Remove Some Bitboard Arrays (#1963)

This is non-functional. These 5 arrays are simple enough to calculate real-time and maintaining an array for them does not help. Decreases the memory footprint.

This seems a tiny bit slower on my machine, but passed STC well enough. Could someone verify speed?

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 44745 W: 9780 L: 9704 D: 25261
http://tests.stockfishchess.org/tests/view/5c47aa2d0ebc5902bca13fc4

The slowdown is minimal even in 32 bit case (thanks to @mstembera for testing):

Compiled using make build ARCH=x86-32 CXX=i686-w64-mingw32-c++ and benched
This patch only:

```
Results for 40 tests for each version:

            Base      Test      Diff      
    Mean    1455204   1450033   5171      
    StDev   49452     34533     59621     

p-value: 0.465
speedup: -0.004
```

No functional change.

src/bitboard.cpp

@@ -27,15 +27,10 @@ uint8_t PopCnt16[1 << 16];
 int8_t SquareDistance[SQUARE_NB][SQUARE_NB];
 
 Bitboard SquareBB[SQUARE_NB];
-Bitboard FileBB[FILE_NB];
-Bitboard RankBB[RANK_NB];
 Bitboard ForwardRanksBB[COLOR_NB][RANK_NB];
 Bitboard BetweenBB[SQUARE_NB][SQUARE_NB];
 Bitboard LineBB[SQUARE_NB][SQUARE_NB];
 Bitboard DistanceRingBB[SQUARE_NB][8];
-Bitboard ForwardFileBB[COLOR_NB][SQUARE_NB];
-Bitboard PassedPawnMask[COLOR_NB][SQUARE_NB];
-Bitboard PawnAttackSpan[COLOR_NB][SQUARE_NB];
 Bitboard PseudoAttacks[PIECE_TYPE_NB][SQUARE_NB];
 Bitboard PawnAttacks[COLOR_NB][SQUARE_NB];
 
@@ -90,22 +85,8 @@ void Bitboards::init() {
   for (Square s = SQ_A1; s <= SQ_H8; ++s)
       SquareBB[s] = (1ULL << s);
 
-  for (File f = FILE_A; f <= FILE_H; ++f)
-      FileBB[f] = f > FILE_A ? FileBB[f - 1] << 1 : FileABB;
-
-  for (Rank r = RANK_1; r <= RANK_8; ++r)
-      RankBB[r] = r > RANK_1 ? RankBB[r - 1] << 8 : Rank1BB;
-
   for (Rank r = RANK_1; r < RANK_8; ++r)
-      ForwardRanksBB[WHITE][r] = ~(ForwardRanksBB[BLACK][r + 1] = ForwardRanksBB[BLACK][r] | RankBB[r]);
-
-  for (Color c = WHITE; c <= BLACK; ++c)
-      for (Square s = SQ_A1; s <= SQ_H8; ++s)
-      {
-          ForwardFileBB [c][s] = ForwardRanksBB[c][rank_of(s)] & FileBB[file_of(s)];
-          PawnAttackSpan[c][s] = ForwardRanksBB[c][rank_of(s)] & adjacent_files_bb(file_of(s));
-          PassedPawnMask[c][s] = ForwardFileBB [c][s] | PawnAttackSpan[c][s];
-      }
+      ForwardRanksBB[WHITE][r] = ~(ForwardRanksBB[BLACK][r + 1] = ForwardRanksBB[BLACK][r] | rank_bb(r));
 
   for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
       for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)

src/bitboard.h

@@ -63,15 +63,10 @@ constexpr Bitboard Rank8BB = Rank1BB << (8 * 7);
 extern int8_t SquareDistance[SQUARE_NB][SQUARE_NB];
 
 extern Bitboard SquareBB[SQUARE_NB];
-extern Bitboard FileBB[FILE_NB];
-extern Bitboard RankBB[RANK_NB];
 extern Bitboard ForwardRanksBB[COLOR_NB][RANK_NB];
 extern Bitboard BetweenBB[SQUARE_NB][SQUARE_NB];
 extern Bitboard LineBB[SQUARE_NB][SQUARE_NB];
 extern Bitboard DistanceRingBB[SQUARE_NB][8];
-extern Bitboard ForwardFileBB[COLOR_NB][SQUARE_NB];
-extern Bitboard PassedPawnMask[COLOR_NB][SQUARE_NB];
-extern Bitboard PawnAttackSpan[COLOR_NB][SQUARE_NB];
 extern Bitboard PseudoAttacks[PIECE_TYPE_NB][SQUARE_NB];
 extern Bitboard PawnAttacks[COLOR_NB][SQUARE_NB];
 
@@ -142,19 +137,19 @@ inline bool opposite_colors(Square s1, Square s2) {
 /// the given file or rank.
 
 inline Bitboard rank_bb(Rank r) {
-  return RankBB[r];
+  return Rank1BB << (8 * r);
 }
 
 inline Bitboard rank_bb(Square s) {
-  return RankBB[rank_of(s)];
+  return rank_bb(rank_of(s));
 }
 
 inline Bitboard file_bb(File f) {
-  return FileBB[f];
+  return FileABB << f;
 }
 
 inline Bitboard file_bb(Square s) {
-  return FileBB[file_of(s)];
+  return file_bb(file_of(s));
 }
 
 
@@ -194,10 +189,9 @@ constexpr Bitboard double_pawn_attacks_bb(Bitboard b) {
 /// adjacent files of the given one.
 
 inline Bitboard adjacent_files_bb(File f) {
-  return shift<EAST>(FileBB[f]) | shift<WEST>(FileBB[f]);
+  return shift<EAST>(file_bb(f)) | shift<WEST>(file_bb(f));
 }
 
-
 /// between_bb() returns a bitboard representing all the squares between the two
 /// given ones. For instance, between_bb(SQ_C4, SQ_F7) returns a bitboard with
 /// the bits for square d5 and e6 set. If s1 and s2 are not on the same rank, file
@@ -222,26 +216,24 @@ inline Bitboard forward_ranks_bb(Color c, Square s) {
 ///      ForwardFileBB[c][s] = forward_ranks_bb(c, s) & file_bb(s)
 
 inline Bitboard forward_file_bb(Color c, Square s) {
-  return ForwardFileBB[c][s];
+  return ForwardRanksBB[c][rank_of(s)] & file_bb(s);
 }
 
 
 /// pawn_attack_span() returns a bitboard representing all the squares that can be
 /// attacked by a pawn of the given color when it moves along its file, starting
 /// from the given square:
-///      PawnAttackSpan[c][s] = forward_ranks_bb(c, s) & adjacent_files_bb(file_of(s));
 
 inline Bitboard pawn_attack_span(Color c, Square s) {
-  return PawnAttackSpan[c][s];
+  return forward_ranks_bb(c, s) & adjacent_files_bb(file_of(s));
 }
 
 
 /// passed_pawn_mask() returns a bitboard mask which can be used to test if a
 /// pawn of the given color and on the given square is a passed pawn:
-///      PassedPawnMask[c][s] = pawn_attack_span(c, s) | forward_file_bb(c, s)
 
 inline Bitboard passed_pawn_mask(Color c, Square s) {
-  return PassedPawnMask[c][s];
+  return pawn_attack_span(c, s) | forward_file_bb(c, s);
 }