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1 /* Copyright (C) 2012-2018 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 && SIZE(HAVE_ATOMIC_LDST)
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30 UTYPE
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31 SIZE(libat_load) (UTYPE *mptr, int smodel)
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32 {
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33 if (maybe_specialcase_relaxed(smodel))
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34 return __atomic_load_n (mptr, __ATOMIC_RELAXED);
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35 else if (maybe_specialcase_acqrel(smodel))
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36 /* Note that REL and ACQ_REL are not valid for loads. */
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37 return __atomic_load_n (mptr, __ATOMIC_ACQUIRE);
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38 else
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39 return __atomic_load_n (mptr, __ATOMIC_SEQ_CST);
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40 }
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41
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42 #define DONE 1
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43 #endif /* HAVE_ATOMIC_LOAD */
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44
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45
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46 /* If we have compare-and-swap, use it to swap 0 with 0 and as a side
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47 effect load the original value. */
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48 #if !DONE && defined(atomic_compare_exchange_n)
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49 UTYPE
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50 SIZE(libat_load) (UTYPE *mptr, int smodel)
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51 {
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52 UTYPE t = 0;
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53
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54 if (maybe_specialcase_relaxed(smodel))
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55 atomic_compare_exchange_n (mptr, &t, 0, true,
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56 __ATOMIC_RELAXED, __ATOMIC_RELAXED);
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57 else if (maybe_specialcase_acqrel(smodel))
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58 atomic_compare_exchange_n (mptr, &t, 0, true,
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59 __ATOMIC_ACQ_REL, __ATOMIC_ACQ_REL);
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60 else
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61 atomic_compare_exchange_n (mptr, &t, 0, true,
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62 __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
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63
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64 return t;
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65 }
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66
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67 #define DONE 1
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68 #endif /* atomic_compare_exchange_n */
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69
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70
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71 /* Similar, but only assume a word-sized compare-and-swap. */
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72 #if !DONE && N < WORDSIZE && defined(atomic_compare_exchange_w)
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73 UTYPE
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74 SIZE(libat_load) (UTYPE *mptr, int smodel)
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75 {
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76 UWORD shift, t, *wptr;
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77
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78 pre_barrier (smodel);
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79
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80 wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
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81 shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
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82
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83 /* Exchange 0 with 0, placing the old value of *WPTR in T. */
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84 t = 0;
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85 atomic_compare_exchange_w (wptr, &t, 0);
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86
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87 post_barrier (smodel);
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88 return t >> shift;
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89 }
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90
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91 #define DONE 1
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92 #endif /* HAVE_ATOMIC_CAS && N < WORDSIZE */
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93
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94
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95 /* Otherwise, fall back to some sort of protection mechanism. */
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96 #if !DONE
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97 UTYPE
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98 SIZE(libat_load) (UTYPE *mptr, int smodel)
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99 {
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100 UTYPE ret;
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101 UWORD magic;
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102
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103 pre_seq_barrier (smodel);
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104 magic = protect_start (mptr);
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105
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106 ret = *mptr;
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107
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108 protect_end (mptr, magic);
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109 post_seq_barrier (smodel);
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110
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111 return ret;
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112 }
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113 #endif
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114
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115 EXPORT_ALIAS (SIZE(load));
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