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annotate src/atomic/SDL_atomic.c @ 5097:dceec93471e7

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Improvements based on feedback from Anthony Williams
author Sam Lantinga <slouken@libsdl.org>
date Tue, 25 Jan 2011 17:40:06 -0800
parents 0c72ae7b7cb2
children b530ef003506
rev   line source
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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 /* Note that we undefine the atomic operations here, in case they are
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27 defined as compiler intrinsics while building SDL but the library user
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28 doesn't have that compiler. That way we always have a working set of
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29 atomic operations built into the library.
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30 */
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31
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32 /*
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33 If any of the operations are not provided then we must emulate some
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34 of them. That means we need a nice implementation of spin locks
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35 that avoids the "one big lock" problem. We use a vector of spin
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36 locks and pick which one to use based on the address of the operand
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37 of the function.
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38
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39 To generate the index of the lock we first shift by 3 bits to get
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40 rid on the zero bits that result from 32 and 64 bit allignment of
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41 data. We then mask off all but 5 bits and use those 5 bits as an
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42 index into the table.
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43
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44 Picking the lock this way insures that accesses to the same data at
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45 the same time will go to the same lock. OTOH, accesses to different
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46 data have only a 1/32 chance of hitting the same lock. That should
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47 pretty much eliminate the chances of several atomic operations on
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48 different data from waiting on the same "big lock". If it isn't
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49 then the table of locks can be expanded to a new size so long as
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50 the new size is a power of two.
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51
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52 Contributed by Bob Pendleton, bob@pendleton.com
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53 */
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54
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55 static SDL_SpinLock locks[32];
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56
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57 static __inline__ void
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58 enterLock(void *a)
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59 {
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60 uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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61
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62 SDL_AtomicLock(&locks[index]);
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63 }
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64
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65 static __inline__ void
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66 leaveLock(void *a)
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67 {
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68 uintptr_t index = ((((uintptr_t)a) >> 3) & 0x1f);
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69
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70 SDL_AtomicUnlock(&locks[index]);
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71 }
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72
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73 SDL_bool
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74 SDL_AtomicCAS_(SDL_atomic_t *a, int oldval, int newval)
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75 {
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76 SDL_bool retval = SDL_FALSE;
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77
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78 enterLock(a);
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79 if (a->value == oldval) {
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80 a->value = newval;
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81 retval = SDL_TRUE;
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82 }
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83 leaveLock(a);
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84
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85 return retval;
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86 }
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87
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88 SDL_bool
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89 SDL_AtomicCASPtr_(void **a, void *oldval, void *newval)
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90 {
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91 SDL_bool retval = SDL_FALSE;
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92
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93 enterLock(a);
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94 if (*a == oldval) {
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95 *a = newval;
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96 retval = SDL_TRUE;
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97 }
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98 leaveLock(a);
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99
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100 return retval;
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101 }
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102
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103 /* vi: set ts=4 sw=4 expandtab: */