Reformat threads code

Apart from some renaming the biggest change
is the retire of split_point_finished()
replaced by slavesMask flags. As a side
effect we now take also split point lock
when allocation available threads.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.cpp

@@ -573,19 +573,25 @@ namespace {
         thread.maxPly = ss->ply;
 
     // Step 1. Initialize node
-    if (!SpNode)
+    if (SpNode)
+    {
+        tte = NULL;
+        ttMove = excludedMove = MOVE_NONE;
+        sp = ss->sp;
+        threatMove = sp->threatMove;
+        bestValue = sp->bestValue;
+        moveCount = sp->moveCount; // Lock must be held here
+
+        assert(bestValue > -VALUE_INFINITE && moveCount > 0);
+
+        goto split_point_start;
+    }
+    else
     {
         ss->currentMove = ss->bestMove = threatMove = (ss+1)->excludedMove = MOVE_NONE;
         (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
         (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
-    }
+
-    else
-    {
-        sp = ss->sp;
-        tte = NULL;
-        ttMove = excludedMove = MOVE_NONE;
-        threatMove = sp->threatMove;
-        goto split_point_start;
     }
 
     // Step 2. Check for aborted search and immediate draw
@@ -820,14 +826,6 @@ split_point_start: // At split points actual search starts from here
                            && !excludedMove // Recursive singular search is not allowed
                            && (tte->type() & VALUE_TYPE_LOWER)
                            && tte->depth() >= depth - 3 * ONE_PLY;
-    if (SpNode)
-    {
-        lock_grab(sp->lock);
-        bestValue = sp->bestValue;
-        moveCount = sp->moveCount;
-
-        assert(bestValue > -VALUE_INFINITE && moveCount > 0);
-    }
 
     // Step 11. Loop through moves
     // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
@@ -1129,14 +1127,6 @@ split_point_start: // At split points actual search starts from here
         }
     }
 
-    if (SpNode)
-    {
-        // Here we have the lock still grabbed
-        sp->is_slave[pos.thread()] = false;
-        sp->nodes += pos.nodes_searched();
-        lock_release(sp->lock);
-    }
-
     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 
     return bestValue;
@@ -1836,24 +1826,24 @@ void RootMove::insert_pv_in_tt(Position& pos) {
 
 
 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
-/// The parameter 'sp', if non-NULL, is a pointer to an active SplitPoint object
+/// The parameter 'master_sp', if non-NULL, is a pointer to an active SplitPoint
-/// for which the thread is the master.
+/// object for which the thread is the master.
 
-void Thread::idle_loop(SplitPoint* sp) {
+void Thread::idle_loop(SplitPoint* sp_master) {
 
-  while (true)
+  // If this thread is the master of a split point and all slaves have
+  // finished their work at this split point, return from the idle loop.
+  while (!sp_master || sp_master->slavesMask)
   {
       // If we are not searching, wait for a condition to be signaled
       // instead of wasting CPU time polling for work.
       while (   do_sleep
-             || do_terminate
+             || do_exit
-             || (Threads.use_sleeping_threads() && !is_searching))
+             || (!is_searching && Threads.use_sleeping_threads()))
       {
-          assert((!sp && threadID) || Threads.use_sleeping_threads());
+          if (do_exit)
-
-          if (do_terminate)
           {
-              assert(!sp);
+              assert(!sp_master);
               return;
           }
 
@@ -1861,7 +1851,7 @@ void Thread::idle_loop(SplitPoint* sp) {
           lock_grab(sleepLock);
 
           // If we are master and all slaves have finished don't go to sleep
-          if (sp && Threads.split_point_finished(sp))
+          if (sp_master && !sp_master->slavesMask)
           {
               lock_release(sleepLock);
               break;
@@ -1880,46 +1870,42 @@ void Thread::idle_loop(SplitPoint* sp) {
       // If this thread has been assigned work, launch a search
       if (is_searching)
       {
-          assert(!do_terminate);
+          assert(!do_sleep && !do_exit);
 
           // Copy split point position and search stack and call search()
           Stack ss[MAX_PLY_PLUS_2];
-          SplitPoint* tsp = splitPoint;
+          SplitPoint* sp = splitPoint;
-          Position pos(*tsp->pos, threadID);
+          Position pos(*sp->pos, threadID);
 
-          memcpy(ss, tsp->ss - 1, 4 * sizeof(Stack));
+          memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
-          (ss+1)->sp = tsp;
+          (ss+1)->sp = sp;
 
-          if (tsp->nodeType == Root)
+          lock_grab(sp->lock);
-              search<SplitPointRoot>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
+
-          else if (tsp->nodeType == PV)
+          if (sp->nodeType == Root)
-              search<SplitPointPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
+              search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
-          else if (tsp->nodeType == NonPV)
+          else if (sp->nodeType == PV)
-              search<SplitPointNonPV>(pos, ss+1, tsp->alpha, tsp->beta, tsp->depth);
+              search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
+          else if (sp->nodeType == NonPV)
+              search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
           else
               assert(false);
 
           assert(is_searching);
 
+          // We return from search with lock held
+          sp->slavesMask &= ~(1ULL << threadID);
+          sp->nodes += pos.nodes_searched();
+          lock_release(sp->lock);
+
           is_searching = false;
 
           // Wake up master thread so to allow it to return from the idle loop in
           // case we are the last slave of the split point.
           if (   Threads.use_sleeping_threads()
-              && threadID != tsp->master
+              && threadID != sp->master
-              && !Threads[tsp->master].is_searching)
+              && !Threads[sp->master].is_searching)
-              Threads[tsp->master].wake_up();
+              Threads[sp->master].wake_up();
-      }
-
-      // If this thread is the master of a split point and all slaves have
-      // finished their work at this split point, return from the idle loop.
-      if (sp && Threads.split_point_finished(sp))
-      {
-          // Because sp->is_slave[] is reset under lock protection,
-          // be sure sp->lock has been released before to return.
-          lock_grab(sp->lock);
-          lock_release(sp->lock);
-          return;
       }
   }
 }

src/thread.cpp

@@ -97,12 +97,11 @@ bool Thread::is_available_to(int master) const {
 
   // Make a local copy to be sure doesn't become zero under our feet while
   // testing next condition and so leading to an out of bound access.
-  int localActiveSplitPoints = activeSplitPoints;
+  int sp_count = activeSplitPoints;
 
   // No active split points means that the thread is available as a slave for any
   // other thread otherwise apply the "helpful master" concept if possible.
-  if (   !localActiveSplitPoints
+  if (!sp_count || (splitPoints[sp_count - 1].slavesMask & (1ULL << master)))
-      || splitPoints[localActiveSplitPoints - 1].is_slave[master])
       return true;
 
   return false;
@@ -190,9 +189,11 @@ void ThreadsManager::init() {
 
 void ThreadsManager::exit() {
 
+  assert(threads[0].is_searching == false);
+
   for (int i = 0; i <= MAX_THREADS; i++)
   {
-      threads[i].do_terminate = true; // Search must be already finished
+      threads[i].do_exit = true; // Search must be already finished
       threads[i].wake_up();
 
       thread_join(threads[i].handle); // Wait for thread termination
@@ -225,19 +226,6 @@ bool ThreadsManager::available_slave_exists(int master) const {
 }
 
 
-// split_point_finished() checks if all the slave threads of a given split
-// point have finished searching.
-
-bool ThreadsManager::split_point_finished(SplitPoint* sp) const {
-
-  for (int i = 0; i < activeThreads; i++)
-      if (sp->is_slave[i])
-          return false;
-
-  return true;
-}
-
-
 // split() does the actual work of distributing the work at a node between
 // several available threads. If it does not succeed in splitting the node
 // (because no idle threads are available, or because we have no unused split
@@ -274,6 +262,7 @@ Value ThreadsManager::split(Position& pos, Stack* ss, Value alpha, Value beta,
   sp->parent = masterThread.splitPoint;
   sp->master = master;
   sp->is_betaCutoff = false;
+  sp->slavesMask = (1ULL << master);
   sp->depth = depth;
   sp->threatMove = threatMove;
   sp->alpha = alpha;
@@ -286,9 +275,6 @@ Value ThreadsManager::split(Position& pos, Stack* ss, Value alpha, Value beta,
   sp->nodes = 0;
   sp->ss = ss;
 
-  for (i = 0; i < activeThreads; i++)
-      sp->is_slave[i] = false;
-
   // If we are here it means we are not available
   assert(masterThread.is_searching);
 
@@ -298,21 +284,25 @@ Value ThreadsManager::split(Position& pos, Stack* ss, Value alpha, Value beta,
   // is_searching flag. This must be done under lock protection to avoid concurrent
   // allocation of the same slave by another master.
   lock_grab(threadsLock);
+  lock_grab(sp->lock); // To protect sp->slaves_mask
 
-  for (i = 0; !Fake && i < activeThreads && workersCnt < maxThreadsPerSplitPoint; i++)
+  for (i = 0; !Fake && i < activeThreads; i++)
       if (threads[i].is_available_to(master))
       {
-          workersCnt++;
+          sp->slavesMask |= (1ULL << i);
-          sp->is_slave[i] = true;
           threads[i].splitPoint = sp;
 
-          // This makes the slave to exit from idle_loop()
+          // Allocate the slave and make it exit from idle_loop()
           threads[i].is_searching = true;
 
           if (useSleepingThreads)
               threads[i].wake_up();
+
+          if (++workersCnt >= maxThreadsPerSplitPoint)
+              break;
       }
 
+  lock_release(sp->lock);
   lock_release(threadsLock);
 
   // We failed to allocate even one slave, return
@@ -337,15 +327,17 @@ Value ThreadsManager::split(Position& pos, Stack* ss, Value alpha, Value beta,
   // finished. Note that changing state and decreasing activeSplitPoints is done
   // under lock protection to avoid a race with Thread::is_available_to().
   lock_grab(threadsLock);
+  lock_grab(sp->lock); // To protect sp->nodes
+
 
   masterThread.is_searching = true;
   masterThread.activeSplitPoints--;
-
-  lock_release(threadsLock);
-
   masterThread.splitPoint = sp->parent;
   pos.set_nodes_searched(pos.nodes_searched() + sp->nodes);
 
+  lock_release(sp->lock);
+  lock_release(threadsLock);
+
   return sp->bestValue;
 }
 
@@ -360,7 +352,7 @@ extern void check_time();
 
 void Thread::timer_loop() {
 
-  while (!do_terminate)
+  while (!do_exit)
   {
       lock_grab(sleepLock);
       timed_wait(sleepCond, sleepLock, maxPly ? maxPly : INT_MAX);
@@ -396,7 +388,7 @@ void Thread::main_loop() {
       do_sleep = true; // Always return to sleep after a search
       is_searching = false;
 
-      while (do_sleep && !do_terminate)
+      while (do_sleep && !do_exit)
       {
           cond_signal(Threads.sleepCond); // Wake up UI thread if needed
           cond_wait(sleepCond, sleepLock);
@@ -406,7 +398,7 @@ void Thread::main_loop() {
 
       lock_release(sleepLock);
 
-      if (do_terminate)
+      if (do_exit)
           return;
 
       Search::think();

src/thread.h

@@ -50,12 +50,12 @@ struct SplitPoint {
 
   // Shared data
   Lock lock;
+  volatile uint64_t slavesMask;
   volatile int64_t nodes;
   volatile Value alpha;
   volatile Value bestValue;
   volatile int moveCount;
   volatile bool is_betaCutoff;
-  volatile bool is_slave[MAX_THREADS];
 };
 
 
@@ -69,7 +69,7 @@ struct Thread {
   void wake_up();
   bool cutoff_occurred() const;
   bool is_available_to(int master) const;
-  void idle_loop(SplitPoint* sp);
+  void idle_loop(SplitPoint* sp_master);
   void main_loop();
   void timer_loop();
 
@@ -85,7 +85,7 @@ struct Thread {
   volatile int activeSplitPoints;
   volatile bool is_searching;
   volatile bool do_sleep;
-  volatile bool do_terminate;
+  volatile bool do_exit;
 };
 
 
@@ -110,7 +110,6 @@ public:
   void set_size(int cnt);
   void read_uci_options();
   bool available_slave_exists(int master) const;
-  bool split_point_finished(SplitPoint* sp) const;
   void set_timer(int msec);
   void wait_for_stop_or_ponderhit();
   void stop_thinking();