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1 /* Copyright (C) 2009-2020 Free Software Foundation, Inc.
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2 Contributed by Richard Henderson <rth@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 /* Exceptions can exist in three phases: (1) after having been allocated by
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30 __cxa_allocate_exception but before being handed off to __cxa_throw,
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31 (2) when they are in flight, so between __cxa_throw and __cxa_begin_catch,
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32 and (3) when they are being handled (between __cxa_begin_catch and
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33 __cxa_end_catch). Note that when an exception is re-thrown in (3), it is
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34 not moving back to (2) but handled as a special case of (3) by the EH
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35 runtime.
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36
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37 We can get aborts in all three phases, for example in (1) during
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38 construction of the exception object, or in (2) in destructors called
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39 while unwinding the stack. The transaction that created an exception
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40 object can only commit in phase (3) by re-throwing the exception; it cannot
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41 commit in other phases because throw expressions and catch clauses are
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42 properly nested wrt transactions and because the compiler wraps
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43 transaction bodies in a try/catch-all construct.
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44
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45 We handle phase (1) by dealing with exception objects similar to how we
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46 deal with other (de)allocations, which also ensures that we can have more
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47 than one exception object allocated at the same time (e.g., if the
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48 throw expression itself throws an exception and thus calls
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49 __cxa_allocate_exception). However, on the call to __cxa_begin_catch
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50 we hand off the exception to the special handling of phase (3) and
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51 remove the undo log entry of the allocation. Note that if the allocation
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52 happened outside of this transaction, we do not need to do anything.
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53
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54 When an exception reaches phase (2) due to a call to __cxa_throw, the count
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55 of uncaught exceptions is incremented. We roll back this effect by saving
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56 and restoring this number in the structure returned from __cxa_get_globals.
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57 This also takes care of increments of this count when re-throwing an
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58 exception.
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59
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60 For phase (3), we keep track of the number of times __cxa_begin_catch
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61 has been called without a matching call to __cxa_end_catch. This count
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62 is then used by __cxa_tm_cleanup to roll back the exception handling state
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63 by calling __cxa_end_catch for the exceptions that have not been finished
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64 yet (without running destructors though because we roll back the memory
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65 anyway).
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66 Once an exception that was allocated in this transaction enters phase (3),
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67 it does not need to be deallocated on abort anymore because the calls to
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68 __cxa_end_catch will take care of that.
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69
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70 We require all code executed by the transaction to be transaction_safe (or
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71 transaction_pure, or to have wrappers) if the transaction is to be rolled
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72 back. However, we take care to not require this for transactions that
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73 just commit; this way, transactions that enter serial mode and then call
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74 uninstrumented code continue to work.
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75 */
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76
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77 /* Everything from libstdc++ is weak, to avoid requiring that library
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78 to be linked into plain C applications using libitm.so. */
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79
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80 #define WEAK __attribute__((weak))
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81
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82 extern "C" {
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83
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84 struct __cxa_eh_globals
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85 {
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86 void * caughtExceptions;
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87 unsigned int uncaughtExceptions;
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88 };
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89
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90 extern void *__cxa_allocate_exception (size_t) WEAK;
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91 extern void __cxa_free_exception (void *) WEAK;
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145
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92 extern void __cxa_throw (void *, void *, void (*) (void *)) WEAK;
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111
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93 extern void *__cxa_begin_catch (void *) WEAK;
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94 extern void __cxa_end_catch (void) WEAK;
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95 extern void __cxa_tm_cleanup (void *, void *, unsigned int) WEAK;
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96 extern __cxa_eh_globals *__cxa_get_globals (void) WEAK;
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97
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98 #if !defined (HAVE_ELF_STYLE_WEAKREF)
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99 void *__cxa_allocate_exception (size_t) { return NULL; }
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100 void __cxa_free_exception (void *) { return; }
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101 void __cxa_throw (void *, void *, void (*) (void *)) { return; }
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102 void *__cxa_begin_catch (void *) { return NULL; }
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103 void __cxa_end_catch (void) { return; }
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104 void __cxa_tm_cleanup (void *, void *, unsigned int) { return; }
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105 void _Unwind_DeleteException (_Unwind_Exception *) { return; }
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106 __cxa_eh_globals *__cxa_get_globals (void) { return NULL; }
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107 #endif /* HAVE_ELF_STYLE_WEAKREF */
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108
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109 }
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110
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111 static void
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112 free_any_exception (void *exc_ptr)
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113 {
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114 // The exception could be in phase (2) and thus calling just
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115 // _cxa_free_exception might not be sufficient.
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116 __cxa_tm_cleanup (NULL, exc_ptr, 0);
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117 }
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118
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119 void *
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120 _ITM_cxa_allocate_exception (size_t size)
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121 {
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122 void *r = __cxa_allocate_exception (size);
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123 gtm_thr()->record_allocation (r, free_any_exception);
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124 return r;
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125 }
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126
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127 void
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128 _ITM_cxa_free_exception (void *exc_ptr)
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129 {
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130 // __cxa_free_exception can be called from user code directly if
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131 // construction of an exception object throws another exception, in which
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132 // case we need to roll back the initial exception. We handle this similar
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133 // to dead allocations in that we deallocate the exception on both commit
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134 // and abort of an outermost transaction.
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135 gtm_thr()->forget_allocation (exc_ptr, free_any_exception);
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136 }
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137
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138 void
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139 _ITM_cxa_throw (void *obj, void *tinfo, void (*dest) (void *))
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140 {
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141 // This used to be instrumented, but does not need to be anymore.
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142 __cxa_throw (obj, tinfo, dest);
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143 }
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144
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145 void *
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146 _ITM_cxa_begin_catch (void *exc_ptr)
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147 {
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148 // If this exception object has been allocated by this transaction, we
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149 // discard the undo log entry for the allocation; we are entering phase (3)
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150 // now and will handle this exception specially.
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151 // Note that this exception cannot have been allocated in a parent
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152 // transaction or enclosing nontransactional block because an atomic block
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153 // cannot contain just a catch clause but not the associated try clause.
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154 // The exception can have been allocated in a nested transaction, in which
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155 // case the commit of the nested transaction will have inserted the undo
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156 // log entry of the allocation in our undo log.
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157 // The exception can also have been allocated in a nested nontransactional
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158 // block, but then this transaction cannot abort anymore; functions that
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159 // are marked transaction_pure, for example, must not side-step the
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160 // transactional exception handling we implement here.
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161 gtm_thread *t = gtm_thr ();
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162 t->discard_allocation (exc_ptr);
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163 // Keep track of the number of unfinished catch handlers.
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164 t->cxa_catch_count++;
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165 return __cxa_begin_catch (exc_ptr);
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166 }
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167
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168 void
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169 _ITM_cxa_end_catch (void)
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170 {
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171 // Keep track of the number of unfinished catch handlers.
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172 gtm_thr()->cxa_catch_count--;
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173 __cxa_end_catch ();
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174 }
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175
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176 void
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177 GTM::gtm_thread::init_cpp_exceptions ()
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178 {
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179 // Only save and restore the number of uncaught exceptions if this is
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180 // actually used in the program.
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181 if (__cxa_get_globals != NULL && __cxa_get_globals () != 0)
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182 cxa_uncaught_count_ptr = &__cxa_get_globals ()->uncaughtExceptions;
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183 else
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184 cxa_uncaught_count_ptr = 0;
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185 }
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186
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187 void
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188 GTM::gtm_thread::revert_cpp_exceptions (gtm_transaction_cp *cp)
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189 {
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190 if (cp)
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191 {
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192 // If rolling back a nested transaction, only clean up incompletely
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193 // caught exceptions since the last checkpoint.
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194 assert (cxa_catch_count >= cp->cxa_catch_count);
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195 uint32_t catch_count = cxa_catch_count - cp->cxa_catch_count;
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196 if (catch_count)
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197 {
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198 __cxa_tm_cleanup (NULL, NULL, catch_count);
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199 cxa_catch_count = cp->cxa_catch_count;
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200 }
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201 }
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202 else
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203 {
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204 // Both cxa_catch_count and cxa_unthrown are maximal because EH regions
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205 // and transactions are properly nested.
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206 if (cxa_catch_count)
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207 {
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208 __cxa_tm_cleanup (NULL, NULL, cxa_catch_count);
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209 cxa_catch_count = 0;
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210 }
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211 }
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212 // Reset the number of uncaught exceptions. Any allocations for these
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213 // exceptions have been rolled back already, if necessary.
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214 if (cxa_uncaught_count_ptr != 0)
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215 *cxa_uncaught_count_ptr = cxa_uncaught_count;
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216 // Always reset eh_in_flight because it just contains the argument provided
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217 // to _ITM_commitTransactionEH.
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218 eh_in_flight = NULL;
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219 }
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