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_GC_SHENANDOAH_SHENANDOAHFORWARDING_HPP

    25 #define SHARE_GC_SHENANDOAH_SHENANDOAHFORWARDING_HPP

    26 

    27 #include "oops/oop.hpp"

    28 #include "utilities/globalDefinitions.hpp"

    29 

    30 class ShenandoahForwarding {

    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 forwarding pointer (to self).

    74    */

    75   static inline void initialize(oop obj);

    76 

    77   /* Gets forwardee from the given object.

    78    */

    79   static inline oop get_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_forwardee_raw(oop obj, HeapWord* update);

    92 

    93   /* Returns the raw value from forwardee slot.

    94    */

    95   static inline HeapWord* get_forwardee_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_forwardee_raw_unchecked(oop obj);

   101 

   102 private:

   103   static inline HeapWord** forward_ptr_addr(oop obj);

   104 };

   105 

   106 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHFORWARDING_HPP