Do less SEE pruning in qsearch

Current master prunes all moves with negative SEE values in qsearch.
This patch sets constant negative threshold thus allowing some moves with negative SEE values to be searched.
Value of threshold is completely arbitrary and can be tweaked - also it as function of depth can be tried.
Original idea by author of Alexandria engine.

Passed STC
https://tests.stockfishchess.org/tests/view/63d79a59a67dd929a5564976
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 34864 W: 9392 L: 9086 D: 16386
Ptnml(0-2): 113, 3742, 9429, 4022, 126

Passed LTC
https://tests.stockfishchess.org/tests/view/63d8074aa67dd929a5565bc2
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 91616 W: 24532 L: 24126 D: 42958
Ptnml(0-2): 32, 8840, 27662, 9238, 36

closes https://github.com/official-stockfish/Stockfish/pull/4376

Bench: 4010877

diff --git a/src/search.cpp b/src/search.cpp
index 7d618a65b56442e595fbf75dc9fc4cd9dd8ffedb..b9ca3961053d435b2207ebe30d1ae6970ecb4ac3 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1556,9 +1556,9 @@ moves_loop: // When in check, search starts here
           }
       }
 
-      // Do not search moves with negative SEE values (~5 Elo)
+      // Do not search moves with bad enough SEE values (~5 Elo)
       if (    bestValue > VALUE_TB_LOSS_IN_MAX_PLY
-          && !pos.see_ge(move))
+          && !pos.see_ge(move, Value(-108)))
           continue;
 
       // Speculative prefetch as early as possible