libgomp/ordered.c

   1 /* Copyright (C) 2005-2013 Free Software Foundation, Inc.
   2    Contributed by Richard Henderson <rth@redhat.com>.
   3
   4    This file is part of the GNU OpenMP Library (libgomp).
   5
   6    Libgomp is free software; you can redistribute it and/or modify it
   7    under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3, or (at your option)
   9    any later version.
  10
  11    Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
  12    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  13    FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
  14    more details.
  15
  16    Under Section 7 of GPL version 3, you are granted additional
  17    permissions described in the GCC Runtime Library Exception, version
  18    3.1, as published by the Free Software Foundation.
  19
  20    You should have received a copy of the GNU General Public License and
  21    a copy of the GCC Runtime Library Exception along with this program;
  22    see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23    <http://www.gnu.org/licenses/>.  */
  24
  25 /* This file handles the ORDERED construct.  */
  26
  27 #include "libgomp.h"
  28
  29
  30 /* This function is called when first allocating an iteration block.  That
  31    is, the thread is not currently on the queue.  The work-share lock must
  32    be held on entry.  */
  33
  34 void
  35 gomp_ordered_first (void)
  36 {
  37   struct gomp_thread *thr = gomp_thread ();
  38   struct gomp_team *team = thr->ts.team;
  39   struct gomp_work_share *ws = thr->ts.work_share;
  40   unsigned index;
  41
  42   /* Work share constructs can be orphaned.  */
  43   if (team == NULL || team->nthreads == 1)
  44     return;
  45
  46   index = ws->ordered_cur + ws->ordered_num_used;
  47   if (index >= team->nthreads)
  48     index -= team->nthreads;
  49   ws->ordered_team_ids[index] = thr->ts.team_id;
  50
  51   /* If this is the first and only thread in the queue, then there is
  52      no one to release us when we get to our ordered section.  Post to
  53      our own release queue now so that we won't block later.  */
  54   if (ws->ordered_num_used++ == 0)
  55     gomp_sem_post (team->ordered_release[thr->ts.team_id]);
  56 }
  57
  58 /* This function is called when completing the last iteration block.  That
  59    is, there are no more iterations to perform and so the thread should be
  60    removed from the queue entirely.  Because of the way ORDERED blocks are
  61    managed, it follows that we currently own access to the ORDERED block,
  62    and should now pass it on to the next thread.  The work-share lock must
  63    be held on entry.  */
  64
  65 void
  66 gomp_ordered_last (void)
  67 {
  68   struct gomp_thread *thr = gomp_thread ();
  69   struct gomp_team *team = thr->ts.team;
  70   struct gomp_work_share *ws = thr->ts.work_share;
  71   unsigned next_id;
  72
  73   /* Work share constructs can be orphaned.  */
  74   if (team == NULL || team->nthreads == 1)
  75     return;
  76
  77   /* We're no longer the owner.  */
  78   ws->ordered_owner = -1;
  79
  80   /* If we're not the last thread in the queue, then wake the next.  */
  81   if (--ws->ordered_num_used > 0)
  82     {
  83       unsigned next = ws->ordered_cur + 1;
  84       if (next == team->nthreads)
  85         next = 0;
  86       ws->ordered_cur = next;
  87
  88       next_id = ws->ordered_team_ids[next];
  89       gomp_sem_post (team->ordered_release[next_id]);
  90     }
  91 }
  92
  93
  94 /* This function is called when allocating a subsequent allocation block.
  95    That is, we're done with the current iteration block and we're allocating
  96    another.  This is the logical combination of a call to gomp_ordered_last
  97    followed by a call to gomp_ordered_first.  The work-share lock must be
  98    held on entry. */
  99
 100 void
 101 gomp_ordered_next (void)
 102 {
 103   struct gomp_thread *thr = gomp_thread ();
 104   struct gomp_team *team = thr->ts.team;
 105   struct gomp_work_share *ws = thr->ts.work_share;
 106   unsigned index, next_id;
 107
 108   /* Work share constructs can be orphaned.  */
 109   if (team == NULL || team->nthreads == 1)
 110     return;
 111
 112   /* We're no longer the owner.  */
 113   ws->ordered_owner = -1;
 114
 115   /* If there's only one thread in the queue, that must be us.  */
 116   if (ws->ordered_num_used == 1)
 117     {
 118       /* We have a similar situation as in gomp_ordered_first
 119          where we need to post to our own release semaphore.  */
 120       gomp_sem_post (team->ordered_release[thr->ts.team_id]);
 121       return;
 122     }
 123
 124   /* If the queue is entirely full, then we move ourself to the end of
 125      the queue merely by incrementing ordered_cur.  Only if it's not
 126      full do we have to write our id.  */
 127   if (ws->ordered_num_used < team->nthreads)
 128     {
 129       index = ws->ordered_cur + ws->ordered_num_used;
 130       if (index >= team->nthreads)
 131         index -= team->nthreads;
 132       ws->ordered_team_ids[index] = thr->ts.team_id;
 133     }
 134
 135   index = ws->ordered_cur + 1;
 136   if (index == team->nthreads)
 137     index = 0;
 138   ws->ordered_cur = index;
 139
 140   next_id = ws->ordered_team_ids[index];
 141   gomp_sem_post (team->ordered_release[next_id]);
 142 }
 143
 144
 145 /* This function is called when a statically scheduled loop is first
 146    being created.  */
 147
 148 void
 149 gomp_ordered_static_init (void)
 150 {
 151   struct gomp_thread *thr = gomp_thread ();
 152   struct gomp_team *team = thr->ts.team;
 153
 154   if (team == NULL || team->nthreads == 1)
 155     return;
 156
 157   gomp_sem_post (team->ordered_release[0]);
 158 }
 159
 160 /* This function is called when a statically scheduled loop is moving to
 161    the next allocation block.  Static schedules are not first come first
 162    served like the others, so we're to move to the numerically next thread,
 163    not the next thread on a list.  The work-share lock should *not* be held
 164    on entry.  */
 165
 166 void
 167 gomp_ordered_static_next (void)
 168 {
 169   struct gomp_thread *thr = gomp_thread ();
 170   struct gomp_team *team = thr->ts.team;
 171   struct gomp_work_share *ws = thr->ts.work_share;
 172   unsigned id = thr->ts.team_id;
 173
 174   if (team == NULL || team->nthreads == 1)
 175     return;
 176
 177   ws->ordered_owner = -1;
 178
 179   /* This thread currently owns the lock.  Increment the owner.  */
 180   if (++id == team->nthreads)
 181     id = 0;
 182   ws->ordered_team_ids[0] = id;
 183   gomp_sem_post (team->ordered_release[id]);
 184 }
 185
 186 /* This function is called when we need to assert that the thread owns the
 187    ordered section.  Due to the problem of posted-but-not-waited semaphores,
 188    this needs to happen before completing a loop iteration.  */
 189
 190 void
 191 gomp_ordered_sync (void)
 192 {
 193   struct gomp_thread *thr = gomp_thread ();
 194   struct gomp_team *team = thr->ts.team;
 195   struct gomp_work_share *ws = thr->ts.work_share;
 196
 197   /* Work share constructs can be orphaned.  But this clearly means that
 198      we are the only thread, and so we automatically own the section.  */
 199   if (team == NULL || team->nthreads == 1)
 200     return;
 201
 202   /* ??? I believe it to be safe to access this data without taking the
 203      ws->lock.  The only presumed race condition is with the previous
 204      thread on the queue incrementing ordered_cur such that it points
 205      to us, concurrently with our check below.  But our team_id is
 206      already present in the queue, and the other thread will always
 207      post to our release semaphore.  So the two cases are that we will
 208      either win the race an momentarily block on the semaphore, or lose
 209      the race and find the semaphore already unlocked and so not block.
 210      Either way we get correct results.
 211      However, there is an implicit flush on entry to an ordered region,
 212      so we do need to have a barrier here.  If we were taking a lock
 213      this could be MEMMODEL_RELEASE since the acquire would be coverd
 214      by the lock.  */
 215
 216   __atomic_thread_fence (MEMMODEL_ACQ_REL);
 217   if (ws->ordered_owner != thr->ts.team_id)
 218     {
 219       gomp_sem_wait (team->ordered_release[thr->ts.team_id]);
 220       ws->ordered_owner = thr->ts.team_id;
 221     }
 222 }
 223
 224 /* This function is called by user code when encountering the start of an
 225    ORDERED block.  We must check to see if the current thread is at the
 226    head of the queue, and if not, block.  */
 227
 228 #ifdef HAVE_ATTRIBUTE_ALIAS
 229 extern void GOMP_ordered_start (void)
 230         __attribute__((alias ("gomp_ordered_sync")));
 231 #else
 232 void
 233 GOMP_ordered_start (void)
 234 {
 235   gomp_ordered_sync ();
 236 }
 237 #endif
 238
 239 /* This function is called by user code when encountering the end of an
 240    ORDERED block.  With the current ORDERED implementation there's nothing
 241    for us to do.
 242
 243    However, the current implementation has a flaw in that it does not allow
 244    the next thread into the ORDERED section immediately after the current
 245    thread exits the ORDERED section in its last iteration.  The existance
 246    of this function allows the implementation to change.  */
 247
 248 void
 249 GOMP_ordered_end (void)
 250 {
 251 }