jdk-24/src/hotspot/share/oops/markWord.hpp

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#ifndef SHARE_OOPS_MARKWORD_HPP
#define SHARE_OOPS_MARKWORD_HPP

#include "metaprogramming/integralConstant.hpp"
#include "metaprogramming/primitiveConversions.hpp"
#include "oops/oopsHierarchy.hpp"
#include "runtime/globals.hpp"

// The markWord describes the header of an object.
//
// Bit-format of an object header (most significant first, big endian layout below):
//
//  32 bits:
//  --------
//             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
//             JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
//
//  64 bits:
//  --------
//  unused:25 hash:31 -->| unused_gap:1   age:4    biased_lock:1 lock:2 (normal object)
//  JavaThread*:54 epoch:2 unused_gap:1   age:4    biased_lock:1 lock:2 (biased object)
//
//  - 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 (32bVM) or 256 bytes (64bVM))
//    to make room for the age bits & the epoch bits (used in support of
//    biased locking).
//
//    [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 markWord {
 private:
  uintptr_t _value;

 public:
  explicit markWord(uintptr_t value) : _value(value) {}

  markWord() { /* uninitialized */}

  // It is critical for performance that this class be trivially
  // destructable, copyable, and assignable.

  static markWord from_pointer(void* ptr) {
    return markWord((uintptr_t)ptr);
  }
  void* to_pointer() const {
    return (void*)_value;
  }

  bool operator==(const markWord& other) const {
    return _value == other._value;
  }
  bool operator!=(const markWord& other) const {
    return !operator==(other);
  }

  // Conversion
  uintptr_t value() const { return _value; }

  // Constants
  static const int age_bits                       = 4;
  static const int lock_bits                      = 2;
  static const int biased_lock_bits               = 1;
  static const int max_hash_bits                  = BitsPerWord - age_bits - lock_bits - biased_lock_bits;
  static const int hash_bits                      = max_hash_bits > 31 ? 31 : max_hash_bits;
  static const int unused_gap_bits                = LP64_ONLY(1) NOT_LP64(0);
  static const int epoch_bits                     = 2;

  // The biased locking code currently requires that the age bits be
  // contiguous to the lock bits.
  static const int lock_shift                     = 0;
  static const int biased_lock_shift              = lock_bits;
  static const int age_shift                      = lock_bits + biased_lock_bits;
  static const int unused_gap_shift               = age_shift + age_bits;
  static const int hash_shift                     = unused_gap_shift + unused_gap_bits;
  static const int epoch_shift                    = hash_shift;

  static const uintptr_t lock_mask                = right_n_bits(lock_bits);
  static const uintptr_t lock_mask_in_place       = lock_mask << lock_shift;
  static const uintptr_t biased_lock_mask         = right_n_bits(lock_bits + biased_lock_bits);
  static const uintptr_t biased_lock_mask_in_place= biased_lock_mask << lock_shift;
  static const uintptr_t biased_lock_bit_in_place = 1 << biased_lock_shift;
  static const uintptr_t age_mask                 = right_n_bits(age_bits);
  static const uintptr_t age_mask_in_place        = age_mask << age_shift;
  static const uintptr_t epoch_mask               = right_n_bits(epoch_bits);
  static const uintptr_t epoch_mask_in_place      = epoch_mask << epoch_shift;

  static const uintptr_t hash_mask                = right_n_bits(hash_bits);
  static const uintptr_t hash_mask_in_place       = hash_mask << hash_shift;

  // Alignment of JavaThread pointers encoded in object header required by biased locking
  static const size_t biased_lock_alignment       = 2 << (epoch_shift + epoch_bits);

  static const uintptr_t locked_value             = 0;
  static const uintptr_t unlocked_value           = 1;
  static const uintptr_t monitor_value            = 2;
  static const uintptr_t marked_value             = 3;
  static const uintptr_t biased_lock_pattern      = 5;

  static const uintptr_t no_hash                  = 0 ;  // no hash value assigned
  static const uintptr_t no_hash_in_place         = (address_word)no_hash << hash_shift;
  static const uintptr_t no_lock_in_place         = unlocked_value;

  static const uint max_age                       = age_mask;

  static const int max_bias_epoch                 = epoch_mask;

  // Creates a markWord with all bits set to zero.
  static markWord zero() { return markWord(uintptr_t(0)); }

  // 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*) 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);
  }
  markWord set_bias_epoch(int epoch) {
    assert(has_bias_pattern(), "should not call this otherwise");
    assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
    return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
  }
  markWord incr_bias_epoch() {
    return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
  }
  // Prototype mark for initialization
  static markWord biased_locking_prototype() {
    return markWord( 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 markWord 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 markWord INFLATING() { return zero(); }    // inflate-in-progress

  // Should this header be preserved during GC?
  template <typename KlassProxy>
  inline bool must_be_preserved(KlassProxy klass) 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.
  template <typename KlassProxy>
  inline bool must_be_preserved_for_promotion_failure(KlassProxy klass) const;

  // WARNING: The following routines are used EXCLUSIVELY by
  // synchronization functions. They are not really gc safe.
  // They must get updated if markWord layout get changed.
  markWord set_unlocked() const {
    return markWord(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);
  }
  markWord displaced_mark_helper() const {
    assert(has_displaced_mark_helper(), "check");
    uintptr_t ptr = (value() & ~monitor_value);
    return *(markWord*)ptr;
  }
  void set_displaced_mark_helper(markWord m) const {
    assert(has_displaced_mark_helper(), "check");
    uintptr_t ptr = (value() & ~monitor_value);
    ((markWord*)ptr)->_value = m._value;
  }
  markWord copy_set_hash(intptr_t hash) const {
    uintptr_t tmp = value() & (~hash_mask_in_place);
    tmp |= ((hash & hash_mask) << hash_shift);
    return markWord(tmp);
  }
  // it is only used to be stored into BasicLock as the
  // indicator that the lock is using heavyweight monitor
  static markWord unused_mark() {
    return markWord(marked_value);
  }
  // the following two functions create the markWord to be
  // stored into object header, it encodes monitor info
  static markWord encode(BasicLock* lock) {
    return from_pointer(lock);
  }
  static markWord encode(ObjectMonitor* monitor) {
    uintptr_t tmp = (uintptr_t) monitor;
    return markWord(tmp | monitor_value);
  }
  static markWord encode(JavaThread* thread, uint age, int bias_epoch) {
    uintptr_t tmp = (uintptr_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 markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
  }

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

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

  uint     age()           const { return mask_bits(value() >> age_shift, age_mask); }
  markWord set_age(uint v) const {
    assert((v & ~age_mask) == 0, "shouldn't overflow age field");
    return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
  }
  markWord incr_age()      const { return age() == max_age ? markWord(_value) : 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 markWord prototype() {
    return markWord( no_hash_in_place | no_lock_in_place );
  }

  // Helper function for restoration of unmarked mark oops during GC
  static inline markWord prototype_for_klass(const Klass* klass);

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

  // Prepare address of oop for placement into mark
  inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(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 (void*)clear_lock_bits().value(); }
};

// Support atomic operations.
template<>
struct PrimitiveConversions::Translate<markWord> : public TrueType {
  typedef markWord Value;
  typedef uintptr_t Decayed;

  static Decayed decay(const Value& x) { return x.value(); }
  static Value recover(Decayed x) { return Value(x); }
};

#endif // SHARE_OOPS_MARKWORD_HPP