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1 /* Copyright (C) 2012-2018 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 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 /* Accesses with a power-of-two size are not lock-free if we don't have an
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28 integer type of this size or if they are not naturally aligned. They
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29 are lock-free if such a naturally aligned access is always lock-free
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30 according to the compiler, which requires that both atomic loads and CAS
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31 are available.
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32 In all other cases, we fall through to LARGER (see below). */
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33 #define EXACT(N) \
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34 do { \
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35 if (!C2(HAVE_INT,N)) break; \
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36 if ((uintptr_t)ptr & (N - 1)) break; \
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37 if (__atomic_always_lock_free(N, 0)) return true; \
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38 if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
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39 if (C2(FAST_ATOMIC_LDST_,N)) return true; \
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40 } while (0)
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41
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42
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43 /* We next check to see if an access of a larger size is lock-free. We use
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44 a similar check as in EXACT, except that we also check that the alignment
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45 of the access is so that the data to be accessed is completely covered
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46 by the larger access. */
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47 #define LARGER(N) \
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48 do { \
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49 uintptr_t r = (uintptr_t)ptr & (N - 1); \
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50 if (!C2(HAVE_INT,N)) break; \
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51 if (!C2(FAST_ATOMIC_LDST_,N)) break; \
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52 if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
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53 if (r + n <= N) return true; \
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54 } while (0)
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55
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56
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57 /* Note that this can return that a size/alignment is not lock-free even if
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58 all the operations that we use to implement the respective accesses provide
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59 lock-free forward progress as specified in C++14: Users likely expect
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60 "lock-free" to also mean "fast", which is why we do not return true if, for
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61 example, we implement loads with this size/alignment using a CAS. */
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62 bool
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63 libat_is_lock_free (size_t n, void *ptr)
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64 {
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65 switch (n)
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66 {
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67 case 0: return true;
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68 case 1: EXACT(1); goto L4;
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69 case 2: EXACT(2); goto L4;
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70 case 4: EXACT(4); goto L8;
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71 case 8: EXACT(8); goto L16;
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72 case 16: EXACT(16); break;
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73
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74 case 3: L4: LARGER(4); /* FALLTHRU */
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75 case 5 ... 7: L8: LARGER(8); /* FALLTHRU */
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76 case 9 ... 15: L16: LARGER(16); break;
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77 }
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78
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79 return false;
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80 }
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81
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82 EXPORT_ALIAS (is_lock_free);
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