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. */
  48   static inline int word_offset() {
  49     return -1; // exactly one HeapWord
  50   }
  51 
  52   /* Offset from the object start, in bytes. */
  53   static inline int byte_offset() {
  54     return -HeapWordSize; // exactly one HeapWord
  55   }
  56 
  57   /* Allocated size, in HeapWords. */
  58   static inline uint word_size() {
  59     return (uint) MinObjAlignment;
  60   }
  61 
  62   /* Allocated size, in bytes */
  63   static inline uint byte_size() {
  64     return (uint) MinObjAlignmentInBytes;
  65   }
  66 
  67   /* Assert basic stuff once at startup. */
  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