Simplify away improvement term in null move search

passed STC:
https://tests.stockfishchess.org/tests/view/649c0d2edc7002ce609d33b5
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 271104 W: 72181 L: 72217 D: 126706
Ptnml(0-2): 691, 30042, 74129, 29992, 698

passed LTC:
https://tests.stockfishchess.org/tests/view/649d0dd7dc7002ce609d4efa
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 183120 W: 49469 L: 49418 D: 84233
Ptnml(0-2): 84, 17636, 56063, 17699, 78

closes https://github.com/official-stockfish/Stockfish/pull/4650

Bench: 2642851

diff --git a/src/search.cpp b/src/search.cpp
index 8bd5ec9b48bc4b48b244a97bb2d2a92a93dc6d4f..656558f87c472821f20898965dd0be30e8d47a1e 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -779,7 +779,7 @@ namespace {
         && (ss-1)->statScore < 17329
         &&  eval >= beta
         &&  eval >= ss->staticEval
-        &&  ss->staticEval >= beta - 21 * depth - improvement / 13 + 258
+        &&  ss->staticEval >= beta - 21 * depth + 258
         && !excludedMove
         &&  pos.non_pawn_material(us)
         &&  ss->ply >= thisThread->nmpMinPly