Avoid searching TT twice for the same key/position during probe() and store().

Just keep the pointer and remove code from tt.cpp

STC
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 13620 W: 2810 L: 2665 D: 8145

LTC
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 13021 W: 2238 L: 2073 D: 8710STC http://tests.stockfishchess.org/tests/view/548436860ebc59331739b90c

STC 4MB
ELO: 2.41 +-2.2 (95%) LOS: 98.6%
Total: 40000 W: 8175 L: 7897 D: 23928

LTC 16MB
ELO: 1.78 +-2.0 (95%) LOS: 96.1%
Total: 39683 W: 6763 L: 6560 D: 26360

Resolves #151

Bench: 8116521

src/search.cpp

@@ -457,7 +457,8 @@ namespace {
 
     Move pv[MAX_PLY+1], quietsSearched[64];
     StateInfo st;
-    const TTEntry *tte;
+    TTEntry* tte;
+    bool ttHit;
     SplitPoint* splitPoint;
     Key posKey;
     Move ttMove, move, excludedMove, bestMove;
@@ -477,6 +478,7 @@ namespace {
         bestMove   = splitPoint->bestMove;
         bestValue  = splitPoint->bestValue;
         tte = NULL;
+        ttHit = false;
         ttMove = excludedMove = MOVE_NONE;
         ttValue = VALUE_NONE;
 
@@ -522,13 +524,13 @@ namespace {
     // TT value, so we use a different position key in case of an excluded move.
     excludedMove = ss->excludedMove;
     posKey = excludedMove ? pos.exclusion_key() : pos.key();
-    tte = TT.probe(posKey);
+    tte = TT.probe(posKey, ttHit);
-    ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
+    ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
-    ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
+    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
 
     // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
     if (  !PvNode
-        && tte
+        && ttHit
         && tte->depth() >= depth
         && ttValue != VALUE_NONE // Only in case of TT access race
         && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
@@ -564,9 +566,9 @@ namespace {
                        : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
                                         :  VALUE_DRAW + 2 * v * drawScore;
 
-                TT.store(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
+                tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
                           std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
-                         MOVE_NONE, VALUE_NONE);
+                          MOVE_NONE, VALUE_NONE, TT.get_generation());
 
                 return value;
             }
@@ -580,7 +582,7 @@ namespace {
         goto moves_loop;
     }
 
-    else if (tte)
+    else if (ttHit)
     {
         // Never assume anything on values stored in TT
         if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
@@ -596,7 +598,7 @@ namespace {
         eval = ss->staticEval =
         (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
 
-        TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
+        tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.get_generation());
     }
 
     if (ss->skipEarlyPruning)
@@ -718,8 +720,8 @@ namespace {
         search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
         ss->skipEarlyPruning = false;
 
-        tte = TT.probe(posKey);
+        tte = TT.probe(posKey, ttHit);
-        ttMove = tte ? tte->move() : MOVE_NONE;
+        ttMove = ttHit ? tte->move() : MOVE_NONE;
     }
 
 moves_loop: // When in check and at SpNode search starts from here
@@ -1079,10 +1081,10 @@ moves_loop: // When in check and at SpNode search starts from here
     else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
         update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
 
-    TT.store(posKey, value_to_tt(bestValue, ss->ply),
+    tte->save(posKey, value_to_tt(bestValue, ss->ply),
               bestValue >= beta ? BOUND_LOWER :
               PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
-             depth, bestMove, ss->staticEval);
+              depth, bestMove, ss->staticEval, TT.get_generation());
 
     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 
@@ -1107,7 +1109,8 @@ moves_loop: // When in check and at SpNode search starts from here
 
     Move pv[MAX_PLY+1];
     StateInfo st;
-    const TTEntry* tte;
+    TTEntry* tte;
+    bool ttHit;
     Key posKey;
     Move ttMove, move, bestMove;
     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
@@ -1138,12 +1141,12 @@ moves_loop: // When in check and at SpNode search starts from here
 
     // Transposition table lookup
     posKey = pos.key();
-    tte = TT.probe(posKey);
+    tte = TT.probe(posKey, ttHit);
-    ttMove = tte ? tte->move() : MOVE_NONE;
+    ttMove = ttHit ? tte->move() : MOVE_NONE;
-    ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
+    ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
 
     if (  !PvNode
-        && tte
+        && ttHit
         && tte->depth() >= ttDepth
         && ttValue != VALUE_NONE // Only in case of TT access race
         && (ttValue >= beta ? (tte->bound() &  BOUND_LOWER)
@@ -1161,7 +1164,7 @@ moves_loop: // When in check and at SpNode search starts from here
     }
     else
     {
-        if (tte)
+        if (ttHit)
         {
             // Never assume anything on values stored in TT
             if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
@@ -1179,9 +1182,9 @@ moves_loop: // When in check and at SpNode search starts from here
         // Stand pat. Return immediately if static value is at least beta
         if (bestValue >= beta)
         {
-            if (!tte)
+            if (!ttHit)
-                TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
+                tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
-                         DEPTH_NONE, MOVE_NONE, ss->staticEval);
+                          DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.get_generation());
 
             return bestValue;
         }
@@ -1279,8 +1282,8 @@ moves_loop: // When in check and at SpNode search starts from here
               }
               else // Fail high
               {
-                  TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
+                  tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
-                           ttDepth, move, ss->staticEval);
+                            ttDepth, move, ss->staticEval, TT.get_generation());
 
                   return value;
               }
@@ -1293,9 +1296,9 @@ moves_loop: // When in check and at SpNode search starts from here
     if (InCheck && bestValue == -VALUE_INFINITE)
         return mated_in(ss->ply); // Plies to mate from the root
 
-    TT.store(posKey, value_to_tt(bestValue, ss->ply),
+    tte->save(posKey, value_to_tt(bestValue, ss->ply),
               PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
-             ttDepth, bestMove, ss->staticEval);
+              ttDepth, bestMove, ss->staticEval, TT.get_generation());
 
     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 
@@ -1469,15 +1472,15 @@ moves_loop: // When in check and at SpNode search starts from here
 void RootMove::insert_pv_in_tt(Position& pos) {
 
   StateInfo state[MAX_PLY], *st = state;
-  const TTEntry* tte;
   size_t idx = 0;
 
   for ( ; idx < pv.size(); ++idx)
   {
-      tte = TT.probe(pos.key());
+      bool ttHit;
+      TTEntry* tte = TT.probe(pos.key(), ttHit);
 
-      if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
+      if (!ttHit || tte->move() != pv[idx]) // Don't overwrite correct entries
-          TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
+          tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE, TT.get_generation());
 
       assert(MoveList<LEGAL>(pos).contains(pv[idx]));
 

src/tt.cpp

@@ -54,7 +54,7 @@ void TranspositionTable::resize(size_t mbSize) {
 
 
 /// TranspositionTable::clear() overwrites the entire transposition table
-/// with zeroes. It is called whenever the table is resized, or when the
+/// with zeros. It is called whenever the table is resized, or when the
 /// user asks the program to clear the table (from the UCI interface).
 
 void TranspositionTable::clear() {
@@ -64,47 +64,28 @@ void TranspositionTable::clear() {
 
 
 /// TranspositionTable::probe() looks up the current position in the
-/// transposition table. Returns a pointer to the TTEntry or NULL if
+/// transposition table. It returns true and a pointer to the TTEntry if
-/// position is not found.
+/// the position is found. Otherwise, it returns false and a pointer to an empty or
-
+/// least valuable TTEntry to be replaced later. A TTEntry t1 is considered
-const TTEntry* TranspositionTable::probe(const Key key) const {
-
-  TTEntry* const tte = first_entry(key);
-  const uint16_t key16 = key >> 48;
-
-  for (unsigned i = 0; i < TTClusterSize; ++i)
-      if (tte[i].key16 == key16)
-      {
-          tte[i].genBound8 = uint8_t(generation | tte[i].bound()); // Refresh
-          return &tte[i];
-      }
-
-  return NULL;
-}
-
-
-/// TranspositionTable::store() writes a new entry containing position key and
-/// valuable information of current position. The lowest order bits of position
-/// key are used to decide in which cluster the position will be placed.
-/// When a new entry is written and there are no empty entries available in the
-/// cluster, it replaces the least valuable of the entries. A TTEntry t1 is considered
 /// to be more valuable than a TTEntry t2 if t1 is from the current search and t2
 /// is from a previous search, or if the depth of t1 is bigger than the depth of t2.
 
-void TranspositionTable::store(const Key key, Value v, Bound b, Depth d, Move m, Value statV) {
+TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
 
   TTEntry* const tte = first_entry(key);
   const uint16_t key16 = key >> 48;  // Use the high 16 bits as key inside the cluster
 
   for (unsigned i = 0; i < TTClusterSize; ++i)
-      if (!tte[i].key16 || tte[i].key16 == key16) // Empty or overwrite old
+      if (!tte[i].key16 || tte[i].key16 == key16)
       {
-          // Save preserving any existing ttMove
+          if (tte[i].key16)
-          tte[i].save(key16, v, b, d, m ? m : tte[i].move(), generation, statV);
+              tte[i].genBound8 = uint8_t(generation | tte[i].bound()); // Refresh
-          return;
+
+          found = (bool)tte[i].key16;
+          return &tte[i];
       }
 
-  // Implement replace strategy
+  // Find an entry to be replaced according to the replacement strategy
   TTEntry* replace = tte;
   for (unsigned i = 1; i < TTClusterSize; ++i)
       if (  ((  tte[i].genBound8 & 0xFC) == generation || tte[i].bound() == BOUND_EXACT)
@@ -112,5 +93,6 @@ void TranspositionTable::store(const Key key, Value v, Bound b, Depth d, Move m,
           - (tte[i].depth8 < replace->depth8) < 0)
           replace = &tte[i];
 
-  replace->save(key16, v, b, d, m, generation, statV);
+  found = false;
+  return replace;
 }

src/tt.h

@@ -41,19 +41,21 @@ struct TTEntry {
   Depth depth() const      { return (Depth)depth8; }
   Bound bound() const      { return (Bound)(genBound8 & 0x3); }
 
-private:
+  void save(Key k, Value v, Bound b, Depth d, Move m, Value ev, uint8_t g) {
-  friend class TranspositionTable;
 
-  void save(uint16_t k, Value v, Bound b, Depth d, Move m, uint8_t g, Value ev) {
+    k >>= 48;
-
+    if (m || k != key16) // preserve any existing ttMove
-    key16     = (uint16_t)k;
         move16 = (uint16_t)m;
+    key16      = (uint16_t)k;
     value16    = (int16_t)v;
     evalValue  = (int16_t)ev;
     genBound8  = (uint8_t)(g | b);
     depth8     = (int8_t)d;
   }
 
+private:
+  friend class TranspositionTable;
+
   uint16_t key16;
   uint16_t move16;
   int16_t  value16;
@@ -67,7 +69,7 @@ private:
 /// 3 x TTEntry (3 x 10 bytes)
 /// padding     (2 bytes)
 
-const unsigned TTClusterSize = 3;
+static const unsigned TTClusterSize = 3;
 
 struct TTCluster {
   TTEntry entry[TTClusterSize];
@@ -85,12 +87,11 @@ class TranspositionTable {
 public:
  ~TranspositionTable() { free(mem); }
   void new_search() { generation += 4; } // Lower 2 bits are used by Bound
-
+  uint8_t get_generation() const { return generation; }
-  const TTEntry* probe(const Key key) const;
+  TTEntry* probe(const Key key, bool& found) const;
   TTEntry* first_entry(const Key key) const;
   void resize(size_t mbSize);
   void clear();
-  void store(const Key key, Value v, Bound type, Depth d, Move m, Value statV);
 
 private:
   size_t clusterCount;