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Unroll least valuable attacker loop in SEE 

This allows to reduce the scanning for new X-ray attacks
according to the capturing piece type.

It seems to be just a very small speed increase in MSVC 64
bit and gcc 32 bit, I guess cache issues value more than some
instruction less to execute (as usual).

No functional change.
sf_2.3.1_base
Marco Costalba 2012-09-01 11:45:14 +02:00
parent 6436e75858
commit 9cdca7516c
1 changed files with 35 additions and 13 deletions
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48
src/position.cpp
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@ -102,6 +102,37 @@ void init() {
} // namespace Zobrist
/// next_attacker() is an helper function used by see() to locate the least
/// valuable attacker for the side to move, remove the attacker we just found
/// from the 'occupied' bitboard and scan for new X-ray attacks behind it.
template<PieceType Pt> static FORCE_INLINE
PieceType next_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
Bitboard& occupied, Bitboard& attackers) {
const PieceType NextPt = PieceType((int)Pt + 1);
if (!(stmAttackers & bb[Pt]))
return next_attacker<NextPt>(bb, to, stmAttackers, occupied, attackers);
Bitboard b = stmAttackers & bb[Pt];
occupied ^= b & ~(b - 1);
if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
if (Pt == ROOK || Pt == QUEEN)
attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
return Pt;
}
template<> FORCE_INLINE
PieceType next_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
return KING; // No need to update bitboards, it is the last cycle
}
/// CheckInfo c'tor
CheckInfo::CheckInfo(const Position& pos) {
@ -1218,7 +1249,7 @@ int Position::see_sign(Move m) const {
int Position::see(Move m) const {
Square from, to;
Bitboard occupied, attackers, stmAttackers, b;
Bitboard occupied, attackers, stmAttackers;
int swapList[32], slIndex = 1;
PieceType captured;
Color stm;
@ -1274,19 +1305,10 @@ int Position::see(Move m) const {
swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[Mg][captured];
slIndex++;
// Locate the least valuable attacker for the side to move. The loop
// below looks like it is potentially infinite, but it isn't. We know
// that the side to move still has at least one attacker left.
for (captured = PAWN; (b = stmAttackers & pieces(captured)) == 0; captured++)
assert(captured < KING);
// Locate and remove from 'occupied' the next least valuable attacker
captured = next_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
// Remove the attacker we just found from the 'occupied' bitboard,
// and scan for new X-ray attacks behind the attacker.
occupied ^= (b & (~b + 1));
attackers |= (attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN))
| (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN));
attackers &= occupied; // Cut out pieces we've already done
attackers &= occupied; // Remove the just found attacker
stm = ~stm;
stmAttackers = attackers & pieces(stm);