1 /*
  2  * Copyright Amazon.com Inc. 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 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHCARDTABLE_HPP
 26 #define SHARE_GC_SHENANDOAH_SHENANDOAHCARDTABLE_HPP
 27 
 28 #include "gc/shared/cardTable.hpp"
 29 #include "memory/virtualspace.hpp"
 30 #include "oops/oopsHierarchy.hpp"
 31 #include "utilities/macros.hpp"
 32 
 33 #define ShenandoahMinCardSizeInBytes 128
 34 
 35 class ShenandoahCardTable: public CardTable {
 36   friend class VMStructs;
 37 
 38 private:
 39   // We maintain two copies of the card table to facilitate concurrent remembered set scanning
 40   // and concurrent clearing of stale remembered set information.  During the init_mark safepoint,
 41   // we copy the contents of _write_byte_map to _read_byte_map and clear _write_byte_map.
 42   //
 43   // Concurrent remembered set scanning reads from _read_byte_map while concurrent mutator write
 44   // barriers are overwriting cards of the _write_byte_map with DIRTY codes.  Concurrent remembered
 45   // set scanning also overwrites cards of the _write_byte_map with DIRTY codes whenever it discovers
 46   // interesting pointers.
 47   //
 48   // During a concurrent update-references phase, we scan the _write_byte_map concurrently to find
 49   // all old-gen references that may need to be updated.
 50   //
 51   // In a future implementation, we may swap the values of _read_byte_map and _write_byte_map during
 52   // the init-mark safepoint to avoid the need for bulk STW copying and initialization.  Doing so
 53   // requires a change to the implementation of mutator write barriers as the address of the card
 54   // table is currently in-lined and hard-coded.
 55   CardValue* _read_byte_map;
 56   CardValue* _write_byte_map;
 57   CardValue* _read_byte_map_base;
 58   CardValue* _write_byte_map_base;
 59 
 60 public:
 61   explicit ShenandoahCardTable(MemRegion whole_heap) : CardTable(whole_heap),
 62     _read_byte_map(nullptr), _write_byte_map(nullptr),
 63     _read_byte_map_base(nullptr), _write_byte_map_base(nullptr) {}
 64 
 65   void initialize();
 66 
 67   bool is_in_young(const void* obj) const override;
 68 
 69   CardValue* read_byte_for(const void* p);
 70 
 71   size_t last_valid_index();
 72 
 73   CardValue* swap_read_and_write_tables() {
 74     swap(_read_byte_map, _write_byte_map);
 75     swap(_read_byte_map_base, _write_byte_map_base);
 76 
 77     _byte_map = _write_byte_map;
 78     _byte_map_base = _write_byte_map_base;
 79 
 80     return _byte_map_base;
 81   }
 82 
 83   CardValue* read_byte_map() {
 84     return _read_byte_map;
 85   }
 86 
 87   CardValue* read_byte_map_base() {
 88     return _read_byte_map_base;
 89   }
 90 
 91   CardValue* write_byte_map() {
 92     return _write_byte_map;
 93   }
 94 
 95   CardValue* write_byte_map_base() {
 96     return _write_byte_map_base;
 97   }
 98 
 99 private:
100   void initialize(const ReservedSpace& card_table);
101 };
102 
103 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHCARDTABLE_HPP