Speedup and simplify pinners and blockers

To compute dicovered check or pinned pieces we use some bitwise
operators that are not really needed because already accounted for
at the caller site.

For instance in evaluation we compute:

     pos.pinned_pieces(Us) & s

Where pinned_pieces() is:

     st->blockersForKing[c] & pieces(c)

So in this case the & operator with pieces(c) is useless,
given the outer '& s'.

There are many places where we can use the naked blockersForKing[]
instead of the full pinned_pieces() or discovered_check_candidates().

This path is simpler than original and gives around 1% speed up for me.
Also tested for speed by mstembera and snicolet (neutral in both cases).

No functional change.

src/evaluate.cpp

@@ -306,7 +306,7 @@ namespace {
           : Pt ==   ROOK ? attacks_bb<  ROOK>(s, pos.pieces() ^ pos.pieces(QUEEN) ^ pos.pieces(Us, ROOK))
                          : pos.attacks_from<Pt>(s);
 
-        if (pos.pinned_pieces(Us) & s)
+        if (pos.blockers_for_king(Us) & s)
             b &= LineBB[pos.square<KING>(Us)][s];
 
         attackedBy2[Us] |= attackedBy[Us][ALL_PIECES] & b;
@@ -392,8 +392,8 @@ namespace {
         if (Pt == QUEEN)
         {
             // Penalty if any relative pin or discovered attack against the queen
-            Bitboard pinners;
-            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s, pinners))
+            Bitboard queenPinners;
+            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s, queenPinners))
                 score -= WeakQueen;
         }
     }
@@ -413,7 +413,7 @@ namespace {
                                        : AllSquares ^ Rank1BB ^ Rank2BB ^ Rank3BB);
 
     const Square ksq = pos.square<KING>(Us);
-    Bitboard weak, b, b1, b2, safe, unsafeChecks;
+    Bitboard weak, b, b1, b2, safe, unsafeChecks, pinned;
 
     // King shelter and enemy pawns storm
     Score score = pe->king_safety<Us>(pos, ksq);
@@ -464,11 +464,12 @@ namespace {
         // Unsafe or occupied checking squares will also be considered, as long as
         // the square is in the attacker's mobility area.
         unsafeChecks &= mobilityArea[Them];
+        pinned = pos.blockers_for_king(Us) & pos.pieces(Us);
 
         kingDanger +=        kingAttackersCount[Them] * kingAttackersWeight[Them]
                      + 102 * kingAdjacentZoneAttacksCount[Them]
                      + 191 * popcount(kingRing[Us] & weak)
-                     + 143 * popcount(pos.pinned_pieces(Us) | unsafeChecks)
+                     + 143 * popcount(pinned | unsafeChecks)
                      - 848 * !pos.count<QUEEN>(Them)
                      -   9 * mg_value(score) / 8
                      +  40;

src/movegen.cpp

@@ -135,7 +135,7 @@ namespace {
             // if the pawn is not on the same file as the enemy king, because we
             // don't generate captures. Note that a possible discovery check
             // promotion has been already generated amongst the captures.
-            Bitboard dcCandidates = pos.discovered_check_candidates();
+            Bitboard dcCandidates = pos.blockers_for_king(Them);
             if (pawnsNotOn7 & dcCandidates)
             {
                 Bitboard dc1 = shift<Up>(pawnsNotOn7 & dcCandidates) & emptySquares & ~file_bb(ksq);
@@ -241,7 +241,7 @@ namespace {
                 && !(PseudoAttacks[Pt][from] & target & pos.check_squares(Pt)))
                 continue;
 
-            if (pos.discovered_check_candidates() & from)
+            if (pos.blockers_for_king(~us) & from)
                 continue;
         }
 
@@ -336,7 +336,7 @@ ExtMove* generate<QUIET_CHECKS>(const Position& pos, ExtMove* moveList) {
   assert(!pos.checkers());
 
   Color us = pos.side_to_move();
-  Bitboard dc = pos.discovered_check_candidates();
+  Bitboard dc = pos.blockers_for_king(~us) & pos.pieces(us);
 
   while (dc)
   {
@@ -403,8 +403,9 @@ ExtMove* generate<EVASIONS>(const Position& pos, ExtMove* moveList) {
 template<>
 ExtMove* generate<LEGAL>(const Position& pos, ExtMove* moveList) {
 
-  Bitboard pinned = pos.pinned_pieces(pos.side_to_move());
-  Square ksq = pos.square<KING>(pos.side_to_move());
+  Color us = pos.side_to_move();
+  Bitboard pinned = pos.blockers_for_king(us) & pos.pieces(us);
+  Square ksq = pos.square<KING>(us);
   ExtMove* cur = moveList;
 
   moveList = pos.checkers() ? generate<EVASIONS    >(pos, moveList)

src/position.cpp

@@ -464,7 +464,7 @@ const string Position::fen() const {
 
 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
 
-  Bitboard result = 0;
+  Bitboard blockers = 0;
   pinners = 0;
 
   // Snipers are sliders that attack 's' when a piece is removed
@@ -476,14 +476,14 @@ Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners
     Square sniperSq = pop_lsb(&snipers);
     Bitboard b = between_bb(s, sniperSq) & pieces();
 
-    if (!more_than_one(b))
+    if (b && !more_than_one(b))
     {
-        result |= b;
+        blockers |= b;
         if (b & pieces(color_of(piece_on(s))))
             pinners |= sniperSq;
     }
   }
-  return result;
+  return blockers;
 }
 
 
@@ -540,7 +540,7 @@ bool Position::legal(Move m) const {
 
   // A non-king move is legal if and only if it is not pinned or it
   // is moving along the ray towards or away from the king.
-  return   !(pinned_pieces(us) & from)
+  return   !(blockers_for_king(us) & from)
         ||  aligned(from, to_sq(m), square<KING>(us));
 }
 
@@ -632,7 +632,7 @@ bool Position::gives_check(Move m) const {
       return true;
 
   // Is there a discovered check?
-  if (   (discovered_check_candidates() & from)
+  if (   (st->blockersForKing[~sideToMove] & from)
       && !aligned(from, to, square<KING>(~sideToMove)))
       return true;
 

src/position.h

@@ -105,8 +105,7 @@ public:
 
   // Checking
   Bitboard checkers() const;
-  Bitboard discovered_check_candidates() const;
-  Bitboard pinned_pieces(Color c) const;
+  Bitboard blockers_for_king(Color c) const;
   Bitboard check_squares(PieceType pt) const;
 
   // Attacks to/from a given square
@@ -296,12 +295,8 @@ inline Bitboard Position::checkers() const {
   return st->checkersBB;
 }
 
-inline Bitboard Position::discovered_check_candidates() const {
-  return st->blockersForKing[~sideToMove] & pieces(sideToMove);
-}
-
-inline Bitboard Position::pinned_pieces(Color c) const {
-  return st->blockersForKing[c] & pieces(c);
+inline Bitboard Position::blockers_for_king(Color c) const {
+  return st->blockersForKing[c];
 }
 
 inline Bitboard Position::check_squares(PieceType pt) const {

src/search.cpp

@@ -108,6 +108,13 @@ namespace {
   void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
   void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
 
+  inline bool gives_check(const Position& pos, Move move) {
+    Color us = pos.side_to_move();
+    return  type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
+          ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
+          : pos.gives_check(move);
+  }
+
   // perft() is our utility to verify move generation. All the leaf nodes up
   // to the given depth are generated and counted, and the sum is returned.
   template<bool Root>
@@ -823,10 +830,7 @@ moves_loop: // When in check, search starts from here
       extension = DEPTH_ZERO;
       captureOrPromotion = pos.capture_or_promotion(move);
       movedPiece = pos.moved_piece(move);
-
-      givesCheck =  type_of(move) == NORMAL && !pos.discovered_check_candidates()
-                  ? pos.check_squares(type_of(movedPiece)) & to_sq(move)
-                  : pos.gives_check(move);
+      givesCheck = gives_check(pos, move);
 
       moveCountPruning =   depth < 16 * ONE_PLY
                         && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
@@ -1238,9 +1242,7 @@ moves_loop: // When in check, search starts from here
     {
       assert(is_ok(move));
 
-      givesCheck =  type_of(move) == NORMAL && !pos.discovered_check_candidates()
-                  ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
-                  : pos.gives_check(move);
+      givesCheck = gives_check(pos, move);
 
       moveCount++;