sync-barrier.hpp

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/*
  SYNC-BARRIER.hpp  -  N-fold synchronisation point with yield-wait

  Copyright (C)         Lumiera.org
    2023,               Hermann Vosseler <Ichthyostega@web.de>

  This program 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 2 of
  the License, or (at your option) any later version.

  This program 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.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

*/


#ifndef LIB_SYNC_BARRIER_H
#define LIB_SYNC_BARRIER_H


#include "lib/error.hpp"
#include "lib/nocopy.hpp"

#include <thread>
#include <atomic>


namespace lib {
  
  
  class SyncBarrier
    : util::NonCopyable
    {
      std::atomic_int latch_;
      
    public:
      explicit
      SyncBarrier (uint nFold =2)
        : latch_{int(nFold)}
        {
          REQUIRE (nFold >= 2, "Pointless to sync less than two participants.");
          ENSURE  (nFold < 100'000, "Danger territory.... sync 100k Threads??");
        }
      
      void
      sync()
        {
          size_t level = latch_.fetch_add(-1, std::memory_order_acq_rel);
          if (1 < level)
            do std::this_thread::yield();
            while (0 < latch_.load (std::memory_order_relaxed));
          else
            latch_.store (0, std::memory_order_relaxed);
        }             //  prevent spurious calls from wrapping
    };
  
  
  
} // namespace lib
#endif /*LIB_SYNC_BARRIER_H*/