libgomp/ordered.c

   1 /* Copyright (C) 2005-2015 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 file handles the ORDERED construct.  */
  27
  28 #include "libgomp.h"
  29
  30
  31 /* This function is called when first allocating an iteration block.  That
  32    is, the thread is not currently on the queue.  The work-share lock must
  33    be held on entry.  */
  34
  35 void
  36 gomp_ordered_first (void)
  37 {
  38   struct gomp_thread *thr = gomp_thread ();
  39   struct gomp_team *team = thr->ts.team;
  40   struct gomp_work_share *ws = thr->ts.work_share;
  41   unsigned index;
  42
  43   /* Work share constructs can be orphaned.  */
  44   if (team == NULL || team->nthreads == 1)
  45     return;
  46
  47   index = ws->ordered_cur + ws->ordered_num_used;
  48   if (index >= team->nthreads)
  49     index -= team->nthreads;
  50   ws->ordered_team_ids[index] = thr->ts.team_id;
  51
  52   /* If this is the first and only thread in the queue, then there is
  53      no one to release us when we get to our ordered section.  Post to
  54      our own release queue now so that we won't block later.  */
  55   if (ws->ordered_num_used++ == 0)
  56     gomp_sem_post (team->ordered_release[thr->ts.team_id]);
  57 }
  58
  59 /* This function is called when completing the last iteration block.  That
  60    is, there are no more iterations to perform and so the thread should be
  61    removed from the queue entirely.  Because of the way ORDERED blocks are
  62    managed, it follows that we currently own access to the ORDERED block,
  63    and should now pass it on to the next thread.  The work-share lock must
  64    be held on entry.  */
  65
  66 void
  67 gomp_ordered_last (void)
  68 {
  69   struct gomp_thread *thr = gomp_thread ();
  70   struct gomp_team *team = thr->ts.team;
  71   struct gomp_work_share *ws = thr->ts.work_share;
  72   unsigned next_id;
  73
  74   /* Work share constructs can be orphaned.  */
  75   if (team == NULL || team->nthreads == 1)
  76     return;
  77
  78   /* We're no longer the owner.  */
  79   ws->ordered_owner = -1;
  80
  81   /* If we're not the last thread in the queue, then wake the next.  */
  82   if (--ws->ordered_num_used > 0)
  83     {
  84       unsigned next = ws->ordered_cur + 1;
  85       if (next == team->nthreads)
  86         next = 0;
  87       ws->ordered_cur = next;
  88
  89       next_id = ws->ordered_team_ids[next];
  90       gomp_sem_post (team->ordered_release[next_id]);
  91     }
  92 }
  93
  94
  95 /* This function is called when allocating a subsequent allocation block.
  96    That is, we're done with the current iteration block and we're allocating
  97    another.  This is the logical combination of a call to gomp_ordered_last
  98    followed by a call to gomp_ordered_first.  The work-share lock must be
  99    held on entry. */
 100
 101 void
 102 gomp_ordered_next (void)
 103 {
 104   struct gomp_thread *thr = gomp_thread ();
 105   struct gomp_team *team = thr->ts.team;
 106   struct gomp_work_share *ws = thr->ts.work_share;
 107   unsigned index, next_id;
 108
 109   /* Work share constructs can be orphaned.  */
 110   if (team == NULL || team->nthreads == 1)
 111     return;
 112
 113   /* We're no longer the owner.  */
 114   ws->ordered_owner = -1;
 115
 116   /* If there's only one thread in the queue, that must be us.  */
 117   if (ws->ordered_num_used == 1)
 118     {
 119       /* We have a similar situation as in gomp_ordered_first
 120          where we need to post to our own release semaphore.  */
 121       gomp_sem_post (team->ordered_release[thr->ts.team_id]);
 122       return;
 123     }
 124
 125   /* If the queue is entirely full, then we move ourself to the end of
 126      the queue merely by incrementing ordered_cur.  Only if it's not
 127      full do we have to write our id.  */
 128   if (ws->ordered_num_used < team->nthreads)
 129     {
 130       index = ws->ordered_cur + ws->ordered_num_used;
 131       if (index >= team->nthreads)
 132         index -= team->nthreads;
 133       ws->ordered_team_ids[index] = thr->ts.team_id;
 134     }
 135
 136   index = ws->ordered_cur + 1;
 137   if (index == team->nthreads)
 138     index = 0;
 139   ws->ordered_cur = index;
 140
 141   next_id = ws->ordered_team_ids[index];
 142   gomp_sem_post (team->ordered_release[next_id]);
 143 }
 144
 145
 146 /* This function is called when a statically scheduled loop is first
 147    being created.  */
 148
 149 void
 150 gomp_ordered_static_init (void)
 151 {
 152   struct gomp_thread *thr = gomp_thread ();
 153   struct gomp_team *team = thr->ts.team;
 154
 155   if (team == NULL || team->nthreads == 1)
 156     return;
 157
 158   gomp_sem_post (team->ordered_release[0]);
 159 }
 160
 161 /* This function is called when a statically scheduled loop is moving to
 162    the next allocation block.  Static schedules are not first come first
 163    served like the others, so we're to move to the numerically next thread,
 164    not the next thread on a list.  The work-share lock should *not* be held
 165    on entry.  */
 166
 167 void
 168 gomp_ordered_static_next (void)
 169 {
 170   struct gomp_thread *thr = gomp_thread ();
 171   struct gomp_team *team = thr->ts.team;
 172   struct gomp_work_share *ws = thr->ts.work_share;
 173   unsigned id = thr->ts.team_id;
 174
 175   if (team == NULL || team->nthreads == 1)
 176     return;
 177
 178   ws->ordered_owner = -1;
 179
 180   /* This thread currently owns the lock.  Increment the owner.  */
 181   if (++id == team->nthreads)
 182     id = 0;
 183   ws->ordered_team_ids[0] = id;
 184   gomp_sem_post (team->ordered_release[id]);
 185 }
 186
 187 /* This function is called when we need to assert that the thread owns the
 188    ordered section.  Due to the problem of posted-but-not-waited semaphores,
 189    this needs to happen before completing a loop iteration.  */
 190
 191 void
 192 gomp_ordered_sync (void)
 193 {
 194   struct gomp_thread *thr = gomp_thread ();
 195   struct gomp_team *team = thr->ts.team;
 196   struct gomp_work_share *ws = thr->ts.work_share;
 197
 198   /* Work share constructs can be orphaned.  But this clearly means that
 199      we are the only thread, and so we automatically own the section.  */
 200   if (team == NULL || team->nthreads == 1)
 201     return;
 202
 203   /* ??? I believe it to be safe to access this data without taking the
 204      ws->lock.  The only presumed race condition is with the previous
 205      thread on the queue incrementing ordered_cur such that it points
 206      to us, concurrently with our check below.  But our team_id is
 207      already present in the queue, and the other thread will always
 208      post to our release semaphore.  So the two cases are that we will
 209      either win the race an momentarily block on the semaphore, or lose
 210      the race and find the semaphore already unlocked and so not block.
 211      Either way we get correct results.
 212      However, there is an implicit flush on entry to an ordered region,
 213      so we do need to have a barrier here.  If we were taking a lock
 214      this could be MEMMODEL_RELEASE since the acquire would be coverd
 215      by the lock.  */
 216
 217   __atomic_thread_fence (MEMMODEL_ACQ_REL);
 218   if (ws->ordered_owner != thr->ts.team_id)
 219     {
 220       gomp_sem_wait (team->ordered_release[thr->ts.team_id]);
 221       ws->ordered_owner = thr->ts.team_id;
 222     }
 223 }
 224
 225 /* This function is called by user code when encountering the start of an
 226    ORDERED block.  We must check to see if the current thread is at the
 227    head of the queue, and if not, block.  */
 228
 229 #ifdef HAVE_ATTRIBUTE_ALIAS
 230 extern void GOMP_ordered_start (void)
 231         __attribute__((alias ("gomp_ordered_sync")));
 232 #else
 233 void
 234 GOMP_ordered_start (void)
 235 {
 236   gomp_ordered_sync ();
 237 }
 238 #endif
 239
 240 /* This function is called by user code when encountering the end of an
 241    ORDERED block.  With the current ORDERED implementation there's nothing
 242    for us to do.
 243
 244    However, the current implementation has a flaw in that it does not allow
 245    the next thread into the ORDERED section immediately after the current
 246    thread exits the ORDERED section in its last iteration.  The existance
 247    of this function allows the implementation to change.  */
 248
 249 void
 250 GOMP_ordered_end (void)
 251 {
 252 }