Add eval cache infrastructure

With this patch series we want to introduce a per-thread
evaluation cache to store node evaluation and do not
rely anymore on the TT table for this.

This patch just introduces the infrastructure.

No functional change.

src/evaluate.cpp

@@ -244,7 +244,7 @@ namespace {
   Score evaluate_pieces_of_color(const Position& pos, EvalInfo& ei, Score& mobility);
 
   template<Color Us, bool Trace>
-  Score evaluate_king(const Position& pos, EvalInfo& ei, Value margins[]);
+  Score evaluate_king(const Position& pos, EvalInfo& ei, int16_t margins[]);
 
   template<Color Us>
   Score evaluate_threats(const Position& pos, EvalInfo& ei);
@@ -364,12 +364,26 @@ Value do_evaluate(const Position& pos, Value& margin) {
   assert(!pos.in_check());
 
   EvalInfo ei;
-  Value margins[COLOR_NB];
   Score score, mobilityWhite, mobilityBlack;
 
+  Key key = pos.key();
+  Eval::Entry* e = pos.this_thread()->evalTable[key];
+
+  // If e->key matches the position's hash key, it means that we have analysed
+  // this node before, and we can simply return the information we found the last
+  // time instead of recomputing it.
+  if (e->key == key)
+  {
+      margin = Value(e->margins[pos.side_to_move()]);
+      return e->value;
+  }
+
+  // Otherwise we overwrite current content with this node info.
+  e->key = key;
+
   // margins[] store the uncertainty estimation of position's evaluation
   // that typically is used by the search for pruning decisions.
-  margins[WHITE] = margins[BLACK] = VALUE_ZERO;
+  e->margins[WHITE] = e->margins[BLACK] = VALUE_ZERO;
 
   // Initialize score by reading the incrementally updated scores included
   // in the position object (material + piece square tables) and adding
@@ -385,7 +399,8 @@ Value do_evaluate(const Position& pos, Value& margin) {
   if (ei.mi->specialized_eval_exists())
   {
       margin = VALUE_ZERO;
-      return ei.mi->evaluate(pos);
+      e->value = ei.mi->evaluate(pos);
+      return e->value;
   }
 
   // Probe the pawn hash table
@@ -404,8 +419,8 @@ Value do_evaluate(const Position& pos, Value& margin) {
 
   // Evaluate kings after all other pieces because we need complete attack
   // information when computing the king safety evaluation.
-  score +=  evaluate_king<WHITE, Trace>(pos, ei, margins)
-          - evaluate_king<BLACK, Trace>(pos, ei, margins);
+  score +=  evaluate_king<WHITE, Trace>(pos, ei, e->margins)
+          - evaluate_king<BLACK, Trace>(pos, ei, e->margins);
 
   // Evaluate tactical threats, we need full attack information including king
   score +=  evaluate_threats<WHITE>(pos, ei)
@@ -451,7 +466,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
            sf = ScaleFactor(50);
   }
 
-  margin = margins[pos.side_to_move()];
+  margin = Value(e->margins[pos.side_to_move()]);
   Value v = interpolate(score, ei.mi->game_phase(), sf);
 
   // In case of tracing add all single evaluation contributions for both white and black
@@ -468,8 +483,8 @@ Value do_evaluate(const Position& pos, Value& margin) {
       Score b = make_score(ei.mi->space_weight() * evaluate_space<BLACK>(pos, ei), 0);
       trace_add(SPACE, apply_weight(w, Weights[Space]), apply_weight(b, Weights[Space]));
       trace_add(TOTAL, score);
-      TraceStream << "\nUncertainty margin: White: " << to_cp(margins[WHITE])
-                  << ", Black: " << to_cp(margins[BLACK])
+      TraceStream << "\nUncertainty margin: White: " << to_cp(Value(e->margins[WHITE]))
+                  << ", Black: " << to_cp(Value(e->margins[BLACK]))
                   << "\nScaling: " << std::noshowpos
                   << std::setw(6) << 100.0 * ei.mi->game_phase() / 128.0 << "% MG, "
                   << std::setw(6) << 100.0 * (1.0 - ei.mi->game_phase() / 128.0) << "% * "
@@ -477,7 +492,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
                   << "Total evaluation: " << to_cp(v);
   }
 
-  return pos.side_to_move() == WHITE ? v : -v;
+  return e->value = pos.side_to_move() == WHITE ? v : -v;
 }
 
 
@@ -752,7 +767,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
   // evaluate_king<>() assigns bonuses and penalties to a king of a given color
 
   template<Color Us, bool Trace>
-  Score evaluate_king(const Position& pos, EvalInfo& ei, Value margins[]) {
+  Score evaluate_king(const Position& pos, EvalInfo& ei, int16_t margins[]) {
 
     const Color Them = (Us == WHITE ? BLACK : WHITE);
 
@@ -852,7 +867,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
         // be very big, and so capturing a single attacking piece can therefore
         // result in a score change far bigger than the value of the captured piece.
         score -= KingDangerTable[Us == Search::RootColor][attackUnits];
-        margins[Us] += mg_value(KingDangerTable[Us == Search::RootColor][attackUnits]);
+        margins[Us] += int16_t(mg_value(KingDangerTable[Us == Search::RootColor][attackUnits]));
     }
 
     if (Trace)

src/evaluate.h

@@ -20,6 +20,7 @@
 #if !defined(EVALUATE_H_INCLUDED)
 #define EVALUATE_H_INCLUDED
 
+#include "misc.h"
 #include "types.h"
 
 class Position;
@@ -30,6 +31,16 @@ extern void init();
 extern Value evaluate(const Position& pos, Value& margin);
 extern std::string trace(const Position& pos);
 
+const int TableSize = 262144;
+
+struct Entry {
+  Key key;
+  Value value;
+  int16_t margins[2];
+};
+
+struct Table : HashTable<Entry, TableSize> {};
+
 }
 
 #endif // !defined(EVALUATE_H_INCLUDED)

src/thread.h

@@ -22,6 +22,7 @@
 
 #include <vector>
 
+#include "evaluate.h"
 #include "material.h"
 #include "movepick.h"
 #include "pawns.h"
@@ -108,6 +109,7 @@ public:
   void wait_for_stop_or_ponderhit();
 
   SplitPoint splitPoints[MAX_SPLITPOINTS_PER_THREAD];
+  Eval::Table evalTable;
   MaterialTable materialTable;
   PawnTable pawnTable;
   size_t idx;