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 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

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 * 2 along with this work; if not, write to the Free Software Foundation,

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// This class describes operations to implement Store-Free Biased

// Locking. The high-level properties of the scheme are similar to

// IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased

// locking mechanisms. The principal difference is in the handling of

// recursive locking which is how this technique achieves a more

// efficient fast path than these other schemes.

//

// The basic observation is that in HotSpot's current fast locking

// scheme, recursive locking (in the fast path) causes no update to

// the object header. The recursion is described simply by stack

// records containing a specific value (NULL). Only the last unlock by

// a given thread causes an update to the object header.

//

// This observation, coupled with the fact that HotSpot only compiles

// methods for which monitor matching is obeyed (and which therefore

// can not throw IllegalMonitorStateException), implies that we can

// completely eliminate modifications to the object header for

// recursive locking in compiled code, and perform similar recursion

// checks and throwing of IllegalMonitorStateException in the

// interpreter with little or no impact on the performance of the fast

// path.

//

// The basic algorithm is as follows (note, see below for more details

// and information). A pattern in the low three bits is reserved in

// the object header to indicate whether biasing of a given object's

// lock is currently being done or is allowed at all.  If the bias

// pattern is present, the contents of the rest of the header are

// either the JavaThread* of the thread to which the lock is biased,

// or NULL, indicating that the lock is "anonymously biased". The

// first thread which locks an anonymously biased object biases the

// lock toward that thread. If another thread subsequently attempts to

// lock the same object, the bias is revoked.

//

// Because there are no updates to the object header at all during

// recursive locking while the lock is biased, the biased lock entry

// code is simply a test of the object header's value. If this test

// succeeds, the lock has been acquired by the thread. If this test

// fails, a bit test is done to see whether the bias bit is still

// set. If not, we fall back to HotSpot's original CAS-based locking

// scheme. If it is set, we attempt to CAS in a bias toward this

// thread. The latter operation is expected to be the rarest operation

// performed on these locks. We optimistically expect the biased lock

// entry to hit most of the time, and want the CAS-based fallthrough

// to occur quickly in the situations where the bias has been revoked.

//

// Revocation of the lock's bias is fairly straightforward. We want to

// restore the object's header and stack-based BasicObjectLocks and

// BasicLocks to the state they would have been in had the object been

// locked by HotSpot's usual fast locking scheme. To do this, we bring

// the system to a safepoint and walk the stack of the thread toward

// which the lock is biased. We find all of the lock records on the

// stack corresponding to this object, in particular the first /

// "highest" record. We fill in the highest lock record with the

// object's displaced header (which is a well-known value given that

// we don't maintain an identity hash nor age bits for the object

// while it's in the biased state) and all other lock records with 0,

// the value for recursive locks. When the safepoint is released, the

// formerly-biased thread and all other threads revert back to

// HotSpot's CAS-based locking.

//

// This scheme can not handle transfers of biases of single objects

// from thread to thread efficiently, but it can handle bulk transfers

// of such biases, which is a usage pattern showing up in some

// applications and benchmarks. We implement "bulk rebias" and "bulk

// revoke" operations using a "bias epoch" on a per-data-type basis.

// If too many bias revocations are occurring for a particular data

// type, the bias epoch for the data type is incremented at a

// safepoint, effectively meaning that all previous biases are

// invalid. The fast path locking case checks for an invalid epoch in

// the object header and attempts to rebias the object with a CAS if

// found, avoiding safepoints or bulk heap sweeps (the latter which

// was used in a prior version of this algorithm and did not scale

// well). If too many bias revocations persist, biasing is completely

// disabled for the data type by resetting the prototype header to the

// unbiased markOop. The fast-path locking code checks to see whether

// the instance's bias pattern differs from the prototype header's and

// causes the bias to be revoked without reaching a safepoint or,

// again, a bulk heap sweep.

// Biased locking counters

class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC {

 private:

  int _total_entry_count;

  int _biased_lock_entry_count;

  int _anonymously_biased_lock_entry_count;

  int _rebiased_lock_entry_count;

  int _revoked_lock_entry_count;

  int _fast_path_entry_count;

  int _slow_path_entry_count;

 public:

  BiasedLockingCounters() :

    _total_entry_count(0),

    _biased_lock_entry_count(0),

    _anonymously_biased_lock_entry_count(0),

    _rebiased_lock_entry_count(0),

    _revoked_lock_entry_count(0),

    _fast_path_entry_count(0),

    _slow_path_entry_count(0) {}

  int slow_path_entry_count(); // Compute this field if necessary

  int* total_entry_count_addr()                   { return &_total_entry_count; }

  int* biased_lock_entry_count_addr()             { return &_biased_lock_entry_count; }

  int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }

  int* rebiased_lock_entry_count_addr()           { return &_rebiased_lock_entry_count; }

  int* revoked_lock_entry_count_addr()            { return &_revoked_lock_entry_count; }

  int* fast_path_entry_count_addr()               { return &_fast_path_entry_count; }

  int* slow_path_entry_count_addr()               { return &_slow_path_entry_count; }

  bool nonzero() { return _total_entry_count > 0; }

  void print_on(outputStream* st);

  void print() { print_on(tty); }

};

class BiasedLocking : AllStatic {

private:

  static BiasedLockingCounters _counters;

public:

  static int* total_entry_count_addr();

  static int* biased_lock_entry_count_addr();

  static int* anonymously_biased_lock_entry_count_addr();

  static int* rebiased_lock_entry_count_addr();

  static int* revoked_lock_entry_count_addr();

  static int* fast_path_entry_count_addr();

  static int* slow_path_entry_count_addr();

  enum Condition {

    NOT_BIASED = 1,

    BIAS_REVOKED = 2,

    BIAS_REVOKED_AND_REBIASED = 3

  };

  // This initialization routine should only be called once and

  // schedules a PeriodicTask to turn on biased locking a few seconds

  // into the VM run to avoid startup time regressions

  static void init();

  // This provides a global switch for leaving biased locking disabled

  // for the first part of a run and enabling it later

  static bool enabled();

  // This should be called by JavaThreads to revoke the bias of an object

  static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);

  // These do not allow rebiasing; they are used by deoptimization to

  // ensure that monitors on the stack can be migrated

  static void revoke(GrowableArray<Handle>* objs);

  static void revoke_at_safepoint(Handle obj);

  static void revoke_at_safepoint(GrowableArray<Handle>* objs);

  static void print_counters() { _counters.print(); }

  static BiasedLockingCounters* counters() { return &_counters; }

  // These routines are GC-related and should not be called by end

  // users. GCs which do not do preservation of mark words do not need

  // to call these routines.

  static void preserve_marks();

  static void restore_marks();

};