Store distinct upper and lower bound scores

This is more complex than what I'd like but I
was unable to split in small chunks.

Here we add 2 slots to TTEntry (valueUpper and depthUpper)
so that sizeof(TTEntry) returns to the original 16 bytes
and we can pack exactly 4 entries in a 64 bytes cache line.

Now we save an upper bound score alongside a lower (exact)
score. The idea is to increase TT cut-offs rates becuase
there is now an higher probability for a node to use TT info.

This patch is highly experimental and probably needs further
steps as is hinted by an unrealistic bench number:

bench: 2022385

src/search.cpp

@@ -482,7 +482,7 @@ namespace {
     Key posKey;
     Move ttMove, move, excludedMove, bestMove, threatMove;
     Depth ext, newDepth;
-    Value bestValue, value, ttValue;
+    Value bestValue, value, ttValue, ttValueUpper;
     Value eval, nullValue, futilityValue;
     bool inCheck, givesCheck, pvMove, singularExtensionNode;
     bool captureOrPromotion, dangerous, doFullDepthSearch;
@@ -544,33 +544,45 @@ namespace {
     tte = TT.probe(posKey);
     ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
     ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
+    ttValueUpper = tte ? value_from_tt(tte->value_upper(), ss->ply) : VALUE_NONE;
 
     // At PV nodes we check for exact scores, while at non-PV nodes we check for
     // a fail high/low. Biggest advantage at probing at PV nodes is to have a
     // smooth experience in analysis mode. We don't probe at Root nodes otherwise
     // we should also update RootMoveList to avoid bogus output.
-    if (   !RootNode
+    if (!RootNode && tte)
-        && tte
-        && tte->depth() >= depth
-        && ttValue != VALUE_NONE // Only in case of TT access race
-        && (           PvNode ?  tte->type() == BOUND_EXACT
-            : ttValue >= beta ? (tte->type() & BOUND_LOWER)
-                              : (tte->type() & BOUND_UPPER)))
     {
-        TT.refresh(tte);
+        // Fail High
-        ss->currentMove = ttMove; // Can be MOVE_NONE
+        if (  (tte->type() & BOUND_LOWER)
-
+            && ttValue >= beta
-        if (    ttValue >= beta
+            && tte->depth() >= depth
-            &&  ttMove
+            && ttValue != VALUE_NONE) // Only in case of TT access race
+        {
+            // Update killers, we assume ttMove caused a cut-off
+            if (    ttMove
                 && !pos.is_capture_or_promotion(ttMove)
                 &&  ttMove != ss->killers[0])
             {
                 ss->killers[1] = ss->killers[0];
                 ss->killers[0] = ttMove;
             }
+            TT.refresh(tte);
+            ss->currentMove = ttMove; // Can be MOVE_NONE
             return ttValue;
         }
 
+        // Fail Low
+        if (  (tte->type() & BOUND_UPPER)
+            && ttValueUpper < beta
+            && tte->depth_upper() >= depth
+            && ttValueUpper != VALUE_NONE) // Only in case of TT access race
+        {
+            TT.refresh(tte);
+            ss->currentMove = ttMove; // Can be MOVE_NONE
+            return ttValueUpper;
+        }
+    }
+
     // Step 5. Evaluate the position statically and update parent's gain statistics
     if (inCheck)
         ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
@@ -1089,7 +1101,7 @@ split_point_start: // At split points actual search starts from here
     const TTEntry* tte;
     Key posKey;
     Move ttMove, move, bestMove;
-    Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
+    Value bestValue, value, ttValue, ttValueUpper, futilityValue, futilityBase, oldAlpha;
     bool givesCheck, enoughMaterial, evasionPrunable, fromNull;
     Depth ttDepth;
 
@@ -1111,23 +1123,36 @@ split_point_start: // At split points actual search starts from here
     tte = TT.probe(posKey);
     ttMove = tte ? tte->move() : MOVE_NONE;
     ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
+    ttValueUpper = tte ? value_from_tt(tte->value_upper(),ss->ply) : VALUE_NONE;
 
     // Decide whether or not to include checks, this fixes also the type of
     // TT entry depth that we are going to use. Note that in qsearch we use
     // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
     ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
                                                   : DEPTH_QS_NO_CHECKS;
-    if (   tte
+    if (tte)
+    {
+        // Fail High
+        if (  (tte->type() & BOUND_LOWER)
+            && ttValue >= beta
             && tte->depth() >= ttDepth
-        && ttValue != VALUE_NONE // Only in case of TT access race
+            && ttValue != VALUE_NONE) // Only in case of TT access race
-        && (           PvNode ?  tte->type() == BOUND_EXACT
-            : ttValue >= beta ? (tte->type() & BOUND_LOWER)
-                              : (tte->type() & BOUND_UPPER)))
         {
             ss->currentMove = ttMove; // Can be MOVE_NONE
             return ttValue;
         }
 
+        // Fail Low
+        if (  (tte->type() & BOUND_UPPER)
+            && ttValueUpper < beta
+            && tte->depth_upper() >= ttDepth
+            && ttValueUpper != VALUE_NONE) // Only in case of TT access race
+        {
+            ss->currentMove = ttMove; // Can be MOVE_NONE
+            return ttValueUpper;
+        }
+    }
+
     // Evaluate the position statically
     if (InCheck)
     {

src/tt.cpp

@@ -82,7 +82,7 @@ void TranspositionTable::clear() {
 /// 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 posKey, Value v, Bound t, Depth d, Move m) {
+void TranspositionTable::store(const Key posKey, Value v, Bound b, Depth d, Move m) {
 
   int c1, c2, c3;
   TTEntry *tte, *replace;
@@ -92,13 +92,16 @@ void TranspositionTable::store(const Key posKey, Value v, Bound t, Depth d, Move
 
   for (int i = 0; i < ClusterSize; i++, tte++)
   {
-      if (!tte->key() || tte->key() == posKey32) // Empty or overwrite old
+      if (!tte->key())
+          tte->save(posKey32, v, b, d, m, generation);
+
+      if (tte->key() == posKey32)
       {
           // Preserve any existing ttMove
           if (m == MOVE_NONE)
               m = tte->move();
 
-          tte->save(posKey32, v, t, d, m, generation);
+          tte->update(v, b, d, m, generation);
           return;
       }
 
@@ -110,7 +113,7 @@ void TranspositionTable::store(const Key posKey, Value v, Bound t, Depth d, Move
       if (c1 + c2 + c3 > 0)
           replace = tte;
   }
-  replace->save(posKey32, v, t, d, m, generation);
+  replace->save(posKey32, v, b, d, m, generation);
 }
 
 

src/tt.h

@@ -50,15 +50,39 @@ public:
     move16      = (uint16_t)m;
     bound       = (uint8_t)b;
     generation8 = (uint8_t)g;
-    value16      = (int16_t)v;
+    valueUpper = (int16_t)(b & BOUND_UPPER ? v : VALUE_NONE);
-    depth16      = (int16_t)d;
+    depthUpper = (int16_t)(b & BOUND_UPPER ? d : DEPTH_NONE);
+    valueLower = (int16_t)(b & BOUND_LOWER ? v : VALUE_NONE);
+    depthLower = (int16_t)(b & BOUND_LOWER ? d : DEPTH_NONE);
   }
+
+  void update(Value v, Bound b, Depth d, Move m, int g) {
+
+    move16      = (uint16_t)m;
+    bound      |= (uint8_t)b;
+    generation8 = (uint8_t)g;
+
+    if (b & BOUND_UPPER)
+    {
+        valueUpper = (int16_t)v;
+        depthUpper = (int16_t)d;
+    }
+
+    if (b & BOUND_LOWER)
+    {
+        valueLower = (int16_t)v;
+        depthLower = (int16_t)d;
+    }
+  }
+
   void set_generation(int g) { generation8 = (uint8_t)g; }
 
   uint32_t key() const              { return key32; }
-  Depth depth() const               { return (Depth)depth16; }
+  Depth depth() const               { return (Depth)depthLower; }
+  Depth depth_upper() const         { return (Depth)depthUpper; }
   Move move() const                 { return (Move)move16; }
-  Value value() const               { return (Value)value16; }
+  Value value() const               { return (Value)valueLower; }
+  Value value_upper() const         { return (Value)valueUpper; }
   Bound type() const                { return (Bound)bound; }
   int generation() const            { return (int)generation8; }
 
@@ -66,7 +90,7 @@ private:
   uint32_t key32;
   uint16_t move16;
   uint8_t bound, generation8;
-  int16_t value16, depth16;
+  int16_t valueLower, depthLower, valueUpper, depthUpper;
 };
 
 
@@ -96,7 +120,7 @@ public:
   ~TranspositionTable();
   void set_size(size_t mbSize);
   void clear();
-  void store(const Key posKey, Value v, Bound type, Depth d, Move m);
+  void store(const Key posKey, Value v, Bound b, Depth d, Move m);
   TTEntry* probe(const Key posKey) const;
   void new_search();
   TTEntry* first_entry(const Key posKey) const;

src/types.h

@@ -163,7 +163,7 @@ enum Bound {
   BOUND_NONE  = 0,
   BOUND_UPPER = 1,
   BOUND_LOWER = 2,
-  BOUND_EXACT = BOUND_UPPER | BOUND_LOWER
+  BOUND_EXACT = BOUND_UPPER | BOUND_LOWER | 4
 };
 
 enum Value {