From 6a8e46d53efca6dd56a396b32f43bef9f95127d6 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Mon, 30 Mar 2009 08:54:09 +0100
Subject: [PATCH] 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 | 17 ++++++++++++++---
 src/tt.cpp     |  5 +++--
 src/tt.h       |  7 +++++--
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 434587a8..022f3a9d 100644
--- a/src/search.cpp
+++ b/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
diff --git a/src/tt.cpp b/src/tt.cpp
index 5c842db8..d7b3b7d5 100644
--- a/src/tt.cpp
+++ b/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;
 }
 
 
diff --git a/src/tt.h b/src/tt.h
index 76ecad81..a820d94a 100644
--- a/src/tt.h
+++ b/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[]);
-- 
2.39.2