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Flag critical search tree in hash table

Introducing new concept, saving principal lines into the transposition table
to generate a "critical search tree" which we can reuse later for intelligent
pruning/extension decisions.

For instance in this patch we just reduce reduction for these lines. But a lot
of other ideas are possible.

To go further : tune some parameters, how to add or remove lines from the
critical search tree, how to use these lines in search choices, etc.

STC :
LLR: 2.94 (-2.94,2.94) [0.50,4.50]
Total: 59761 W: 13321 L: 12863 D: 33577 +2.23 ELO
http://tests.stockfishchess.org/tests/view/5c34da5d0ebc596a450c53d3

LTC :
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 26826 W: 4439 L: 4191 D: 18196 +2.9 ELO
http://tests.stockfishchess.org/tests/view/5c35ceb00ebc596a450c65b2

Special thanks to Miguel Lahoz for his help in transposition table in/out.

Bench: 3399866
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MJZ1977 authored and snicolet committed Jan 9, 2019
1 parent f69106f commit 70880b8e247c94d0a16a2fb50b41827726e00742
Showing with 31 additions and 16 deletions.
  1. +19 −7 src/search.cpp
  2. +6 −6 src/tt.cpp
  3. +5 −3 src/tt.h
  4. +1 −0 tests/instrumented.sh
@@ -577,7 +577,7 @@ namespace {
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
bool ttHit, inCheck, givesCheck, improving;
bool ttHit, pvHit, inCheck, givesCheck, improving;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
Piece movedPiece;
int moveCount, captureCount, quietCount;
@@ -643,6 +643,7 @@ namespace {
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
: ttHit ? tte->move() : MOVE_NONE;
pvHit = ttHit ? tte->pv_hit() : false;

// At non-PV nodes we check for an early TT cutoff
if ( !PvNode
@@ -676,6 +677,11 @@ namespace {
return ttValue;
}

if ( depth > 6 * ONE_PLY
&& !excludedMove
&& PvNode)
pvHit = true;

// Step 5. Tablebases probe
if (!rootNode && TB::Cardinality)
{
@@ -709,7 +715,7 @@ namespace {
if ( b == BOUND_EXACT
|| (b == BOUND_LOWER ? value >= beta : value <= alpha))
{
tte->save(posKey, value_to_tt(value, ss->ply), b,
tte->save(posKey, value_to_tt(value, ss->ply), pvHit, b,
std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
MOVE_NONE, VALUE_NONE);

@@ -760,7 +766,7 @@ namespace {
else
ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;

tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
tte->save(posKey, VALUE_NONE, pvHit, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
}

// Step 7. Razoring (~2 Elo)
@@ -875,6 +881,7 @@ namespace {
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = ttHit ? tte->move() : MOVE_NONE;
pvHit = ttHit ? tte->pv_hit() : false;
}

moves_loop: // When in check, search starts from here
@@ -1035,6 +1042,10 @@ namespace {
{
Depth r = reduction<PvNode>(improving, depth, moveCount);

// Decrease reduction if position is or has been on the PV
if (pvHit && !PvNode)
r -= ONE_PLY;

// Decrease reduction if opponent's move count is high (~10 Elo)
if ((ss-1)->moveCount > 15)
r -= ONE_PLY;
@@ -1217,7 +1228,7 @@ namespace {
bestValue = std::min(bestValue, maxValue);

if (!excludedMove)
tte->save(posKey, value_to_tt(bestValue, ss->ply),
tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
bestValue >= beta ? BOUND_LOWER :
PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
depth, bestMove, pureStaticEval);
@@ -1247,7 +1258,7 @@ namespace {
Move ttMove, move, bestMove;
Depth ttDepth;
Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
bool ttHit, inCheck, givesCheck, evasionPrunable;
bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
int moveCount;

if (PvNode)
@@ -1281,6 +1292,7 @@ namespace {
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = ttHit ? tte->move() : MOVE_NONE;
pvHit = ttHit ? tte->pv_hit() : false;

if ( !PvNode
&& ttHit
@@ -1318,7 +1330,7 @@ namespace {
if (bestValue >= beta)
{
if (!ttHit)
tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
DEPTH_NONE, MOVE_NONE, ss->staticEval);

return bestValue;
@@ -1429,7 +1441,7 @@ namespace {
if (inCheck && bestValue == -VALUE_INFINITE)
return mated_in(ss->ply); // Plies to mate from the root

tte->save(posKey, value_to_tt(bestValue, ss->ply),
tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
bestValue >= beta ? BOUND_LOWER :
PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
ttDepth, bestMove, ss->staticEval);
@@ -31,7 +31,7 @@
TranspositionTable TT; // Our global transposition table

/// TTEntry::save saves a TTEntry
void TTEntry::save(Key k, Value v, Bound b, Depth d, Move m, Value ev) {
void TTEntry::save(Key k, Value v, bool PvNode, Bound b, Depth d, Move m, Value ev) {

assert(d / ONE_PLY * ONE_PLY == d);

@@ -47,7 +47,7 @@ void TTEntry::save(Key k, Value v, Bound b, Depth d, Move m, Value ev) {
key16 = (uint16_t)(k >> 48);
value16 = (int16_t)v;
eval16 = (int16_t)ev;
genBound8 = (uint8_t)(TT.generation8 | b);
genBound8 = (uint8_t)(TT.generation8 | PvNode << 2 | b);
depth8 = (int8_t)(d / ONE_PLY);
}
}
@@ -122,7 +122,7 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
for (int i = 0; i < ClusterSize; ++i)
if (!tte[i].key16 || tte[i].key16 == key16)
{
tte[i].genBound8 = uint8_t(generation8 | tte[i].bound()); // Refresh
tte[i].genBound8 = uint8_t(generation8 | tte[i].pv_hit() << 2 | tte[i].bound()); // Refresh

return found = (bool)tte[i].key16, &tte[i];
}
@@ -131,11 +131,11 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
TTEntry* replace = tte;
for (int i = 1; i < ClusterSize; ++i)
// Due to our packed storage format for generation and its cyclic
// nature we add 259 (256 is the modulus plus 3 to keep the lowest
// nature we add 263 (263 is the modulus plus 7 to keep the lowest
// two bound bits from affecting the result) to calculate the entry
// age correctly even after generation8 overflows into the next cycle.
if ( replace->depth8 - ((259 + generation8 - replace->genBound8) & 0xFC) * 2
> tte[i].depth8 - ((259 + generation8 - tte[i].genBound8) & 0xFC) * 2)
if ( replace->depth8 - ((263 + generation8 - replace->genBound8) & 0xF8)
> tte[i].depth8 - ((263 + generation8 - tte[i].genBound8) & 0xF8))
replace = &tte[i];

return found = false, replace;
@@ -30,7 +30,8 @@
/// move 16 bit
/// value 16 bit
/// eval value 16 bit
/// generation 6 bit
/// generation 5 bit
/// PvNode 1 bit
/// bound type 2 bit
/// depth 8 bit

@@ -40,8 +41,9 @@ struct TTEntry {
Value value() const { return (Value)value16; }
Value eval() const { return (Value)eval16; }
Depth depth() const { return (Depth)(depth8 * int(ONE_PLY)); }
bool pv_hit() const { return (bool)(genBound8 & 0x4); }
Bound bound() const { return (Bound)(genBound8 & 0x3); }
void save(Key k, Value v, Bound b, Depth d, Move m, Value ev);
void save(Key k, Value v, bool PvNode, Bound b, Depth d, Move m, Value ev);

private:
friend class TranspositionTable;
@@ -76,7 +78,7 @@ class TranspositionTable {

public:
~TranspositionTable() { free(mem); }
void new_search() { generation8 += 4; } // Lower 2 bits are used by Bound
void new_search() { generation8 += 8; } // Lower 3 bits are used by PV flag and Bound
TTEntry* probe(const Key key, bool& found) const;
int hashfull() const;
void resize(size_t mbSize);
@@ -45,6 +45,7 @@ race:TTEntry::bound
race:TTEntry::save
race:TTEntry::value
race:TTEntry::eval
race:TTEntry::pv_hit
race:TranspositionTable::probe
race:TranspositionTable::hashfull

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