Do not modify alpha in split()

When calling split or we immediately return because unable to
find available slaves, or we start searching on _all_ the moves
of the node or until a cut-off occurs, so that when returning
from split we immediately leave the moves loop.

Because of this we don't need to change alpha inside split() and
we can use a signature similar to search() so to better clarify
that split() is actually a search on the remaining node's moves.

No functional change with faked split.

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

src/search.cpp

@@ -1228,8 +1228,8 @@ split_point_start: // At split points actual search starts from here
           && Threads.available_slave_exists(pos.thread())
           && !StopRequest
           && !thread.cutoff_occurred())
-          Threads.split<FakeSplit>(pos, ss, &alpha, beta, &bestValue, depth,
-                                   threatMove, moveCount, &mp, NT);
+          bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, depth,
+                                               threatMove, moveCount, &mp, NT);
     }
 
     // Step 20. Check for mate and stalemate

src/thread.cpp

@@ -238,13 +238,13 @@ bool ThreadsManager::available_slave_exists(int master) const {
 // call search().When all threads have returned from search() then split() returns.
 
 template <bool Fake>
-void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const Value beta,
-                           Value* bestValue, Depth depth, Move threatMove,
-                           int moveCount, MovePicker* mp, int nodeType) {
+Value ThreadsManager::split(Position& pos, SearchStack* ss, Value alpha, Value beta,
+                            Value bestValue, Depth depth, Move threatMove,
+                            int moveCount, MovePicker* mp, int nodeType) {
   assert(pos.is_ok());
-  assert(*bestValue >= -VALUE_INFINITE);
-  assert(*bestValue <= *alpha);
-  assert(*alpha < beta);
+  assert(bestValue >= -VALUE_INFINITE);
+  assert(bestValue <= alpha);
+  assert(alpha < beta);
   assert(beta <= VALUE_INFINITE);
   assert(depth > DEPTH_ZERO);
   assert(pos.thread() >= 0 && pos.thread() < activeThreads);
@@ -255,7 +255,7 @@ void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const V
 
   // If we already have too many active split points, don't split
   if (masterThread.activeSplitPoints >= MAX_ACTIVE_SPLIT_POINTS)
-      return;
+      return bestValue;
 
   // Pick the next available split point object from the split point stack
   SplitPoint& splitPoint = masterThread.splitPoints[masterThread.activeSplitPoints];
@@ -266,10 +266,10 @@ void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const V
   splitPoint.is_betaCutoff = false;
   splitPoint.depth = depth;
   splitPoint.threatMove = threatMove;
-  splitPoint.alpha = *alpha;
+  splitPoint.alpha = alpha;
   splitPoint.beta = beta;
   splitPoint.nodeType = nodeType;
-  splitPoint.bestValue = *bestValue;
+  splitPoint.bestValue = bestValue;
   splitPoint.mp = mp;
   splitPoint.moveCount = moveCount;
   splitPoint.pos = &pos;
@@ -301,12 +301,12 @@ void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const V
 
   // We failed to allocate even one slave, return
   if (!Fake && !booked)
-      return;
+      return bestValue;
 
   masterThread.activeSplitPoints++;
   masterThread.splitPoint = &splitPoint;
 
-  // Tell the threads that they have work to do. This will make them leave
+  // Tell the threads that they have some work to do. This will make them leave
   // their idle loop.
   for (i = 0; i < activeThreads; i++)
       if (i == master || splitPoint.is_slave[i])
@@ -328,9 +328,8 @@ void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const V
   idle_loop(master, &splitPoint);
 
   // We have returned from the idle loop, which means that all threads are
-  // finished. Update alpha and bestValue, and return. Note that changing
-  // state and decreasing activeSplitPoints is done under lock protection
-  // to avoid a race with Thread::is_available_to().
+  // 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);
 
   masterThread.state = Thread::SEARCHING;
@@ -339,11 +338,10 @@ void ThreadsManager::split(Position& pos, SearchStack* ss, Value* alpha, const V
 
   lock_release(&threadsLock);
 
-  *alpha = splitPoint.alpha;
-  *bestValue = splitPoint.bestValue;
   pos.set_nodes_searched(pos.nodes_searched() + splitPoint.nodes);
+  return splitPoint.bestValue;
 }
 
 // Explicit template instantiations
-template void ThreadsManager::split<false>(Position&, SearchStack*, Value*, const Value, Value*, Depth, Move, int, MovePicker*, int);
-template void ThreadsManager::split<true>(Position&, SearchStack*, Value*, const Value, Value*, Depth, Move, int, MovePicker*, int);
+template Value ThreadsManager::split<false>(Position&, SearchStack*, Value, Value, Value, Depth, Move, int, MovePicker*, int);
+template Value ThreadsManager::split<true>(Position&, SearchStack*, Value, Value, Value, Depth, Move, int, MovePicker*, int);

src/thread.h

@@ -115,8 +115,8 @@ public:
   void idle_loop(int threadID, SplitPoint* sp);
 
   template <bool Fake>
-  void split(Position& pos, SearchStack* ss, Value* alpha, const Value beta, Value* bestValue,
-             Depth depth, Move threatMove, int moveCount, MovePicker* mp, int nodeType);
+  Value split(Position& pos, SearchStack* ss, Value alpha, Value beta, Value bestValue,
+              Depth depth, Move threatMove, int moveCount, MovePicker* mp, int nodeType);
 private:
   Thread threads[MAX_THREADS];
   Lock threadsLock;