PV-Nodes likely to fail low

Do not decrease reduction at pv-nodes which are likely to fail low.

The idea of this patch can be described as following: during the search, if a node
on the principal variation was re-searched in non-pv search and this re-search got
a value which was much lower than alpha, then we can assume that this pv-node is
likely to fail low again, and we can reduce more aggressively at this node.

Passed STC
https://tests.stockfishchess.org/tests/view/6023a5fa7f517a561bc49638
LLR: 2.95 (-2.94,2.94) {-0.25,1.25}
Total: 70288 W: 6443 L: 6223 D: 57622
Ptnml(0-2): 239, 5022, 24436, 5174, 273

Passed LTC
https://tests.stockfishchess.org/tests/view/6023f2617f517a561bc49661
LLR: 2.94 (-2.94,2.94) {0.25,1.25}
Total: 105656 W: 4048 L: 3748 D: 97860
Ptnml(0-2): 67, 3312, 45761, 3630, 58

Closes https://github.com/official-stockfish/Stockfish/pull/3349

Bench: 3766422

diff --git a/src/search.cpp b/src/search.cpp
index 80c3e4c245fe5e5be672fb5587d6275e0ddbdcba..d77ab691d62b34d68e678801c120ae4d607fbbf5 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