vm/oops/markOop.hpp

/*
 * Copyright 1997-2006 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 *
 */

// The markOop describes the header of an object.
//
// Note that the mark is not a real oop but just a word.
// It is placed in the oop hierarchy for historical reasons.
//
// Bit-format of an object header (most significant first):
//
//
//  unused:0/25 hash:25/31 age:4 biased_lock:1 lock:2 = 32/64 bits
//
//  - hash contains the identity hash value: largest value is
//    31 bits, see os::random().  Also, 64-bit vm's require
//    a hash value no bigger than 32 bits because they will not
//    properly generate a mask larger than that: see library_call.cpp
//    and c1_CodePatterns_sparc.cpp.
//
//  - the biased lock pattern is used to bias a lock toward a given
//    thread. When this pattern is set in the low three bits, the lock
//    is either biased toward a given thread or "anonymously" biased,
//    indicating that it is possible for it to be biased. When the
//    lock is biased toward a given thread, locking and unlocking can
//    be performed by that thread without using atomic operations.
//    When a lock's bias is revoked, it reverts back to the normal
//    locking scheme described below.
//
//    Note that we are overloading the meaning of the "unlocked" state
//    of the header. Because we steal a bit from the age we can
//    guarantee that the bias pattern will never be seen for a truly
//    unlocked object.
//
//    Note also that the biased state contains the age bits normally
//    contained in the object header. Large increases in scavenge
//    times were seen when these bits were absent and an arbitrary age
//    assigned to all biased objects, because they tended to consume a
//    significant fraction of the eden semispaces and were not
//    promoted promptly, causing an increase in the amount of copying
//    performed. The runtime system aligns all JavaThread* pointers to
//    a very large value (currently 128 bytes) to make room for the
//    age bits when biased locking is enabled.
//
//    [JavaThread* | epoch | age | 1 | 01]       lock is biased toward given thread
//    [0           | epoch | age | 1 | 01]       lock is anonymously biased
//
//  - the two lock bits are used to describe three states: locked/unlocked and monitor.
//
//    [ptr             | 00]  locked             ptr points to real header on stack
//    [header      | 0 | 01]  unlocked           regular object header
//    [ptr             | 10]  monitor            inflated lock (header is wapped out)
//    [ptr             | 11]  marked             used by markSweep to mark an object
//                                               not valid at any other time
//
//    We assume that stack/thread pointers have the lowest two bits cleared.

class BasicLock;
class ObjectMonitor;
class JavaThread;

class markOopDesc: public oopDesc {
 private:
  // Conversion
  uintptr_t value() const { return (uintptr_t) this; }

 public:
  // Constants
  enum { age_bits                 = 4,
         lock_bits                = 2,
         biased_lock_bits         = 1,
         max_hash_bits            = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
         hash_bits                = max_hash_bits > 31 ? 31 : max_hash_bits,
         epoch_bits               = 2
  };

  // The biased locking code currently requires that the age bits be
  // contiguous to the lock bits. Class data sharing would prefer the
  // hash bits to be lower down to provide more random hash codes for
  // shared read-only symbolOop objects, because these objects' mark
  // words are set to their own address with marked_value in the lock
  // bit, and using lower bits would make their identity hash values
  // more random. However, the performance decision was made in favor
  // of the biased locking code.

  enum { lock_shift               = 0,
         biased_lock_shift        = lock_bits,
         age_shift                = lock_bits + biased_lock_bits,
         hash_shift               = lock_bits + biased_lock_bits + age_bits,
         epoch_shift              = hash_shift
  };

  enum { lock_mask                = right_n_bits(lock_bits),
         lock_mask_in_place       = lock_mask << lock_shift,
         biased_lock_mask         = right_n_bits(lock_bits + biased_lock_bits),
         biased_lock_mask_in_place= biased_lock_mask << lock_shift,
         biased_lock_bit_in_place = 1 << biased_lock_shift,
         age_mask                 = right_n_bits(age_bits),
         age_mask_in_place        = age_mask << age_shift,
         epoch_mask               = right_n_bits(epoch_bits),
         epoch_mask_in_place      = epoch_mask << epoch_shift
#ifndef _WIN64
         ,hash_mask               = right_n_bits(hash_bits),
         hash_mask_in_place       = (address_word)hash_mask << hash_shift
#endif
  };

  // Alignment of JavaThread pointers encoded in object header required by biased locking
  enum { biased_lock_alignment    = 2 << (epoch_shift + epoch_bits)
  };

#ifdef _WIN64
    // These values are too big for Win64
    const static uintptr_t hash_mask = right_n_bits(hash_bits);
    const static uintptr_t hash_mask_in_place  =
                            (address_word)hash_mask << hash_shift;
#endif

  enum { locked_value             = 0,
         unlocked_value           = 1,
         monitor_value            = 2,
         marked_value             = 3,
         biased_lock_pattern      = 5
  };

  enum { no_hash                  = 0 };  // no hash value assigned

  enum { no_hash_in_place         = (address_word)no_hash << hash_shift,
         no_lock_in_place         = unlocked_value
  };

  enum { max_age                  = age_mask };

  enum { max_bias_epoch           = epoch_mask };

  // Biased Locking accessors.
  // These must be checked by all code which calls into the
  // ObjectSynchronizer and other code. The biasing is not understood
  // by the lower-level CAS-based locking code, although the runtime
  // fixes up biased locks to be compatible with it when a bias is
  // revoked.
  bool has_bias_pattern() const {
    return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
  }
  JavaThread* biased_locker() const {
    assert(has_bias_pattern(), "should not call this otherwise");
    return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place))));
  }
  // Indicates that the mark has the bias bit set but that it has not
  // yet been biased toward a particular thread
  bool is_biased_anonymously() const {
    return (has_bias_pattern() && (biased_locker() == NULL));
  }
  // Indicates epoch in which this bias was acquired. If the epoch
  // changes due to too many bias revocations occurring, the biases
  // from the previous epochs are all considered invalid.
  int bias_epoch() const {
    assert(has_bias_pattern(), "should not call this otherwise");
    return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
  }
  markOop set_bias_epoch(int epoch) {
    assert(has_bias_pattern(), "should not call this otherwise");
    assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
    return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
  }
  markOop incr_bias_epoch() {
    return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
  }
  // Prototype mark for initialization
  static markOop biased_locking_prototype() {
    return markOop( biased_lock_pattern );
  }

  // lock accessors (note that these assume lock_shift == 0)
  bool is_locked()   const {
    return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
  }
  bool is_unlocked() const {
    return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
  }
  bool is_marked()   const {
    return (mask_bits(value(), lock_mask_in_place) == marked_value);
  }
  bool is_neutral()  const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }

  // Special temporary state of the markOop while being inflated.
  // Code that looks at mark outside a lock need to take this into account.
  bool is_being_inflated() const { return (value() == 0); }

  // Distinguished markword value - used when inflating over
  // an existing stacklock.  0 indicates the markword is "BUSY".
  // Lockword mutators that use a LD...CAS idiom should always
  // check for and avoid overwriting a 0 value installed by some
  // other thread.  (They should spin or block instead.  The 0 value
  // is transient and *should* be short-lived).
  static markOop INFLATING() { return (markOop) 0; }    // inflate-in-progress

  // Should this header be preserved during GC?
  inline bool must_be_preserved(oop obj_containing_mark) const;
  inline bool must_be_preserved_with_bias(oop obj_containing_mark) const;

  // Should this header (including its age bits) be preserved in the
  // case of a promotion failure during scavenge?
  // Note that we special case this situation. We want to avoid
  // calling BiasedLocking::preserve_marks()/restore_marks() (which
  // decrease the number of mark words that need to be preserved
  // during GC) during each scavenge. During scavenges in which there
  // is no promotion failure, we actually don't need to call the above
  // routines at all, since we don't mutate and re-initialize the
  // marks of promoted objects using init_mark(). However, during
  // scavenges which result in promotion failure, we do re-initialize
  // the mark words of objects, meaning that we should have called
  // these mark word preservation routines. Currently there's no good
  // place in which to call them in any of the scavengers (although
  // guarded by appropriate locks we could make one), but the
  // observation is that promotion failures are quite rare and
  // reducing the number of mark words preserved during them isn't a
  // high priority.
  inline bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const;
  inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const;

  // Should this header be preserved during a scavenge where CMS is
  // the old generation?
  // (This is basically the same body as must_be_preserved_for_promotion_failure(),
  // but takes the klassOop as argument instead)
  inline bool must_be_preserved_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const;
  inline bool must_be_preserved_with_bias_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const;

  // WARNING: The following routines are used EXCLUSIVELY by
  // synchronization functions. They are not really gc safe.
  // They must get updated if markOop layout get changed.
  markOop set_unlocked() const {
    return markOop(value() | unlocked_value);
  }
  bool has_locker() const {
    return ((value() & lock_mask_in_place) == locked_value);
  }
  BasicLock* locker() const {
    assert(has_locker(), "check");
    return (BasicLock*) value();
  }
  bool has_monitor() const {
    return ((value() & monitor_value) != 0);
  }
  ObjectMonitor* monitor() const {
    assert(has_monitor(), "check");
    // Use xor instead of &~ to provide one extra tag-bit check.
    return (ObjectMonitor*) (value() ^ monitor_value);
  }
  bool has_displaced_mark_helper() const {
    return ((value() & unlocked_value) == 0);
  }
  markOop displaced_mark_helper() const {
    assert(has_displaced_mark_helper(), "check");
    intptr_t ptr = (value() & ~monitor_value);
    return *(markOop*)ptr;
  }
  void set_displaced_mark_helper(markOop m) const {
    assert(has_displaced_mark_helper(), "check");
    intptr_t ptr = (value() & ~monitor_value);
    *(markOop*)ptr = m;
  }
  markOop copy_set_hash(intptr_t hash) const {
    intptr_t tmp = value() & (~hash_mask_in_place);
    tmp |= ((hash & hash_mask) << hash_shift);
    return (markOop)tmp;
  }
  // it is only used to be stored into BasicLock as the
  // indicator that the lock is using heavyweight monitor
  static markOop unused_mark() {
    return (markOop) marked_value;
  }
  // the following two functions create the markOop to be
  // stored into object header, it encodes monitor info
  static markOop encode(BasicLock* lock) {
    return (markOop) lock;
  }
  static markOop encode(ObjectMonitor* monitor) {
    intptr_t tmp = (intptr_t) monitor;
    return (markOop) (tmp | monitor_value);
  }
  static markOop encode(JavaThread* thread, int age, int bias_epoch) {
    intptr_t tmp = (intptr_t) thread;
    assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
    assert(age <= max_age, "age too large");
    assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
    return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
  }

  // used to encode pointers during GC
  markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); }

  // age operations
  markOop set_marked()   { return markOop((value() & ~lock_mask_in_place) | marked_value); }

  int     age()               const { return mask_bits(value() >> age_shift, age_mask); }
  markOop set_age(int v) const {
    assert((v & ~age_mask) == 0, "shouldn't overflow age field");
    return markOop((value() & ~age_mask_in_place) | (((intptr_t)v & age_mask) << age_shift));
  }
  markOop incr_age()          const { return age() == max_age ? markOop(this) : set_age(age() + 1); }

  // hash operations
  intptr_t hash() const {
    return mask_bits(value() >> hash_shift, hash_mask);
  }

  bool has_no_hash() const {
    return hash() == no_hash;
  }

  // Prototype mark for initialization
  static markOop prototype() {
    return markOop( no_hash_in_place | no_lock_in_place );
  }

  // Helper function for restoration of unmarked mark oops during GC
  static inline markOop prototype_for_object(oop obj);

  // Debugging
  void print_on(outputStream* st) const;

  // Prepare address of oop for placement into mark
  inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); }

  // Recover address of oop from encoded form used in mark
  inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); }

  // see the definition in markOop.cpp for the gory details
  bool should_not_be_cached() const;
};