Micro optimize pawn moves generation

It is very rare we have pawns on 7(2) rank, so we
can skip the promotion handling stuff in most cases.

With this patch pawn moves generation is almost 20% faster.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movegen.cpp

@@ -61,7 +61,7 @@ namespace {
   MoveStack* generate_pawn_captures(const Position& pos, MoveStack* mlist);
 
   template<Color Us, SquareDelta Diagonal>
-  MoveStack* generate_pawn_captures_diagonal(MoveStack* mlist, Bitboard pawns, Bitboard enemyPieces);
+  MoveStack* generate_pawn_captures_diagonal(MoveStack* mlist, Bitboard pawns, Bitboard enemyPieces, bool promotion);
 
   template<Color Us>
   MoveStack* generate_pawn_noncaptures(const Position& pos, MoveStack* mlist);
@@ -611,7 +611,7 @@ namespace {
   }
 
   template<Color Us, SquareDelta Diagonal>
-  MoveStack* generate_pawn_captures_diagonal(MoveStack* mlist, Bitboard pawns, Bitboard enemyPieces) {
+  MoveStack* generate_pawn_captures_diagonal(MoveStack* mlist, Bitboard pawns, Bitboard enemyPieces, bool promotion) {
 
     // Calculate our parametrized parameters at compile time
     const Bitboard TRank8BB = (Us == WHITE ? Rank8BB : Rank1BB);
@@ -626,18 +626,21 @@ namespace {
     Bitboard b1 = move_pawns<Us, Diagonal>(pawns) & ~TFileABB & enemyPieces;
 
     // Capturing promotions
+    if (promotion)
+    {
         Bitboard b2 = b1 & TRank8BB;
+        b1 &= ~TRank8BB;
         while (b2)
         {
             to = pop_1st_bit(&b2);
             (*mlist++).move = make_promotion_move(to - TTDELTA_NE, to, QUEEN);
         }
+    }
 
     // Capturing non-promotions
-    b2 = b1 & ~TRank8BB;
+    while (b1)
-    while (b2)
     {
-        to = pop_1st_bit(&b2);
+        to = pop_1st_bit(&b1);
         (*mlist++).move = make_move(to - TTDELTA_NE, to);
     }
     return mlist;
@@ -649,23 +652,28 @@ namespace {
     // Calculate our parametrized parameters at compile time
     const Color Them = (Us == WHITE ? BLACK : WHITE);
     const Bitboard TRank8BB = (Us == WHITE ? Rank8BB : Rank1BB);
+    const Bitboard TRank7BB = (Us == WHITE ? Rank7BB : Rank2BB);
     const SquareDelta TDELTA_N = (Us == WHITE ? DELTA_N : DELTA_S);
 
     Square to;
     Bitboard pawns = pos.pawns(Us);
     Bitboard enemyPieces = pos.pieces_of_color(opposite_color(Us));
+    bool possiblePromotion = (pawns & TRank7BB);
 
     // Standard captures and capturing promotions in both directions
-    mlist = generate_pawn_captures_diagonal<Us, DELTA_NE>(mlist, pawns, enemyPieces);
+    mlist = generate_pawn_captures_diagonal<Us, DELTA_NE>(mlist, pawns, enemyPieces, possiblePromotion);
-    mlist = generate_pawn_captures_diagonal<Us, DELTA_NW>(mlist, pawns, enemyPieces);
+    mlist = generate_pawn_captures_diagonal<Us, DELTA_NW>(mlist, pawns, enemyPieces, possiblePromotion);
 
     // Non-capturing promotions
+    if (possiblePromotion)
+    {
         Bitboard b1 = move_pawns<Us, DELTA_N>(pawns) & pos.empty_squares() & TRank8BB;
         while (b1)
         {
             to = pop_1st_bit(&b1);
             (*mlist++).move = make_promotion_move(to - TDELTA_N, to, QUEEN);
         }
+    }
 
     // En passant captures
     if (pos.ep_square() != SQ_NONE)
@@ -673,7 +681,7 @@ namespace {
         assert(Us != WHITE || square_rank(pos.ep_square()) == RANK_6);
         assert(Us != BLACK || square_rank(pos.ep_square()) == RANK_3);
 
-        b1 = pawns & pos.pawn_attacks(Them, pos.ep_square());
+        Bitboard b1 = pawns & pos.pawn_attacks(Them, pos.ep_square());
         assert(b1 != EmptyBoardBB);
 
         while (b1)
@@ -690,6 +698,7 @@ namespace {
 
     // Calculate our parametrized parameters at compile time
     const Bitboard TRank8BB = (Us == WHITE ? Rank8BB : Rank1BB);
+    const Bitboard TRank7BB = (Us == WHITE ? Rank7BB : Rank2BB);
     const Bitboard TRank3BB = (Us == WHITE ? Rank3BB : Rank6BB);
     const SquareDelta TDELTA_NE = (Us == WHITE ? DELTA_NE : DELTA_SE);
     const SquareDelta TDELTA_NW = (Us == WHITE ? DELTA_NW : DELTA_SW);
@@ -698,9 +707,12 @@ namespace {
     Bitboard b1, b2;
     Square to;
     Bitboard pawns = pos.pawns(Us);
-    Bitboard enemyPieces = pos.pieces_of_color(opposite_color(Us));
     Bitboard emptySquares = pos.empty_squares();
 
+    if (pawns & TRank7BB) // There is some promotion candidate ?
+    {
+         Bitboard enemyPieces = pos.pieces_of_color(opposite_color(Us));
+
         // Underpromotion captures in the a1-h8 (a8-h1 for black) direction
         b1 = move_pawns<Us, DELTA_NE>(pawns) & ~FileABB & enemyPieces & TRank8BB;
         while (b1)
@@ -721,17 +733,19 @@ namespace {
             (*mlist++).move = make_promotion_move(to - TDELTA_NW, to, KNIGHT);
         }
 
-    // Single pawn pushes
+        // Underpromotion pawn pushes
-    b1 = move_pawns<Us, DELTA_N>(pawns) & emptySquares;
+        b1 = move_pawns<Us, DELTA_N>(pawns) & emptySquares & TRank8BB;
-    b2 = b1 & TRank8BB;
+        while (b1)
-    while (b2)
         {
-        to = pop_1st_bit(&b2);
+            to = pop_1st_bit(&b1);
             (*mlist++).move = make_promotion_move(to - TDELTA_N, to, ROOK);
             (*mlist++).move = make_promotion_move(to - TDELTA_N, to, BISHOP);
             (*mlist++).move = make_promotion_move(to - TDELTA_N, to, KNIGHT);
         }
-    b2 = b1 & ~TRank8BB;
+    }
+
+    // Single pawn pushes
+    b2 = b1 = move_pawns<Us, DELTA_N>(pawns) & emptySquares & ~TRank8BB;
     while (b2)
     {
         to = pop_1st_bit(&b2);