Rewrite how threads are spawned

Instead of creating a running std::thread and
returning, wait in Thread c'tor that the native
thread of execution goes to sleep in idle_loop().

In this way we can simplify how search is started,
because when main thread is idle we are sure also
all other threads will be idle, in any case, even
at thread creation and startup.

After lazy smp went in, we can simpify and rewrite
a lot of logic that is now no more needed. This is
hopefully the final big cleanup.

Tested for no regression at 5+0.1 with 3 threads:
LLR: 2.95 (-2.94,2.94) [-5.00,0.00]
Total: 17411 W: 3248 L: 3198 D: 10965

No functional change.

src/thread.cpp

@@ -29,52 +29,46 @@ using namespace Search;
 
 ThreadPool Threads; // Global object
 
-// Thread constructor makes some init and launches the thread that will go to
-// sleep in idle_loop().
+/// Thread constructor launch the thread and then wait until it goes to sleep
+/// in idle_loop().
 
 Thread::Thread() {
 
-  searching = true; // Avoid a race with start_thinking()
-  exit = resetCalls = false;
+  resetCalls = exit = false;
   maxPly = callsCnt = 0;
   history.clear();
   counterMoves.clear();
-  idx = Threads.size(); // Starts from 0
-  std::thread::operator=(std::thread(&Thread::idle_loop, this));
+  idx = Threads.size(); // Start from 0
+
+  std::unique_lock<Mutex> lk(mutex);
+  searching = true;
+  nativeThread = std::thread(&Thread::idle_loop, this);
+  sleepCondition.wait(lk, [&]{ return !searching; });
 }
 
 
-// Thread destructor waits for thread termination before deleting
+/// Thread destructor wait for thread termination before returning
 
 Thread::~Thread() {
 
   mutex.lock();
-  exit = true; // Search must be already finished
+  exit = true;
+  sleepCondition.notify_one();
   mutex.unlock();
-
-  notify_one();
-  std::thread::join(); // Wait for thread termination
+  nativeThread.join();
 }
 
 
-// Thread::join() waits for the thread to finish searching
-void Thread::join() {
+/// Thread::wait_for_search_finished() wait on sleep condition until not searching
+
+void Thread::wait_for_search_finished() {
 
   std::unique_lock<Mutex> lk(mutex);
   sleepCondition.wait(lk, [&]{ return !searching; });
 }
 
 
-// Thread::notify_one() wakes up the thread when there is some work to do
-
-void Thread::notify_one() {
-
-  std::unique_lock<Mutex> lk(mutex);
-  sleepCondition.notify_one();
-}
-
-
-// Thread::wait() set the thread to sleep until 'condition' turns true
+/// Thread::wait() wait on sleep condition until condition is true
 
 void Thread::wait(std::atomic_bool& condition) {
 
@@ -83,7 +77,20 @@ void Thread::wait(std::atomic_bool& condition) {
 }
 
 
-// Thread::idle_loop() is where the thread is parked when it has no work to do
+/// Thread::start_searching() wake up the thread that will start the search
+
+void Thread::start_searching(bool resume) {
+
+  std::unique_lock<Mutex> lk(mutex);
+
+  if (!resume)
+      searching = true;
+
+  sleepCondition.notify_one();
+}
+
+
+/// Thread::idle_loop() is where the thread is parked when it has no work to do
 
 void Thread::idle_loop() {
 
@@ -95,22 +102,22 @@ void Thread::idle_loop() {
 
       while (!searching && !exit)
       {
-          sleepCondition.notify_one(); // Wake up main thread if needed
+          sleepCondition.notify_one(); // Wake up any waiting thread
           sleepCondition.wait(lk);
       }
 
       lk.unlock();
 
-      if (!exit && searching)
+      if (!exit)
           search();
   }
 }
 
 
-// ThreadPool::init() is called at startup to create and launch requested threads,
-// that will go immediately to sleep. We cannot use a constructor 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.
+/// ThreadPool::init() create and launch requested threads, that will go
+/// immediately to sleep. We cannot use a constructor 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() {
 
@@ -119,27 +126,24 @@ void ThreadPool::init() {
 }
 
 
-// ThreadPool::exit() terminates the threads before the program exits. Cannot be
-// done in destructor because threads must be terminated before freeing us.
+/// ThreadPool::exit() terminate threads before the program exits. Cannot be
+/// done in destructor because threads must be terminated before deleting any
+/// static objects, so while still in main().
 
 void ThreadPool::exit() {
 
-  for (Thread* th : *this)
-      delete th;
-
-  clear(); // Get rid of stale pointers
+  while (size())
+      delete back(), pop_back();
 }
 
 
-// ThreadPool::read_uci_options() updates internal threads parameters from the
-// corresponding UCI options and creates/destroys threads to match the requested
-// number. Thread objects are dynamically allocated to avoid creating all possible
-// threads in advance (which include pawns and material tables), even if only a
-// few are to be used.
+/// ThreadPool::read_uci_options() updates internal threads parameters from the
+/// corresponding UCI options and creates/destroys threads to match requested
+/// number. Thread objects are dynamically allocated.
 
 void ThreadPool::read_uci_options() {
 
-  size_t requested  = Options["Threads"];
+  size_t requested = Options["Threads"];
 
   assert(requested > 0);
 
@@ -147,31 +151,28 @@ void ThreadPool::read_uci_options() {
       push_back(new Thread);
 
   while (size() > requested)
-  {
-      delete back();
-      pop_back();
-  }
+      delete back(), pop_back();
 }
 
 
-// ThreadPool::nodes_searched() returns the number of nodes searched
+/// ThreadPool::nodes_searched() return the number of nodes searched
 
 int64_t ThreadPool::nodes_searched() {
 
   int64_t nodes = 0;
-  for (Thread *th : *this)
+  for (Thread* th : *this)
       nodes += th->rootPos.nodes_searched();
   return nodes;
 }
 
 
-// ThreadPool::start_thinking() wakes up the main thread sleeping in
-// MainThread::idle_loop() and starts a new search, then returns immediately.
+/// ThreadPool::start_thinking() wake up the main thread sleeping in idle_loop()
+/// and start a new search, then return immediately.
 
 void ThreadPool::start_thinking(const Position& pos, const LimitsType& limits,
                                 StateStackPtr& states) {
-  for (Thread* th : Threads)
-      th->join();
+
+  main()->wait_for_search_finished();
 
   Signals.stopOnPonderhit = Signals.firstRootMove = false;
   Signals.stop = Signals.failedLowAtRoot = false;
@@ -190,6 +191,5 @@ void ThreadPool::start_thinking(const Position& pos, const LimitsType& limits,
           || std::count(limits.searchmoves.begin(), limits.searchmoves.end(), m))
           main()->rootMoves.push_back(RootMove(m));
 
-  main()->searching = true;
-  main()->notify_one(); // Wake up main thread: 'searching' must be already set
+  main()->start_searching();
 }