X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fsearch.cpp;h=7c797bef4087a2e62d58a1fe88359763fbe139e9;hp=9cdc7046ef764bba82ed87ccecccdb091aa3a128;hb=4231d99ab408674115623f42f7ff89f3f189ca23;hpb=3368d0328591b2741ca32e57cfa0a35a7144fdd1 diff --git a/src/search.cpp b/src/search.cpp index 9cdc7046..9f005587 100644 --- a/src/search.cpp +++ b/src/search.cpp @@ -1,6 +1,6 @@ /* Stockfish, a UCI chess playing engine derived from Glaurung 2.1 - Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file) + Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file) Stockfish is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by @@ -35,6 +35,8 @@ #include "uci.h" #include "syzygy/tbprobe.h" +namespace Stockfish { + namespace Search { LimitsType Limits; @@ -57,23 +59,19 @@ using namespace Search; namespace { // Different node types, used as a template parameter - enum NodeType { NonPV, PV }; - - constexpr uint64_t TtHitAverageWindow = 4096; - constexpr uint64_t TtHitAverageResolution = 1024; + enum NodeType { NonPV, PV, Root }; - // Razor and futility margins - constexpr int RazorMargin = 527; + // Futility margin Value futility_margin(Depth d, bool improving) { - return Value(227 * (d - improving)); + return Value(214 * (d - improving)); } // Reductions lookup table, initialized at startup int Reductions[MAX_MOVES]; // [depth or moveNumber] - Depth reduction(bool i, Depth d, int mn) { + Depth reduction(bool i, Depth d, int mn, bool rangeReduction) { int r = Reductions[d] * Reductions[mn]; - return (r + 570) / 1024 + (!i && r > 1018); + return (r + 534) / 1024 + (!i && r > 904) + rangeReduction; } constexpr int futility_move_count(bool improving, Depth depth) { @@ -82,14 +80,38 @@ namespace { // History and stats update bonus, based on depth int stat_bonus(Depth d) { - return d > 15 ? 27 : 17 * d * d + 133 * d - 134; + return std::min((6 * d + 229) * d - 215 , 2000); } - // Add a small random component to draw evaluations to avoid 3fold-blindness + // Add a small random component to draw evaluations to avoid 3-fold blindness Value value_draw(Thread* thisThread) { return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1); } + // Check if the current thread is in a search explosion + ExplosionState search_explosion(Thread* thisThread) { + + uint64_t nodesNow = thisThread->nodes; + bool explosive = thisThread->doubleExtensionAverage[WHITE].is_greater(2, 100) + || thisThread->doubleExtensionAverage[BLACK].is_greater(2, 100); + + if (explosive) + thisThread->nodesLastExplosive = nodesNow; + else + thisThread->nodesLastNormal = nodesNow; + + if ( explosive + && thisThread->state == EXPLOSION_NONE + && nodesNow - thisThread->nodesLastNormal > 6000000) + thisThread->state = MUST_CALM_DOWN; + + if ( thisThread->state == MUST_CALM_DOWN + && nodesNow - thisThread->nodesLastExplosive > 6000000) + thisThread->state = EXPLOSION_NONE; + + return thisThread->state; + } + // Skill structure is used to implement strength limit struct Skill { explicit Skill(int l) : level(l) {} @@ -101,53 +123,10 @@ namespace { Move best = MOVE_NONE; }; - // Breadcrumbs are used to mark nodes as being searched by a given thread - struct Breadcrumb { - std::atomic thread; - std::atomic key; - }; - std::array breadcrumbs; - - // ThreadHolding structure keeps track of which thread left breadcrumbs at the given - // node for potential reductions. A free node will be marked upon entering the moves - // loop by the constructor, and unmarked upon leaving that loop by the destructor. - struct ThreadHolding { - explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) { - location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr; - otherThread = false; - owning = false; - if (location) - { - // See if another already marked this location, if not, mark it ourselves - Thread* tmp = (*location).thread.load(std::memory_order_relaxed); - if (tmp == nullptr) - { - (*location).thread.store(thisThread, std::memory_order_relaxed); - (*location).key.store(posKey, std::memory_order_relaxed); - owning = true; - } - else if ( tmp != thisThread - && (*location).key.load(std::memory_order_relaxed) == posKey) - otherThread = true; - } - } - - ~ThreadHolding() { - if (owning) // Free the marked location - (*location).thread.store(nullptr, std::memory_order_relaxed); - } - - bool marked() { return otherThread; } - - private: - Breadcrumb* location; - bool otherThread, owning; - }; - - template + template Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode); - template + template Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0); Value value_to_tt(Value v, int ply); @@ -164,6 +143,8 @@ namespace { uint64_t perft(Position& pos, Depth depth) { StateInfo st; + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); + uint64_t cnt, nodes = 0; const bool leaf = (depth == 2); @@ -192,7 +173,7 @@ namespace { void Search::init() { for (int i = 1; i < MAX_MOVES; ++i) - Reductions[i] = int((24.8 + std::log(Threads.size())) * std::log(i)); + Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i)); } @@ -225,7 +206,7 @@ void MainThread::search() { Time.init(Limits, us, rootPos.game_ply()); TT.new_search(); - Eval::verify_NNUE(); + Eval::NNUE::verify(); if (rootMoves.empty()) { @@ -293,7 +274,7 @@ void Thread::search() { // To allow access to (ss-7) up to (ss+2), the stack must be oversized. // The former is needed to allow update_continuation_histories(ss-1, ...), // which accesses its argument at ss-6, also near the root. - // The latter is needed for statScores and killer initialization. + // The latter is needed for statScore and killer initialization. Stack stack[MAX_PLY+10], *ss = stack+7; Move pv[MAX_PLY+1]; Value bestValue, alpha, beta, delta; @@ -308,6 +289,9 @@ void Thread::search() { for (int i = 7; i > 0; i--) (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel + for (int i = 0; i <= MAX_PLY + 2; ++i) + (ss+i)->ply = i; + ss->pv = pv; bestValue = delta = alpha = -VALUE_INFINITE; @@ -335,7 +319,7 @@ void Thread::search() { // for match (TC 60+0.6) results spanning a wide range of k values. PRNG rng(now()); double floatLevel = Options["UCI_LimitStrength"] ? - Utility::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) : + std::clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) : double(Options["Skill Level"]); int intLevel = int(floatLevel) + ((floatLevel - int(floatLevel)) * 1024 > rng.rand() % 1024 ? 1 : 0); @@ -347,21 +331,14 @@ void Thread::search() { multiPV = std::max(multiPV, (size_t)4); multiPV = std::min(multiPV, rootMoves.size()); - ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2; - int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns + doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0% + doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0% - // In analysis mode, adjust contempt in accordance with user preference - if (Limits.infinite || Options["UCI_AnalyseMode"]) - ct = Options["Analysis Contempt"] == "Off" ? 0 - : Options["Analysis Contempt"] == "Both" ? ct - : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct - : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct - : ct; - - // Evaluation score is from the white point of view - contempt = (us == WHITE ? make_score(ct, ct / 2) - : -make_score(ct, ct / 2)); + nodesLastExplosive = nodes; + nodesLastNormal = nodes; + state = EXPLOSION_NONE; + trend = SCORE_ZERO; int searchAgainCounter = 0; @@ -403,15 +380,15 @@ void Thread::search() { if (rootDepth >= 4) { Value prev = rootMoves[pvIdx].previousScore; - delta = Value(19); + delta = Value(17); alpha = std::max(prev - delta,-VALUE_INFINITE); beta = std::min(prev + delta, VALUE_INFINITE); - // Adjust contempt based on root move's previousScore (dynamic contempt) - int dct = ct + (110 - ct / 2) * prev / (abs(prev) + 140); + // Adjust trend based on root move's previousScore (dynamic contempt) + int tr = 113 * prev / (abs(prev) + 147); - contempt = (us == WHITE ? make_score(dct, dct / 2) - : -make_score(dct, dct / 2)); + trend = (us == WHITE ? make_score(tr, tr / 2) + : -make_score(tr, tr / 2)); } // Start with a small aspiration window and, in the case of a fail @@ -421,7 +398,7 @@ void Thread::search() { while (true) { Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter); - bestValue = ::search(rootPos, ss, alpha, beta, adjustedDepth, false); + bestValue = Stockfish::search(rootPos, ss, alpha, beta, adjustedDepth, false); // Bring the best move to the front. It is critical that sorting // is done with a stable algorithm because all the values but the @@ -462,10 +439,7 @@ void Thread::search() { ++failedHighCnt; } else - { - ++rootMoves[pvIdx].bestMoveCount; break; - } delta += delta / 4 + 5; @@ -506,13 +480,13 @@ void Thread::search() { && !Threads.stop && !mainThread->stopOnPonderhit) { - double fallingEval = (296 + 6 * (mainThread->bestPreviousScore - bestValue) - + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 725.0; - fallingEval = Utility::clamp(fallingEval, 0.5, 1.5); + double fallingEval = (318 + 6 * (mainThread->bestPreviousScore - bestValue) + + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0; + fallingEval = std::clamp(fallingEval, 0.5, 1.5); // If the bestMove is stable over several iterations, reduce time accordingly - timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.92 : 0.95; - double reduction = (1.47 + mainThread->previousTimeReduction) / (2.22 * timeReduction); + timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95; + double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction); // Use part of the gained time from a previous stable move for the current move for (Thread* th : Threads) @@ -520,12 +494,16 @@ void Thread::search() { totBestMoveChanges += th->bestMoveChanges; th->bestMoveChanges = 0; } - double bestMoveInstability = 1 + totBestMoveChanges / Threads.size(); + double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth) + * totBestMoveChanges / Threads.size(); + double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability; - double totalTime = rootMoves.size() == 1 ? 0 : - Time.optimum() * fallingEval * reduction * bestMoveInstability; + // Cap used time in case of a single legal move for a better viewer experience in tournaments + // yielding correct scores and sufficiently fast moves. + if (rootMoves.size() == 1) + totalTime = std::min(500.0, totalTime); - // Stop the search if we have exceeded the totalTime, at least 1ms search + // Stop the search if we have exceeded the totalTime if (Time.elapsed() > totalTime) { // If we are allowed to ponder do not stop the search now but @@ -537,7 +515,7 @@ void Thread::search() { } else if ( Threads.increaseDepth && !mainThread->ponder - && Time.elapsed() > totalTime * 0.56) + && Time.elapsed() > totalTime * 0.58) Threads.increaseDepth = false; else Threads.increaseDepth = true; @@ -563,17 +541,26 @@ namespace { // search<>() is the main search function for both PV and non-PV nodes - template + template Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) { - constexpr bool PvNode = NT == PV; - const bool rootNode = PvNode && ss->ply == 0; + Thread* thisThread = pos.this_thread(); + + // Step 0. Limit search explosion + if ( ss->ply > 10 + && search_explosion(thisThread) == MUST_CALM_DOWN + && depth > (ss-1)->depth) + depth = (ss-1)->depth; + + constexpr bool PvNode = nodeType != NonPV; + constexpr bool rootNode = nodeType == Root; + const Depth maxNextDepth = rootNode ? depth : depth + 1; // Check if we have an upcoming move which draws by repetition, or // if the opponent had an alternative move earlier to this position. - if ( pos.rule50_count() >= 3 + if ( !rootNode + && pos.rule50_count() >= 3 && alpha < VALUE_DRAW - && !rootNode && pos.has_game_cycle(ss->ply)) { alpha = value_draw(pos.this_thread()); @@ -583,7 +570,7 @@ namespace { // Dive into quiescence search when the depth reaches zero if (depth <= 0) - return qsearch(pos, ss, alpha, beta); + return qsearch(pos, ss, alpha, beta); assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE); assert(PvNode || (alpha == beta - 1)); @@ -592,25 +579,26 @@ namespace { Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64]; StateInfo st; + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); + TTEntry* tte; Key posKey; Move ttMove, move, excludedMove, bestMove; Depth extension, newDepth; - Value bestValue, value, ttValue, eval, maxValue, probcutBeta; - bool ttHit, ttPv, formerPv, givesCheck, improving, didLMR, priorCapture; + Value bestValue, value, ttValue, eval, maxValue, probCutBeta; + bool givesCheck, improving, didLMR, priorCapture; bool captureOrPromotion, doFullDepthSearch, moveCountPruning, - ttCapture, singularQuietLMR; + ttCapture, singularQuietLMR, noLMRExtension; Piece movedPiece; - int moveCount, captureCount, quietCount; + int moveCount, captureCount, quietCount, bestMoveCount, improvement; // Step 1. Initialize node - Thread* thisThread = pos.this_thread(); - ss->inCheck = pos.checkers(); - priorCapture = pos.captured_piece(); - Color us = pos.side_to_move(); - moveCount = captureCount = quietCount = ss->moveCount = 0; - bestValue = -VALUE_INFINITE; - maxValue = VALUE_INFINITE; + ss->inCheck = pos.checkers(); + priorCapture = pos.captured_piece(); + Color us = pos.side_to_move(); + moveCount = bestMoveCount = captureCount = quietCount = ss->moveCount = 0; + bestValue = -VALUE_INFINITE; + maxValue = VALUE_INFINITE; // Check for the available remaining time if (thisThread == Threads.main()) @@ -643,19 +631,22 @@ namespace { assert(0 <= ss->ply && ss->ply < MAX_PLY); - (ss+1)->ply = ss->ply + 1; + (ss+1)->ttPv = false; (ss+1)->excludedMove = bestMove = MOVE_NONE; - (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE; - Square prevSq = to_sq((ss-1)->currentMove); + (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE; + ss->doubleExtensions = (ss-1)->doubleExtensions; + ss->depth = depth; + Square prevSq = to_sq((ss-1)->currentMove); + + // Update the running average statistics for double extensions + thisThread->doubleExtensionAverage[us].update(ss->depth > (ss-1)->depth); // Initialize statScore to zero for the grandchildren of the current position. // So statScore is shared between all grandchildren and only the first grandchild // starts with statScore = 0. Later grandchildren start with the last calculated // statScore of the previous grandchild. This influences the reduction rules in // LMR which are based on the statScore of parent position. - if (rootNode) - (ss+4)->statScore = 0; - else + if (!rootNode) (ss+2)->statScore = 0; // Step 4. Transposition table lookup. We don't want the score of a partial @@ -663,27 +654,25 @@ namespace { // position key in case of an excluded move. excludedMove = ss->excludedMove; posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove); - tte = TT.probe(posKey, ttHit); - ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; + tte = TT.probe(posKey, ss->ttHit); + ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0] - : ttHit ? tte->move() : MOVE_NONE; - ttPv = PvNode || (ttHit && tte->is_pv()); - formerPv = ttPv && !PvNode; + : ss->ttHit ? tte->move() : MOVE_NONE; + ttCapture = ttMove && pos.capture_or_promotion(ttMove); + if (!excludedMove) + ss->ttPv = PvNode || (ss->ttHit && tte->is_pv()); - if ( ttPv + // Update low ply history for previous move if we are near root and position is or has been in PV + if ( ss->ttPv && depth > 12 && ss->ply - 1 < MAX_LPH && !priorCapture && is_ok((ss-1)->currentMove)) thisThread->lowPlyHistory[ss->ply - 1][from_to((ss-1)->currentMove)] << stat_bonus(depth - 5); - // thisThread->ttHitAverage can be used to approximate the running average of ttHit - thisThread->ttHitAverage = (TtHitAverageWindow - 1) * thisThread->ttHitAverage / TtHitAverageWindow - + TtHitAverageResolution * ttHit; - // At non-PV nodes we check for an early TT cutoff if ( !PvNode - && ttHit + && ss->ttHit && tte->depth() >= depth && ttValue != VALUE_NONE // Possible in case of TT access race && (ttValue >= beta ? (tte->bound() & BOUND_LOWER) @@ -694,7 +683,8 @@ namespace { { if (ttValue >= beta) { - if (!pos.capture_or_promotion(ttMove)) + // Bonus for a quiet ttMove that fails high + if (!ttCapture) update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth); // Extra penalty for early quiet moves of the previous ply @@ -702,7 +692,7 @@ namespace { update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1)); } // Penalty for a quiet ttMove that fails low - else if (!pos.capture_or_promotion(ttMove)) + else if (!ttCapture) { int penalty = -stat_bonus(depth); thisThread->mainHistory[us][from_to(ttMove)] << penalty; @@ -710,6 +700,8 @@ namespace { } } + // Partial workaround for the graph history interaction problem + // For high rule50 counts don't produce transposition table cutoffs. if (pos.rule50_count() < 90) return ttValue; } @@ -748,7 +740,7 @@ namespace { if ( b == BOUND_EXACT || (b == BOUND_LOWER ? value >= beta : value <= alpha)) { - tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b, + tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b, std::min(MAX_PLY - 1, depth + 6), MOVE_NONE, VALUE_NONE); @@ -774,15 +766,17 @@ namespace { // Skip early pruning when in check ss->staticEval = eval = VALUE_NONE; improving = false; + improvement = 0; goto moves_loop; } - else if (ttHit) + else if (ss->ttHit) { // Never assume anything about values stored in TT ss->staticEval = eval = tte->eval(); if (eval == VALUE_NONE) ss->staticEval = eval = evaluate(pos); + // Randomize draw evaluation if (eval == VALUE_DRAW) eval = value_draw(thisThread); @@ -793,41 +787,49 @@ namespace { } else { + // In case of null move search use previous static eval with a different sign if ((ss-1)->currentMove != MOVE_NULL) - { - int bonus = -(ss-1)->statScore / 512; - - ss->staticEval = eval = evaluate(pos) + bonus; - } + ss->staticEval = eval = evaluate(pos); else - ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo; + ss->staticEval = eval = -(ss-1)->staticEval; - tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval); + // Save static evaluation into transposition table + if(!excludedMove) + tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval); + } + + // Use static evaluation difference to improve quiet move ordering + if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture) + { + int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval), -1000, 1000); + thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus; } - // Step 7. Razoring (~1 Elo) - if ( !rootNode // The required rootNode PV handling is not available in qsearch - && depth == 1 - && eval <= alpha - RazorMargin) - return qsearch(pos, ss, alpha, beta); + // Set up the improvement variable, which is the difference between the current + // static evaluation and the previous static evaluation at our turn (if we were + // in check at our previous move we look at the move prior to it). The improvement + // margin and the improving flag are used in various pruning heuristics. + improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval + : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval + : 200; - improving = (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval > (ss-4)->staticEval - || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval > (ss-2)->staticEval; + improving = improvement > 0; - // Step 8. Futility pruning: child node (~50 Elo) + // Step 7. Futility pruning: child node (~50 Elo). + // The depth condition is important for mate finding. if ( !PvNode - && depth < 8 + && depth < 9 && eval - futility_margin(depth, improving) >= beta && eval < VALUE_KNOWN_WIN) // Do not return unproven wins return eval; - // Step 9. Null move search with verification search (~40 Elo) + // Step 8. Null move search with verification search (~40 Elo) if ( !PvNode && (ss-1)->currentMove != MOVE_NULL - && (ss-1)->statScore < 23824 + && (ss-1)->statScore < 23767 && eval >= beta && eval >= ss->staticEval - && ss->staticEval >= beta - 28 * depth - 28 * improving + 94 * ttPv + 200 + && ss->staticEval >= beta - 20 * depth - improvement / 15 + 168 * ss->ttPv + 177 && !excludedMove && pos.non_pawn_material(us) && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor)) @@ -835,7 +837,7 @@ namespace { assert(eval - beta >= 0); // Null move dynamic reduction based on depth and value - Depth R = (737 + 77 * depth) / 246 + std::min(int(eval - beta) / 192, 3); + Depth R = std::min(int(eval - beta) / 205, 3) + depth / 3 + 4; ss->currentMove = MOVE_NULL; ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0]; @@ -852,7 +854,7 @@ namespace { if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY) nullValue = beta; - if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13)) + if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14)) return nullValue; assert(!thisThread->nmpMinPly); // Recursive verification is not allowed @@ -871,30 +873,29 @@ namespace { } } - probcutBeta = beta + 176 - 49 * improving; + probCutBeta = beta + 209 - 44 * improving; - // Step 10. ProbCut (~10 Elo) + // Step 9. ProbCut (~4 Elo) // If we have a good enough capture and a reduced search returns a value // much above beta, we can (almost) safely prune the previous move. if ( !PvNode && depth > 4 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY - && !( ttHit + // if value from transposition table is lower than probCutBeta, don't attempt probCut + // there and in further interactions with transposition table cutoff depth is set to depth - 3 + // because probCut search has depth set to depth - 4 but we also do a move before it + // so effective depth is equal to depth - 3 + && !( ss->ttHit && tte->depth() >= depth - 3 && ttValue != VALUE_NONE - && ttValue < probcutBeta)) + && ttValue < probCutBeta)) { - if ( ttHit - && tte->depth() >= depth - 3 - && ttValue != VALUE_NONE - && ttValue >= probcutBeta - && ttMove - && pos.capture_or_promotion(ttMove)) - return probcutBeta; - - assert(probcutBeta < VALUE_INFINITE); - MovePicker mp(pos, ttMove, probcutBeta - ss->staticEval, &captureHistory); + assert(probCutBeta < VALUE_INFINITE); + + MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory); int probCutCount = 0; + bool ttPv = ss->ttPv; + ss->ttPv = false; while ( (move = mp.next_move()) != MOVE_NONE && probCutCount < 2 + 2 * cutNode) @@ -915,17 +916,18 @@ namespace { pos.do_move(move, st); // Perform a preliminary qsearch to verify that the move holds - value = -qsearch(pos, ss+1, -probcutBeta, -probcutBeta+1); + value = -qsearch(pos, ss+1, -probCutBeta, -probCutBeta+1); // If the qsearch held, perform the regular search - if (value >= probcutBeta) - value = -search(pos, ss+1, -probcutBeta, -probcutBeta+1, depth - 4, !cutNode); + if (value >= probCutBeta) + value = -search(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode); pos.undo_move(move); - if (value >= probcutBeta) + if (value >= probCutBeta) { - if ( !(ttHit + // if transposition table doesn't have equal or more deep info write probCut data into it + if ( !(ss->ttHit && tte->depth() >= depth - 3 && ttValue != VALUE_NONE)) tte->save(posKey, value_to_tt(value, ss->ply), ttPv, @@ -934,19 +936,38 @@ namespace { return value; } } + ss->ttPv = ttPv; } - // Step 11. Internal iterative deepening (~1 Elo) - if (depth >= 7 && !ttMove) - { - search(pos, ss, alpha, beta, depth - 7, cutNode); + // Step 10. If the position is not in TT, decrease depth by 2 or 1 depending on node type + if ( PvNode + && depth >= 6 + && !ttMove) + depth -= 2; - tte = TT.probe(posKey, ttHit); - ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; - ttMove = ttHit ? tte->move() : MOVE_NONE; - } + if ( cutNode + && depth >= 9 + && !ttMove) + depth--; + +moves_loop: // When in check, search starts here + + int rangeReduction = 0; + + // Step 11. A small Probcut idea, when we are in check + probCutBeta = beta + 409; + 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; -moves_loop: // When in check, search starts from here const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr , (ss-4)->continuationHistory, @@ -963,11 +984,14 @@ moves_loop: // When in check, search starts from here ss->ply); value = bestValue; - singularQuietLMR = moveCountPruning = false; - ttCapture = ttMove && pos.capture_or_promotion(ttMove); + singularQuietLMR = moveCountPruning = noLMRExtension = false; - // Mark this node as being searched - ThreadHolding th(thisThread, posKey, ss->ply); + // Indicate PvNodes that will probably fail low if the node was searched + // at a depth equal or greater than the current depth, and the result of this search was a fail low. + bool likelyFailLow = PvNode + && ttMove + && (tte->bound() & BOUND_UPPER) + && tte->depth() >= depth; // Step 12. Loop through all pseudo-legal moves until no moves remain // or a beta cutoff occurs. @@ -1007,7 +1031,7 @@ moves_loop: // When in check, search starts from here // Calculate new depth for this move newDepth = depth - 1; - // Step 13. Pruning at shallow depth (~200 Elo) + // Step 13. Pruning at shallow depth (~200 Elo). Depth conditions are important for mate finding. if ( !rootNode && pos.non_pawn_material(us) && bestValue > VALUE_TB_LOSS_IN_MAX_PLY) @@ -1016,51 +1040,38 @@ moves_loop: // When in check, search starts from here moveCountPruning = moveCount >= futility_move_count(improving, depth); // Reduced depth of the next LMR search - int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0); + int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0); - if ( !captureOrPromotion - && !givesCheck) + if ( captureOrPromotion + || givesCheck) { - // Countermoves based pruning (~20 Elo) - if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1) - && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold - && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold) + // Capture history based pruning when the move doesn't give check + if ( !givesCheck + && lmrDepth < 1 + && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0) continue; - // Futility pruning: parent node (~5 Elo) - if ( lmrDepth < 8 - && !ss->inCheck - && ss->staticEval + 284 + 188 * lmrDepth <= alpha - && (*contHist[0])[movedPiece][to_sq(move)] - + (*contHist[1])[movedPiece][to_sq(move)] - + (*contHist[3])[movedPiece][to_sq(move)] - + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 28388) - continue; - - // Prune moves with negative SEE (~20 Elo) - if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 17)) * lmrDepth * lmrDepth))) + // SEE based pruning + if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo) continue; } else { - // Capture history based pruning when the move doesn't give check - if ( !givesCheck - && lmrDepth < 1 - && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0) + // Continuation history based pruning (~20 Elo) + if (lmrDepth < 5 + && (*contHist[0])[movedPiece][to_sq(move)] + + (*contHist[1])[movedPiece][to_sq(move)] + + (*contHist[3])[movedPiece][to_sq(move)] < -3000 * depth + 3000) continue; - // Futility pruning for captures - if ( !givesCheck - && lmrDepth < 6 - && !(PvNode && abs(bestValue) < 2) - && PieceValue[MG][type_of(movedPiece)] >= PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] - && !ss->inCheck - && ss->staticEval + 267 + 391 * lmrDepth - + PieceValue[MG][type_of(pos.piece_on(to_sq(move)))] <= alpha) + // Futility pruning: parent node (~5 Elo) + if ( !ss->inCheck + && lmrDepth < 8 + && ss->staticEval + 172 + 145 * lmrDepth <= alpha) continue; - // See based pruning - if (!pos.see_ge(move, Value(-202) * depth)) // (~25 Elo) + // Prune moves with negative SEE (~20 Elo) + if (!pos.see_ge(move, Value(-21 * lmrDepth * lmrDepth - 21 * lmrDepth))) continue; } } @@ -1072,17 +1083,18 @@ moves_loop: // When in check, search starts from here // then that move is singular and should be extended. To verify this we do // a reduced search on all the other moves but the ttMove and if the // result is lower than ttValue minus a margin, then we will extend the ttMove. - if ( depth >= 6 + if ( !rootNode + && depth >= 7 && move == ttMove - && !rootNode && !excludedMove // Avoid recursive singular search /* && ttValue != VALUE_NONE Already implicit in the next condition */ && abs(ttValue) < VALUE_KNOWN_WIN && (tte->bound() & BOUND_LOWER) && tte->depth() >= depth - 3) { - Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2; - Depth singularDepth = (depth - 1 + 3 * formerPv) / 2; + Value singularBeta = ttValue - 3 * depth; + Depth singularDepth = (depth - 1) / 2; + ss->excludedMove = move; value = search(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode); ss->excludedMove = MOVE_NONE; @@ -1091,6 +1103,15 @@ moves_loop: // When in check, search starts from here { extension = 1; singularQuietLMR = !ttCapture; + + // Avoid search explosion by limiting the number of double extensions + if ( !PvNode + && value < singularBeta - 75 + && ss->doubleExtensions <= 6) + { + extension = 2; + noLMRExtension = true; + } } // Multi-cut pruning @@ -1101,47 +1122,33 @@ moves_loop: // When in check, search starts from here else if (singularBeta >= beta) return singularBeta; - // If the eval of ttMove is greater than beta we try also if there is another - // move that pushes it over beta, if so also produce a cutoff. + // If the eval of ttMove is greater than beta, we reduce it (negative extension) else if (ttValue >= beta) - { - ss->excludedMove = move; - value = search(pos, ss, beta - 1, beta, (depth + 3) / 2, cutNode); - ss->excludedMove = MOVE_NONE; - - if (value >= beta) - return beta; - } + extension = -2; } - // Check extension (~2 Elo) - else if ( givesCheck - && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move))) + // Capture extensions for PvNodes and cutNodes + else if ( (PvNode || cutNode) + && captureOrPromotion + && moveCount != 1) extension = 1; - // Passed pawn extension - else if ( move == ss->killers[0] - && pos.advanced_pawn_push(move) - && pos.pawn_passed(us, to_sq(move))) + // Check extensions + else if ( givesCheck + && depth > 6 + && abs(ss->staticEval) > 100) extension = 1; - // Last captures extension - else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg - && pos.non_pawn_material() <= 2 * RookValueMg) + // Quiet ttMove extensions + else if ( PvNode + && move == ttMove + && move == ss->killers[0] + && (*contHist[0])[movedPiece][to_sq(move)] >= 10000) extension = 1; - // Castling extension - if (type_of(move) == CASTLING) - extension = 1; - - // Late irreversible move extension - if ( move == ttMove - && pos.rule50_count() > 80 - && (captureOrPromotion || type_of(movedPiece) == PAWN)) - extension = 2; - // Add extension to new depth newDepth += extension; + ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2); // Speculative prefetch as early as possible prefetch(TT.first_entry(pos.key_after(move))); @@ -1156,106 +1163,85 @@ moves_loop: // When in check, search starts from here // Step 15. Make the move pos.do_move(move, st, givesCheck); - // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be - // re-searched at full depth. + // Step 16. Late moves reduction / extension (LMR, ~200 Elo) + // We use various heuristics for the sons of a node after the first son has + // been searched. In general we would like to reduce them, but there are many + // cases where we extend a son if it has good chances to be "interesting". if ( depth >= 3 && moveCount > 1 + 2 * rootNode - && (!rootNode || thisThread->best_move_count(move) == 0) && ( !captureOrPromotion - || moveCountPruning - || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha - || cutNode - || thisThread->ttHitAverage < 415 * TtHitAverageResolution * TtHitAverageWindow / 1024)) + || (cutNode && (ss-1)->moveCount > 1) + || !ss->ttPv) + && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3)) { - Depth r = reduction(improving, depth, moveCount); + Depth r = reduction(improving, depth, moveCount, rangeReduction > 2); - // Decrease reduction at non-check cut nodes for second move at low depths - if ( cutNode - && depth <= 10 - && moveCount <= 2 - && !ss->inCheck) + // Decrease reduction if on the PV (~2 Elo) + if ( PvNode + && bestMoveCount <= 3) r--; - // Decrease reduction if the ttHit running average is large - if (thisThread->ttHitAverage > 473 * TtHitAverageResolution * TtHitAverageWindow / 1024) - r--; - - // Reduction if other threads are searching this position - if (th.marked()) - r++; - - // Decrease reduction if position is or has been on the PV (~10 Elo) - if (ttPv) + // Decrease reduction if position is or has been on the PV + // and node is not likely to fail low. (~3 Elo) + if ( ss->ttPv + && !likelyFailLow) r -= 2; - if (moveCountPruning && !formerPv) + // Increase reduction at root and non-PV nodes when the best move does not change frequently + if ( (rootNode || !PvNode) + && thisThread->bestMoveChanges <= 2) r++; - // Decrease reduction if opponent's move count is high (~5 Elo) + // Decrease reduction if opponent's move count is high (~1 Elo) if ((ss-1)->moveCount > 13) r--; - // Decrease reduction if ttMove has been singularly extended (~3 Elo) + // Decrease reduction if ttMove has been singularly extended (~1 Elo) if (singularQuietLMR) - r -= 1 + formerPv; + r--; - if (!captureOrPromotion) - { - // Increase reduction if ttMove is a capture (~5 Elo) - if (ttCapture) - r++; - - // Increase reduction for cut nodes (~10 Elo) - if (cutNode) - r += 2; - - // Decrease reduction for moves that escape a capture. Filter out - // castling moves, because they are coded as "king captures rook" and - // hence break make_move(). (~2 Elo) - else if ( type_of(move) == NORMAL - && !pos.see_ge(reverse_move(move))) - r -= 2 + ttPv - (type_of(movedPiece) == PAWN); - - ss->statScore = thisThread->mainHistory[us][from_to(move)] - + (*contHist[0])[movedPiece][to_sq(move)] - + (*contHist[1])[movedPiece][to_sq(move)] - + (*contHist[3])[movedPiece][to_sq(move)] - - 4826; - - // Decrease/increase reduction by comparing opponent's stat score (~10 Elo) - if (ss->statScore >= -100 && (ss-1)->statScore < -112) - r--; - - else if ((ss-1)->statScore >= -125 && ss->statScore < -138) - r++; - - // Decrease/increase reduction for moves with a good/bad history (~30 Elo) - r -= ss->statScore / 14615; - } - else - { - // Increase reduction for captures/promotions if late move and at low depth - if (depth < 8 && moveCount > 2) - r++; - - // Unless giving check, this capture is likely bad - if ( !givesCheck - && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 211 * depth <= alpha) - r++; - } + // Increase reduction for cut nodes (~3 Elo) + if (cutNode && move != ss->killers[0]) + r += 2; + + // Increase reduction if ttMove is a capture (~3 Elo) + if (ttCapture) + r++; + + ss->statScore = thisThread->mainHistory[us][from_to(move)] + + (*contHist[0])[movedPiece][to_sq(move)] + + (*contHist[1])[movedPiece][to_sq(move)] + + (*contHist[3])[movedPiece][to_sq(move)] + - 4923; + + // Decrease/increase reduction for moves with a good/bad history (~30 Elo) + r -= ss->statScore / 14721; + + // In general we want to cap the LMR depth search at newDepth. But if reductions + // are really negative and movecount is low, we allow this move to be searched + // deeper than the first move (this may lead to hidden double extensions if + // newDepth got its own extension before). + int deeper = r >= -1 ? 0 + : noLMRExtension ? 0 + : moveCount <= 5 ? 1 + : (depth > 6 && PvNode) ? 1 + : 0; - Depth d = Utility::clamp(newDepth - r, 1, newDepth); + Depth d = std::clamp(newDepth - r, 1, newDepth + deeper); value = -search(pos, ss+1, -(alpha+1), -alpha, d, true); - doFullDepthSearch = value > alpha && d != newDepth; + // Range reductions (~3 Elo) + if (ss->staticEval - value < 30 && depth > 7) + rangeReduction++; + // If the son is reduced and fails high it will be re-searched at full depth + doFullDepthSearch = value > alpha && d < newDepth; didLMR = true; } else { doFullDepthSearch = !PvNode || moveCount > 1; - didLMR = false; } @@ -1264,14 +1250,12 @@ moves_loop: // When in check, search starts from here { value = -search(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode); + // If the move passed LMR update its stats if (didLMR && !captureOrPromotion) { int bonus = value > alpha ? stat_bonus(newDepth) : -stat_bonus(newDepth); - if (move == ss->killers[0]) - bonus += bonus / 4; - update_continuation_histories(ss, movedPiece, to_sq(move), bonus); } } @@ -1284,7 +1268,8 @@ moves_loop: // When in check, search starts from here (ss+1)->pv = pv; (ss+1)->pv[0] = MOVE_NONE; - value = -search(pos, ss+1, -beta, -alpha, newDepth, false); + value = -search(pos, ss+1, -beta, -alpha, + std::min(maxNextDepth, newDepth), false); } // Step 18. Undo move @@ -1316,10 +1301,11 @@ moves_loop: // When in check, search starts from here for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m) rm.pv.push_back(*m); - // We record how often the best move has been changed in each - // iteration. This information is used for time management: when - // the best move changes frequently, we allocate some more time. - if (moveCount > 1) + // We record how often the best move has been changed in each iteration. + // This information is used for time management and LMR. In MultiPV mode, + // we must take care to only do this for the first PV line. + if ( moveCount > 1 + && !thisThread->pvIdx) ++thisThread->bestMoveChanges; } else @@ -1341,16 +1327,19 @@ moves_loop: // When in check, search starts from here update_pv(ss->pv, move, (ss+1)->pv); if (PvNode && value < beta) // Update alpha! Always alpha < beta + { alpha = value; + bestMoveCount++; + } else { assert(value >= beta); // Fail high - ss->statScore = 0; break; } } } + // If the move is worse than some previously searched move, remember it to update its stats later if (move != bestMove) { if (captureOrPromotion && captureCount < 32) @@ -1377,9 +1366,11 @@ moves_loop: // When in check, search starts from here assert(moveCount || !ss->inCheck || excludedMove || !MoveList(pos).size()); if (!moveCount) - bestValue = excludedMove ? alpha - : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW; + bestValue = excludedMove ? alpha : + ss->inCheck ? mated_in(ss->ply) + : VALUE_DRAW; + // If there is a move which produces search value greater than alpha we update stats of searched moves else if (bestMove) update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq, quietsSearched, quietCount, capturesSearched, captureCount, depth); @@ -1387,13 +1378,23 @@ moves_loop: // When in check, search starts from here // Bonus for prior countermove that caused the fail low else if ( (depth >= 3 || PvNode) && !priorCapture) - update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth)); + update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + (PvNode || cutNode))); if (PvNode) bestValue = std::min(bestValue, maxValue); + // If no good move is found and the previous position was ttPv, then the previous + // opponent move is probably good and the new position is added to the search tree. + if (bestValue <= alpha) + ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3); + // Otherwise, a counter move has been found and if the position is the last leaf + // in the search tree, remove the position from the search tree. + else if (depth > 3) + ss->ttPv = ss->ttPv && (ss+1)->ttPv; + + // Write gathered information in transposition table if (!excludedMove && !(rootNode && thisThread->pvIdx)) - tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv, + tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv, bestValue >= beta ? BOUND_LOWER : PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER, depth, bestMove, ss->staticEval); @@ -1406,10 +1407,11 @@ moves_loop: // When in check, search starts from here // qsearch() is the quiescence search function, which is called by the main search // function with zero depth, or recursively with further decreasing depth per call. - template + template Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) { - constexpr bool PvNode = NT == PV; + static_assert(nodeType != Root); + constexpr bool PvNode = nodeType == PV; assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE); assert(PvNode || (alpha == beta - 1)); @@ -1417,12 +1419,14 @@ moves_loop: // When in check, search starts from here Move pv[MAX_PLY+1]; StateInfo st; + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); + TTEntry* tte; Key posKey; Move ttMove, move, bestMove; Depth ttDepth; Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha; - bool ttHit, pvHit, givesCheck, captureOrPromotion; + bool pvHit, givesCheck, captureOrPromotion; int moveCount; if (PvNode) @@ -1433,7 +1437,6 @@ moves_loop: // When in check, search starts from here } Thread* thisThread = pos.this_thread(); - (ss+1)->ply = ss->ply + 1; bestMove = MOVE_NONE; ss->inCheck = pos.checkers(); moveCount = 0; @@ -1452,13 +1455,13 @@ moves_loop: // When in check, search starts from here : DEPTH_QS_NO_CHECKS; // Transposition table lookup posKey = pos.key(); - tte = TT.probe(posKey, ttHit); - ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; - ttMove = ttHit ? tte->move() : MOVE_NONE; - pvHit = ttHit && tte->is_pv(); + tte = TT.probe(posKey, ss->ttHit); + ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; + ttMove = ss->ttHit ? tte->move() : MOVE_NONE; + pvHit = ss->ttHit && tte->is_pv(); if ( !PvNode - && ttHit + && ss->ttHit && tte->depth() >= ttDepth && ttValue != VALUE_NONE // Only in case of TT access race && (ttValue >= beta ? (tte->bound() & BOUND_LOWER) @@ -1473,7 +1476,7 @@ moves_loop: // When in check, search starts from here } else { - if (ttHit) + if (ss->ttHit) { // Never assume anything about values stored in TT if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE) @@ -1485,14 +1488,16 @@ moves_loop: // When in check, search starts from here bestValue = ttValue; } else + // In case of null move search use previous static eval with a different sign ss->staticEval = bestValue = (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) - : -(ss-1)->staticEval + 2 * Tempo; + : -(ss-1)->staticEval; // Stand pat. Return immediately if static value is at least beta if (bestValue >= beta) { - if (!ttHit) + // Save gathered info in transposition table + if (!ss->ttHit) tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER, DEPTH_NONE, MOVE_NONE, ss->staticEval); @@ -1502,7 +1507,7 @@ moves_loop: // When in check, search starts from here if (PvNode && bestValue > alpha) alpha = bestValue; - futilityBase = bestValue + 141; + futilityBase = bestValue + 155; } const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, @@ -1511,7 +1516,7 @@ moves_loop: // When in check, search starts from here // Initialize a MovePicker object for the current position, and prepare // to search the moves. Because the depth is <= 0 here, only captures, - // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS) + // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS) // will be generated. MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, @@ -1523,18 +1528,24 @@ moves_loop: // When in check, search starts from here { assert(is_ok(move)); + // Check for legality + if (!pos.legal(move)) + continue; + givesCheck = pos.gives_check(move); captureOrPromotion = pos.capture_or_promotion(move); moveCount++; - // Futility pruning - if ( !ss->inCheck + // Futility pruning and moveCount pruning + if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY && !givesCheck && futilityBase > -VALUE_KNOWN_WIN - && !pos.advanced_pawn_push(move)) + && type_of(move) != PROMOTION) { - assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push + + if (moveCount > 2) + continue; futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))]; @@ -1552,28 +1563,29 @@ moves_loop: // When in check, search starts from here } // Do not search moves with negative SEE values - if ( !ss->inCheck && !pos.see_ge(move)) + if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY + && !pos.see_ge(move)) continue; // Speculative prefetch as early as possible prefetch(TT.first_entry(pos.key_after(move))); - // Check for legality just before making the move - if (!pos.legal(move)) - { - moveCount--; - continue; - } - ss->currentMove = move; ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck] [captureOrPromotion] [pos.moved_piece(move)] [to_sq(move)]; + // Continuation history based pruning + if ( !captureOrPromotion + && bestValue > VALUE_TB_LOSS_IN_MAX_PLY + && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold + && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold) + continue; + // Make and search the move pos.do_move(move, st, givesCheck); - value = -qsearch(pos, ss+1, -beta, -alpha, depth - 1); + value = -qsearch(pos, ss+1, -beta, -alpha, depth - 1); pos.undo_move(move); assert(value > -VALUE_INFINITE && value < VALUE_INFINITE); @@ -1601,8 +1613,13 @@ moves_loop: // When in check, search starts from here // All legal moves have been searched. A special case: if we're in check // and no legal moves were found, it is checkmate. if (ss->inCheck && bestValue == -VALUE_INFINITE) + { + assert(!MoveList(pos).size()); + return mated_in(ss->ply); // Plies to mate from the root + } + // Save gathered info in transposition table tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, bestValue >= beta ? BOUND_LOWER : PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER, @@ -1686,9 +1703,10 @@ moves_loop: // When in check, search starts from here if (!pos.capture_or_promotion(bestMove)) { + // Increase stats for the best move in case it was a quiet move update_quiet_stats(pos, ss, bestMove, bonus2, depth); - // Decrease all the non-best quiet moves + // Decrease stats for all non-best quiet moves for (int i = 0; i < quietCount; ++i) { thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2; @@ -1696,14 +1714,16 @@ moves_loop: // When in check, search starts from here } } else + // Increase stats for the best move in case it was a capture move captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1; - // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted - if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0])) + // Extra penalty for a quiet early move that was not a TT move or + // main killer move in previous ply when it gets refuted. + if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0])) && !pos.captured_piece()) update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1); - // Decrease all the non-best capture moves + // Decrease stats for all non-best capture moves for (int i = 0; i < captureCount; ++i) { moved_piece = pos.moved_piece(capturesSearched[i]); @@ -1720,6 +1740,7 @@ moves_loop: // When in check, search starts from here for (int i : {1, 2, 4, 6}) { + // Only update first 2 continuation histories if we are in check if (ss->inCheck && i > 2) break; if (is_ok((ss-i)->currentMove)) @@ -1732,6 +1753,7 @@ moves_loop: // When in check, search starts from here void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) { + // Update killers if (ss->killers[0] != move) { ss->killers[1] = ss->killers[0]; @@ -1743,17 +1765,20 @@ moves_loop: // When in check, search starts from here thisThread->mainHistory[us][from_to(move)] << bonus; update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus); + // Penalty for reversed move in case of moved piece not being a pawn if (type_of(pos.moved_piece(move)) != PAWN) thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus; + // Update countermove history if (is_ok((ss-1)->currentMove)) { Square prevSq = to_sq((ss-1)->currentMove); thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move; } + // Update low ply history if (depth > 11 && ss->ply < MAX_LPH) - thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 6); + thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7); } // When playing with strength handicap, choose best move among a set of RootMoves @@ -1842,12 +1867,15 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { { bool updated = rootMoves[i].score != -VALUE_INFINITE; - if (depth == 1 && !updated) + if (depth == 1 && !updated && i > 0) continue; - Depth d = updated ? depth : depth - 1; + Depth d = updated ? depth : std::max(1, depth - 1); Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore; + if (v == -VALUE_INFINITE) + v = VALUE_ZERO; + bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY; v = tb ? rootMoves[i].tbScore : v; @@ -1892,6 +1920,8 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { bool RootMove::extract_ponder_from_tt(Position& pos) { StateInfo st; + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); + bool ttHit; assert(pv.size() == 1); @@ -1945,7 +1975,7 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) { if (RootInTB) { // Sort moves according to TB rank - std::sort(rootMoves.begin(), rootMoves.end(), + std::stable_sort(rootMoves.begin(), rootMoves.end(), [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } ); // Probe during search only if DTZ is not available and we are winning @@ -1959,3 +1989,5 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) { m.tbRank = 0; } } + +} // namespace Stockfish