Stockfish/src/thread.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
  Copyright (C) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad

  Stockfish 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 of the License, or
  (at your option) any later version.

  Stockfish 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, see <http://www.gnu.org/licenses/>.
*/

#include <algorithm> // For std::count
#include <cassert>

#include "movegen.h"
#include "search.h"
#include "thread.h"
#include "syzygy/tbprobe.h"

ThreadPool Threads; // Global object


/// Thread constructor launches the thread and waits until it goes to sleep
/// in idle_loop(). Note that 'searching' and 'exit' should be alredy set.

Thread::Thread(size_t n) : idx(n), stdThread(&Thread::idle_loop, this) {

  wait_for_search_finished();
}


/// Thread destructor wakes up the thread in idle_loop() and waits
/// for its termination. Thread should be already waiting.

Thread::~Thread() {

  assert(!searching);

  exit = true;
  start_searching();
  stdThread.join();
}


/// Thread::start_searching() wakes up the thread that will start the search

void Thread::start_searching() {

  std::lock_guard<Mutex> lk(mutex);
  searching = true;
  cv.notify_one(); // Wake up the thread in idle_loop()
}


/// Thread::wait_for_search_finished() blocks on the condition variable
/// until the thread has finished searching.

void Thread::wait_for_search_finished() {

  std::unique_lock<Mutex> lk(mutex);
  cv.wait(lk, [&]{ return !searching; });
}


/// Thread::idle_loop() is where the thread is parked, blocked on the
/// condition variable, when it has no work to do.

void Thread::idle_loop() {

  WinProcGroup::bindThisThread(idx);

  while (true)
  {
      std::unique_lock<Mutex> lk(mutex);
      searching = false;
      cv.notify_one(); // Wake up anyone waiting for search finished
      cv.wait(lk, [&]{ return searching; });

      if (exit)
          return;

      lk.unlock();

      search();
  }
}


/// ThreadPool::init() creates and launches the threads that will go
/// immediately to sleep in idle_loop. We cannot use the c'tor because
/// Threads is a static object and we need a fully initialized engine at
/// this point due to allocation of Endgames in the Thread constructor.

void ThreadPool::init(size_t requested) {

  push_back(new MainThread(0));
  set(requested);
}


/// ThreadPool::exit() terminates threads before the program exits. Cannot be
/// done in the destructor because threads must be terminated before deleting
/// any static object, so before main() returns.

void ThreadPool::exit() {

  main()->wait_for_search_finished();
  set(0);
}


/// ThreadPool::set() creates/destroys threads to match the requested number

void ThreadPool::set(size_t requested) {

  while (size() < requested)
      push_back(new Thread(size()));

  while (size() > requested)
      delete back(), pop_back();
}


/// ThreadPool::start_thinking() wakes up main thread waiting in idle_loop() and
/// returns immediately. Main thread will wake up other threads and start the search.

void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
                                const Search::LimitsType& limits, bool ponderMode) {

  main()->wait_for_search_finished();

  stopOnPonderhit = stop = false;
  ponder = ponderMode;
  Search::Limits = limits;
  Search::RootMoves rootMoves;

  for (const auto& m : MoveList<LEGAL>(pos))
      if (   limits.searchmoves.empty()
          || std::count(limits.searchmoves.begin(), limits.searchmoves.end(), m))
          rootMoves.emplace_back(m);

  if (!rootMoves.empty())
      Tablebases::filter_root_moves(pos, rootMoves);

  // After ownership transfer 'states' becomes empty, so if we stop the search
  // and call 'go' again without setting a new position states.get() == NULL.
  assert(states.get() || setupStates.get());

  if (states.get())
      setupStates = std::move(states); // Ownership transfer, states is now empty

  // We use Position::set() to set root position across threads. So we
  // need to save and later to restore st->previous, cleared by set().
  // Note that setupStates is shared by threads but is accessed in read-only mode.
  StateInfo* previous = setupStates->back().previous;

  for (Thread* th : Threads)
  {
      th->nodes = th->tbHits = 0;
      th->rootDepth = th->completedDepth = DEPTH_ZERO;
      th->rootMoves = rootMoves;
      th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
  }

  setupStates->back().previous = previous;

  main()->start_searching();
}