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Small optimization in generate_evasions()

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Find squares attacked by slider checkers, we will
remove them from king evasions set so to avoid a couple
of cycles in the slow king evasions legality check loop.

Not a biggie, but now generate_evasions() is faster then
generate_non_captures(), before was slower.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2009-02-07 13:11:52 +01:00
parent 1156eb865b
commit 4573d618e4
1 changed files with 34 additions and 17 deletions
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51
src/movegen.cpp
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@ -249,34 +249,51 @@ int generate_evasions(const Position& pos, MoveStack* mlist) {
assert(pos.piece_on(ksq) == king_of_color(us));
// Generate evasions for king
Bitboard b1 = pos.piece_attacks<KING>(ksq) & ~pos.pieces_of_color(us);
// The bitboard of occupied pieces without our king
Bitboard b2 = pos.occupied_squares();
clear_bit(&b2, ksq);
// Find squares attacked by slider checkers, we will
// remove them from king evasions set so to avoid a couple
// of cycles in the slow king evasions legality check loop.
Bitboard checkers = pos.checkers();
Bitboard checkersAttacks = EmptyBoardBB;
Bitboard b = checkers & (pos.queens() | pos.bishops());
while (b)
{
from = pop_1st_bit(&b);
checkersAttacks |= bishop_attacks_bb(from, b2);
}
b = checkers & (pos.queens() | pos.rooks());
while (b)
{
from = pop_1st_bit(&b);
checkersAttacks |= rook_attacks_bb(from, b2);
}
// Generate evasions for king
Bitboard b1 = pos.piece_attacks<KING>(ksq) & ~pos.pieces_of_color(us) & ~checkersAttacks;
while (b1)
{
to = pop_1st_bit(&b1);
to = pop_1st_bit(&b1);
// Make sure 'to' is not attacked by the other side. This is a bit ugly,
// because we can't use Position::square_is_attacked. Instead we use
// the low-level bishop_attacks_bb and rook_attacks_bb with the bitboard
// b2 (the occupied squares with the king removed) in order to test whether
// the king will remain in check on the destination square.
if (!( (pos.piece_attacks<KNIGHT>(to) & pos.knights(them))
|| (pos.pawn_attacks(us, to) & pos.pawns(them))
|| (bishop_attacks_bb(to, b2) & pos.bishops_and_queens(them))
|| (rook_attacks_bb(to, b2) & pos.rooks_and_queens(them))
|| (pos.piece_attacks<KING>(to) & pos.kings(them))))
(*mlist++).move = make_move(ksq, to);
// Make sure 'to' is not attacked by the other side. This is a bit ugly,
// because we can't use Position::square_is_attacked. Instead we use
// the low-level bishop_attacks_bb and rook_attacks_bb with the bitboard
// b2 (the occupied squares with the king removed) in order to test whether
// the king will remain in check on the destination square.
if (!( (pos.piece_attacks<KNIGHT>(to) & pos.knights(them))
|| (pos.pawn_attacks(us, to) & pos.pawns(them))
|| (bishop_attacks_bb(to, b2) & pos.bishops_and_queens(them))
|| (rook_attacks_bb(to, b2) & pos.rooks_and_queens(them))
|| (pos.piece_attacks<KING>(to) & pos.kings(them))))
(*mlist++).move = make_move(ksq, to);
}
// Generate evasions for other pieces only if not double check. We use a
// simple bit twiddling hack here rather than calling count_1s in order to
// save some time (we know that pos.checkers() has at most two nonzero bits).
Bitboard checkers = pos.checkers();
if (!(checkers & (checkers - 1))) // Only one bit set?
{
Square checksq = first_1(checkers);