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1 /* Copyright (C) 2005-2020 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 Offloading and Multi Processing Library
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5 (libgomp).
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6
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7 Libgomp is free software; you can redistribute it and/or modify it
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8 under the terms of the GNU General Public License as published by
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9 the Free Software Foundation; either version 3, or (at your option)
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10 any later version.
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11
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12 Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
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13 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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14 FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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15 more details.
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16
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17 Under Section 7 of GPL version 3, you are granted additional
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18 permissions described in the GCC Runtime Library Exception, version
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19 3.1, as published by the Free Software Foundation.
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20
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21 You should have received a copy of the GNU General Public License and
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22 a copy of the GCC Runtime Library Exception along with this program;
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23 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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24 <http://www.gnu.org/licenses/>. */
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25
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26 /* This is a Linux specific implementation of a semaphore synchronization
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27 mechanism for libgomp. This type is private to the library. This
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28 counting semaphore implementation uses atomic instructions and the
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29 futex syscall, and a single 32-bit int to store semaphore state.
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30 The low 31 bits are the count, the top bit is a flag set when some
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31 threads may be waiting. */
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32
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33 #ifndef GOMP_SEM_H
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34 #define GOMP_SEM_H 1
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35
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36 #include <limits.h> /* For INT_MIN */
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37
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38 typedef int gomp_sem_t;
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39 #define SEM_WAIT INT_MIN
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40 #define SEM_INC 1
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41
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42 extern void gomp_sem_wait_slow (gomp_sem_t *, int);
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43 extern void gomp_sem_post_slow (gomp_sem_t *);
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44
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45 static inline void
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46 gomp_sem_init (gomp_sem_t *sem, int value)
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47 {
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48 *sem = value * SEM_INC;
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49 }
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50
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51 static inline void
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52 gomp_sem_destroy (gomp_sem_t *sem)
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53 {
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54 }
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55
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56 static inline void
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57 gomp_sem_wait (gomp_sem_t *sem)
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58 {
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59 int count = *sem;
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60
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61 while ((count & ~SEM_WAIT) != 0)
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62 if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true,
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63 MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
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64 return;
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65 gomp_sem_wait_slow (sem, count);
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66 }
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67
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68 static inline void
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69 gomp_sem_post (gomp_sem_t *sem)
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70 {
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71 int count = *sem;
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72
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73 /* Clear SEM_WAIT here so that if there are no more waiting threads
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74 we transition back to the uncontended state that does not make
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75 futex syscalls. If there are waiting threads then when one is
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76 awoken it will set SEM_WAIT again, so other waiting threads are
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77 woken on a future gomp_sem_post. Furthermore, the awoken thread
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78 will wake other threads in case gomp_sem_post was called again
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79 before it had time to set SEM_WAIT. */
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80 while (!__atomic_compare_exchange_n (sem, &count,
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81 (count + SEM_INC) & ~SEM_WAIT, true,
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82 MEMMODEL_RELEASE, MEMMODEL_RELAXED))
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83 continue;
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84
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85 if (__builtin_expect (count & SEM_WAIT, 0))
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86 gomp_sem_post_slow (sem);
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87 }
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88 #endif /* GOMP_SEM_H */
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