Store only unchanged static evaluations in TT

A recent commit introduced a decrease of the static evaluation of
an inner node dependent on the previous stat score, which finally
was also stored in the transposition table. Now only the unchanged
static evaluation are stored there.

Remark:
For the case that a static evaluation can be retrieved from the
transposition table the value is now used unchanged. Another test
which also applies the modification in this case failed:
http://tests.stockfishchess.org/tests/view/5b7af6df0ebc5902bdbae2f6

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 6707 W: 1547 L: 1383 D: 3777
http://tests.stockfishchess.org/tests/view/5b7a92df0ebc5902bdbadcf3

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 36203 W: 6046 L: 5781 D: 24376
http://tests.stockfishchess.org/tests/view/5b7abaa10ebc5902bdbadfa9

Closes https://github.com/official-stockfish/Stockfish/pull/1742

Bench: 4457440

diff --git a/src/search.cpp b/src/search.cpp
index 3d9c718b9df304c77c37885af263adb9f9b95727..bda4c4c508afa206d38d8ab6d6862a5b7be02bdb 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -556,7 +556,7 @@ namespace {
     Key posKey;
     Move ttMove, move, excludedMove, bestMove;
     Depth extension, newDepth;
     Key posKey;
     Move ttMove, move, excludedMove, bestMove;
     Depth extension, newDepth;

-    Value bestValue, value, ttValue, eval, maxValue;
+    Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;

     bool ttHit, inCheck, givesCheck, improving;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
     Piece movedPiece;
     bool ttHit, inCheck, givesCheck, improving;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
     Piece movedPiece;


@@ -705,15 +705,15 @@ namespace {
     // Step 6. Static evaluation of the position
     if (inCheck)
     {
     // Step 6. Static evaluation of the position
     if (inCheck)
     {

-        ss->staticEval = eval = VALUE_NONE;
+        ss->staticEval = pureStaticEval = eval = VALUE_NONE;

         improving = false;
         goto moves_loop;  // Skip early pruning when in check
     }
     else if (ttHit)
     {
         // Never assume anything on values stored in TT
         improving = false;
         goto moves_loop;  // Skip early pruning when in check
     }
     else if (ttHit)
     {
         // Never assume anything on values stored in TT

-        if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
-            eval = ss->staticEval = evaluate(pos) - 10 * ((ss-1)->statScore > 0);
+        if ((ss->staticEval = pureStaticEval = eval = tte->eval()) == VALUE_NONE)
+            eval = ss->staticEval = (pureStaticEval = evaluate(pos)) - 10 * ((ss-1)->statScore > 0);

 
         // Can ttValue be used as a better position evaluation?
         if (    ttValue != VALUE_NONE
 
         // Can ttValue be used as a better position evaluation?
         if (    ttValue != VALUE_NONE


@@ -726,10 +726,10 @@ namespace {
         int malus = p > 0 ? (p + 5000) / 1024 :
                     p < 0 ? (p - 5000) / 1024 : 0;
 
         int malus = p > 0 ? (p + 5000) / 1024 :
                     p < 0 ? (p - 5000) / 1024 : 0;
 

-        ss->staticEval = eval = (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) - malus
-                                                                 : -(ss-1)->staticEval + 2 * Eval::Tempo;
+        ss->staticEval = eval = (ss-1)->currentMove != MOVE_NULL ? (pureStaticEval = evaluate(pos)) - malus
+                                                                 : (pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo);

 
 

-        tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
+        tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);

     }
 
     // Step 7. Razoring (~2 Elo)
     }
 
     // Step 7. Razoring (~2 Elo)


@@ -1179,7 +1179,7 @@ moves_loop: // When in check, search starts from here
         tte->save(posKey, value_to_tt(bestValue, ss->ply),
                   bestValue >= beta ? BOUND_LOWER :
                   PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
         tte->save(posKey, value_to_tt(bestValue, ss->ply),
                   bestValue >= beta ? BOUND_LOWER :
                   PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,

-                  depth, bestMove, ss->staticEval);
+                  depth, bestMove, pureStaticEval);

 
     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
 
 
     assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);