Use reference instead of pointer for pop_lsb() signature

This patch changes the pop_lsb() signature from Square pop_lsb(Bitboard*) to
Square pop_lsb(Bitboard&). This is more idomatic for C++ style signatures.

Passed a non-regression STC test:
LLR: 2.93 (-2.94,2.94) {-1.25,0.25}
Total: 21280 W: 1928 L: 1847 D: 17505
Ptnml(0-2): 71, 1427, 7558, 1518, 66
https://tests.stockfishchess.org/tests/view/6053a1e22433018de7a38e2f

We have verified that the generated binary is identical on gcc-10.

Closes https://github.com/official-stockfish/Stockfish/pull/3404

No functional change.

src/bitbase.cpp

@@ -150,8 +150,8 @@ namespace {
     Bitboard b = attacks_bb<KING>(ksq[stm]);
 
     while (b)
-        r |= stm == WHITE ? db[index(BLACK, ksq[BLACK] , pop_lsb(&b), psq)]
+        r |= stm == WHITE ? db[index(BLACK, ksq[BLACK] , pop_lsb(b), psq)]
-                          : db[index(WHITE, pop_lsb(&b),  ksq[WHITE], psq)];
+                          : db[index(WHITE, pop_lsb(b),  ksq[WHITE], psq)];
 
     if (stm == WHITE)
     {

src/bitboard.h

@@ -428,10 +428,10 @@ inline Bitboard least_significant_square_bb(Bitboard b) {
 
 /// pop_lsb() finds and clears the least significant bit in a non-zero bitboard
 
-inline Square pop_lsb(Bitboard* b) {
+inline Square pop_lsb(Bitboard& b) {
-  assert(*b);
+  assert(b);
-  const Square s = lsb(*b);
+  const Square s = lsb(b);
-  *b &= *b - 1;
+  b &= b - 1;
   return s;
 }
 

src/evaluate.cpp

@@ -397,7 +397,7 @@ namespace {
     attackedBy[Us][Pt] = 0;
 
     while (b1) {
-        Square s = pop_lsb(&b1);
+        Square s = pop_lsb(b1);
 
         // Find attacked squares, including x-ray attacks for bishops and rooks
         b = Pt == BISHOP ? attacks_bb<BISHOP>(s, pos.pieces() ^ pos.pieces(QUEEN))
@@ -658,11 +658,11 @@ namespace {
     {
         b = (defended | weak) & (attackedBy[Us][KNIGHT] | attackedBy[Us][BISHOP]);
         while (b)
-            score += ThreatByMinor[type_of(pos.piece_on(pop_lsb(&b)))];
+            score += ThreatByMinor[type_of(pos.piece_on(pop_lsb(b)))];
 
         b = weak & attackedBy[Us][ROOK];
         while (b)
-            score += ThreatByRook[type_of(pos.piece_on(pop_lsb(&b)))];
+            score += ThreatByRook[type_of(pos.piece_on(pop_lsb(b)))];
 
         if (weak & attackedBy[Us][KING])
             score += ThreatByKing;
@@ -760,7 +760,7 @@ namespace {
 
     while (b)
     {
-        Square s = pop_lsb(&b);
+        Square s = pop_lsb(b);
 
         assert(!(pos.pieces(Them, PAWN) & forward_file_bb(Us, s + Up)));
 

src/movegen.cpp

@@ -102,13 +102,13 @@ namespace {
 
         while (b1)
         {
-            Square to = pop_lsb(&b1);
+            Square to = pop_lsb(b1);
             *moveList++ = make_move(to - Up, to);
         }
 
         while (b2)
         {
-            Square to = pop_lsb(&b2);
+            Square to = pop_lsb(b2);
             *moveList++ = make_move(to - Up - Up, to);
         }
     }
@@ -127,13 +127,13 @@ namespace {
         Bitboard b3 = shift<Up     >(pawnsOn7) & emptySquares;
 
         while (b1)
-            moveList = make_promotions<Type, UpRight>(moveList, pop_lsb(&b1), ksq);
+            moveList = make_promotions<Type, UpRight>(moveList, pop_lsb(b1), ksq);
 
         while (b2)
-            moveList = make_promotions<Type, UpLeft >(moveList, pop_lsb(&b2), ksq);
+            moveList = make_promotions<Type, UpLeft >(moveList, pop_lsb(b2), ksq);
 
         while (b3)
-            moveList = make_promotions<Type, Up     >(moveList, pop_lsb(&b3), ksq);
+            moveList = make_promotions<Type, Up     >(moveList, pop_lsb(b3), ksq);
     }
 
     // Standard and en passant captures
@@ -144,13 +144,13 @@ namespace {
 
         while (b1)
         {
-            Square to = pop_lsb(&b1);
+            Square to = pop_lsb(b1);
             *moveList++ = make_move(to - UpRight, to);
         }
 
         while (b2)
         {
-            Square to = pop_lsb(&b2);
+            Square to = pop_lsb(b2);
             *moveList++ = make_move(to - UpLeft, to);
         }
 
@@ -167,7 +167,7 @@ namespace {
             assert(b1);
 
             while (b1)
-                *moveList++ = make<EN_PASSANT>(pop_lsb(&b1), pos.ep_square());
+                *moveList++ = make<EN_PASSANT>(pop_lsb(b1), pos.ep_square());
         }
     }
 
@@ -183,14 +183,14 @@ namespace {
     Bitboard bb = piecesToMove & pos.pieces(Pt);
 
     while (bb) {
-        Square from = pop_lsb(&bb);
+        Square from = pop_lsb(bb);
 
         Bitboard b = attacks_bb<Pt>(from, pos.pieces()) & target;
         if constexpr (Checks)
             b &= pos.check_squares(Pt);
 
         while (b)
-            *moveList++ = make_move(from, pop_lsb(&b));
+            *moveList++ = make_move(from, pop_lsb(b));
     }
 
     return moveList;
@@ -236,7 +236,7 @@ namespace {
         Square ksq = pos.square<KING>(Us);
         Bitboard b = attacks_bb<KING>(ksq) & target;
         while (b)
-            *moveList++ = make_move(ksq, pop_lsb(&b));
+            *moveList++ = make_move(ksq, pop_lsb(b));
 
         if ((Type != CAPTURES) && pos.can_castle(Us & ANY_CASTLING))
             for (CastlingRights cr : { Us & KING_SIDE, Us & QUEEN_SIDE } )
@@ -286,7 +286,7 @@ ExtMove* generate<QUIET_CHECKS>(const Position& pos, ExtMove* moveList) {
 
   while (dc)
   {
-     Square from = pop_lsb(&dc);
+     Square from = pop_lsb(dc);
      PieceType pt = type_of(pos.piece_on(from));
 
      Bitboard b = attacks_bb(pt, from, pos.pieces()) & ~pos.pieces();
@@ -295,7 +295,7 @@ ExtMove* generate<QUIET_CHECKS>(const Position& pos, ExtMove* moveList) {
          b &= ~attacks_bb<QUEEN>(pos.square<KING>(~us));
 
      while (b)
-         *moveList++ = make_move(from, pop_lsb(&b));
+         *moveList++ = make_move(from, pop_lsb(b));
   }
 
   return us == WHITE ? generate_all<WHITE, QUIET_CHECKS>(pos, moveList)
@@ -319,12 +319,12 @@ ExtMove* generate<EVASIONS>(const Position& pos, ExtMove* moveList) {
   // the king evasions in order to skip known illegal moves, which avoids any
   // useless legality checks later on.
   while (sliders)
-      sliderAttacks |= line_bb(ksq, pop_lsb(&sliders)) & ~pos.checkers();
+      sliderAttacks |= line_bb(ksq, pop_lsb(sliders)) & ~pos.checkers();
 
   // Generate evasions for king, capture and non capture moves
   Bitboard b = attacks_bb<KING>(ksq) & ~pos.pieces(us) & ~sliderAttacks;
   while (b)
-      *moveList++ = make_move(ksq, pop_lsb(&b));
+      *moveList++ = make_move(ksq, pop_lsb(b));
 
   if (more_than_one(pos.checkers()))
       return moveList; // Double check, only a king move can save the day

src/nnue/features/half_kp.cpp

@@ -41,7 +41,7 @@ namespace Stockfish::Eval::NNUE::Features {
     Square ksq = orient(perspective, pos.square<KING>(perspective));
     Bitboard bb = pos.pieces() & ~pos.pieces(KING);
     while (bb) {
-      Square s = pop_lsb(&bb);
+      Square s = pop_lsb(bb);
       active->push_back(make_index(perspective, s, pos.piece_on(s), ksq));
     }
   }

src/pawns.cpp

@@ -110,7 +110,7 @@ namespace {
 
     // Loop through all pawns of the current color and score each pawn
     while (b) {
-        s = pop_lsb(&b);
+        s = pop_lsb(b);
 
         assert(pos.piece_on(s) == make_piece(Us, PAWN));
 
@@ -290,7 +290,7 @@ Score Entry::do_king_safety(const Position& pos) {
   if (pawns & attacks_bb<KING>(ksq))
       minPawnDist = 1;
   else while (pawns)
-      minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(&pawns)));
+      minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(pawns)));
 
   return shelter - make_score(0, 16 * minPawnDist);
 }

src/position.cpp

@@ -73,7 +73,7 @@ std::ostream& operator<<(std::ostream& os, const Position& pos) {
      << std::setfill(' ') << std::dec << "\nCheckers: ";
 
   for (Bitboard b = pos.checkers(); b; )
-      os << UCI::square(pop_lsb(&b)) << " ";
+      os << UCI::square(pop_lsb(b)) << " ";
 
   if (    int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
       && !pos.can_castle(ANY_CASTLING))
@@ -359,7 +359,7 @@ void Position::set_state(StateInfo* si) const {
 
   for (Bitboard b = pieces(); b; )
   {
-      Square s = pop_lsb(&b);
+      Square s = pop_lsb(b);
       Piece pc = piece_on(s);
       si->key ^= Zobrist::psq[pc][s];
 
@@ -476,7 +476,7 @@ Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners
 
   while (snipers)
   {
-    Square sniperSq = pop_lsb(&snipers);
+    Square sniperSq = pop_lsb(snipers);
     Bitboard b = between_bb(s, sniperSq) & occupancy;
 
     if (b && !more_than_one(b))

src/syzygy/tbprobe.cpp

@@ -711,7 +711,7 @@ Ret do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* resu
 
         leadPawns = b = pos.pieces(color_of(pc), PAWN);
         do
-            squares[size++] = pop_lsb(&b) ^ flipSquares;
+            squares[size++] = pop_lsb(b) ^ flipSquares;
         while (b);
 
         leadPawnsCnt = size;
@@ -731,7 +731,7 @@ Ret do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* resu
     // directly map them to the correct color and square.
     b = pos.pieces() ^ leadPawns;
     do {
-        Square s = pop_lsb(&b);
+        Square s = pop_lsb(b);
         squares[size] = s ^ flipSquares;
         pieces[size++] = Piece(pos.piece_on(s) ^ flipColor);
     } while (b);