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1 /* Copyright (C) 2011-2020 Free Software Foundation, Inc.
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2 Contributed by Torvald Riegel <triegel@redhat.com>.
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3
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4 This file is part of the GNU Transactional Memory Library (libitm).
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5
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6 Libitm is free software; you can redistribute it and/or modify it
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7 under the terms of the GNU General Public License as published by
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8 the Free Software Foundation; either version 3 of the License, or
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9 (at your option) any later version.
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10
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11 Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
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12 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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13 FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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14 more details.
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15
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16 Under Section 7 of GPL version 3, you are granted additional
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17 permissions described in the GCC Runtime Library Exception, version
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18 3.1, as published by the Free Software Foundation.
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19
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20 You should have received a copy of the GNU General Public License and
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21 a copy of the GCC Runtime Library Exception along with this program;
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22 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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23 <http://www.gnu.org/licenses/>. */
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24
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25 #include "libitm_i.h"
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26
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27 using namespace GTM;
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28
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29 namespace {
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30
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31 // This group consists of all TM methods that synchronize via just a single
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32 // global lock (or ownership record).
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33 struct gl_mg : public method_group
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34 {
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35 static const gtm_word LOCK_BIT = (~(gtm_word)0 >> 1) + 1;
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36 // We can't use the full bitrange because ~0 in gtm_thread::shared_state has
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37 // special meaning.
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38 static const gtm_word VERSION_MAX = (~(gtm_word)0 >> 1) - 1;
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39 static bool is_locked(gtm_word l) { return l & LOCK_BIT; }
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40 static gtm_word set_locked(gtm_word l) { return l | LOCK_BIT; }
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41 static gtm_word clear_locked(gtm_word l) { return l & ~LOCK_BIT; }
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42
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43 // The global ownership record.
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44 // No tail-padding necessary (the virtual functions aren't used frequently).
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45 atomic<gtm_word> orec __attribute__((aligned(HW_CACHELINE_SIZE)));
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46
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47 virtual void init()
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48 {
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49 // This store is only executed while holding the serial lock, so relaxed
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50 // memory order is sufficient here.
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51 orec.store(0, memory_order_relaxed);
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52 }
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53 virtual void fini() { }
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54 };
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55
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56 static gl_mg o_gl_mg;
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57
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58
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59 // The global lock, write-through TM method.
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60 // Acquires the orec eagerly before the first write, and then writes through.
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61 // Reads abort if the global orec's version number changed or if it is locked.
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62 // Currently, writes require undo-logging to prevent deadlock between the
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63 // serial lock and the global orec (writer txn acquires orec, reader txn
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64 // upgrades to serial and waits for all other txns, writer tries to upgrade to
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65 // serial too but cannot, writer cannot abort either, deadlock). We could
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66 // avoid this if the serial lock would allow us to prevent other threads from
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67 // going to serial mode, but this probably is too much additional complexity
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68 // just to optimize this TM method.
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69 // gtm_thread::shared_state is used to store a transaction's current
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70 // snapshot time (or commit time). The serial lock uses ~0 for inactive
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71 // transactions and 0 for active ones. Thus, we always have a meaningful
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72 // timestamp in shared_state that can be used to implement quiescence-based
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73 // privatization safety. This even holds if a writing transaction has the
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74 // lock bit set in its shared_state because this is fine for both the serial
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75 // lock (the value will be smaller than ~0) and privatization safety (we
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76 // validate that no other update transaction comitted before we acquired the
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77 // orec, so we have the most recent timestamp and no other transaction can
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78 // commit until we have committed).
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79 // However, we therefore depend on shared_state not being modified by the
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80 // serial lock during upgrades to serial mode, which is ensured by
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81 // gtm_thread::serialirr_mode by not calling gtm_rwlock::write_upgrade_finish
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82 // before we have committed or rolled back.
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83 class gl_wt_dispatch : public abi_dispatch
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84 {
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85 protected:
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86 static void pre_write(const void *addr, size_t len,
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87 gtm_thread *tx = gtm_thr())
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88 {
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89 gtm_word v = tx->shared_state.load(memory_order_relaxed);
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90 if (unlikely(!gl_mg::is_locked(v)))
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91 {
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92 // Check for and handle version number overflow.
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93 if (unlikely(v >= gl_mg::VERSION_MAX))
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94 tx->restart(RESTART_INIT_METHOD_GROUP);
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95
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96 // This validates that we have a consistent snapshot, which is also
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97 // for making privatization safety work (see the class' comments).
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98 // Note that this check here will be performed by the subsequent CAS
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99 // again, so relaxed memory order is fine.
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100 gtm_word now = o_gl_mg.orec.load(memory_order_relaxed);
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101 if (now != v)
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102 tx->restart(RESTART_VALIDATE_WRITE);
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103
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104 // CAS global orec from our snapshot time to the locked state.
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105 // We need acquire memory order here to synchronize with other
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106 // (ownership) releases of the orec. We do not need acq_rel order
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107 // because whenever another thread reads from this CAS'
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108 // modification, then it will abort anyway and does not rely on
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109 // any further happens-before relation to be established.
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110 // Also note that unlike in ml_wt's increase of the global time
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111 // base (remember that the global orec is used as time base), we do
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112 // not need require memory order here because we do not need to make
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113 // prior orec acquisitions visible to other threads that try to
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114 // extend their snapshot time.
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115 if (!o_gl_mg.orec.compare_exchange_strong (now, gl_mg::set_locked(now),
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116 memory_order_acquire))
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117 tx->restart(RESTART_LOCKED_WRITE);
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118
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119 // We use an explicit fence here to avoid having to use release
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120 // memory order for all subsequent data stores. This fence will
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121 // synchronize with loads of the data with acquire memory order. See
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122 // validate() for why this is necessary.
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123 // Adding require memory order to the prior CAS is not sufficient,
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124 // at least according to the Batty et al. formalization of the
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125 // memory model.
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126 atomic_thread_fence(memory_order_release);
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127
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128 // Set shared_state to new value.
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129 tx->shared_state.store(gl_mg::set_locked(now), memory_order_release);
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130 }
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131
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132 tx->undolog.log(addr, len);
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133 }
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134
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135 static void validate(gtm_thread *tx = gtm_thr())
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136 {
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137 // Check that snapshot is consistent. We expect the previous data load to
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138 // have acquire memory order, or be atomic and followed by an acquire
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139 // fence.
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140 // As a result, the data load will synchronize with the release fence
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141 // issued by the transactions whose data updates the data load has read
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142 // from. This forces the orec load to read from a visible sequence of side
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143 // effects that starts with the other updating transaction's store that
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144 // acquired the orec and set it to locked.
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145 // We therefore either read a value with the locked bit set (and restart)
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146 // or read an orec value that was written after the data had been written.
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147 // Either will allow us to detect inconsistent reads because it will have
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148 // a higher/different value.
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149 gtm_word l = o_gl_mg.orec.load(memory_order_relaxed);
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150 if (l != tx->shared_state.load(memory_order_relaxed))
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151 tx->restart(RESTART_VALIDATE_READ);
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152 }
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153
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154 template <typename V> static V load(const V* addr, ls_modifier mod)
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155 {
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156 // Read-for-write should be unlikely, but we need to handle it or will
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157 // break later WaW optimizations.
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158 if (unlikely(mod == RfW))
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159 {
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160 pre_write(addr, sizeof(V));
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161 return *addr;
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162 }
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163 if (unlikely(mod == RaW))
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164 return *addr;
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165
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166 // We do not have acquired the orec, so we need to load a value and then
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167 // validate that this was consistent.
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168 // This needs to have acquire memory order (see validate()).
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169 // Alternatively, we can put an acquire fence after the data load but this
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170 // is probably less efficient.
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171 // FIXME We would need an atomic load with acquire memory order here but
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172 // we can't just forge an atomic load for nonatomic data because this
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173 // might not work on all implementations of atomics. However, we need
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174 // the acquire memory order and we can only establish this if we link
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175 // it to the matching release using a reads-from relation between atomic
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176 // loads. Also, the compiler is allowed to optimize nonatomic accesses
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177 // differently than atomic accesses (e.g., if the load would be moved to
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178 // after the fence, we potentially don't synchronize properly anymore).
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179 // Instead of the following, just use an ordinary load followed by an
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180 // acquire fence, and hope that this is good enough for now:
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181 // V v = atomic_load_explicit((atomic<V>*)addr, memory_order_acquire);
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182 V v = *addr;
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183 atomic_thread_fence(memory_order_acquire);
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184 validate();
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185 return v;
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186 }
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187
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188 template <typename V> static void store(V* addr, const V value,
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189 ls_modifier mod)
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190 {
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191 if (likely(mod != WaW))
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192 pre_write(addr, sizeof(V));
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193 // FIXME We would need an atomic store here but we can't just forge an
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194 // atomic load for nonatomic data because this might not work on all
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195 // implementations of atomics. However, we need this store to link the
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196 // release fence in pre_write() to the acquire operation in load, which
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197 // is only guaranteed if we have a reads-from relation between atomic
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198 // accesses. Also, the compiler is allowed to optimize nonatomic accesses
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199 // differently than atomic accesses (e.g., if the store would be moved
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200 // to before the release fence in pre_write(), things could go wrong).
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201 // atomic_store_explicit((atomic<V>*)addr, value, memory_order_relaxed);
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202 *addr = value;
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203 }
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204
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205 public:
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206 static void memtransfer_static(void *dst, const void* src, size_t size,
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207 bool may_overlap, ls_modifier dst_mod, ls_modifier src_mod)
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208 {
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209 gtm_thread *tx = gtm_thr();
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210 if (dst_mod != WaW && dst_mod != NONTXNAL)
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211 pre_write(dst, size, tx);
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212 // We need at least undo-logging for an RfW src region because we might
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213 // subsequently write there with WaW.
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214 if (src_mod == RfW)
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215 pre_write(src, size, tx);
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216
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217 // FIXME We should use atomics here (see store()). Let's just hope that
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218 // memcpy/memmove are good enough.
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219 if (!may_overlap)
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220 ::memcpy(dst, src, size);
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221 else
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222 ::memmove(dst, src, size);
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223
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224 if (src_mod != RfW && src_mod != RaW && src_mod != NONTXNAL
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225 && dst_mod != WaW)
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226 validate(tx);
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227 }
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228
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229 static void memset_static(void *dst, int c, size_t size, ls_modifier mod)
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230 {
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231 if (mod != WaW)
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232 pre_write(dst, size);
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233 // FIXME We should use atomics here (see store()). Let's just hope that
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234 // memset is good enough.
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235 ::memset(dst, c, size);
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236 }
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237
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238 virtual gtm_restart_reason begin_or_restart()
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239 {
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240 // We don't need to do anything for nested transactions.
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241 gtm_thread *tx = gtm_thr();
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242 if (tx->parent_txns.size() > 0)
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243 return NO_RESTART;
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244
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245 // Spin until global orec is not locked.
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246 // TODO This is not necessary if there are no pure loads (check txn props).
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247 unsigned i = 0;
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248 gtm_word v;
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249 while (1)
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250 {
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251 // We need acquire memory order here so that this load will
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252 // synchronize with the store that releases the orec in trycommit().
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253 // In turn, this makes sure that subsequent data loads will read from
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254 // a visible sequence of side effects that starts with the most recent
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255 // store to the data right before the release of the orec.
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256 v = o_gl_mg.orec.load(memory_order_acquire);
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257 if (!gl_mg::is_locked(v))
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258 break;
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259 // TODO need method-specific max spin count
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260 if (++i > gtm_spin_count_var)
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261 return RESTART_VALIDATE_READ;
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262 cpu_relax();
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263 }
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264
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265 // Everything is okay, we have a snapshot time.
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266 // We don't need to enforce any ordering for the following store. There
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267 // are no earlier data loads in this transaction, so the store cannot
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268 // become visible before those (which could lead to the violation of
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269 // privatization safety). The store can become visible after later loads
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270 // but this does not matter because the previous value will have been
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271 // smaller or equal (the serial lock will set shared_state to zero when
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272 // marking the transaction as active, and restarts enforce immediate
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273 // visibility of a smaller or equal value with a barrier (see
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274 // rollback()).
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275 tx->shared_state.store(v, memory_order_relaxed);
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276 return NO_RESTART;
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277 }
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278
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279 virtual bool trycommit(gtm_word& priv_time)
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280 {
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281 gtm_thread* tx = gtm_thr();
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282 gtm_word v = tx->shared_state.load(memory_order_relaxed);
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283
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284 // Release the orec but do not reset shared_state, which will be modified
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285 // by the serial lock right after our commit anyway. Also, resetting
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286 // shared state here would interfere with the serial lock's use of this
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287 // location.
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288 if (gl_mg::is_locked(v))
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289 {
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290 // Release the global orec, increasing its version number / timestamp.
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291 // See begin_or_restart() for why we need release memory order here.
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292 v = gl_mg::clear_locked(v) + 1;
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293 o_gl_mg.orec.store(v, memory_order_release);
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294 }
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295
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296 // Need to ensure privatization safety. Every other transaction must have
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297 // a snapshot time that is at least as high as our commit time (i.e., our
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298 // commit must be visible to them). Because of proxy privatization, we
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299 // must ensure that even if we are a read-only transaction. See
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300 // ml_wt_dispatch::trycommit() for details: We can't get quite the same
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301 // set of problems because we just use one orec and thus, for example,
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302 // there cannot be concurrent writers -- but we can still get pending
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303 // loads to privatized data when not ensuring privatization safety, which
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304 // is problematic if the program unmaps the privatized memory.
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305 priv_time = v;
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306 return true;
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307 }
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308
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309 virtual void rollback(gtm_transaction_cp *cp)
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310 {
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311 // We don't do anything for rollbacks of nested transactions.
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312 if (cp != 0)
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313 return;
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314
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315 gtm_thread *tx = gtm_thr();
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316 gtm_word v = tx->shared_state.load(memory_order_relaxed);
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317
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318 // Release lock and increment version number to prevent dirty reads.
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319 // Also reset shared state here, so that begin_or_restart() can expect a
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320 // value that is correct wrt. privatization safety.
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321 if (gl_mg::is_locked(v))
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322 {
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323 // With our rollback, global time increases.
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324 v = gl_mg::clear_locked(v) + 1;
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325
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326 // First reset the timestamp published via shared_state. Release
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327 // memory order will make this happen after undoing prior data writes.
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328 // This must also happen before we actually release the global orec
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329 // next, so that future update transactions in other threads observe
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330 // a meaningful snapshot time for our transaction; otherwise, they
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331 // could read a shared_store value with the LOCK_BIT set, which can
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332 // break privatization safety because it's larger than the actual
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333 // snapshot time. Note that we only need to consider other update
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334 // transactions because only those will potentially privatize data.
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335 tx->shared_state.store(v, memory_order_release);
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336
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337 // Release the global orec, increasing its version number / timestamp.
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338 // See begin_or_restart() for why we need release memory order here,
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339 // and we also need it to make future update transactions read the
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340 // prior update to shared_state too (update transactions acquire the
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341 // global orec with acquire memory order).
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342 o_gl_mg.orec.store(v, memory_order_release);
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343 }
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344
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345 }
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346
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347 virtual bool snapshot_most_recent()
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348 {
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349 // This is the same check as in validate() except that we do not restart
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350 // on failure but simply return the result.
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351 return o_gl_mg.orec.load(memory_order_relaxed)
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352 == gtm_thr()->shared_state.load(memory_order_relaxed);
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353 }
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354
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355
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356 CREATE_DISPATCH_METHODS(virtual, )
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357 CREATE_DISPATCH_METHODS_MEM()
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358
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359 gl_wt_dispatch() : abi_dispatch(false, true, false, false, 0, &o_gl_mg)
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360 { }
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361 };
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362
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363 } // anon namespace
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364
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365 static const gl_wt_dispatch o_gl_wt_dispatch;
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366
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367 abi_dispatch *
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368 GTM::dispatch_gl_wt ()
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369 {
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370 return const_cast<gl_wt_dispatch *>(&o_gl_wt_dispatch);
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371 }
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