Simplify away SEE verification

After 4 simplificatons over PR#4453 the idea does not yield significant
improvement anymore. Maybe also
https://tests.stockfishchess.org/tests/view/640c88092644b62c3394c1c5 was
a fluke.

Passed non-regression bounds:

STC:
https://tests.stockfishchess.org/tests/view/64705389c079b6583146d873
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 131936 W: 35040 L: 34930 D: 61966
Ptnml(0-2): 336, 14559, 36035, 14735, 303

LTC:
https://tests.stockfishchess.org/tests/view/6471a2ade549d9cf2fb213cd
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 407700 W: 109999 L: 110164 D: 187537
Ptnml(0-2): 279, 39913, 123689, 39632, 337

closes https://github.com/official-stockfish/Stockfish/pull/4595

bench: 2675974

diff --git a/src/position.cpp b/src/position.cpp
index 2a9d798ff7d0d85afa60c75f35646b7e4b399ef6..af274d3f2c64183e61eaa163a121112082b8ede1 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1061,7 +1061,7 @@ Key Position::key_after(Move m) const {
 /// SEE value of move is greater or equal to the given threshold. We'll use an
 /// algorithm similar to alpha-beta pruning with a null window.
 
-bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
+bool Position::see_ge(Move m, Value threshold) const {
 
   assert(is_ok(m));
 
@@ -1080,7 +1080,7 @@ bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
       return true;
 
   assert(color_of(piece_on(from)) == sideToMove);
-  occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
+  Bitboard occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
   Color stm = sideToMove;
   Bitboard attackers = attackers_to(to, occupied);
   Bitboard stmAttackers, bb;
@@ -1111,43 +1111,43 @@ bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
       // the bitboard 'attackers' any X-ray attackers behind it.
       if ((bb = stmAttackers & pieces(PAWN)))
       {
-          occupied ^= least_significant_square_bb(bb);
           if ((swap = PawnValueMg - swap) < res)
               break;
+          occupied ^= least_significant_square_bb(bb);
 
           attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
       }
 
       else if ((bb = stmAttackers & pieces(KNIGHT)))
       {
-          occupied ^= least_significant_square_bb(bb);
           if ((swap = KnightValueMg - swap) < res)
               break;
+          occupied ^= least_significant_square_bb(bb);
       }
 
       else if ((bb = stmAttackers & pieces(BISHOP)))
       {
-          occupied ^= least_significant_square_bb(bb);
           if ((swap = BishopValueMg - swap) < res)
               break;
+          occupied ^= least_significant_square_bb(bb);
 
           attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
       }
 
       else if ((bb = stmAttackers & pieces(ROOK)))
       {
-          occupied ^= least_significant_square_bb(bb);
           if ((swap = RookValueMg - swap) < res)
               break;
+          occupied ^= least_significant_square_bb(bb);
 
           attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
       }
 
       else if ((bb = stmAttackers & pieces(QUEEN)))
       {
-          occupied ^= least_significant_square_bb(bb);
           if ((swap = QueenValueMg - swap) < res)
               break;
+          occupied ^= least_significant_square_bb(bb);
 
           attackers |=  (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
                       | (attacks_bb<ROOK  >(to, occupied) & pieces(ROOK  , QUEEN));
@@ -1162,11 +1162,6 @@ bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
   return bool(res);
 }
 
-bool Position::see_ge(Move m, Value threshold) const {
-    Bitboard occupied;
-    return see_ge(m, occupied, threshold);
-}
-
 
 /// Position::is_draw() tests whether the position is drawn by 50-move rule
 /// or by repetition. It does not detect stalemates.

diff --git a/src/position.h b/src/position.h
index a736f3e677bd0463f7e7db93d1b9af1f9b153954..780d463cc6475060755843f88570f00221802079 100644 (file)
--- a/src/position.h
+++ b/src/position.h
@@ -143,7 +143,6 @@ public:
   void undo_null_move();
 
   // Static Exchange Evaluation
-  bool see_ge(Move m, Bitboard& occupied, Value threshold = VALUE_ZERO) const;
   bool see_ge(Move m, Value threshold = VALUE_ZERO) const;
 
   // Accessing hash keys

diff --git a/src/search.cpp b/src/search.cpp
index 16122315032d2303ec48d4ae3ebf2c1b48d5b78a..1e82203a7775f7e4232f5b27f469667c6049b7c6 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -989,25 +989,9 @@ moves_loop: // When in check, search starts here
                    + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 7 < alpha)
                   continue;
 
-              Bitboard occupied;
               // SEE based pruning (~11 Elo)
-              if (!pos.see_ge(move, occupied, Value(-212) * depth))
-              {
-                  // Don't prune the move if opponent King/Queen/Rook gets a discovered attack during or after the exchanges
-                  Bitboard leftEnemies = pos.pieces(~us, KING, QUEEN, ROOK);
-                  Bitboard attacks = 0;
-                  occupied |= to_sq(move);
-                  while (leftEnemies && !attacks)
-                  {
-                      Square sq = pop_lsb(leftEnemies);
-                      attacks = pos.attackers_to(sq, occupied) & pos.pieces(us) & occupied;
-                      // Exclude Queen/Rook(s) which were already threatened before SEE (opponent King can't be in check when it's our turn)
-                      if (attacks && sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us)))
-                          attacks = 0;
-                  }
-                  if (!attacks)
+              if (!pos.see_ge(move, Value(-205) * depth))
                       continue;
-              }
           }
           else
           {