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1 /*
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2 * Copyright 2005-2006 Sun Microsystems, Inc. All Rights Reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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20 * CA 95054 USA or visit www.sun.com if you need additional information or
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21 * have any questions.
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22 *
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23 */
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24
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25 // This class describes operations to implement Store-Free Biased
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26 // Locking. The high-level properties of the scheme are similar to
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27 // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased
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28 // locking mechanisms. The principal difference is in the handling of
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29 // recursive locking which is how this technique achieves a more
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30 // efficient fast path than these other schemes.
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31 //
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32 // The basic observation is that in HotSpot's current fast locking
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33 // scheme, recursive locking (in the fast path) causes no update to
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34 // the object header. The recursion is described simply by stack
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35 // records containing a specific value (NULL). Only the last unlock by
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36 // a given thread causes an update to the object header.
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37 //
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38 // This observation, coupled with the fact that HotSpot only compiles
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39 // methods for which monitor matching is obeyed (and which therefore
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40 // can not throw IllegalMonitorStateException), implies that we can
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41 // completely eliminate modifications to the object header for
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42 // recursive locking in compiled code, and perform similar recursion
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43 // checks and throwing of IllegalMonitorStateException in the
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44 // interpreter with little or no impact on the performance of the fast
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45 // path.
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46 //
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47 // The basic algorithm is as follows (note, see below for more details
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48 // and information). A pattern in the low three bits is reserved in
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49 // the object header to indicate whether biasing of a given object's
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50 // lock is currently being done or is allowed at all. If the bias
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51 // pattern is present, the contents of the rest of the header are
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52 // either the JavaThread* of the thread to which the lock is biased,
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53 // or NULL, indicating that the lock is "anonymously biased". The
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54 // first thread which locks an anonymously biased object biases the
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55 // lock toward that thread. If another thread subsequently attempts to
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56 // lock the same object, the bias is revoked.
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57 //
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58 // Because there are no updates to the object header at all during
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59 // recursive locking while the lock is biased, the biased lock entry
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60 // code is simply a test of the object header's value. If this test
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61 // succeeds, the lock has been acquired by the thread. If this test
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62 // fails, a bit test is done to see whether the bias bit is still
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63 // set. If not, we fall back to HotSpot's original CAS-based locking
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64 // scheme. If it is set, we attempt to CAS in a bias toward this
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65 // thread. The latter operation is expected to be the rarest operation
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66 // performed on these locks. We optimistically expect the biased lock
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67 // entry to hit most of the time, and want the CAS-based fallthrough
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68 // to occur quickly in the situations where the bias has been revoked.
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69 //
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70 // Revocation of the lock's bias is fairly straightforward. We want to
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71 // restore the object's header and stack-based BasicObjectLocks and
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72 // BasicLocks to the state they would have been in had the object been
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73 // locked by HotSpot's usual fast locking scheme. To do this, we bring
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74 // the system to a safepoint and walk the stack of the thread toward
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75 // which the lock is biased. We find all of the lock records on the
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76 // stack corresponding to this object, in particular the first /
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77 // "highest" record. We fill in the highest lock record with the
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78 // object's displaced header (which is a well-known value given that
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79 // we don't maintain an identity hash nor age bits for the object
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80 // while it's in the biased state) and all other lock records with 0,
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81 // the value for recursive locks. When the safepoint is released, the
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82 // formerly-biased thread and all other threads revert back to
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83 // HotSpot's CAS-based locking.
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84 //
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85 // This scheme can not handle transfers of biases of single objects
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86 // from thread to thread efficiently, but it can handle bulk transfers
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87 // of such biases, which is a usage pattern showing up in some
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88 // applications and benchmarks. We implement "bulk rebias" and "bulk
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89 // revoke" operations using a "bias epoch" on a per-data-type basis.
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90 // If too many bias revocations are occurring for a particular data
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91 // type, the bias epoch for the data type is incremented at a
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92 // safepoint, effectively meaning that all previous biases are
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93 // invalid. The fast path locking case checks for an invalid epoch in
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94 // the object header and attempts to rebias the object with a CAS if
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95 // found, avoiding safepoints or bulk heap sweeps (the latter which
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96 // was used in a prior version of this algorithm and did not scale
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97 // well). If too many bias revocations persist, biasing is completely
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98 // disabled for the data type by resetting the prototype header to the
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99 // unbiased markOop. The fast-path locking code checks to see whether
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100 // the instance's bias pattern differs from the prototype header's and
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101 // causes the bias to be revoked without reaching a safepoint or,
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102 // again, a bulk heap sweep.
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103
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104 // Biased locking counters
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105 class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC {
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106 private:
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107 int _total_entry_count;
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108 int _biased_lock_entry_count;
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109 int _anonymously_biased_lock_entry_count;
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110 int _rebiased_lock_entry_count;
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111 int _revoked_lock_entry_count;
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112 int _fast_path_entry_count;
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113 int _slow_path_entry_count;
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114
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115 public:
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116 BiasedLockingCounters() :
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117 _total_entry_count(0),
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118 _biased_lock_entry_count(0),
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119 _anonymously_biased_lock_entry_count(0),
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120 _rebiased_lock_entry_count(0),
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121 _revoked_lock_entry_count(0),
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122 _fast_path_entry_count(0),
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123 _slow_path_entry_count(0) {}
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124
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125 int slow_path_entry_count(); // Compute this field if necessary
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126
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127 int* total_entry_count_addr() { return &_total_entry_count; }
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128 int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; }
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129 int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }
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130 int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; }
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131 int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; }
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132 int* fast_path_entry_count_addr() { return &_fast_path_entry_count; }
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133 int* slow_path_entry_count_addr() { return &_slow_path_entry_count; }
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134
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135 bool nonzero() { return _total_entry_count > 0; }
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136
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137 void print_on(outputStream* st);
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138 void print() { print_on(tty); }
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139 };
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140
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141
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142 class BiasedLocking : AllStatic {
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143 private:
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144 static BiasedLockingCounters _counters;
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145
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146 public:
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147 static int* total_entry_count_addr();
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148 static int* biased_lock_entry_count_addr();
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149 static int* anonymously_biased_lock_entry_count_addr();
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150 static int* rebiased_lock_entry_count_addr();
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151 static int* revoked_lock_entry_count_addr();
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152 static int* fast_path_entry_count_addr();
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153 static int* slow_path_entry_count_addr();
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154
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155 enum Condition {
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156 NOT_BIASED = 1,
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157 BIAS_REVOKED = 2,
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158 BIAS_REVOKED_AND_REBIASED = 3
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159 };
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160
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161 // This initialization routine should only be called once and
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162 // schedules a PeriodicTask to turn on biased locking a few seconds
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163 // into the VM run to avoid startup time regressions
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164 static void init();
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165
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166 // This provides a global switch for leaving biased locking disabled
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167 // for the first part of a run and enabling it later
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168 static bool enabled();
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169
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170 // This should be called by JavaThreads to revoke the bias of an object
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171 static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);
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172
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173 // These do not allow rebiasing; they are used by deoptimization to
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174 // ensure that monitors on the stack can be migrated
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175 static void revoke(GrowableArray<Handle>* objs);
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176 static void revoke_at_safepoint(Handle obj);
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177 static void revoke_at_safepoint(GrowableArray<Handle>* objs);
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178
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179 static void print_counters() { _counters.print(); }
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180 static BiasedLockingCounters* counters() { return &_counters; }
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181
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182 // These routines are GC-related and should not be called by end
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183 // users. GCs which do not do preservation of mark words do not need
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184 // to call these routines.
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185 static void preserve_marks();
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186 static void restore_marks();
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187 };
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