Remember when TT value equals static evaluation value

When the stored TT value equals the static value set a
proper flag so to not call evaluation() if we hit the
same position again but use the stored TT value instead.

This is another trick to avoid calling costly evaluation()
in qsearch.

No functional change.

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

src/search.cpp

@@ -1455,7 +1455,7 @@ namespace {
     if (isCheck)
         staticValue = -VALUE_INFINITE;
 
-    else if (tte && tte->type() == VALUE_TYPE_EVAL)
+    else if (tte && (tte->type() == VALUE_TYPE_EVAL || tte->staticValue()))
     {
         // Use the cached evaluation score if possible
         assert(tte->value() == evaluate(pos, ei, threadID));
@@ -1569,10 +1569,21 @@ namespace {
     if (!pvNode)
     {
         Depth d = (depth == Depth(0) ? Depth(0) : Depth(-1));
+        Value v = value_to_tt(bestValue, ply);
+        TTEntry* e;
         if (bestValue < beta)
-            TT.store(pos, value_to_tt(bestValue, ply), d, MOVE_NONE, VALUE_TYPE_UPPER);
+            e = TT.store(pos, v, d, MOVE_NONE, VALUE_TYPE_UPPER);
         else
-            TT.store(pos, value_to_tt(bestValue, ply), d, MOVE_NONE, VALUE_TYPE_LOWER);
+            e = TT.store(pos, v, d, MOVE_NONE, VALUE_TYPE_LOWER);
+
+        assert(e && e == TT.retrieve(pos));
+        assert(!e->staticValue());
+
+        // If the just stored value happens to be equal to the static evaluation
+        // score then set the flag, so to avoid calling evaluation() next time we
+        // hit this position.
+        if (staticValue == v && !ei.futilityMargin)
+            e->setStaticValue();
     }
 
     // Update killers only for good check moves

src/tt.cpp

@@ -103,7 +103,7 @@ void TranspositionTable::clear() {
 /// 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 Position &pos, Value v, Depth d,
+TTEntry* TranspositionTable::store(const Position &pos, Value v, Depth d,
                                Move m, ValueType type) {
   TTEntry *tte, *replace;
 
@@ -116,7 +116,7 @@ void TranspositionTable::store(const Position &pos, Value v, Depth d,
             m = tte->move();
 
         *tte = TTEntry(pos.get_key(), v, type, d, m, generation);
-        return;
+        return tte;
     }
     else if (i == 0)  // replace would be a no-op in this common case
         continue;
@@ -130,6 +130,7 @@ void TranspositionTable::store(const Position &pos, Value v, Depth d,
   }
   *replace = TTEntry(pos.get_key(), v, type, d, m, generation);
   writes++;
+  return replace;
 }
 
 

src/tt.h

@@ -46,7 +46,8 @@
 /// the 32 bits of the data field are so defined
 ///
 /// bit  0-16: move
-/// bit 17-19: not used
+/// bit    17: stored value equals static value
+/// bit 18-19: not used
 /// bit 20-22: value type
 /// bit 23-31: generation
 
@@ -61,6 +62,8 @@ public:
   Value value() const { return Value(value_); }
   ValueType type() const { return ValueType((data >> 20) & 7); }
   int generation() const { return (data >> 23); }
+  bool staticValue() const { return ((data >> 17) & 1); }
+  void setStaticValue() { data |= (1 << 17); }
 
 private:
   Key key_;
@@ -80,7 +83,7 @@ public:
   ~TranspositionTable();
   void set_size(unsigned mbSize);
   void clear();
-  void store(const Position &pos, Value v, Depth d, Move m, ValueType type);
+  TTEntry* store(const Position &pos, Value v, Depth d, Move m, ValueType type);
   TTEntry* retrieve(const Position &pos) const;
   void new_search();
   void insert_pv(const Position &pos, Move pv[]);