Do more reductions at Pv nodes with low delta

This patch increases reduction for PvNodes that have their delta (difference between beta and alpha) significantly reduced compared to what it was at root.

passed STC
https://tests.stockfishchess.org/tests/view/617f9063af49befdeee40226
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 220840 W: 55752 L: 55150 D: 109938
Ptnml(0-2): 583, 24982, 58712, 25536, 607

passed LTC
https://tests.stockfishchess.org/tests/view/61815de959e71df00dcc42ed
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 79000 W: 19937 L: 19562 D: 39501
Ptnml(0-2): 36, 8190, 22674, 8563, 37

closes https://github.com/official-stockfish/Stockfish/pull/3774

bench: 6717808

diff --git a/src/search.cpp b/src/search.cpp
index a712ce87bb77feca94399d739367e7049e98f3b1..46c95ef0979d89e8d856e5c79e9d902b224d19dd 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -625,6 +625,8 @@ namespace {
         if (alpha >= beta)
             return alpha;
     }
+    else
+        thisThread->rootDelta = beta - alpha;
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
@@ -1167,6 +1169,10 @@ moves_loop: // When in check, search starts here
               && bestMoveCount <= 3)
               r--;
 
+          // Increases reduction for PvNodes that have small window
+          if (PvNode && beta - alpha < thisThread->rootDelta / 4)
+              r++;
+
           // Decrease reduction if position is or has been on the PV
           // and node is not likely to fail low. (~3 Elo)
           if (   ss->ttPv

diff --git a/src/thread.h b/src/thread.h
index e04d303a271996ee39f2335997e58e6fd4c3bcf7..fae866c4f30d21657f1ae396f5277a0b83cf50ee 100644 (file)
--- a/src/thread.h
+++ b/src/thread.h
@@ -72,6 +72,7 @@ public:
   StateInfo rootState;
   Search::RootMoves rootMoves;
   Depth rootDepth, completedDepth;
+  Value rootDelta;
   CounterMoveHistory counterMoves;
   ButterflyHistory mainHistory;
   LowPlyHistory lowPlyHistory;