Introduce least_significant_square_bb()

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

diff --git a/src/bitboard.h b/src/bitboard.h
index e14fe0df7d30c2d3bec0d6d83e2ee833fa61c0c9..1b6af3ead1afedbaba67526b32cd3f6762ffed4f 100644 (file)
--- a/src/bitboard.h
+++ b/src/bitboard.h
@@ -414,6 +414,13 @@ inline Square msb(Bitboard b) {
 
 #endif
 
 
 #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
 
 
 /// pop_lsb() finds and clears the least significant bit in a non-zero bitboard
 

diff --git a/src/position.cpp b/src/position.cpp
index 17b165b9afb6f9a4115afc03d0762e98076993b4..f4739413da44906e4b03c9bd54283f0d8e9251c4 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1109,7 +1109,7 @@ bool Position::see_ge(Move m, Value threshold) const {
           if ((swap = PawnValueMg - swap) < res)
               break;
 
           if ((swap = PawnValueMg - swap) < res)
               break;
 

-          occupied ^= lsb(bb);
+          occupied ^= least_significant_square_bb(bb);

           attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
       }
 
           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;
 
           if ((swap = KnightValueMg - swap) < res)
               break;
 

-          occupied ^= lsb(bb);
+          occupied ^= least_significant_square_bb(bb);

       }
 
       else if ((bb = stmAttackers & pieces(BISHOP)))
       }
 
       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;
 
           if ((swap = BishopValueMg - swap) < res)
               break;
 

-          occupied ^= lsb(bb);
+          occupied ^= least_significant_square_bb(bb);

           attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
       }
 
           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;
 
           if ((swap = RookValueMg - swap) < res)
               break;
 

-          occupied ^= lsb(bb);
+          occupied ^= least_significant_square_bb(bb);

           attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
       }
 
           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;
 
           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));
       }
           attackers |=  (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
                       | (attacks_bb<ROOK  >(to, occupied) & pieces(ROOK  , QUEEN));
       }