Revert so called "fromNull patch"

Revert patch c581b7ea3627482

Seems a regression after testing from Gary:
ELO: 7.24 +- 99%: 17.03 95%: 12.93
LOS: 97.86%
Wins: 439 Losses: 381 Draws: 1962

And mine:
After 5410 games at 15"+0.05
Wins: 936 Losses: 1141 Draws: 3333  ELO -13

Moreover we know that there is a regression in the range
of patches which include the fromNull patch.

Probably this is not the only regression since 2.3.1 and
perhaps the idea under fromNull is good, but at the moment,
while in deep regression hunting, better to be on the safe
side and revert it entirely.

My guess on why this is a regression is that using the
negated evaluation of previous ply in case of null search
fails to take in account the king safety asymmetry between
the two colors. This is of course just a guess.

bench 5503830

diff --git a/src/search.cpp b/src/search.cpp
index 744d84f442e682808839cf5b1195b1555f821e2b..dc82d34d312b4d3966984f685fa447ef68ede848 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1109,7 +1109,7 @@ split_point_start: // At split points actual search starts from here
     Key posKey;
     Move ttMove, move, bestMove;
     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
     Key posKey;
     Move ttMove, move, bestMove;
     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;

-    bool givesCheck, enoughMaterial, evasionPrunable, fromNull;
+    bool givesCheck, enoughMaterial, evasionPrunable;

     Depth ttDepth;
 
     // To flag BOUND_EXACT a node with eval above alpha and no available moves
     Depth ttDepth;
 
     // To flag BOUND_EXACT a node with eval above alpha and no available moves


@@ -1118,7 +1118,6 @@ split_point_start: // At split points actual search starts from here
 
     ss->currentMove = bestMove = MOVE_NONE;
     ss->ply = (ss-1)->ply + 1;
 
     ss->currentMove = bestMove = MOVE_NONE;
     ss->ply = (ss-1)->ply + 1;

-    fromNull = (ss-1)->currentMove == MOVE_NULL;

 
     // Check for an instant draw or maximum ply reached
     if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)
 
     // Check for an instant draw or maximum ply reached
     if (pos.is_draw<false, false>() || ss->ply > MAX_PLY)


@@ -1156,13 +1155,7 @@ split_point_start: // At split points actual search starts from here
     }
     else
     {
     }
     else
     {

-        if (fromNull)
-        {
-            // Approximated score. Real one is slightly higher due to tempo
-            ss->staticEval = bestValue = -(ss-1)->staticEval;
-            ss->evalMargin = VALUE_ZERO;
-        }
-        else if (tte)
+        if (tte)

         {
             // Never assume anything on values stored in TT
             if (  (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE
         {
             // Never assume anything on values stored in TT
             if (  (ss->staticEval = bestValue = tte->static_value()) == VALUE_NONE


@@ -1206,7 +1199,6 @@ split_point_start: // At split points actual search starts from here
       // Futility pruning
       if (   !PvNode
           && !InCheck
       // Futility pruning
       if (   !PvNode
           && !InCheck

-          && !fromNull

           && !givesCheck
           &&  move != ttMove
           &&  enoughMaterial
           && !givesCheck
           &&  move != ttMove
           &&  enoughMaterial