1 /*
  2  * Copyright (c) 2013, 2019, Red Hat, Inc. 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_SHENANDOAHFORWARDING_HPP
 26 #define SHARE_GC_SHENANDOAH_SHENANDOAHFORWARDING_HPP
 27 
 28 #include "oops/markWord.hpp"
 29 #include "oops/oop.hpp"
 30 #include "utilities/globalDefinitions.hpp"
 31 
 32 class ShenandoahForwarding {
 33 private:
 34   static const uintptr_t FWDED_HASH_TRANSITION = 0b111;
 35 
 36 public:
 37   /* Gets forwardee from the given object. For a self-forwarded object
 38    * (evacuation failure), returns the object itself.
 39    */
 40   static inline oop get_forwardee(oop obj);
 41 
 42   /* Gets forwardee from the given object. Only from mutator thread.
 43    * For a self-forwarded object, returns the object itself.
 44    */
 45   static inline oop get_forwardee_mutator(oop obj);
 46 
 47   /* Returns the raw value from forwardee slot. For a self-forwarded
 48    * object, returns the object itself.
 49    */
 50   static inline oop get_forwardee_raw(oop obj);
 51 
 52   /* Returns the raw value from forwardee slot without any checks.
 53    * Used for quick verification. For a self-forwarded object,
 54    * returns the object itself.
 55    */
 56   static inline oop get_forwardee_raw_unchecked(oop obj);
 57 
 58   /**
 59    * Returns true if the object is forwarded (including self-forwarded),
 60    * false otherwise.
 61    */
 62   static inline bool is_forwarded(oop obj);
 63   static inline bool is_forwarded(markWord m);
 64 
 65   /**
 66    * Returns true iff the mark word's lock bits are marked_value (0b11),
 67    * i.e. the upper bits encode a real forwardee pointer. This covers both
 68    * normal-forwarded (0b011) and forward-expanded (0b111) states, and
 69    * excludes self-forwarded states (0b100, 0b101, 0b110) whose upper bits
 70    * still hold the original klass/hash/age metadata.
 71    *
 72    * Do NOT use markWord::is_marked() for this purpose -- it also returns
 73    * true for self-forwarded objects.
 74    */
 75   static inline bool has_forwardee(markWord m);
 76 
 77   /**
 78    * Returns true iff obj has been self-forwarded (i.e. evacuation has
 79    * failed for this object in the current cycle).
 80    */
 81   static inline bool is_self_forwarded(oop obj);
 82 
 83   /* Tries to atomically update forwardee in $holder object to $update.
 84    * Assumes $holder points at itself.
 85    * Asserts $holder is in from-space.
 86    * Asserts $update is in to-space.
 87    *
 88    * Returns the new object 'update' upon success, or
 89    * the new forwardee that a competing thread installed. If another
 90    * thread self-forwarded the object, returns the object itself.
 91    */
 92   static inline oop try_update_forwardee(oop obj, oop update);
 93 
 94   /* Tries to atomically self-forward obj. Used by the evacuation path
 95    * when the copy allocation fails: the failing thread installs the
 96    * self-forwarded bit so other threads see the object as "already
 97    * handled" and return it unchanged.
 98    *
 99    * Returns nullptr on success (we installed the self-forward), or
100    * the winning forwardee when another thread raced ahead (either a
101    * real forwardee pointing at a copy, or obj itself if the winner
102    * also self-forwarded).
103    */
104   static inline oop try_forward_to_self(oop obj, markWord old_mark);
105 
106   static inline size_t size(oop obj);
107   static inline Klass* klass(oop obj);
108 };
109 
110 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHFORWARDING_HPP