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1 /*
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2 * Copyright 1997-2006 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 // The markOop describes the header of an object.
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26 //
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27 // Note that the mark is not a real oop but just a word.
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28 // It is placed in the oop hierarchy for historical reasons.
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29 //
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30 // Bit-format of an object header (most significant first):
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31 //
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32 //
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33 // unused:0/25 hash:25/31 age:4 biased_lock:1 lock:2 = 32/64 bits
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34 //
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35 // - hash contains the identity hash value: largest value is
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36 // 31 bits, see os::random(). Also, 64-bit vm's require
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37 // a hash value no bigger than 32 bits because they will not
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38 // properly generate a mask larger than that: see library_call.cpp
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39 // and c1_CodePatterns_sparc.cpp.
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40 //
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41 // - the biased lock pattern is used to bias a lock toward a given
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42 // thread. When this pattern is set in the low three bits, the lock
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43 // is either biased toward a given thread or "anonymously" biased,
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44 // indicating that it is possible for it to be biased. When the
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45 // lock is biased toward a given thread, locking and unlocking can
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46 // be performed by that thread without using atomic operations.
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47 // When a lock's bias is revoked, it reverts back to the normal
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48 // locking scheme described below.
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49 //
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50 // Note that we are overloading the meaning of the "unlocked" state
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51 // of the header. Because we steal a bit from the age we can
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52 // guarantee that the bias pattern will never be seen for a truly
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53 // unlocked object.
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54 //
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55 // Note also that the biased state contains the age bits normally
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56 // contained in the object header. Large increases in scavenge
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57 // times were seen when these bits were absent and an arbitrary age
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58 // assigned to all biased objects, because they tended to consume a
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59 // significant fraction of the eden semispaces and were not
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60 // promoted promptly, causing an increase in the amount of copying
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61 // performed. The runtime system aligns all JavaThread* pointers to
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62 // a very large value (currently 128 bytes) to make room for the
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63 // age bits when biased locking is enabled.
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64 //
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65 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
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66 // [0 | epoch | age | 1 | 01] lock is anonymously biased
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67 //
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68 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
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69 //
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70 // [ptr | 00] locked ptr points to real header on stack
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71 // [header | 0 | 01] unlocked regular object header
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72 // [ptr | 10] monitor inflated lock (header is wapped out)
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73 // [ptr | 11] marked used by markSweep to mark an object
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74 // not valid at any other time
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75 //
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76 // We assume that stack/thread pointers have the lowest two bits cleared.
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77
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78 class BasicLock;
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79 class ObjectMonitor;
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80 class JavaThread;
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81
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82 class markOopDesc: public oopDesc {
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83 private:
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84 // Conversion
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85 uintptr_t value() const { return (uintptr_t) this; }
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86
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87 public:
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88 // Constants
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89 enum { age_bits = 4,
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90 lock_bits = 2,
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91 biased_lock_bits = 1,
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92 max_hash_bits = BitsPerOop - age_bits - lock_bits - biased_lock_bits,
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93 hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits,
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94 epoch_bits = 2
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95 };
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96
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97 // The biased locking code currently requires that the age bits be
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98 // contiguous to the lock bits. Class data sharing would prefer the
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99 // hash bits to be lower down to provide more random hash codes for
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100 // shared read-only symbolOop objects, because these objects' mark
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101 // words are set to their own address with marked_value in the lock
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102 // bit, and using lower bits would make their identity hash values
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103 // more random. However, the performance decision was made in favor
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104 // of the biased locking code.
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105
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106 enum { lock_shift = 0,
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107 biased_lock_shift = lock_bits,
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108 age_shift = lock_bits + biased_lock_bits,
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109 hash_shift = lock_bits + biased_lock_bits + age_bits,
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110 epoch_shift = hash_shift
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111 };
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112
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113 enum { lock_mask = right_n_bits(lock_bits),
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114 lock_mask_in_place = lock_mask << lock_shift,
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115 biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits),
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116 biased_lock_mask_in_place= biased_lock_mask << lock_shift,
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117 biased_lock_bit_in_place = 1 << biased_lock_shift,
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118 age_mask = right_n_bits(age_bits),
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119 age_mask_in_place = age_mask << age_shift,
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120 epoch_mask = right_n_bits(epoch_bits),
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121 epoch_mask_in_place = epoch_mask << epoch_shift
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122 #ifndef _WIN64
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123 ,hash_mask = right_n_bits(hash_bits),
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124 hash_mask_in_place = (address_word)hash_mask << hash_shift
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125 #endif
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126 };
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127
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128 // Alignment of JavaThread pointers encoded in object header required by biased locking
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129 enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits)
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130 };
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131
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132 #ifdef _WIN64
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133 // These values are too big for Win64
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134 const static uintptr_t hash_mask = right_n_bits(hash_bits);
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135 const static uintptr_t hash_mask_in_place =
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136 (address_word)hash_mask << hash_shift;
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137 #endif
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138
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139 enum { locked_value = 0,
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140 unlocked_value = 1,
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141 monitor_value = 2,
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142 marked_value = 3,
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143 biased_lock_pattern = 5
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144 };
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145
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146 enum { no_hash = 0 }; // no hash value assigned
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147
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148 enum { no_hash_in_place = (address_word)no_hash << hash_shift,
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149 no_lock_in_place = unlocked_value
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150 };
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151
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152 enum { max_age = age_mask };
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153
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154 enum { max_bias_epoch = epoch_mask };
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155
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156 // Biased Locking accessors.
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157 // These must be checked by all code which calls into the
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158 // ObjectSynchronizer and other code. The biasing is not understood
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159 // by the lower-level CAS-based locking code, although the runtime
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160 // fixes up biased locks to be compatible with it when a bias is
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161 // revoked.
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162 bool has_bias_pattern() const {
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163 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
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164 }
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165 JavaThread* biased_locker() const {
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166 assert(has_bias_pattern(), "should not call this otherwise");
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167 return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place))));
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168 }
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169 // Indicates that the mark has the bias bit set but that it has not
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170 // yet been biased toward a particular thread
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171 bool is_biased_anonymously() const {
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172 return (has_bias_pattern() && (biased_locker() == NULL));
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173 }
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174 // Indicates epoch in which this bias was acquired. If the epoch
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175 // changes due to too many bias revocations occurring, the biases
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176 // from the previous epochs are all considered invalid.
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177 int bias_epoch() const {
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178 assert(has_bias_pattern(), "should not call this otherwise");
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179 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
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180 }
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181 markOop set_bias_epoch(int epoch) {
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182 assert(has_bias_pattern(), "should not call this otherwise");
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183 assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
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184 return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
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185 }
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186 markOop incr_bias_epoch() {
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187 return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
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188 }
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189 // Prototype mark for initialization
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190 static markOop biased_locking_prototype() {
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191 return markOop( biased_lock_pattern );
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192 }
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193
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194 // lock accessors (note that these assume lock_shift == 0)
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195 bool is_locked() const {
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196 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
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197 }
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198 bool is_unlocked() const {
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199 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
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200 }
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201 bool is_marked() const {
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202 return (mask_bits(value(), lock_mask_in_place) == marked_value);
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203 }
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204 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
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205
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206 // Special temporary state of the markOop while being inflated.
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207 // Code that looks at mark outside a lock need to take this into account.
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208 bool is_being_inflated() const { return (value() == 0); }
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209
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210 // Distinguished markword value - used when inflating over
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211 // an existing stacklock. 0 indicates the markword is "BUSY".
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212 // Lockword mutators that use a LD...CAS idiom should always
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213 // check for and avoid overwriting a 0 value installed by some
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214 // other thread. (They should spin or block instead. The 0 value
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215 // is transient and *should* be short-lived).
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216 static markOop INFLATING() { return (markOop) 0; } // inflate-in-progress
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217
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218 // Should this header be preserved during GC?
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219 bool must_be_preserved(oop obj_containing_mark) const {
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220 if (!UseBiasedLocking)
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221 return (!is_unlocked() || !has_no_hash());
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222 return must_be_preserved_with_bias(obj_containing_mark);
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223 }
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224 inline bool must_be_preserved_with_bias(oop obj_containing_mark) const;
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225
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226 // Should this header (including its age bits) be preserved in the
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227 // case of a promotion failure during scavenge?
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228 // Note that we special case this situation. We want to avoid
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229 // calling BiasedLocking::preserve_marks()/restore_marks() (which
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230 // decrease the number of mark words that need to be preserved
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231 // during GC) during each scavenge. During scavenges in which there
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232 // is no promotion failure, we actually don't need to call the above
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233 // routines at all, since we don't mutate and re-initialize the
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234 // marks of promoted objects using init_mark(). However, during
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235 // scavenges which result in promotion failure, we do re-initialize
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236 // the mark words of objects, meaning that we should have called
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237 // these mark word preservation routines. Currently there's no good
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238 // place in which to call them in any of the scavengers (although
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239 // guarded by appropriate locks we could make one), but the
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240 // observation is that promotion failures are quite rare and
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241 // reducing the number of mark words preserved during them isn't a
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242 // high priority.
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243 bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const {
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244 if (!UseBiasedLocking)
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245 return (this != prototype());
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246 return must_be_preserved_with_bias_for_promotion_failure(obj_containing_mark);
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247 }
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248 inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const;
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249
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250 // Should this header be preserved during a scavenge where CMS is
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251 // the old generation?
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252 // (This is basically the same body as must_be_preserved_for_promotion_failure(),
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253 // but takes the klassOop as argument instead)
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254 bool must_be_preserved_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const {
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255 if (!UseBiasedLocking)
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256 return (this != prototype());
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257 return must_be_preserved_with_bias_for_cms_scavenge(klass_of_obj_containing_mark);
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258 }
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259 inline bool must_be_preserved_with_bias_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const;
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260
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261 // WARNING: The following routines are used EXCLUSIVELY by
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262 // synchronization functions. They are not really gc safe.
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263 // They must get updated if markOop layout get changed.
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264 markOop set_unlocked() const {
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265 return markOop(value() | unlocked_value);
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266 }
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267 bool has_locker() const {
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268 return ((value() & lock_mask_in_place) == locked_value);
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269 }
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270 BasicLock* locker() const {
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271 assert(has_locker(), "check");
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272 return (BasicLock*) value();
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273 }
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274 bool has_monitor() const {
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275 return ((value() & monitor_value) != 0);
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276 }
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277 ObjectMonitor* monitor() const {
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278 assert(has_monitor(), "check");
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279 // Use xor instead of &~ to provide one extra tag-bit check.
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280 return (ObjectMonitor*) (value() ^ monitor_value);
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281 }
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282 bool has_displaced_mark_helper() const {
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283 return ((value() & unlocked_value) == 0);
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284 }
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285 markOop displaced_mark_helper() const {
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286 assert(has_displaced_mark_helper(), "check");
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287 intptr_t ptr = (value() & ~monitor_value);
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288 return *(markOop*)ptr;
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289 }
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290 void set_displaced_mark_helper(markOop m) const {
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291 assert(has_displaced_mark_helper(), "check");
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292 intptr_t ptr = (value() & ~monitor_value);
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293 *(markOop*)ptr = m;
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294 }
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295 markOop copy_set_hash(intptr_t hash) const {
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296 intptr_t tmp = value() & (~hash_mask_in_place);
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297 tmp |= ((hash & hash_mask) << hash_shift);
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298 return (markOop)tmp;
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299 }
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300 // it is only used to be stored into BasicLock as the
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301 // indicator that the lock is using heavyweight monitor
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302 static markOop unused_mark() {
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303 return (markOop) marked_value;
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304 }
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305 // the following two functions create the markOop to be
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306 // stored into object header, it encodes monitor info
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307 static markOop encode(BasicLock* lock) {
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308 return (markOop) lock;
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309 }
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310 static markOop encode(ObjectMonitor* monitor) {
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311 intptr_t tmp = (intptr_t) monitor;
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312 return (markOop) (tmp | monitor_value);
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313 }
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314 static markOop encode(JavaThread* thread, int age, int bias_epoch) {
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315 intptr_t tmp = (intptr_t) thread;
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316 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
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317 assert(age <= max_age, "age too large");
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318 assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
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319 return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
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320 }
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321
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322 // used to encode pointers during GC
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323 markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); }
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324
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325 // age operations
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326 markOop set_marked() { return markOop((value() & ~lock_mask_in_place) | marked_value); }
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327
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328 int age() const { return mask_bits(value() >> age_shift, age_mask); }
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329 markOop set_age(int v) const {
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330 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
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331 return markOop((value() & ~age_mask_in_place) | (((intptr_t)v & age_mask) << age_shift));
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332 }
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333 markOop incr_age() const { return age() == max_age ? markOop(this) : set_age(age() + 1); }
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334
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335 // hash operations
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336 intptr_t hash() const {
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337 return mask_bits(value() >> hash_shift, hash_mask);
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338 }
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339
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340 bool has_no_hash() const {
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341 return hash() == no_hash;
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342 }
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343
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344 // Prototype mark for initialization
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345 static markOop prototype() {
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346 return markOop( no_hash_in_place | no_lock_in_place );
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347 }
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348
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349 // Helper function for restoration of unmarked mark oops during GC
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350 static inline markOop prototype_for_object(oop obj);
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351
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352 // Debugging
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353 void print_on(outputStream* st) const;
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354
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355 // Prepare address of oop for placement into mark
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356 inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); }
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357
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358 // Recover address of oop from encoded form used in mark
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359 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); }
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360 };
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