Clarify when forcing the moves loop

In some cases we want to go direcly to the moves loop
without checking for early return. The patch make this
logic more clear and consistent.

Tested for no regression, passed STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 25282 W: 5136 L: 5022 D: 15124

and LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 72007 W: 12133 L: 12095 D: 47779

bench: 9316798

src/search.cpp

@@ -514,7 +514,7 @@ namespace {
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
     ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
-    (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
+    (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
 
     // Step 4. Transposition table lookup
@@ -599,6 +599,9 @@ namespace {
         TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
     }
 
+    if (ss->skipEarlyPruning)
+        goto moves_loop;
+
     if (   !pos.captured_piece_type()
         &&  ss->staticEval != VALUE_NONE
         && (ss-1)->staticEval != VALUE_NONE
@@ -629,7 +632,6 @@ namespace {
 
     // Step 7. Futility pruning: child node (skipped when in check)
     if (   !PvNode
-        && !ss->skipNullMove
         &&  depth < 7 * ONE_PLY
         &&  eval - futility_margin(depth) >= beta
         &&  eval < VALUE_KNOWN_WIN  // Do not return unproven wins
@@ -638,7 +640,6 @@ namespace {
 
     // Step 8. Null move search with verification search (is omitted in PV nodes)
     if (   !PvNode
-        && !ss->skipNullMove
         &&  depth >= 2 * ONE_PLY
         &&  eval >= beta
         &&  pos.non_pawn_material(pos.side_to_move()))
@@ -651,10 +652,10 @@ namespace {
         Depth R = (3 + depth / 4 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
 
         pos.do_null_move(st);
-        (ss+1)->skipNullMove = true;
+        (ss+1)->skipEarlyPruning = 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)->skipEarlyPruning = false;
         pos.undo_null_move();
 
         if (nullValue >= beta)
@@ -667,10 +668,10 @@ namespace {
                 return nullValue;
 
             // Do verification search at high depths
-            ss->skipNullMove = true;
+            ss->skipEarlyPruning = true;
             Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
                                         :  search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
-            ss->skipNullMove = false;
+            ss->skipEarlyPruning = false;
 
             if (v >= beta)
                 return nullValue;
@@ -683,7 +684,6 @@ namespace {
     // prune the previous move.
     if (   !PvNode
         &&  depth >= 5 * ONE_PLY
-        && !ss->skipNullMove
         &&  abs(beta) < VALUE_MATE_IN_MAX_PLY)
     {
         Value rbeta = std::min(beta + 200, VALUE_INFINITE);
@@ -714,9 +714,9 @@ namespace {
         && (PvNode || ss->staticEval + 256 >= beta))
     {
         Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
-        ss->skipNullMove = true;
+        ss->skipEarlyPruning = true;
         search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
-        ss->skipNullMove = false;
+        ss->skipEarlyPruning = false;
 
         tte = TT.probe(posKey);
         ttMove = tte ? tte->move() : MOVE_NONE;
@@ -816,9 +816,9 @@ namespace {
       {
           Value rBeta = ttValue - 2 * depth / ONE_PLY;
           ss->excludedMove = move;
-          ss->skipNullMove = true;
+          ss->skipEarlyPruning = true;
           value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
-          ss->skipNullMove = false;
+          ss->skipEarlyPruning = false;
           ss->excludedMove = MOVE_NONE;
 
           if (value < rBeta)

src/search.h

@@ -46,7 +46,7 @@ struct Stack {
   Move killers[2];
   Depth reduction;
   Value staticEval;
-  bool skipNullMove;
+  bool skipEarlyPruning;
 };