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IIR on cutnodes if there is a ttMove but the ttBound is upper

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If there is an upper bound stored in the transposition table, but we still have a ttMove, the upperbound indicates that the last time the ttMove was tried, it failed low. This fail low indicates that the ttMove may not be good, so this patch introduces a depth reduction of one for cutnodes with such ttMoves.

Passed STC:
https://tests.stockfishchess.org/tests/view/663be4d1ca93dad645f7f45f
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 139424 W: 35900 L: 35433 D: 68091
Ptnml(0-2): 425, 16357, 35743, 16700, 487

Passed LTC:
https://tests.stockfishchess.org/tests/view/663bec95ca93dad645f7f5c8
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 129690 W: 32902 L: 32390 D: 64398
Ptnml(0-2): 63, 14304, 35610, 14794, 74

closes https://github.com/official-stockfish/Stockfish/pull/5227

bench 2257437
This commit is contained in:
rn5f107s2 2024-05-08 22:08:56 +02:00 • committed by Joost VandeVondele
parent d1b8d8bab3
commit db147fe258
1 changed files with 2 additions and 2 deletions
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4
src/search.cpp
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@ -828,8 +828,8 @@ Value Search::Worker::search(
return qsearch<PV>(pos, ss, alpha, beta);
// For cutNodes without a ttMove, we decrease depth by 2 if depth is high enough.
if (cutNode && depth >= 8 && !ttMove)
depth -= 2;
if (cutNode && depth >= 8 && (!ttMove || tte->bound() == BOUND_UPPER))
depth -= 1 + !ttMove;
// Step 11. ProbCut (~10 Elo)
// If we have a good enough capture (or queen promotion) and a reduced search returns a value