Use null move when depth >= 2 plies

Passed both short TC:
LLR: 2.96 (-2.94,2.94)
Total: 23725 W: 5031 L: 4855 D: 13839

And long TC:
LLR: 2.96 (-2.94,2.94)
Total: 15730 W: 2939 L: 2754 D: 10037

bench: 4729333

diff --git a/src/search.cpp b/src/search.cpp
index 2a7cde2b5fe848b7be3c778c263fc5d730da9a5b..9ae28d3c55870b554a5681782442dbbc3569b3c3 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -663,7 +663,7 @@ namespace {
     // Step 8. Null move search with verification search (is omitted in PV nodes)
     if (   !PvNode
         && !ss->skipNullMove
-        &&  depth > ONE_PLY
+        &&  depth >= 2 * ONE_PLY
         &&  eval >= beta
         &&  abs(beta) < VALUE_MATE_IN_MAX_PLY
         &&  pos.non_pawn_material(pos.side_to_move()))