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Introduce ProbCut for check evasions

Browse source 
The idea of this patch can be described as follows: if we are in check
and the transposition table move is a capture that returns a value
far above beta, we can assume that the opponent just blundered a piece
by giving check, and we return the transposition table value. This is
similar to the usual probCut logic for quiet moves, but with a different
threshold.

Passed STC
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 33440 W: 3056 L: 2891 D: 27493
Ptnml(0-2): 110, 2338, 11672, 2477, 123
https://tests.stockfishchess.org/tests/view/602cd1087f517a561bc49bda

Passed LTC
LLR: 2.98 (-2.94,2.94) {0.25,1.25}
Total: 10072 W: 401 L: 309 D: 9362
Ptnml(0-2): 2, 288, 4365, 378, 3
https://tests.stockfishchess.org/tests/view/602ceea57f517a561bc49bf0

The committed version has an additional fix to never return unproven wins
in the tablebase range or the mate range. This fix passed tests for non-
regression at STC and LTC:

STC:
LLR: 2.93 (-2.94,2.94) {-1.25,0.25}
Total: 26240 W: 2354 L: 2280 D: 21606
Ptnml(0-2): 85, 1763, 9372, 1793, 107
https://tests.stockfishchess.org/tests/view/602d86a87f517a561bc49c7a

LTC:
LLR: 2.95 (-2.94,2.94) {-0.75,0.25}
Total: 35304 W: 1299 L: 1256 D: 32749
Ptnml(0-2): 14, 1095, 15395, 1130, 18
https://tests.stockfishchess.org/tests/view/602d98d17f517a561bc49c83

Closes https://github.com/official-stockfish/Stockfish/pull/3362

Bench: 3830215
This commit is contained in:
Vizvezdenec 2021-02-20 22:48:08 +01:00 • committed by Stéphane Nicolet
parent 6294db7514
commit 7c30091a92
1 changed files with 26 additions and 10 deletions
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36
src/search.cpp
View file

@ -969,6 +969,23 @@ namespace {
moves_loop: // When in check, search starts from here
ttCapture = ttMove && pos.capture_or_promotion(ttMove);
// Step 11. A small Probcut idea, when we are in check
probCutBeta = beta + 400;
if ( ss->inCheck
&& !PvNode
&& depth >= 4
&& ttCapture
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3
&& ttValue >= probCutBeta
&& abs(ttValue) <= VALUE_KNOWN_WIN
&& abs(beta) <= VALUE_KNOWN_WIN
)
return probCutBeta;
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
nullptr , (ss-4)->continuationHistory,
nullptr , (ss-6)->continuationHistory };
@ -985,12 +1002,11 @@ moves_loop: // When in check, search starts from here
value = bestValue;
singularQuietLMR = moveCountPruning = false;
ttCapture = ttMove && pos.capture_or_promotion(ttMove);
// Mark this node as being searched
ThreadHolding th(thisThread, posKey, ss->ply);
// Step 11. Loop through all pseudo-legal moves until no moves remain
// Step 12. Loop through all pseudo-legal moves until no moves remain
// or a beta cutoff occurs.
while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
{
@ -1036,7 +1052,7 @@ moves_loop: // When in check, search starts from here
// Calculate new depth for this move
newDepth = depth - 1;
// Step 12. Pruning at shallow depth (~200 Elo)
// Step 13. Pruning at shallow depth (~200 Elo)
if ( !rootNode
&& pos.non_pawn_material(us)
&& bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
@ -1084,7 +1100,7 @@ moves_loop: // When in check, search starts from here
}
}
// Step 13. Extensions (~75 Elo)
// Step 14. Extensions (~75 Elo)
// Singular extension search (~70 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
@ -1156,10 +1172,10 @@ moves_loop: // When in check, search starts from here
[movedPiece]
[to_sq(move)];
// Step 14. Make the move
// Step 15. Make the move
pos.do_move(move, st, givesCheck);
// Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
// Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
// re-searched at full depth.
if ( depth >= 3
&& moveCount > 1 + 2 * rootNode
@ -1266,7 +1282,7 @@ moves_loop: // When in check, search starts from here
didLMR = false;
}
// Step 16. Full depth search when LMR is skipped or fails high
// Step 17. Full depth search when LMR is skipped or fails high
if (doFullDepthSearch)
{
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
@ -1293,12 +1309,12 @@ moves_loop: // When in check, search starts from here
std::min(maxNextDepth, newDepth), false);
}
// Step 17. Undo move
// Step 18. Undo move
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
// Step 18. Check for a new best move
// Step 19. Check for a new best move
// Finished searching the move. If a stop occurred, the return value of
// the search cannot be trusted, and we return immediately without
// updating best move, PV and TT.
@ -1375,7 +1391,7 @@ moves_loop: // When in check, search starts from here
return VALUE_DRAW;
*/
// Step 19. Check for mate and stalemate
// Step 20. Check for mate and stalemate
// All legal moves have been searched and if there are no legal moves, it
// must be a mate or a stalemate. If we are in a singular extension search then
// return a fail low score.