X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fsearch.cpp;h=28049e87dcd8dd49a4f21242c88bf8ffa1d9a1bd;hp=d854cd95d1d5dc75b13b698b0f95cf0de7485191;hb=673841301b0cc6ed78c4db3e6ec2a0b9a010c8cb;hpb=66da1e802c22b1952c2a07af65c6224d0e01b3df diff --git a/src/search.cpp b/src/search.cpp index d854cd95..28049e87 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 = 510; + // Futility margin Value futility_margin(Depth d, bool improving) { - return Value(234 * (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 + 503) / 1024 + (!i && r > 915); + 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 > 13 ? 29 : 17 * d * d + 134 * 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); @@ -164,7 +143,7 @@ namespace { uint64_t perft(Position& pos, Depth depth) { StateInfo st; - ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize); + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); uint64_t cnt, nodes = 0; const bool leaf = (depth == 2); @@ -194,7 +173,7 @@ namespace { void Search::init() { for (int i = 1; i < MAX_MOVES; ++i) - Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i))); + Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i)); } @@ -295,7 +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; @@ -310,6 +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; @@ -349,21 +331,15 @@ 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 - // 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; + ttHitAverage.set(50, 100); // initialize the running average at 50% + doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0% + doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0% - // 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; @@ -409,21 +385,21 @@ void Thread::search() { 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 + (113 - ct / 2) * prev / (abs(prev) + 147); + // 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 // high/low, re-search with a bigger window until we don't fail // high/low anymore. - failedHighCnt = 0; + int failedHighCnt = 0; 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 @@ -519,8 +495,8 @@ void Thread::search() { totBestMoveChanges += th->bestMoveChanges; th->bestMoveChanges = 0; } - double bestMoveInstability = 1 + 2 * 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; // Cap used time in case of a single legal move for a better viewer experience in tournaments @@ -566,18 +542,26 @@ 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()); @@ -587,7 +571,7 @@ 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)); @@ -596,27 +580,26 @@ namespace { Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64]; StateInfo st; - ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize); + 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 formerPv, givesCheck, improving, didLMR, priorCapture; + 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; // 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()) @@ -649,11 +632,15 @@ namespace { assert(0 <= ss->ply && ss->ply < MAX_PLY); - (ss+1)->ply = ss->ply + 1; - (ss+1)->ttPv = false; + (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 @@ -674,8 +661,8 @@ namespace { : ss->ttHit ? tte->move() : MOVE_NONE; if (!excludedMove) ss->ttPv = PvNode || (ss->ttHit && tte->is_pv()); - formerPv = ss->ttPv && !PvNode; + // 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 @@ -683,9 +670,8 @@ namespace { && 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 * ss->ttHit; + // running average of ttHit + thisThread->ttHitAverage.update(ss->ttHit); // At non-PV nodes we check for an early TT cutoff if ( !PvNode @@ -700,6 +686,7 @@ namespace { { if (ttValue >= beta) { + // Bonus for a quiet ttMove that fails high if (!pos.capture_or_promotion(ttMove)) update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth); @@ -716,6 +703,8 @@ 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; } @@ -789,6 +778,7 @@ namespace { if (eval == VALUE_NONE) ss->staticEval = eval = evaluate(pos); + // Randomize draw evaluation if (eval == VALUE_DRAW) eval = value_draw(thisThread); @@ -799,38 +789,47 @@ namespace { } else { + // In case of null move search use previous static eval with a different sign if ((ss-1)->currentMove != MOVE_NULL) ss->staticEval = eval = evaluate(pos); else - ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo; + ss->staticEval = eval = -(ss-1)->staticEval; + // Save static evaluation into transposition table + if(!excludedMove) tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval); } - // 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); + // 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; + } + // Set up improving flag that is used in various pruning heuristics + // We define position as improving if static evaluation of position is better + // Than the previous static evaluation at our turn + // In case of us being in check at our previous move we look at move prior to it improving = (ss-2)->staticEval == VALUE_NONE ? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE : ss->staticEval > (ss-2)->staticEval; - // 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 < 22977 + && (ss-1)->statScore < 23767 && eval >= beta && eval >= ss->staticEval - && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 168 + && ss->staticEval >= beta - 20 * depth - 22 * improving + 168 * ss->ttPv + 177 && !excludedMove && pos.non_pawn_material(us) && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor)) @@ -838,7 +837,7 @@ namespace { assert(eval - beta >= 0); // Null move dynamic reduction based on depth and value - Depth R = (1015 + 85 * depth) / 256 + std::min(int(eval - beta) / 191, 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]; @@ -874,9 +873,9 @@ namespace { } } - probCutBeta = beta + 183 - 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 @@ -891,17 +890,8 @@ namespace { && ttValue != VALUE_NONE && ttValue < probCutBeta)) { - // if ttMove is a capture and value from transposition table is good enough produce probCut - // cutoff without digging into actual probCut search - if ( ss->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); int probCutCount = 0; bool ttPv = ss->ttPv; @@ -949,13 +939,36 @@ namespace { ss->ttPv = ttPv; } - // Step 11. If the position is not in TT, decrease depth by 2 + // 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; -moves_loop: // When in check, search starts from here + if ( cutNode + && depth >= 9 + && !ttMove) + depth--; + +moves_loop: // When in check, search starts here + + ttCapture = ttMove && pos.capture_or_promotion(ttMove); + 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; + const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr , (ss-4)->continuationHistory, @@ -972,11 +985,14 @@ 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. @@ -1016,7 +1032,7 @@ 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) @@ -1025,41 +1041,38 @@ 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) - continue; - - // Futility pruning: parent node (~5 Elo) - if ( lmrDepth < 7 - && !ss->inCheck - && ss->staticEval + 266 + 170 * 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 < 27376) + // 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; - // Prune moves with negative SEE (~20 Elo) - if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * 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; - // SEE based pruning - if (!pos.see_ge(move, Value(-213) * depth)) // (~25 Elo) + // Futility pruning: parent node (~5 Elo) + if ( !ss->inCheck + && lmrDepth < 8 + && ss->staticEval + 172 + 145 * lmrDepth <= alpha) + continue; + + // Prune moves with negative SEE (~20 Elo) + if (!pos.see_ge(move, Value(-21 * lmrDepth * lmrDepth - 21 * lmrDepth))) continue; } } @@ -1071,17 +1084,18 @@ 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 >= 7 + 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; @@ -1090,6 +1104,15 @@ 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 @@ -1100,37 +1123,33 @@ 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; - // Last captures extension - else if ( PieceValue[EG][pos.captured_piece()] > PawnValueEg - && pos.non_pawn_material() <= 2 * RookValueMg) + // Check extensions + else if ( givesCheck + && depth > 6 + && abs(ss->staticEval) > 100) extension = 1; - // Late irreversible move extension - if ( move == ttMove - && pos.rule50_count() > 80 - && (captureOrPromotion || type_of(movedPiece) == PAWN)) - extension = 2; + // Quiet ttMove extensions + else if ( PvNode + && move == ttMove + && move == ss->killers[0] + && (*contHist[0])[movedPiece][to_sq(move)] >= 10000) + extension = 1; // 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))); @@ -1145,105 +1164,89 @@ 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 && ( !captureOrPromotion - || moveCountPruning - || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha - || cutNode - || thisThread->ttHitAverage < 432 * 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 if the ttHit running average is large - if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024) + // Decrease reduction if on the PV (~2 Elo) + if ( PvNode + && bestMoveCount <= 3) r--; - // Increase reduction if other threads are searching this position - if (th.marked()) - r++; + // Decrease reduction if the ttHit running average is large (~0 Elo) + if (thisThread->ttHitAverage.is_greater(537, 1024)) + r--; - // Decrease reduction if position is or has been on the PV (~10 Elo) - if (ss->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; // Increase reduction at root and non-PV nodes when the best move does not change frequently - if ((rootNode || !PvNode) && depth > 10 && thisThread->bestMoveChanges <= 2) - r++; - - if (moveCountPruning && !formerPv) + 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--; - if (!captureOrPromotion) - { - // Increase reduction if ttMove is a capture (~5 Elo) - if (ttCapture) - r++; - - // Increase reduction at root if failing high - r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0; - - // 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 + ss->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)] - - 5287; - - // Decrease/increase reduction by comparing opponent's stat score (~10 Elo) - if (ss->statScore >= -105 && (ss-1)->statScore < -103) - r--; - - else if ((ss-1)->statScore >= -122 && ss->statScore < -129) - r++; - - // Decrease/increase reduction for moves with a good/bad history (~30 Elo) - r -= ss->statScore / 14884; - } - else - { - // Increase reduction for captures/promotions if late move and at low depth - if (depth < 8 && moveCount > 2) - r++; + // Increase reduction for cut nodes (~3 Elo) + if (cutNode && move != ss->killers[0]) + r += 2; - // Unless giving check, this capture is likely bad - if ( !givesCheck - && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha) - r++; - } + // 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; - Depth d = std::clamp(newDepth - r, 1, newDepth); + // 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 = 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; } @@ -1252,6 +1255,7 @@ 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) @@ -1302,10 +1306,11 @@ 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 @@ -1327,16 +1332,19 @@ 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) @@ -1363,9 +1371,11 @@ 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); @@ -1373,7 +1383,7 @@ 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); @@ -1387,6 +1397,7 @@ moves_loop: // When in check, search starts from here 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), ss->ttPv, bestValue >= beta ? BOUND_LOWER : @@ -1401,10 +1412,11 @@ 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)); @@ -1412,7 +1424,7 @@ moves_loop: // When in check, search starts from here Move pv[MAX_PLY+1]; StateInfo st; - ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize); + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); TTEntry* tte; Key posKey; @@ -1430,7 +1442,6 @@ 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; @@ -1482,13 +1493,15 @@ 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) { + // 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); @@ -1508,7 +1521,7 @@ 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, @@ -1520,20 +1533,22 @@ 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 + // 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 - // moveCount pruning if (moveCount > 2) continue; @@ -1554,27 +1569,19 @@ moves_loop: // When in check, search starts from here // Do not search moves with negative SEE values if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY - && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move)) && !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)]; - // CounterMove based pruning + // Continuation history based pruning if ( !captureOrPromotion && bestValue > VALUE_TB_LOSS_IN_MAX_PLY && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold @@ -1583,7 +1590,7 @@ moves_loop: // When in check, search starts from here // 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); @@ -1617,6 +1624,7 @@ moves_loop: // When in check, search starts from here 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, @@ -1700,9 +1708,10 @@ 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; @@ -1710,14 +1719,16 @@ 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 early move that was not a TT move or main killer move in previous ply when it gets refuted + // 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]); @@ -1734,6 +1745,7 @@ 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)) @@ -1746,6 +1758,7 @@ 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]; @@ -1757,15 +1770,18 @@ 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 - 7); } @@ -1909,7 +1925,7 @@ 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::kCacheLineSize); + ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize); bool ttHit; @@ -1978,3 +1994,5 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) { m.tbRank = 0; } } + +} // namespace Stockfish