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1 /* Copyright (C) 2012-2020 Free Software Foundation, Inc.
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111
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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 Atomic Library (libatomic).
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5
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6 Libatomic 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 Libatomic 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 "libatomic_i.h"
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26
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27
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28 /* If we support the builtin, just use it. */
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29 #if !DONE && defined(atomic_compare_exchange_n)
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30 bool
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31 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
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32 int smodel, int fmodel UNUSED)
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33 {
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34 if (maybe_specialcase_relaxed(smodel))
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35 return atomic_compare_exchange_n (mptr, eptr, newval, false,
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36 __ATOMIC_RELAXED, __ATOMIC_RELAXED);
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37 else if (maybe_specialcase_acqrel(smodel))
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38 return atomic_compare_exchange_n (mptr, eptr, newval, false,
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39 __ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
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40 else
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41 return atomic_compare_exchange_n (mptr, eptr, newval, false,
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42 __ATOMIC_SEQ_CST, __ATOMIC_RELAXED);
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43 }
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44
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45 #define DONE 1
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46 #endif /* HAVE_ATOMIC_CAS */
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47
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48
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49 /* If this type is not larger than word-sized, fall back to a word-sized
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50 compare-and-swap loop, possibly assisted by the OS. */
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51 #if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
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52 bool
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53 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
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54 int smodel, int fmodel)
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55 {
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56 UWORD mask, shift, weval, woldval, wnewval, t, *wptr;
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57
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58 pre_barrier (smodel);
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59
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60 if (N < WORDSIZE)
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61 {
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62 wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
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63 shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
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64 mask = SIZE(MASK) << shift;
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65 }
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66 else
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67 {
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68 wptr = (UWORD *)mptr;
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69 shift = 0;
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70 mask = -1;
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71 }
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72
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73 weval = (UWORD)*eptr << shift;
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74 wnewval = (UWORD)newval << shift;
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75 woldval = __atomic_load_n (wptr, __ATOMIC_RELAXED);
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76 do
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77 {
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78 if ((woldval & mask) != weval)
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79 goto failure;
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80 t = (woldval & ~mask) | wnewval;
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81 }
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82 while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
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83 __ATOMIC_RELAXED, __ATOMIC_RELAXED));
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84 post_barrier (smodel);
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85 return true;
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86
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87 failure:
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88 *eptr = woldval >> shift;
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89 post_barrier (fmodel);
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90 return false;
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91 }
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92
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93 #define DONE 1
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94 #endif /* HAVE_ATOMIC_CAS && N <= WORDSIZE */
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95
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96
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97 /* Otherwise, fall back to some sort of protection mechanism. */
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98 #if !DONE
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99 bool
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100 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
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101 int smodel, int fmodel UNUSED)
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102 {
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103 UTYPE oldval;
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104 UWORD magic;
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105 bool ret;
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106
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107 pre_seq_barrier (smodel);
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108 magic = protect_start (mptr);
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109
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110 oldval = *mptr;
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111 ret = (oldval == *eptr);
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112 if (ret)
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113 *mptr = newval;
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114 else
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115 *eptr = oldval;
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116
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117 protect_end (mptr, magic);
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118 post_seq_barrier (smodel);
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119
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120 return ret;
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121 }
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122 #endif
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123
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124 EXPORT_ALIAS (SIZE(compare_exchange));
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