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Simplify limiting extensions.

Browse source 
Replace the current method for limiting extensions to avoid search getting stuck
with a much simpler method.

the test position in 73018a0337
can still be searched without stuck search.

fixes #3815 where the search now makes progress with rootDepth

shows robust behavior in a d10 search for 1M positions.

passed STC
https://tests.stockfishchess.org/tests/view/61e303e3babab931824dfb18
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 57568 W: 15449 L: 15327 D: 26792
Ptnml(0-2): 243, 6211, 15779, 6283, 268

passed LTC
https://tests.stockfishchess.org/tests/view/61e3586cbabab931824e091c
LLR: 2.96 (-2.94,2.94) <-2.25,0.25>
Total: 128200 W: 34632 L: 34613 D: 58955
Ptnml(0-2): 124, 12559, 38710, 12588, 119

closes https://github.com/official-stockfish/Stockfish/pull/3899

Bench: 4550528
This commit is contained in:
J. Oster 2022-01-15 18:18:52 +01:00 • committed by Joost VandeVondele
parent 77cf5704b6
commit 9083050be6
2 changed files with 59 additions and 100 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

152
src/search.cpp
View file

@ -88,30 +88,6 @@ namespace {
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. If we have an uci_elo then
// we convert it to a suitable fractional skill level using anchoring to CCRL Elo
// (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
@ -327,16 +303,11 @@ void Thread::search() {
multiPV = std::min(multiPV, rootMoves.size());
doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0%
doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0%
complexityAverage.set(232, 1);
nodesLastExplosive = nodes;
nodesLastNormal = nodes;
state = EXPLOSION_NONE;
trend = SCORE_ZERO;
optimism[ us] = Value(25);
optimism[~us] = -optimism[us];
trend = SCORE_ZERO;
optimism[ us] = Value(25);
optimism[~us] = -optimism[us];
int searchAgainCounter = 0;
@ -548,14 +519,6 @@ namespace {
template <NodeType nodeType>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
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;
@ -596,6 +559,7 @@ namespace {
int moveCount, captureCount, quietCount, bestMoveCount, improvement, complexity;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
ss->inCheck = pos.checkers();
priorCapture = pos.captured_piece();
Color us = pos.side_to_move();
@ -643,9 +607,6 @@ namespace {
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
@ -1077,65 +1038,68 @@ moves_loop: // When in check, search starts here
}
// Step 14. Extensions (~66 Elo)
// Singular extension search (~58 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
// 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 ( !rootNode
&& depth >= 6 + 2 * (PvNode && tte->is_pv())
&& move == ttMove
&& !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)
// We take care to not overdo to avoid search getting stuck.
if (ss->ply < thisThread->rootDepth * 2)
{
Value singularBeta = ttValue - 3 * depth;
Depth singularDepth = (depth - 1) / 2;
ss->excludedMove = move;
value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
ss->excludedMove = MOVE_NONE;
if (value < singularBeta)
// Singular extension search (~58 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
// 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 ( !rootNode
&& depth >= 6 + 2 * (PvNode && tte->is_pv())
&& move == ttMove
&& !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)
{
extension = 1;
Value singularBeta = ttValue - 3 * depth;
Depth singularDepth = (depth - 1) / 2;
// Avoid search explosion by limiting the number of double extensions
if ( !PvNode
&& value < singularBeta - 75
&& ss->doubleExtensions <= 6)
extension = 2;
ss->excludedMove = move;
value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
ss->excludedMove = MOVE_NONE;
if (value < singularBeta)
{
extension = 1;
// Avoid search explosion by limiting the number of double extensions
if ( !PvNode
&& value < singularBeta - 75
&& ss->doubleExtensions <= 6)
extension = 2;
}
// Multi-cut pruning
// Our ttMove is assumed to fail high, and now we failed high also on a reduced
// search without the ttMove. So we assume this expected Cut-node is not singular,
// that multiple moves fail high, and we can prune the whole subtree by returning
// a soft bound.
else if (singularBeta >= beta)
return singularBeta;
// If the eval of ttMove is greater than beta, we reduce it (negative extension)
else if (ttValue >= beta)
extension = -2;
}
// Multi-cut pruning
// Our ttMove is assumed to fail high, and now we failed high also on a reduced
// search without the ttMove. So we assume this expected Cut-node is not singular,
// that multiple moves fail high, and we can prune the whole subtree by returning
// a soft bound.
else if (singularBeta >= beta)
return singularBeta;
// Check extensions (~1 Elo)
else if ( givesCheck
&& depth > 6
&& abs(ss->staticEval) > 100)
extension = 1;
// If the eval of ttMove is greater than beta, we reduce it (negative extension)
else if (ttValue >= beta)
extension = -2;
// Quiet ttMove extensions (~0 Elo)
else if ( PvNode
&& move == ttMove
&& move == ss->killers[0]
&& (*contHist[0])[movedPiece][to_sq(move)] >= 10000)
extension = 1;
}
// Check extensions (~1 Elo)
else if ( givesCheck
&& depth > 6
&& abs(ss->staticEval) > 100)
extension = 1;
// Quiet ttMove extensions (~0 Elo)
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);

7
src/thread.h
View file

@ -60,16 +60,11 @@ public:
Pawns::Table pawnsTable;
Material::Table materialTable;
size_t pvIdx, pvLast;
RunningAverage doubleExtensionAverage[COLOR_NB];
RunningAverage complexityAverage;
uint64_t nodesLastExplosive;
uint64_t nodesLastNormal;
std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;
Value bestValue;
int selDepth, nmpMinPly;
Color nmpColor;
ExplosionState state;
Value optimism[COLOR_NB];
Value bestValue, optimism[COLOR_NB];
Position rootPos;
StateInfo rootState;