Remove coordination between searching threads

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 99c2b09f9ef42b4032c95864e7f4e0e497147ec5..3a0839d62db858b0aa290036c7dce974e72506f9 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -102,49 +102,6 @@ namespace {
     Move best = MOVE_NONE;
   };
 
-  // Breadcrumbs are used to mark nodes as being searched by a given thread
-  struct Breadcrumb {
-    std::atomic<Thread*> thread;
-    std::atomic<Key> key;
-  };
-  std::array<Breadcrumb, 1024> breadcrumbs;
-
-  // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
-  // node for potential reductions. A free node will be marked upon entering the moves
-  // loop by the constructor, and unmarked upon leaving that loop by the destructor.
-  struct ThreadHolding {
-    explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
-       location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
-       otherThread = false;
-       owning = false;
-       if (location)
-       {
-          // See if another already marked this location, if not, mark it ourselves
-          Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
-          if (tmp == nullptr)
-          {
-              (*location).thread.store(thisThread, std::memory_order_relaxed);
-              (*location).key.store(posKey, std::memory_order_relaxed);
-              owning = true;
-          }
-          else if (   tmp != thisThread
-                   && (*location).key.load(std::memory_order_relaxed) == posKey)
-              otherThread = true;
-       }
-    }
-
-    ~ThreadHolding() {
-       if (owning) // Free the marked location
-           (*location).thread.store(nullptr, std::memory_order_relaxed);
-    }
-
-    bool marked() { return otherThread; }
-
-    private:
-    Breadcrumb* location;
-    bool otherThread, owning;
-  };
-
   template <NodeType NT>
   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
 
@@ -1006,8 +963,12 @@ moves_loop: // When in check, search starts from here
     value = bestValue;
     singularQuietLMR = moveCountPruning = false;
 
-    // Mark this node as being searched
-    ThreadHolding th(thisThread, posKey, ss->ply);
+    // Indicate PvNodes that will probably fail low if the node was searched
+    // at a depth equal or greater than the current depth, and the result of this search was a fail low.
+    bool likelyFailLow =    PvNode
+                         && ttMove
+                         && (tte->bound() & BOUND_UPPER)
+                         && tte->depth() >= depth;
 
     // Step 12. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
@@ -1044,14 +1005,6 @@ moves_loop: // When in check, search starts from here
       movedPiece = pos.moved_piece(move);
       givesCheck = pos.gives_check(move);
 
-      // Indicate PvNodes that will probably fail low if node was searched with non-PV search
-      // at depth equal or greater to current depth and result of this search was far below alpha
-      bool likelyFailLow =    PvNode
-                           && ttMove
-                           && (tte->bound() & BOUND_UPPER)
-                           && ttValue < alpha + 200 + 100 * depth
-                           && tte->depth() >= depth;
-
       // Calculate new depth for this move
       newDepth = depth - 1;
 
@@ -1130,6 +1083,8 @@ moves_loop: // When in check, search starts from here
           {
               extension = 1;
               singularQuietLMR = !ttCapture;
+              if (!PvNode && value < singularBeta - 140)
+                  extension = 2;
           }
 
           // Multi-cut pruning
@@ -1153,11 +1108,6 @@ moves_loop: // When in check, search starts from here
           }
       }
 
-      // Check extension (~2 Elo)
-      else if (    givesCheck
-               && (pos.is_discovered_check_on_king(~us, move) || pos.see_ge(move)))
-          extension = 1;
-
       // Add extension to new depth
       newDepth += extension;
 
@@ -1194,10 +1144,6 @@ moves_loop: // When in check, search starts from here
           if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
               r--;
 
-          // Increase reduction if other threads are searching this position
-          if (th.marked())
-              r++;
-
           // Decrease reduction if position is or has been on the PV
           // and node is not likely to fail low. (~10 Elo)
           if (   ss->ttPv
@@ -1210,16 +1156,11 @@ moves_loop: // When in check, search starts from here
               && thisThread->bestMoveChanges <= 2)
               r++;
 
-          // More reductions for late moves if position was not in previous PV
-          if (   moveCountPruning
-              && !formerPv)
-              r++;
-
           // Decrease reduction if opponent's move count is high (~5 Elo)
           if ((ss-1)->moveCount > 13)
               r--;
 
-          // Decrease reduction if ttMove has been singularly extended (~3 Elo)
+          // Decrease reduction if ttMove has been singularly extended (~1 Elo)
           if (singularQuietLMR)
               r--;
 
@@ -1232,7 +1173,7 @@ moves_loop: // When in check, search starts from here
           }
           else
           {
-              // Increase reduction if ttMove is a capture (~5 Elo)
+              // Increase reduction if ttMove is a capture (~3 Elo)
               if (ttCapture)
                   r++;
 
@@ -1243,13 +1184,6 @@ moves_loop: // When in check, search starts from here
               if (cutNode)
                   r += 2;
 
-              // Decrease reduction for moves that escape a capture. Filter out
-              // castling moves, because they are coded as "king captures rook" and
-              // hence break reverse_move() (~2 Elo)
-              else if (    type_of(move) == NORMAL
-                       && !pos.see_ge(reverse_move(move)))
-                  r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
-
               ss->statScore =  thisThread->mainHistory[us][from_to(move)]
                              + (*contHist[0])[movedPiece][to_sq(move)]
                              + (*contHist[1])[movedPiece][to_sq(move)]