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Introduce least_significant_square_bb()

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Introducing least_significant_square_bb(). It is a function that returns a value equal
to square_bb(lsb(bb)), but it uses fewer instruction. It should speed up more on older
processors like armv7-a Clang.

Passed STC:
LLR: 2.93 (-2.94,2.94) {-0.25,1.25}
Total: 213200 W: 19171 L: 18753 D: 175276
Ptnml(0-2): 680, 14513, 75831, 14861, 715
https://tests.stockfishchess.org/tests/view/604bc7632433018de7a38982

Closes https://github.com/official-stockfish/Stockfish/pull/3391

No functional change
This commit is contained in:
bmc4 2021-03-16 20:51:31 +01:00 • committed by Stéphane Nicolet
parent f3b296c2e2
commit 939395729c
2 changed files with 12 additions and 5 deletions
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7
src/bitboard.h
View file

@ -414,6 +414,13 @@ inline Square msb(Bitboard b) {
#endif
/// least_significant_square_bb() returns the bitboard of the least significant
/// square of a non-zero bitboard. It is equivalent to square_bb(lsb(bb)).
inline Bitboard least_significant_square_bb(Bitboard b) {
assert(b);
return b & -b;
}
/// pop_lsb() finds and clears the least significant bit in a non-zero bitboard

10
src/position.cpp
View file

@ -1109,7 +1109,7 @@ bool Position::see_ge(Move m, Value threshold) const {
if ((swap = PawnValueMg - swap) < res)
break;
occupied ^= lsb(bb);
occupied ^= least_significant_square_bb(bb);
attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
}
@ -1118,7 +1118,7 @@ bool Position::see_ge(Move m, Value threshold) const {
if ((swap = KnightValueMg - swap) < res)
break;
occupied ^= lsb(bb);
occupied ^= least_significant_square_bb(bb);
}
else if ((bb = stmAttackers & pieces(BISHOP)))
@ -1126,7 +1126,7 @@ bool Position::see_ge(Move m, Value threshold) const {
if ((swap = BishopValueMg - swap) < res)
break;
occupied ^= lsb(bb);
occupied ^= least_significant_square_bb(bb);
attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
}
@ -1135,7 +1135,7 @@ bool Position::see_ge(Move m, Value threshold) const {
if ((swap = RookValueMg - swap) < res)
break;
occupied ^= lsb(bb);
occupied ^= least_significant_square_bb(bb);
attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
}
@ -1144,7 +1144,7 @@ bool Position::see_ge(Move m, Value threshold) const {
if ((swap = QueenValueMg - swap) < res)
break;
occupied ^= lsb(bb);
occupied ^= least_significant_square_bb(bb);
attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
| (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
}