1  /* Copyright (C) 2005-2023 Free Software Foundation, Inc.
       2     Contributed by Richard Henderson <rth@redhat.com>.
       3  
       4     This file is part of the GNU Offloading and Multi Processing Library
       5     (libgomp).
       6  
       7     Libgomp is free software; you can redistribute it and/or modify it
       8     under the terms of the GNU General Public License as published by
       9     the Free Software Foundation; either version 3, or (at your option)
      10     any later version.
      11  
      12     Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
      13     WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
      14     FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
      15     more details.
      16  
      17     Under Section 7 of GPL version 3, you are granted additional
      18     permissions described in the GCC Runtime Library Exception, version
      19     3.1, as published by the Free Software Foundation.
      20  
      21     You should have received a copy of the GNU General Public License and
      22     a copy of the GCC Runtime Library Exception along with this program;
      23     see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      24     <http://www.gnu.org/licenses/>.  */
      25  
      26  /* This is a Linux specific implementation of a semaphore synchronization
      27     mechanism for libgomp.  This type is private to the library.  This 
      28     counting semaphore implementation uses atomic instructions and the
      29     futex syscall, and a single 32-bit int to store semaphore state.
      30     The low 31 bits are the count, the top bit is a flag set when some
      31     threads may be waiting.  */
      32  
      33  #ifndef GOMP_SEM_H
      34  #define GOMP_SEM_H 1
      35  
      36  typedef int gomp_sem_t;
      37  #define SEM_WAIT (-__INT_MAX__ - 1)
      38  #define SEM_INC 1
      39  
      40  extern void gomp_sem_wait_slow (gomp_sem_t *, int);
      41  extern void gomp_sem_post_slow (gomp_sem_t *);
      42  
      43  static inline void
      44  gomp_sem_init (gomp_sem_t *sem, int value)
      45  {
      46    *sem = value * SEM_INC;
      47  }
      48  
      49  static inline void
      50  gomp_sem_destroy (gomp_sem_t *sem)
      51  {
      52  }
      53  
      54  static inline void
      55  gomp_sem_wait (gomp_sem_t *sem)
      56  {
      57    int count = *sem;
      58  
      59    while ((count & ~SEM_WAIT) != 0)
      60      if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, true,
      61  				     MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
      62        return;
      63    gomp_sem_wait_slow (sem, count);
      64  }
      65  
      66  static inline void
      67  gomp_sem_post (gomp_sem_t *sem)
      68  {
      69    int count = *sem;
      70  
      71    /* Clear SEM_WAIT here so that if there are no more waiting threads
      72       we transition back to the uncontended state that does not make
      73       futex syscalls.  If there are waiting threads then when one is
      74       awoken it will set SEM_WAIT again, so other waiting threads are
      75       woken on a future gomp_sem_post.  Furthermore, the awoken thread
      76       will wake other threads in case gomp_sem_post was called again
      77       before it had time to set SEM_WAIT.  */
      78    while (!__atomic_compare_exchange_n (sem, &count,
      79  				       (count + SEM_INC) & ~SEM_WAIT, true,
      80  				       MEMMODEL_RELEASE, MEMMODEL_RELAXED))
      81      continue;
      82  
      83    if (__builtin_expect (count & SEM_WAIT, 0))
      84      gomp_sem_post_slow (sem);
      85  }
      86  
      87  static inline int
      88  gomp_sem_getcount (gomp_sem_t *sem)
      89  {
      90    int count = __atomic_load_n (sem, MEMMODEL_RELAXED);
      91    if ((count & SEM_WAIT) != 0)
      92      return -1;
      93    return count / SEM_INC;
      94  }
      95  #endif /* GOMP_SEM_H */