89 lines
2.9 KiB
C
89 lines
2.9 KiB
C
/* Copyright (C) 2005-2017 Free Software Foundation, Inc.
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Contributed by Richard Henderson <rth@redhat.com>.
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This file is part of the GNU Offloading and Multi Processing Library
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(libgomp).
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Libgomp is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3, or (at your option)
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any later version.
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Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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/* This is a Linux specific implementation of a semaphore synchronization
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mechanism for libgomp. This type is private to the library. This
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counting semaphore implementation uses atomic instructions and the
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futex syscall, and a single 32-bit int to store semaphore state.
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The low 31 bits are the count, the top bit is a flag set when some
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threads may be waiting. */
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#ifndef GOMP_SEM_H
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#define GOMP_SEM_H 1
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#include <limits.h> /* For INT_MIN */
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typedef int gomp_sem_t;
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#define SEM_WAIT INT_MIN
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#define SEM_INC 1
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extern void gomp_sem_wait_slow (gomp_sem_t *, int);
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extern void gomp_sem_post_slow (gomp_sem_t *);
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static inline void
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gomp_sem_init (gomp_sem_t *sem, int value)
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{
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*sem = value * SEM_INC;
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}
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static inline void
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gomp_sem_destroy (gomp_sem_t *sem)
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{
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}
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static inline void
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gomp_sem_wait (gomp_sem_t *sem)
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{
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int count = *sem;
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while ((count & ~SEM_WAIT) != 0)
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if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true,
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MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
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return;
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gomp_sem_wait_slow (sem, count);
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}
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static inline void
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gomp_sem_post (gomp_sem_t *sem)
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{
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int count = *sem;
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/* Clear SEM_WAIT here so that if there are no more waiting threads
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we transition back to the uncontended state that does not make
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futex syscalls. If there are waiting threads then when one is
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awoken it will set SEM_WAIT again, so other waiting threads are
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woken on a future gomp_sem_post. Furthermore, the awoken thread
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will wake other threads in case gomp_sem_post was called again
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before it had time to set SEM_WAIT. */
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while (!__atomic_compare_exchange_n (sem, &count,
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(count + SEM_INC) & ~SEM_WAIT, true,
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MEMMODEL_RELEASE, MEMMODEL_RELAXED))
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continue;
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if (__builtin_expect (count & SEM_WAIT, 0))
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gomp_sem_post_slow (sem);
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}
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#endif /* GOMP_SEM_H */
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