gcc/libgomp/task.c

/* Copyright (C) 2007-2015 Free Software Foundation, Inc.
   Contributed by Richard Henderson <rth@redhat.com>.

   This file is part of the GNU OpenMP Library (libgomp).

   Libgomp is free software; you can redistribute it and/or modify it
   under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3, or (at your option)
   any later version.

   Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
   more details.

   Under Section 7 of GPL version 3, you are granted additional
   permissions described in the GCC Runtime Library Exception, version
   3.1, as published by the Free Software Foundation.

   You should have received a copy of the GNU General Public License and
   a copy of the GCC Runtime Library Exception along with this program;
   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
   <http://www.gnu.org/licenses/>.  */

/* This file handles the maintainence of tasks in response to task
   creation and termination.  */

#include "libgomp.h"
#include <stdlib.h>
#include <string.h>

typedef struct gomp_task_depend_entry *hash_entry_type;

static inline void *
htab_alloc (size_t size)
{
  return gomp_malloc (size);
}

static inline void
htab_free (void *ptr)
{
  free (ptr);
}

#include "hashtab.h"

static inline hashval_t
htab_hash (hash_entry_type element)
{
  return hash_pointer (element->addr);
}

static inline bool
htab_eq (hash_entry_type x, hash_entry_type y)
{
  return x->addr == y->addr;
}

/* Create a new task data structure.  */

void
gomp_init_task (struct gomp_task *task, struct gomp_task *parent_task,
		struct gomp_task_icv *prev_icv)
{
  task->parent = parent_task;
  task->icv = *prev_icv;
  task->kind = GOMP_TASK_IMPLICIT;
  task->taskwait = NULL;
  task->in_tied_task = false;
  task->final_task = false;
  task->copy_ctors_done = false;
  task->parent_depends_on = false;
  task->children = NULL;
  task->taskgroup = NULL;
  task->dependers = NULL;
  task->depend_hash = NULL;
  task->depend_count = 0;
}

/* Clean up a task, after completing it.  */

void
gomp_end_task (void)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_task *task = thr->task;

  gomp_finish_task (task);
  thr->task = task->parent;
}

static inline void
gomp_clear_parent (struct gomp_task *children)
{
  struct gomp_task *task = children;

  if (task)
    do
      {
	task->parent = NULL;
	task = task->next_child;
      }
    while (task != children);
}

static void gomp_task_maybe_wait_for_dependencies (void **depend);

/* Called when encountering an explicit task directive.  If IF_CLAUSE is
   false, then we must not delay in executing the task.  If UNTIED is true,
   then the task may be executed by any member of the team.  */

void
GOMP_task (void (*fn) (void *), void *data, void (*cpyfn) (void *, void *),
	   long arg_size, long arg_align, bool if_clause, unsigned flags,
	   void **depend)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_team *team = thr->ts.team;

#ifdef HAVE_BROKEN_POSIX_SEMAPHORES
  /* If pthread_mutex_* is used for omp_*lock*, then each task must be
     tied to one thread all the time.  This means UNTIED tasks must be
     tied and if CPYFN is non-NULL IF(0) must be forced, as CPYFN
     might be running on different thread than FN.  */
  if (cpyfn)
    if_clause = false;
  if (flags & 1)
    flags &= ~1;
#endif

  /* If parallel or taskgroup has been cancelled, don't start new tasks.  */
  if (team
      && (gomp_team_barrier_cancelled (&team->barrier)
	  || (thr->task->taskgroup && thr->task->taskgroup->cancelled)))
    return;

  if (!if_clause || team == NULL
      || (thr->task && thr->task->final_task)
      || team->task_count > 64 * team->nthreads)
    {
      struct gomp_task task;

      /* If there are depend clauses and earlier deferred sibling tasks
	 with depend clauses, check if there isn't a dependency.  If there
	 is, we need to wait for them.  There is no need to handle
	 depend clauses for non-deferred tasks other than this, because
	 the parent task is suspended until the child task finishes and thus
	 it can't start further child tasks.  */
      if ((flags & 8) && thr->task && thr->task->depend_hash)
	gomp_task_maybe_wait_for_dependencies (depend);

      gomp_init_task (&task, thr->task, gomp_icv (false));
      task.kind = GOMP_TASK_IFFALSE;
      task.final_task = (thr->task && thr->task->final_task) || (flags & 2);
      if (thr->task)
	{
	  task.in_tied_task = thr->task->in_tied_task;
	  task.taskgroup = thr->task->taskgroup;
	}
      thr->task = &task;
      if (__builtin_expect (cpyfn != NULL, 0))
	{
	  char buf[arg_size + arg_align - 1];
	  char *arg = (char *) (((uintptr_t) buf + arg_align - 1)
				& ~(uintptr_t) (arg_align - 1));
	  cpyfn (arg, data);
	  fn (arg);
	}
      else
	fn (data);
      /* Access to "children" is normally done inside a task_lock
	 mutex region, but the only way this particular task.children
	 can be set is if this thread's task work function (fn)
	 creates children.  So since the setter is *this* thread, we
	 need no barriers here when testing for non-NULL.  We can have
	 task.children set by the current thread then changed by a
	 child thread, but seeing a stale non-NULL value is not a
	 problem.  Once past the task_lock acquisition, this thread
	 will see the real value of task.children.  */
      if (task.children != NULL)
	{
	  gomp_mutex_lock (&team->task_lock);
	  gomp_clear_parent (task.children);
	  gomp_mutex_unlock (&team->task_lock);
	}
      gomp_end_task ();
    }
  else
    {
      struct gomp_task *task;
      struct gomp_task *parent = thr->task;
      struct gomp_taskgroup *taskgroup = parent->taskgroup;
      char *arg;
      bool do_wake;
      size_t depend_size = 0;

      if (flags & 8)
	depend_size = ((uintptr_t) depend[0]
		       * sizeof (struct gomp_task_depend_entry));
      task = gomp_malloc (sizeof (*task) + depend_size
			  + arg_size + arg_align - 1);
      arg = (char *) (((uintptr_t) (task + 1) + depend_size + arg_align - 1)
		      & ~(uintptr_t) (arg_align - 1));
      gomp_init_task (task, parent, gomp_icv (false));
      task->kind = GOMP_TASK_IFFALSE;
      task->in_tied_task = parent->in_tied_task;
      task->taskgroup = taskgroup;
      thr->task = task;
      if (cpyfn)
	{
	  cpyfn (arg, data);
	  task->copy_ctors_done = true;
	}
      else
	memcpy (arg, data, arg_size);
      thr->task = parent;
      task->kind = GOMP_TASK_WAITING;
      task->fn = fn;
      task->fn_data = arg;
      task->final_task = (flags & 2) >> 1;
      gomp_mutex_lock (&team->task_lock);
      /* If parallel or taskgroup has been cancelled, don't start new
	 tasks.  */
      if (__builtin_expect ((gomp_team_barrier_cancelled (&team->barrier)
			     || (taskgroup && taskgroup->cancelled))
			    && !task->copy_ctors_done, 0))
	{
	  gomp_mutex_unlock (&team->task_lock);
	  gomp_finish_task (task);
	  free (task);
	  return;
	}
      if (taskgroup)
	taskgroup->num_children++;
      if (depend_size)
	{
	  size_t ndepend = (uintptr_t) depend[0];
	  size_t nout = (uintptr_t) depend[1];
	  size_t i;
	  hash_entry_type ent;

	  task->depend_count = ndepend;
	  task->num_dependees = 0;
	  if (parent->depend_hash == NULL)
	    parent->depend_hash
	      = htab_create (2 * ndepend > 12 ? 2 * ndepend : 12);
	  for (i = 0; i < ndepend; i++)
	    {
	      task->depend[i].addr = depend[2 + i];
	      task->depend[i].next = NULL;
	      task->depend[i].prev = NULL;
	      task->depend[i].task = task;
	      task->depend[i].is_in = i >= nout;
	      task->depend[i].redundant = false;
	      task->depend[i].redundant_out = false;

	      hash_entry_type *slot
		= htab_find_slot (&parent->depend_hash, &task->depend[i],
				  INSERT);
	      hash_entry_type out = NULL, last = NULL;
	      if (*slot)
		{
		  /* If multiple depends on the same task are the
		     same, all but the first one are redundant.
		     As inout/out come first, if any of them is
		     inout/out, it will win, which is the right
		     semantics.  */
		  if ((*slot)->task == task)
		    {
		      task->depend[i].redundant = true;
		      continue;
		    }
		  for (ent = *slot; ent; ent = ent->next)
		    {
		      if (ent->redundant_out)
			break;

		      last = ent;

		      /* depend(in:...) doesn't depend on earlier
			 depend(in:...).  */
		      if (i >= nout && ent->is_in)
			continue;

		      if (!ent->is_in)
			out = ent;

		      struct gomp_task *tsk = ent->task;
		      if (tsk->dependers == NULL)
			{
			  tsk->dependers
			    = gomp_malloc (sizeof (struct gomp_dependers_vec)
					   + 6 * sizeof (struct gomp_task *));
			  tsk->dependers->n_elem = 1;
			  tsk->dependers->allocated = 6;
			  tsk->dependers->elem[0] = task;
			  task->num_dependees++;
			  continue;
			}
		      /* We already have some other dependency on tsk
			 from earlier depend clause.  */
		      else if (tsk->dependers->n_elem
			       && (tsk->dependers->elem[tsk->dependers->n_elem
							- 1]
				   == task))
			continue;
		      else if (tsk->dependers->n_elem
			       == tsk->dependers->allocated)
			{
			  tsk->dependers->allocated
			    = tsk->dependers->allocated * 2 + 2;
			  tsk->dependers
			    = gomp_realloc (tsk->dependers,
					    sizeof (struct gomp_dependers_vec)
					    + (tsk->dependers->allocated
					       * sizeof (struct gomp_task *)));
			}
		      tsk->dependers->elem[tsk->dependers->n_elem++] = task;
		      task->num_dependees++;
		    }
		  task->depend[i].next = *slot;
		  (*slot)->prev = &task->depend[i];
		}
	      *slot = &task->depend[i];

	      /* There is no need to store more than one depend({,in}out:)
		 task per address in the hash table chain for the purpose
		 of creation of deferred tasks, because each out
		 depends on all earlier outs, thus it is enough to record
		 just the last depend({,in}out:).  For depend(in:), we need
		 to keep all of the previous ones not terminated yet, because
		 a later depend({,in}out:) might need to depend on all of
		 them.  So, if the new task's clause is depend({,in}out:),
		 we know there is at most one other depend({,in}out:) clause
		 in the list (out).  For non-deferred tasks we want to see
		 all outs, so they are moved to the end of the chain,
		 after first redundant_out entry all following entries
		 should be redundant_out.  */
	      if (!task->depend[i].is_in && out)
		{
		  if (out != last)
		    {
		      out->next->prev = out->prev;
		      out->prev->next = out->next;
		      out->next = last->next;
		      out->prev = last;
		      last->next = out;
		      if (out->next)
			out->next->prev = out;
		    }
		  out->redundant_out = true;
		}
	    }
	  if (task->num_dependees)
	    {
	      gomp_mutex_unlock (&team->task_lock);
	      return;
	    }
	}
      if (parent->children)
	{
	  task->next_child = parent->children;
	  task->prev_child = parent->children->prev_child;
	  task->next_child->prev_child = task;
	  task->prev_child->next_child = task;
	}
      else
	{
	  task->next_child = task;
	  task->prev_child = task;
	}
      parent->children = task;
      if (taskgroup)
	{
	  if (taskgroup->children)
	    {
	      task->next_taskgroup = taskgroup->children;
	      task->prev_taskgroup = taskgroup->children->prev_taskgroup;
	      task->next_taskgroup->prev_taskgroup = task;
	      task->prev_taskgroup->next_taskgroup = task;
	    }
	  else
	    {
	      task->next_taskgroup = task;
	      task->prev_taskgroup = task;
	    }
	  taskgroup->children = task;
	}
      if (team->task_queue)
	{
	  task->next_queue = team->task_queue;
	  task->prev_queue = team->task_queue->prev_queue;
	  task->next_queue->prev_queue = task;
	  task->prev_queue->next_queue = task;
	}
      else
	{
	  task->next_queue = task;
	  task->prev_queue = task;
	  team->task_queue = task;
	}
      ++team->task_count;
      ++team->task_queued_count;
      gomp_team_barrier_set_task_pending (&team->barrier);
      do_wake = team->task_running_count + !parent->in_tied_task
		< team->nthreads;
      gomp_mutex_unlock (&team->task_lock);
      if (do_wake)
	gomp_team_barrier_wake (&team->barrier, 1);
    }
}

static inline bool
gomp_task_run_pre (struct gomp_task *child_task, struct gomp_task *parent,
		   struct gomp_taskgroup *taskgroup, struct gomp_team *team)
{
  if (parent)
    {
      if (parent->children == child_task)
	parent->children = child_task->next_child;
      if (__builtin_expect (child_task->parent_depends_on, 0)
	  && parent->taskwait->last_parent_depends_on == child_task)
	{
	  if (child_task->prev_child->kind == GOMP_TASK_WAITING
	      && child_task->prev_child->parent_depends_on)
	    parent->taskwait->last_parent_depends_on = child_task->prev_child;
	  else
	    parent->taskwait->last_parent_depends_on = NULL;
	}
    }
  if (taskgroup && taskgroup->children == child_task)
    taskgroup->children = child_task->next_taskgroup;
  child_task->prev_queue->next_queue = child_task->next_queue;
  child_task->next_queue->prev_queue = child_task->prev_queue;
  if (team->task_queue == child_task)
    {
      if (child_task->next_queue != child_task)
	team->task_queue = child_task->next_queue;
      else
	team->task_queue = NULL;
    }
  child_task->kind = GOMP_TASK_TIED;
  if (--team->task_queued_count == 0)
    gomp_team_barrier_clear_task_pending (&team->barrier);
  if ((gomp_team_barrier_cancelled (&team->barrier)
       || (taskgroup && taskgroup->cancelled))
      && !child_task->copy_ctors_done)
    return true;
  return false;
}

static void
gomp_task_run_post_handle_depend_hash (struct gomp_task *child_task)
{
  struct gomp_task *parent = child_task->parent;
  size_t i;

  for (i = 0; i < child_task->depend_count; i++)
    if (!child_task->depend[i].redundant)
      {
	if (child_task->depend[i].next)
	  child_task->depend[i].next->prev = child_task->depend[i].prev;
	if (child_task->depend[i].prev)
	  child_task->depend[i].prev->next = child_task->depend[i].next;
	else
	  {
	    hash_entry_type *slot
	      = htab_find_slot (&parent->depend_hash, &child_task->depend[i],
				NO_INSERT);
	    if (*slot != &child_task->depend[i])
	      abort ();
	    if (child_task->depend[i].next)
	      *slot = child_task->depend[i].next;
	    else
	      htab_clear_slot (parent->depend_hash, slot);
	  }
      }
}

static size_t
gomp_task_run_post_handle_dependers (struct gomp_task *child_task,
				     struct gomp_team *team)
{
  struct gomp_task *parent = child_task->parent;
  size_t i, count = child_task->dependers->n_elem, ret = 0;
  for (i = 0; i < count; i++)
    {
      struct gomp_task *task = child_task->dependers->elem[i];
      if (--task->num_dependees != 0)
	continue;

      struct gomp_taskgroup *taskgroup = task->taskgroup;
      if (parent)
	{
	  if (parent->children)
	    {
	      /* If parent is in gomp_task_maybe_wait_for_dependencies
		 and it doesn't need to wait for this task, put it after
		 all ready to run tasks it needs to wait for.  */
	      if (parent->taskwait && parent->taskwait->last_parent_depends_on
		  && !task->parent_depends_on)
		{
		  struct gomp_task *last_parent_depends_on
		    = parent->taskwait->last_parent_depends_on;
		  task->next_child = last_parent_depends_on->next_child;
		  task->prev_child = last_parent_depends_on;
		}
	      else
		{
		  task->next_child = parent->children;
		  task->prev_child = parent->children->prev_child;
		  parent->children = task;
		}
	      task->next_child->prev_child = task;
	      task->prev_child->next_child = task;
	    }
	  else
	    {
	      task->next_child = task;
	      task->prev_child = task;
	      parent->children = task;
	    }
	  if (parent->taskwait)
	    {
	      if (parent->taskwait->in_taskwait)
		{
		  parent->taskwait->in_taskwait = false;
		  gomp_sem_post (&parent->taskwait->taskwait_sem);
		}
	      else if (parent->taskwait->in_depend_wait)
		{
		  parent->taskwait->in_depend_wait = false;
		  gomp_sem_post (&parent->taskwait->taskwait_sem);
		}
	      if (parent->taskwait->last_parent_depends_on == NULL
		  && task->parent_depends_on)
		parent->taskwait->last_parent_depends_on = task;
	    }
	}
      if (taskgroup)
	{
	  if (taskgroup->children)
	    {
	      task->next_taskgroup = taskgroup->children;
	      task->prev_taskgroup = taskgroup->children->prev_taskgroup;
	      task->next_taskgroup->prev_taskgroup = task;
	      task->prev_taskgroup->next_taskgroup = task;
	    }
	  else
	    {
	      task->next_taskgroup = task;
	      task->prev_taskgroup = task;
	    }
	  taskgroup->children = task;
	  if (taskgroup->in_taskgroup_wait)
	    {
	      taskgroup->in_taskgroup_wait = false;
	      gomp_sem_post (&taskgroup->taskgroup_sem);
	    }
	}
      if (team->task_queue)
	{
	  task->next_queue = team->task_queue;
	  task->prev_queue = team->task_queue->prev_queue;
	  task->next_queue->prev_queue = task;
	  task->prev_queue->next_queue = task;
	}
      else
	{
	  task->next_queue = task;
	  task->prev_queue = task;
	  team->task_queue = task;
	}
      ++team->task_count;
      ++team->task_queued_count;
      ++ret;
    }
  free (child_task->dependers);
  child_task->dependers = NULL;
  if (ret > 1)
    gomp_team_barrier_set_task_pending (&team->barrier);
  return ret;
}

static inline size_t
gomp_task_run_post_handle_depend (struct gomp_task *child_task,
				  struct gomp_team *team)
{
  if (child_task->depend_count == 0)
    return 0;

  /* If parent is gone already, the hash table is freed and nothing
     will use the hash table anymore, no need to remove anything from it.  */
  if (child_task->parent != NULL)
    gomp_task_run_post_handle_depend_hash (child_task);

  if (child_task->dependers == NULL)
    return 0;

  return gomp_task_run_post_handle_dependers (child_task, team);
}

static inline void
gomp_task_run_post_remove_parent (struct gomp_task *child_task)
{
  struct gomp_task *parent = child_task->parent;
  if (parent == NULL)
    return;
  if (__builtin_expect (child_task->parent_depends_on, 0)
      && --parent->taskwait->n_depend == 0
      && parent->taskwait->in_depend_wait)
    {
      parent->taskwait->in_depend_wait = false;
      gomp_sem_post (&parent->taskwait->taskwait_sem);
    }
  child_task->prev_child->next_child = child_task->next_child;
  child_task->next_child->prev_child = child_task->prev_child;
  if (parent->children != child_task)
    return;
  if (child_task->next_child != child_task)
    parent->children = child_task->next_child;
  else
    {
      /* We access task->children in GOMP_taskwait
	 outside of the task lock mutex region, so
	 need a release barrier here to ensure memory
	 written by child_task->fn above is flushed
	 before the NULL is written.  */
      __atomic_store_n (&parent->children, NULL, MEMMODEL_RELEASE);
      if (parent->taskwait && parent->taskwait->in_taskwait)
	{
	  parent->taskwait->in_taskwait = false;
	  gomp_sem_post (&parent->taskwait->taskwait_sem);
	}
    }
}

static inline void
gomp_task_run_post_remove_taskgroup (struct gomp_task *child_task)
{
  struct gomp_taskgroup *taskgroup = child_task->taskgroup;
  if (taskgroup == NULL)
    return;
  child_task->prev_taskgroup->next_taskgroup = child_task->next_taskgroup;
  child_task->next_taskgroup->prev_taskgroup = child_task->prev_taskgroup;
  if (taskgroup->num_children > 1)
    --taskgroup->num_children;
  else
    {
      /* We access taskgroup->num_children in GOMP_taskgroup_end
	 outside of the task lock mutex region, so
	 need a release barrier here to ensure memory
	 written by child_task->fn above is flushed
	 before the NULL is written.  */
      __atomic_store_n (&taskgroup->num_children, 0, MEMMODEL_RELEASE);
    }
  if (taskgroup->children != child_task)
    return;
  if (child_task->next_taskgroup != child_task)
    taskgroup->children = child_task->next_taskgroup;
  else
    {
      taskgroup->children = NULL;
      if (taskgroup->in_taskgroup_wait)
	{
	  taskgroup->in_taskgroup_wait = false;
	  gomp_sem_post (&taskgroup->taskgroup_sem);
	}
    }
}

void
gomp_barrier_handle_tasks (gomp_barrier_state_t state)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_team *team = thr->ts.team;
  struct gomp_task *task = thr->task;
  struct gomp_task *child_task = NULL;
  struct gomp_task *to_free = NULL;
  int do_wake = 0;

  gomp_mutex_lock (&team->task_lock);
  if (gomp_barrier_last_thread (state))
    {
      if (team->task_count == 0)
	{
	  gomp_team_barrier_done (&team->barrier, state);
	  gomp_mutex_unlock (&team->task_lock);
	  gomp_team_barrier_wake (&team->barrier, 0);
	  return;
	}
      gomp_team_barrier_set_waiting_for_tasks (&team->barrier);
    }

  while (1)
    {
      bool cancelled = false;
      if (team->task_queue != NULL)
	{
	  child_task = team->task_queue;
	  cancelled = gomp_task_run_pre (child_task, child_task->parent,
					 child_task->taskgroup, team);
	  if (__builtin_expect (cancelled, 0))
	    {
	      if (to_free)
		{
		  gomp_finish_task (to_free);
		  free (to_free);
		  to_free = NULL;
		}
	      goto finish_cancelled;
	    }
	  team->task_running_count++;
	  child_task->in_tied_task = true;
	}
      gomp_mutex_unlock (&team->task_lock);
      if (do_wake)
	{
	  gomp_team_barrier_wake (&team->barrier, do_wake);
	  do_wake = 0;
	}
      if (to_free)
	{
	  gomp_finish_task (to_free);
	  free (to_free);
	  to_free = NULL;
	}
      if (child_task)
	{
	  thr->task = child_task;
	  child_task->fn (child_task->fn_data);
	  thr->task = task;
	}
      else
	return;
      gomp_mutex_lock (&team->task_lock);
      if (child_task)
	{
	 finish_cancelled:;
	  size_t new_tasks
	    = gomp_task_run_post_handle_depend (child_task, team);
	  gomp_task_run_post_remove_parent (child_task);
	  gomp_clear_parent (child_task->children);
	  gomp_task_run_post_remove_taskgroup (child_task);
	  to_free = child_task;
	  child_task = NULL;
	  if (!cancelled)
	    team->task_running_count--;
	  if (new_tasks > 1)
	    {
	      do_wake = team->nthreads - team->task_running_count;
	      if (do_wake > new_tasks)
		do_wake = new_tasks;
	    }
	  if (--team->task_count == 0
	      && gomp_team_barrier_waiting_for_tasks (&team->barrier))
	    {
	      gomp_team_barrier_done (&team->barrier, state);
	      gomp_mutex_unlock (&team->task_lock);
	      gomp_team_barrier_wake (&team->barrier, 0);
	      gomp_mutex_lock (&team->task_lock);
	    }
	}
    }
}

/* Called when encountering a taskwait directive.  */

void
GOMP_taskwait (void)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_team *team = thr->ts.team;
  struct gomp_task *task = thr->task;
  struct gomp_task *child_task = NULL;
  struct gomp_task *to_free = NULL;
  struct gomp_taskwait taskwait;
  int do_wake = 0;

  /* The acquire barrier on load of task->children here synchronizes
     with the write of a NULL in gomp_task_run_post_remove_parent.  It is
     not necessary that we synchronize with other non-NULL writes at
     this point, but we must ensure that all writes to memory by a
     child thread task work function are seen before we exit from
     GOMP_taskwait.  */
  if (task == NULL
      || __atomic_load_n (&task->children, MEMMODEL_ACQUIRE) == NULL)
    return;

  memset (&taskwait, 0, sizeof (taskwait));
  gomp_mutex_lock (&team->task_lock);
  while (1)
    {
      bool cancelled = false;
      if (task->children == NULL)
	{
	  bool destroy_taskwait = task->taskwait != NULL;
	  task->taskwait = NULL;
	  gomp_mutex_unlock (&team->task_lock);
	  if (to_free)
	    {
	      gomp_finish_task (to_free);
	      free (to_free);
	    }
	  if (destroy_taskwait)
	    gomp_sem_destroy (&taskwait.taskwait_sem);
	  return;
	}
      if (task->children->kind == GOMP_TASK_WAITING)
	{
	  child_task = task->children;
	  cancelled
	    = gomp_task_run_pre (child_task, task, child_task->taskgroup,
				 team);
	  if (__builtin_expect (cancelled, 0))
	    {
	      if (to_free)
		{
		  gomp_finish_task (to_free);
		  free (to_free);
		  to_free = NULL;
		}
	      goto finish_cancelled;
	    }
	}
      else
	{
	  /* All tasks we are waiting for are already running
	     in other threads.  Wait for them.  */
	  if (task->taskwait == NULL)
	    {
	      taskwait.in_depend_wait = false;
	      gomp_sem_init (&taskwait.taskwait_sem, 0);
	      task->taskwait = &taskwait;
	    }
	  taskwait.in_taskwait = true;
	}
      gomp_mutex_unlock (&team->task_lock);
      if (do_wake)
	{
	  gomp_team_barrier_wake (&team->barrier, do_wake);
	  do_wake = 0;
	}
      if (to_free)
	{
	  gomp_finish_task (to_free);
	  free (to_free);
	  to_free = NULL;
	}
      if (child_task)
	{
	  thr->task = child_task;
	  child_task->fn (child_task->fn_data);
	  thr->task = task;
	}
      else
	gomp_sem_wait (&taskwait.taskwait_sem);
      gomp_mutex_lock (&team->task_lock);
      if (child_task)
	{
	 finish_cancelled:;
	  size_t new_tasks
	    = gomp_task_run_post_handle_depend (child_task, team);
	  child_task->prev_child->next_child = child_task->next_child;
	  child_task->next_child->prev_child = child_task->prev_child;
	  if (task->children == child_task)
	    {
	      if (child_task->next_child != child_task)
		task->children = child_task->next_child;
	      else
		task->children = NULL;
	    }
	  gomp_clear_parent (child_task->children);
	  gomp_task_run_post_remove_taskgroup (child_task);
	  to_free = child_task;
	  child_task = NULL;
	  team->task_count--;
	  if (new_tasks > 1)
	    {
	      do_wake = team->nthreads - team->task_running_count
			- !task->in_tied_task;
	      if (do_wake > new_tasks)
		do_wake = new_tasks;
	    }
	}
    }
}

/* This is like GOMP_taskwait, but we only wait for tasks that the
   upcoming task depends on.  */

static void
gomp_task_maybe_wait_for_dependencies (void **depend)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_task *task = thr->task;
  struct gomp_team *team = thr->ts.team;
  struct gomp_task_depend_entry elem, *ent = NULL;
  struct gomp_taskwait taskwait;
  struct gomp_task *last_parent_depends_on = NULL;
  size_t ndepend = (uintptr_t) depend[0];
  size_t nout = (uintptr_t) depend[1];
  size_t i;
  size_t num_awaited = 0;
  struct gomp_task *child_task = NULL;
  struct gomp_task *to_free = NULL;
  int do_wake = 0;

  gomp_mutex_lock (&team->task_lock);
  for (i = 0; i < ndepend; i++)
    {
      elem.addr = depend[i + 2];
      ent = htab_find (task->depend_hash, &elem);
      for (; ent; ent = ent->next)
	if (i >= nout && ent->is_in)
	  continue;
	else
	  {
	    struct gomp_task *tsk = ent->task;
	    if (!tsk->parent_depends_on)
	      {
		tsk->parent_depends_on = true;
		++num_awaited;
		if (tsk->num_dependees == 0 && tsk->kind == GOMP_TASK_WAITING)
		  {
		    /* If a task we need to wait for is not already
		       running and is ready to be scheduled, move it
		       to front, so that we run it as soon as possible.  */
		    if (last_parent_depends_on)
		      {
			tsk->prev_child->next_child = tsk->next_child;
			tsk->next_child->prev_child = tsk->prev_child;
			tsk->prev_child = last_parent_depends_on;
			tsk->next_child = last_parent_depends_on->next_child;
			tsk->prev_child->next_child = tsk;
			tsk->next_child->prev_child = tsk;
		      }
		    else if (tsk != task->children)
		      {
			tsk->prev_child->next_child = tsk->next_child;
			tsk->next_child->prev_child = tsk->prev_child;
			tsk->prev_child = task->children;
			tsk->next_child = task->children->next_child;
			task->children = tsk;
			tsk->prev_child->next_child = tsk;
			tsk->next_child->prev_child = tsk;
		      }
		    last_parent_depends_on = tsk;
		  }
	      }
	  }
    }
  if (num_awaited == 0)
    {
      gomp_mutex_unlock (&team->task_lock);
      return;
    }

  memset (&taskwait, 0, sizeof (taskwait));
  taskwait.n_depend = num_awaited;
  taskwait.last_parent_depends_on = last_parent_depends_on;
  gomp_sem_init (&taskwait.taskwait_sem, 0);
  task->taskwait = &taskwait;

  while (1)
    {
      bool cancelled = false;
      if (taskwait.n_depend == 0)
	{
	  task->taskwait = NULL;
	  gomp_mutex_unlock (&team->task_lock);
	  if (to_free)
	    {
	      gomp_finish_task (to_free);
	      free (to_free);
	    }
	  gomp_sem_destroy (&taskwait.taskwait_sem);
	  return;
	}
      if (task->children->kind == GOMP_TASK_WAITING)
	{
	  child_task = task->children;
	  cancelled
	    = gomp_task_run_pre (child_task, task, child_task->taskgroup,
				 team);
	  if (__builtin_expect (cancelled, 0))
	    {
	      if (to_free)
		{
		  gomp_finish_task (to_free);
		  free (to_free);
		  to_free = NULL;
		}
	      goto finish_cancelled;
	    }
	}
      else
	/* All tasks we are waiting for are already running
	   in other threads.  Wait for them.  */
	taskwait.in_depend_wait = true;
      gomp_mutex_unlock (&team->task_lock);
      if (do_wake)
	{
	  gomp_team_barrier_wake (&team->barrier, do_wake);
	  do_wake = 0;
	}
      if (to_free)
	{
	  gomp_finish_task (to_free);
	  free (to_free);
	  to_free = NULL;
	}
      if (child_task)
	{
	  thr->task = child_task;
	  child_task->fn (child_task->fn_data);
	  thr->task = task;
	}
      else
	gomp_sem_wait (&taskwait.taskwait_sem);
      gomp_mutex_lock (&team->task_lock);
      if (child_task)
	{
	 finish_cancelled:;
	  size_t new_tasks
	    = gomp_task_run_post_handle_depend (child_task, team);
	  if (child_task->parent_depends_on)
	    --taskwait.n_depend;
	  child_task->prev_child->next_child = child_task->next_child;
	  child_task->next_child->prev_child = child_task->prev_child;
	  if (task->children == child_task)
	    {
	      if (child_task->next_child != child_task)
		task->children = child_task->next_child;
	      else
		task->children = NULL;
	    }
	  gomp_clear_parent (child_task->children);
	  gomp_task_run_post_remove_taskgroup (child_task);
	  to_free = child_task;
	  child_task = NULL;
	  team->task_count--;
	  if (new_tasks > 1)
	    {
	      do_wake = team->nthreads - team->task_running_count
			- !task->in_tied_task;
	      if (do_wake > new_tasks)
		do_wake = new_tasks;
	    }
	}
    }
}

/* Called when encountering a taskyield directive.  */

void
GOMP_taskyield (void)
{
  /* Nothing at the moment.  */
}

void
GOMP_taskgroup_start (void)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_team *team = thr->ts.team;
  struct gomp_task *task = thr->task;
  struct gomp_taskgroup *taskgroup;

  /* If team is NULL, all tasks are executed as
     GOMP_TASK_IFFALSE tasks and thus all children tasks of
     taskgroup and their descendant tasks will be finished
     by the time GOMP_taskgroup_end is called.  */
  if (team == NULL)
    return;
  taskgroup = gomp_malloc (sizeof (struct gomp_taskgroup));
  taskgroup->prev = task->taskgroup;
  taskgroup->children = NULL;
  taskgroup->in_taskgroup_wait = false;
  taskgroup->cancelled = false;
  taskgroup->num_children = 0;
  gomp_sem_init (&taskgroup->taskgroup_sem, 0);
  task->taskgroup = taskgroup;
}

void
GOMP_taskgroup_end (void)
{
  struct gomp_thread *thr = gomp_thread ();
  struct gomp_team *team = thr->ts.team;
  struct gomp_task *task = thr->task;
  struct gomp_taskgroup *taskgroup;
  struct gomp_task *child_task = NULL;
  struct gomp_task *to_free = NULL;
  int do_wake = 0;

  if (team == NULL)
    return;
  taskgroup = task->taskgroup;

  /* The acquire barrier on load of taskgroup->num_children here
     synchronizes with the write of 0 in gomp_task_run_post_remove_taskgroup.
     It is not necessary that we synchronize with other non-0 writes at
     this point, but we must ensure that all writes to memory by a
     child thread task work function are seen before we exit from
     GOMP_taskgroup_end.  */
  if (__atomic_load_n (&taskgroup->num_children, MEMMODEL_ACQUIRE) == 0)
    goto finish;

  gomp_mutex_lock (&team->task_lock);
  while (1)
    {
      bool cancelled = false;
      if (taskgroup->children == NULL)
	{
	  if (taskgroup->num_children)
	    {
	      if (task->children == NULL)
		goto do_wait;
	      child_task = task->children;
            }
          else
	    {
	      gomp_mutex_unlock (&team->task_lock);
	      if (to_free)
		{
		  gomp_finish_task (to_free);
		  free (to_free);
		}
	      goto finish;
	    }
	}
      else
	child_task = taskgroup->children;
      if (child_task->kind == GOMP_TASK_WAITING)
	{
	  cancelled
	    = gomp_task_run_pre (child_task, child_task->parent, taskgroup,
				 team);
	  if (__builtin_expect (cancelled, 0))
	    {
	      if (to_free)
		{
		  gomp_finish_task (to_free);
		  free (to_free);
		  to_free = NULL;
		}
	      goto finish_cancelled;
	    }
	}
      else
	{
	  child_task = NULL;
	 do_wait:
	  /* All tasks we are waiting for are already running
	     in other threads.  Wait for them.  */
	  taskgroup->in_taskgroup_wait = true;
	}
      gomp_mutex_unlock (&team->task_lock);
      if (do_wake)
	{
	  gomp_team_barrier_wake (&team->barrier, do_wake);
	  do_wake = 0;
	}
      if (to_free)
	{
	  gomp_finish_task (to_free);
	  free (to_free);
	  to_free = NULL;
	}
      if (child_task)
	{
	  thr->task = child_task;
	  child_task->fn (child_task->fn_data);
	  thr->task = task;
	}
      else
	gomp_sem_wait (&taskgroup->taskgroup_sem);
      gomp_mutex_lock (&team->task_lock);
      if (child_task)
	{
	 finish_cancelled:;
	  size_t new_tasks
	    = gomp_task_run_post_handle_depend (child_task, team);
	  gomp_task_run_post_remove_parent (child_task);
	  gomp_clear_parent (child_task->children);
	  gomp_task_run_post_remove_taskgroup (child_task);
	  to_free = child_task;
	  child_task = NULL;
	  team->task_count--;
	  if (new_tasks > 1)
	    {
	      do_wake = team->nthreads - team->task_running_count
			- !task->in_tied_task;
	      if (do_wake > new_tasks)
		do_wake = new_tasks;
	    }
	}
    }

 finish:
  task->taskgroup = taskgroup->prev;
  gomp_sem_destroy (&taskgroup->taskgroup_sem);
  free (taskgroup);
}

int
omp_in_final (void)
{
  struct gomp_thread *thr = gomp_thread ();
  return thr->task && thr->task->final_task;
}

ialias (omp_in_final)