Small trivial clean-ups, February 2021

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 80c3e4c245fe5e5be672fb5587d6275e0ddbdcba..b5d21b9d5585c2cfbeed24a6470f9df69c8aa907 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1025,6 +1025,14 @@ 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;
 
@@ -1172,8 +1180,9 @@ moves_loop: // When in check, search starts from here
           if (th.marked())
               r++;
 
-          // Decrease reduction if position is or has been on the PV (~10 Elo)
-          if (ss->ttPv)
+          // Decrease reduction if position is or has been on the PV
+          // and node is not likely to fail low. (~10 Elo)
+          if (ss->ttPv && !likelyFailLow)
               r -= 2;
 
           // Increase reduction at root and non-PV nodes when the best move does not change frequently