Cache evaluation score in qsearch

Instead of just drop evaluation score after stand pat
logic save it in TT so to be reused if the same position
occurs again.

Note that we NEVER use the cached value apart to avoid an
evaluation call, in particulary we never return to caller
after a succesful tt hit.

To accomodate this a new value type VALUE_TYPE_EVAL has been
introduced so that ok_to_use_TT() always returns false.

With this patch we cut about 15% of total evaluation calls.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movepick.cpp

@@ -66,7 +66,7 @@ namespace {
 /// move ordering is at the current node.
 
 MovePicker::MovePicker(const Position& p, bool pv, Move ttm,
-                       const SearchStack& ss, Depth d, EvalInfo* ei) : pos(p) {
+                       const SearchStack& ss, Depth d) : pos(p) {
   pvNode = pv;
   ttMove = ttm;
   mateKiller = (ss.mateKiller == ttm)? MOVE_NONE : ss.mateKiller;
@@ -81,20 +81,15 @@ MovePicker::MovePicker(const Position& p, bool pv, Move ttm,
   // generating them. So avoid generating in case we know are zero.
   Color us = pos.side_to_move();
   Color them = opposite_color(us);
-  bool noCaptures =    ei
-                   && (ei->attackedBy[us][0] & pos.pieces_of_color(them)) == 0
-                   && !ei->mi->specialized_eval_exists()
-                   && (pos.ep_square() == SQ_NONE)
-                   && !pos.has_pawn_on_7th(us);
 
   if (p.is_check())
       phaseIndex = EvasionsPhaseIndex;
   else if (depth > Depth(0))
       phaseIndex = MainSearchPhaseIndex;
   else if (depth == Depth(0))
-      phaseIndex = (noCaptures ? QsearchNoCapturesPhaseIndex : QsearchWithChecksPhaseIndex);
+      phaseIndex = QsearchWithChecksPhaseIndex;
   else
-      phaseIndex = (noCaptures ? NoMovesPhaseIndex : QsearchWithoutChecksPhaseIndex);
+      phaseIndex = QsearchWithoutChecksPhaseIndex;
 
   dc = p.discovered_check_candidates(us);
   pinned = p.pinned_pieces(us);

src/movepick.h

@@ -63,7 +63,7 @@ public:
     PH_STOP
   };
 
-  MovePicker(const Position& p, bool pvnode, Move ttm, const SearchStack& ss, Depth d, EvalInfo* ei = NULL);
+  MovePicker(const Position& p, bool pvnode, Move ttm, const SearchStack& ss, Depth d);
   Move get_next_move();
   Move get_next_move(Lock &lock);
   int number_of_moves() const;

src/search.cpp

@@ -1434,18 +1434,38 @@ namespace {
         return VALUE_DRAW;
 
     // Transposition table lookup, only when not in PV
+    TTEntry* tte = NULL;
     bool pvNode = (beta - alpha != 1);
     if (!pvNode)
     {
-        const TTEntry* tte = TT.retrieve(pos);
+        tte = TT.retrieve(pos);
         if (tte && ok_to_use_TT(tte, depth, beta, ply))
+        {
+            assert(tte->type() != VALUE_TYPE_EVAL);
+
             return value_from_tt(tte->value(), ply);
         }
+    }
 
     // Evaluate the position statically
     EvalInfo ei;
+    Value staticValue;
     bool isCheck = pos.is_check();
-    Value staticValue = (isCheck ? -VALUE_INFINITE : evaluate(pos, ei, threadID));
+
+    if (isCheck)
+        staticValue = -VALUE_INFINITE;
+
+    else if (tte && tte->type() == VALUE_TYPE_EVAL)
+    {
+        // Use the cached evaluation score if possible
+        assert(tte->value() == evaluate(pos, ei, threadID));
+        assert(ei.futilityMargin == Value(0));
+
+        staticValue = tte->value();
+        ei.futilityMargin = Value(0); // manually initialize futilityMargin
+    }
+    else
+        staticValue = evaluate(pos, ei, threadID);
 
     if (ply == PLY_MAX - 1)
         return evaluate(pos, ei, threadID);
@@ -1460,6 +1480,11 @@ namespace {
         TT.store(pos, value_to_tt(bestValue, ply), depth, MOVE_NONE, VALUE_TYPE_EXACT);
         return bestValue;
     }
+    else if (!isCheck && !tte && ei.futilityMargin == 0)
+    {
+        // Store the score to avoid a future costly evaluation() call
+        TT.store(pos, value_to_tt(bestValue, ply), Depth(-127*OnePly), MOVE_NONE, VALUE_TYPE_EVAL);
+    }
 
     if (bestValue > alpha)
         alpha = bestValue;
@@ -1467,7 +1492,7 @@ namespace {
     // Initialize a MovePicker object for the current position, and prepare
     // to search the moves.  Because the depth is <= 0 here, only captures,
     // queen promotions and checks (only if depth == 0) will be generated.
-    MovePicker mp = MovePicker(pos, pvNode, MOVE_NONE, EmptySearchStack, depth, isCheck ? NULL : &ei);
+    MovePicker mp = MovePicker(pos, pvNode, MOVE_NONE, EmptySearchStack, depth);
     Move move;
     int moveCount = 0;
     Bitboard dcCandidates = mp.discovered_check_candidates();

src/tt.cpp

@@ -137,7 +137,7 @@ void TranspositionTable::store(const Position &pos, Value v, Depth d,
 /// transposition table. Returns a pointer to the TTEntry or NULL
 /// if position is not found.
 
-const TTEntry* TranspositionTable::retrieve(const Position &pos) const {
+TTEntry* TranspositionTable::retrieve(const Position &pos) const {
 
   TTEntry *tte = first_entry(pos);
 

src/tt.h

@@ -45,7 +45,7 @@ public:
   Depth depth() const { return Depth(depth_); }
   Move move() const { return Move(data & 0x7FFFF); }
   Value value() const { return Value(value_); }
-  ValueType type() const { return ValueType((data >> 20) & 3); }
+  ValueType type() const { return ValueType((data >> 20) & 7); }
   int generation() const { return (data >> 23); }
 
 private:
@@ -67,7 +67,7 @@ public:
   void set_size(unsigned mbSize);
   void clear();
   void store(const Position &pos, Value v, Depth d, Move m, ValueType type);
-  const TTEntry* retrieve(const Position &pos) const;
+  TTEntry* retrieve(const Position &pos) const;
   void new_search();
   void insert_pv(const Position &pos, Move pv[]);
   int full();

src/value.h

@@ -36,7 +36,8 @@ enum ValueType {
   VALUE_TYPE_NONE = 0,
   VALUE_TYPE_UPPER = 1,  // Upper bound
   VALUE_TYPE_LOWER = 2,  // Lower bound
-  VALUE_TYPE_EXACT = 3   // Exact score
+  VALUE_TYPE_EXACT = 3,  // Exact score
+  VALUE_TYPE_EVAL  = 4   // Evaluation cache
 };