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annotate src/atomic/SDL_atomic.c @ 5003:3a95a2b93eb3

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Updated the atomic API for better use cases
author Sam Lantinga <slouken@libsdl.org>
date Sat, 15 Jan 2011 12:41:59 -0800
parents
children 0c72ae7b7cb2
rev   line source
5003
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1 /*
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2 SDL - Simple DirectMedia Layer
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3 Copyright (C) 1997-2010 Sam Lantinga
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4
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5 This library is free software; you can redistribute it and/or
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6 modify it under the terms of the GNU Lesser General Public
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7 License as published by the Free Software Foundation; either
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8 version 2.1 of the License, or (at your option) any later version.
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9
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10 This library is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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13 Lesser General Public License for more details.
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14
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15 You should have received a copy of the GNU Lesser General Public
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16 License along with this library; if not, write to the Free Software
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17 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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18
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19 Sam Lantinga
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20 slouken@libsdl.org
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21 */
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22 #include "SDL_stdinc.h"
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23
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24 #include "SDL_atomic.h"
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25
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26 /*
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27 If any of the operations are not provided then we must emulate some
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28 of them. That means we need a nice implementation of spin locks
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29 that avoids the "one big lock" problem. We use a vector of spin
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30 locks and pick which one to use based on the address of the operand
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31 of the function.
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32
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33 To generate the index of the lock we first shift by 3 bits to get
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34 rid on the zero bits that result from 32 and 64 bit allignment of
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35 data. We then mask off all but 5 bits and use those 5 bits as an
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36 index into the table.
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37
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38 Picking the lock this way insures that accesses to the same data at
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39 the same time will go to the same lock. OTOH, accesses to different
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40 data have only a 1/32 chance of hitting the same lock. That should
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41 pretty much eliminate the chances of several atomic operations on
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42 different data from waiting on the same "big lock". If it isn't
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43 then the table of locks can be expanded to a new size so long as
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44 the new size is a power of two.
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45
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46 Contributed by Bob Pendleton, bob@pendleton.com
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47 */
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48
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49 static SDL_SpinLock locks[32];
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50
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51 static __inline__ void
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52 enterLock(void *a)
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53 {
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54 uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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55
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56 SDL_AtomicLock(&locks[index]);
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57 }
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58
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59 static __inline__ void
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60 leaveLock(void *a)
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61 {
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62 uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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63
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64 SDL_AtomicUnlock(&locks[index]);
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65 }
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66
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67 #ifndef SDL_AtomicSet
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68 int
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69 SDL_AtomicSet(SDL_atomic_t *a, int value)
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70 {
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71 int oldvalue;
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72
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73 enterLock(a);
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74 oldvalue = a->value;
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75 a->value = value;
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76 leaveLock(a);
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77
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78 return oldvalue;
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79 }
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80 #endif
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81
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82 #ifndef SDL_AtomicGet
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83 int
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84 SDL_AtomicGet(SDL_atomic_t *a)
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85 {
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86 /* Assuming integral reads on this platform, we're safe here since the
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87 functions that set the variable have the necessary memory barriers.
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88 */
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89 return a->value;
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90 }
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91 #endif
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92
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93 #ifndef SDL_AtomicAdd
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94 int
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95 SDL_AtomicAdd(SDL_atomic_t *a, int value)
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96 {
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97 int oldvalue;
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98
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99 enterLock(a);
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100 oldvalue = a->value;
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101 a->value += value;
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102 leaveLock(a);
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103
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104 return oldvalue;
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105 }
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106 #endif
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107
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108 #ifndef SDL_AtomicIncRef
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109 void
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110 SDL_AtomicIncRef(SDL_atomic_t *a)
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111 {
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112 SDL_AtomicAdd(a, 1);
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113 }
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114 #endif
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115
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116 #ifndef SDL_AtomicDecRef
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117 SDL_bool
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118 SDL_AtomicDecRef(SDL_atomic_t *a)
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119 {
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120 return SDL_AtomicAdd(a, -1) == 1;
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121 }
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122 #endif
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123
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124 #ifndef SDL_AtomicCAS
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125 int
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126 SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval)
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127 {
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128 int prevval;
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129
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130 enterLock(a);
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131 prevval = a->value;
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132 if (prevval == oldval) {
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133 a->value = newval;
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134 }
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135 leaveLock(a);
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136
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137 return prevval;
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138 }
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139 #endif
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140
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141 #ifndef SDL_AtomicSetPtr
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142 void
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143 SDL_AtomicSetPtr(void** a, void* value)
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144 {
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145 void *prevval;
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146 do {
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147 prevval = *a;
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148 } while (SDL_AtomicCASPtr(a, prevval, value) != prevval);
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149 }
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150 #endif
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151
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152 #ifndef SDL_AtomicGetPtr
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153 void*
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154 SDL_AtomicGetPtr(void** a)
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155 {
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156 /* Assuming integral reads on this platform, we're safe here since the
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157 functions that set the pointer have the necessary memory barriers.
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158 */
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159 return *a;
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160 }
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161 #endif
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162
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163 #ifndef SDL_AtomicCASPtr
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164 void* SDL_AtomicCASPtr(void **a, void *oldval, void *newval)
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165 {
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166 void *prevval;
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167
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168 enterLock(a);
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169 prevval = *a;
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170 if (*a == oldval) {
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171 *a = newval;
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172 }
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173 leaveLock(a);
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174
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175 return prevval;
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176 }
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177 #endif
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178
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179 /* vi: set ts=4 sw=4 expandtab: */