Adjust NNUE usage based on number of pawns in position

The idea of this patch is that positions are usually more complex and hard to evaluate even if there are more pawns.
This patch adjusts NNUE threshold usage depending on number of pawns in position, if pawn count is <3 we use the
classical evaluation more often, for pawn count = 3 patch the is non-functional,
with pawn count > 3 NNUE evaluation is used more often.

passed STC
https://tests.stockfishchess.org/tests/view/5f2f02d09081672066536b1f
LLR: 2.96 (-2.94,2.94) {-0.50,1.50}
Total: 36520 W: 5011 L: 4823 D: 26686
Ptnml(0-2): 299, 3482, 10548, 3594, 337

passed LTC
https://tests.stockfishchess.org/tests/view/5f2f4c329081672066536b5c
LLR: 2.98 (-2.94,2.94) {0.25,1.75}
Total: 39272 W: 2630 L: 2433 D: 34209
Ptnml(0-2): 53, 2066, 15218, 2229, 70

closes https://github.com/official-stockfish/Stockfish/pull/2960

bench 4084753

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index a642357e3c4904ea0ff5f6160325cd51d52b8b59..ce35c6306c42829db351da8051fd53d536d6aa9a 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -114,7 +114,7 @@ namespace {
   constexpr Value LazyThreshold1 =  Value(1400);
   constexpr Value LazyThreshold2 =  Value(1300);
   constexpr Value SpaceThreshold = Value(12222);
-  constexpr Value NNUEThreshold  =   Value(520);
+  constexpr Value NNUEThreshold  =   Value(460);
 
   // KingAttackWeights[PieceType] contains king attack weights by piece type
   constexpr int KingAttackWeights[PIECE_TYPE_NB] = { 0, 0, 81, 52, 44, 10 };
@@ -945,7 +945,7 @@ Value Eval::evaluate(const Position& pos) {
   {
       Value v = eg_value(pos.psq_score());
       // Take NNUE eval only on balanced positions
-      if (abs(v) < NNUEThreshold)
+      if (abs(v) < NNUEThreshold + 20 * pos.count<PAWN>())
          return NNUE::evaluate(pos) + Tempo;
   }
   return Evaluation<NO_TRACE>(pos).value();