/*+
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 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.+
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.+
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 *+
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 * This code is free software; you can redistribute it and/or modify it+
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 * under the terms of the GNU General Public License version 2 only, as+
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 * published by the Free Software Foundation.+
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 *+
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 * This code is distributed in the hope that it will be useful, but WITHOUT+
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or+
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License+
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 * version 2 for more details (a copy is included in the LICENSE file that+
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 * accompanied this code).+
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 *+
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 * You should have received a copy of the GNU General Public License version+
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 * 2 along with this work; if not, write to the Free Software Foundation,+
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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.+
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 *+
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 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA+
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 * or visit www.oracle.com if you need additional information or have any+
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 * questions.+
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 *+
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 */+
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+
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#ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP+
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#define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP+
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+
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#include "runtime/handles.hpp"+
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#include "utilities/growableArray.hpp"+
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+
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// This class describes operations to implement Store-Free Biased+
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// Locking. The high-level properties of the scheme are similar to+
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// IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased+
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// locking mechanisms. The principal difference is in the handling of+
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// recursive locking which is how this technique achieves a more+
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// efficient fast path than these other schemes.+
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//+
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// The basic observation is that in HotSpot's current fast locking+
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// scheme, recursive locking (in the fast path) causes no update to+
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// the object header. The recursion is described simply by stack+
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// records containing a specific value (NULL). Only the last unlock by+
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// a given thread causes an update to the object header.+
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//+
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// This observation, coupled with the fact that HotSpot only compiles+
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// methods for which monitor matching is obeyed (and which therefore+
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// can not throw IllegalMonitorStateException), implies that we can+
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// completely eliminate modifications to the object header for+
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// recursive locking in compiled code, and perform similar recursion+
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// checks and throwing of IllegalMonitorStateException in the+
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// interpreter with little or no impact on the performance of the fast+
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// path.+
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//+
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// The basic algorithm is as follows (note, see below for more details+
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// and information). A pattern in the low three bits is reserved in+
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// the object header to indicate whether biasing of a given object's+
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// lock is currently being done or is allowed at all.  If the bias+
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// pattern is present, the contents of the rest of the header are+
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// either the JavaThread* of the thread to which the lock is biased,+
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// or NULL, indicating that the lock is "anonymously biased". The+
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// first thread which locks an anonymously biased object biases the+
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// lock toward that thread. If another thread subsequently attempts to+
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// lock the same object, the bias is revoked.+
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//+
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// Because there are no updates to the object header at all during+
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// recursive locking while the lock is biased, the biased lock entry+
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// code is simply a test of the object header's value. If this test+
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// succeeds, the lock has been acquired by the thread. If this test+
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// fails, a bit test is done to see whether the bias bit is still+
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// set. If not, we fall back to HotSpot's original CAS-based locking+
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// scheme. If it is set, we attempt to CAS in a bias toward this+
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// thread. The latter operation is expected to be the rarest operation+
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// performed on these locks. We optimistically expect the biased lock+
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// entry to hit most of the time, and want the CAS-based fallthrough+
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// to occur quickly in the situations where the bias has been revoked.+
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//+
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// Revocation of the lock's bias is fairly straightforward. We want to+
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// restore the object's header and stack-based BasicObjectLocks and+
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// BasicLocks to the state they would have been in had the object been+
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// locked by HotSpot's usual fast locking scheme. To do this, we bring+
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// the system to a safepoint and walk the stack of the thread toward+
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// which the lock is biased. We find all of the lock records on the+
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// stack corresponding to this object, in particular the first /+
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// "highest" record. We fill in the highest lock record with the+
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// object's displaced header (which is a well-known value given that+
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// we don't maintain an identity hash nor age bits for the object+
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// while it's in the biased state) and all other lock records with 0,+
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// the value for recursive locks. When the safepoint is released, the+
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// formerly-biased thread and all other threads revert back to+
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// HotSpot's CAS-based locking.+
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//+
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// This scheme can not handle transfers of biases of single objects+
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// from thread to thread efficiently, but it can handle bulk transfers+
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// of such biases, which is a usage pattern showing up in some+
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// applications and benchmarks. We implement "bulk rebias" and "bulk+
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// revoke" operations using a "bias epoch" on a per-data-type basis.+
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// If too many bias revocations are occurring for a particular data+
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// type, the bias epoch for the data type is incremented at a+
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// safepoint, effectively meaning that all previous biases are+
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// invalid. The fast path locking case checks for an invalid epoch in+
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// the object header and attempts to rebias the object with a CAS if+
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// found, avoiding safepoints or bulk heap sweeps (the latter which+
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// was used in a prior version of this algorithm and did not scale+
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// well). If too many bias revocations persist, biasing is completely+
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// disabled for the data type by resetting the prototype header to the+
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// unbiased markOop. The fast-path locking code checks to see whether+
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// the instance's bias pattern differs from the prototype header's and+
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// causes the bias to be revoked without reaching a safepoint or,+
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// again, a bulk heap sweep.+
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+
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// Biased locking counters+
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class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC {+
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 private:+
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  int _total_entry_count;+
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  int _biased_lock_entry_count;+
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  int _anonymously_biased_lock_entry_count;+
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  int _rebiased_lock_entry_count;+
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  int _revoked_lock_entry_count;+
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  int _fast_path_entry_count;+
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  int _slow_path_entry_count;+
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+
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 public:+
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  BiasedLockingCounters() :+
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    _total_entry_count(0),+
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    _biased_lock_entry_count(0),+
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    _anonymously_biased_lock_entry_count(0),+
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    _rebiased_lock_entry_count(0),+
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    _revoked_lock_entry_count(0),+
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    _fast_path_entry_count(0),+
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    _slow_path_entry_count(0) {}+
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+
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  int slow_path_entry_count(); // Compute this field if necessary+
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+
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  int* total_entry_count_addr()                   { return &_total_entry_count; }+
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  int* biased_lock_entry_count_addr()             { return &_biased_lock_entry_count; }+
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  int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }+
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  int* rebiased_lock_entry_count_addr()           { return &_rebiased_lock_entry_count; }+
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  int* revoked_lock_entry_count_addr()            { return &_revoked_lock_entry_count; }+
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  int* fast_path_entry_count_addr()               { return &_fast_path_entry_count; }+
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  int* slow_path_entry_count_addr()               { return &_slow_path_entry_count; }+
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+
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  bool nonzero() { return _total_entry_count > 0; }+
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+
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  void print_on(outputStream* st);+
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  void print() { print_on(tty); }+
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};+
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+
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+
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class BiasedLocking : AllStatic {+
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private:+
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  static BiasedLockingCounters _counters;+
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+
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public:+
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  static int* total_entry_count_addr();+
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  static int* biased_lock_entry_count_addr();+
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  static int* anonymously_biased_lock_entry_count_addr();+
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  static int* rebiased_lock_entry_count_addr();+
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  static int* revoked_lock_entry_count_addr();+
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  static int* fast_path_entry_count_addr();+
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  static int* slow_path_entry_count_addr();+
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+
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  enum Condition {+
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    NOT_BIASED = 1,+
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    BIAS_REVOKED = 2,+
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    BIAS_REVOKED_AND_REBIASED = 3+
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  };+
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+
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  // This initialization routine should only be called once and+
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  // schedules a PeriodicTask to turn on biased locking a few seconds+
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  // into the VM run to avoid startup time regressions+
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  static void init();+
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+
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  // This provides a global switch for leaving biased locking disabled+
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  // for the first part of a run and enabling it later+
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  static bool enabled();+
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+
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  // This should be called by JavaThreads to revoke the bias of an object+
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  static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);+
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+
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  // These do not allow rebiasing; they are used by deoptimization to+
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  // ensure that monitors on the stack can be migrated+
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  static void revoke(GrowableArray<Handle>* objs);+
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  static void revoke_at_safepoint(Handle obj);+
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  static void revoke_at_safepoint(GrowableArray<Handle>* objs);+
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+
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  static void print_counters() { _counters.print(); }+
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  static BiasedLockingCounters* counters() { return &_counters; }+
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+
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  // These routines are GC-related and should not be called by end+
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  // users. GCs which do not do preservation of mark words do not need+
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  // to call these routines.+
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  static void preserve_marks();+
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  static void restore_marks();+
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};+
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+
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#endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP+
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