1 /*
   2  * Copyright (c) 2013, 2015, Red Hat, Inc. and/or its affiliates.
   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_BROOKSPOINTER_HPP
  25 #define SHARE_VM_GC_SHENANDOAH_BROOKSPOINTER_HPP
  26 
  27 #include "oops/oop.hpp"
  28 #include "utilities/globalDefinitions.hpp"
  29 
  30 class BrooksPointer {
  31 
  32 public:
  33 
  34   /*
  35    * Notes:
  36    *
  37    *  a. It is important to have byte_offset and word_offset return constant
  38    *     expressions, because that will allow to constant-fold forwarding ptr
  39    *     accesses. This is not a problem in JIT compilers that would generate
  40    *     the code once, but it is problematic in GC hotpath code.
  41    *
  42    *  b. With filler object mechanics, we may need to allocate more space for
  43    *     the forwarding ptr to meet alignment requirements for objects. This
  44    *     means *_offset and *_size calls are NOT interchangeable. The accesses
  45    *     to forwarding ptrs should always be via *_offset. Storage size
  46    *     calculations should always be via *_size.
  47    */
  48 
  49   /* Offset from the object start, in HeapWords. */
  50   static inline int word_offset() {
  51     return -1; // exactly one HeapWord
  52   }
  53 
  54   /* Offset from the object start, in bytes. */
  55   static inline int byte_offset() {
  56     return -HeapWordSize; // exactly one HeapWord
  57   }
  58 
  59   /* Allocated size, in HeapWords. */
  60   static inline size_t word_size() {
  61     return MinObjAlignment;
  62   }
  63 
  64   /* Allocated size, in bytes */
  65   static inline size_t byte_size() {
  66     return MinObjAlignmentInBytes;
  67   }
  68 
  69   /* Initializes Brooks pointer (to self).
  70    */
  71   static inline void initialize(oop obj);
  72 
  73   /* Gets forwardee from the given object.
  74    */
  75   static inline oop forwardee(oop obj);
  76 
  77   /* Forcefully sets forwardee in $holder to $update.
  78    */
  79   static inline void set_forwardee(oop obj, oop update);
  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 private:
  98   static inline HeapWord** brooks_ptr_addr(oop obj);
  99 };
 100 
 101 #endif // SHARE_VM_GC_SHENANDOAH_BROOKSPOINTER_HPP