Temporary revert previous patch

Split previous patch in 2 steps: first remove
the MOVE_NULL hack, then retire nullChild.

The first step is a prerequisite
for second one and affects bench.

The second step (next patch) just removes nullChild
without affecting bench.

bench: 8205159

src/search.cpp

@@ -270,6 +270,7 @@ namespace {
     Value bestValue, alpha, beta, delta;
 
     std::memset(ss-2, 0, 5 * sizeof(Stack));
+    (ss-1)->currentMove = MOVE_NULL; // Hack to skip update gains
 
     depth = 0;
     BestMoveChanges = 0;
@@ -469,7 +470,7 @@ namespace {
     bestValue = -VALUE_INFINITE;
     ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
     ss->ply = (ss-1)->ply + 1;
-    (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
+    (ss+1)->skipNullMove = (ss+1)->nullChild = false; (ss+1)->reduction = DEPTH_ZERO;
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
 
     // Used to send selDepth info to GUI
@@ -544,17 +545,14 @@ namespace {
     }
     else
     {
-        eval = ss->staticEval =
-        (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
-
+        eval = ss->staticEval = ss->nullChild ? -(ss-1)->staticEval + 2 * Eval::Tempo : evaluate(pos);
         TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
     }
 
     if (   !pos.captured_piece_type()
         &&  ss->staticEval != VALUE_NONE
         && (ss-1)->staticEval != VALUE_NONE
         && (move = (ss-1)->currentMove) != MOVE_NULL
-        &&  move != MOVE_NONE
         &&  type_of(move) == NORMAL)
     {
         Square to = to_sq(move);
@@ -607,10 +605,10 @@ namespace {
                  + int(eval - beta) / PawnValueMg * ONE_PLY;
 
         pos.do_null_move(st);
-        (ss+1)->skipNullMove = true;
+        (ss+1)->skipNullMove = (ss+1)->nullChild = true;
         nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
                                       : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
-        (ss+1)->skipNullMove = false;
+        (ss+1)->skipNullMove = (ss+1)->nullChild = false;
         pos.undo_null_move();
 
         if (nullValue >= beta)
@@ -1110,8 +1108,7 @@ namespace {
                     bestValue = ttValue;
         }
         else
-            ss->staticEval = bestValue =
-            (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
+            ss->staticEval = bestValue = ss->nullChild ? -(ss-1)->staticEval + 2 * Eval::Tempo : evaluate(pos);
 
         // Stand pat. Return immediately if static value is at least beta
         if (bestValue >= beta)

src/search.h

@@ -46,6 +46,7 @@ struct Stack {
   Depth reduction;
   Value staticEval;
   bool skipNullMove;
+  bool nullChild;
 };