Software Foundation,
16  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
17  *
18  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
19  * or visit www.oracle.com if you need additional information or have any
20  * questions.
21  *
22  */
23 
24 #ifndef SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP
25 #define SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP
26 
27 #include "oops/oop.hpp"
28 #include "utilities/globalDefinitions.hpp"
29 
30 class ShenandoahBrooksPointer {
31   /*
32    * Notes:
33    *
34    *  a. It is important to have byte_offset and word_offset return constant
35    *     expressions, because that will allow to constant-fold forwarding ptr
36    *     accesses. This is not a problem in JIT compilers that would generate
37    *     the code once, but it is problematic in GC hotpath code.
38    *
39    *  b. With filler object mechanics, we may need to allocate more space for
40    *     the forwarding ptr to meet alignment requirements for objects. This
41    *     means *_offset and *_size calls are NOT interchangeable. The accesses
42    *     to forwarding ptrs should always be via *_offset. Storage size
43    *     calculations should always be via *_size.
44    */
45 
46 public:
47   /* Offset from the object start, in HeapWords. */
word_offset()48   static inline int word_offset() {
49     return -1; // exactly one HeapWord
50   }
51 
52   /* Offset from the object start, in bytes. */
byte_offset()53   static inline int byte_offset() {
54     return -HeapWordSize; // exactly one HeapWord
55   }
56 
57   /* Allocated size, in HeapWords. */
word_size()58   static inline uint word_size() {
59     return (uint) MinObjAlignment;
60   }
61 
62   /* Allocated size, in bytes */
byte_size()63   static inline uint byte_size() {
64     return (uint) MinObjAlignmentInBytes;
65   }
66 
67   /* Assert basic stuff once at startup. */
initial_checks()68   static void initial_checks() {
69     guarantee (MinObjAlignment > 0, "sanity, word_size is correct");
70     guarantee (MinObjAlignmentInBytes > 0, "sanity, byte_size is correct");
71   }
72 
73   /* Initializes Brooks pointer (to self).
74    */
75   static inline void initialize(oop obj);
76 
77   /* Gets forwardee from the given object.
78    */
79   static inline oop forwardee(oop obj);
80 
81   /* Tries to atomically update forwardee in $holder object to $update.
82    * Assumes $holder points at itself.
83    * Asserts $holder is in from-space.
84    * Asserts $update is in to-space.
85    */
86   static inline oop try_update_forwardee(oop obj, oop update);
87 
88   /* Sets raw value for forwardee slot.
89    * THIS IS DANGEROUS: USERS HAVE TO INITIALIZE/SET FORWARDEE BACK AFTER THEY ARE DONE.
90    */
91   static inline void set_raw(oop obj, HeapWord* update);
92 
93   /* Returns the raw value from forwardee slot.
94    */
95   static inline HeapWord* get_raw(oop obj);
96 
97   /* Returns the raw value from forwardee slot without any checks.
98    * Used for quick verification.
99    */
100   static inline HeapWord* get_raw_unchecked(oop obj);
101 
102 private:
103   static inline HeapWord** brooks_ptr_addr(oop obj);
104 };
105 
106 #endif // SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP
107