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Optimisation of Position::see and Position::see_sign

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Stephane's patch removes the only usage of Position::see, where the
returned value isn't immediately compared with a value. So I replaced
this function by its optimised and more specific version see_ge. This
function also supersedes the function Position::see_sign.

bool Position::see_ge(Move m, Value v) const;

This function tests if the SEE of a move is greater or equal than a
given value. We use forward iteration on captures instread of backward
one, therefore we don't need the swapList array. Also we stop as soon
as we have enough information to obtain the result, avoiding unnecessary
calls to the min_attacker function.

Speed tests (Windows 7), 20 runs for each engine:
Test engine: mean 866648, st. dev. 5964
Base engine: mean 846751, st. dev. 22846
Speedup: 1.023

Speed test by Stephane Nicolet

Fishtest STC test:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 26040 W: 4675 L: 4442 D: 16923
http://tests.stockfishchess.org/tests/view/57f648990ebc59038170fa03

No functional change.
This commit is contained in:
atumanian
2016-10-06 20:55:10 +03:00
committed by Marco Costalba
parent 1e586288ca
commit 073eed590e
4 changed files with 67 additions and 87 deletions
Download Patch File Download Diff File Expand all files Collapse all files

133
src/position.cpp
View File

@@ -955,102 +955,83 @@ Key Position::key_after(Move m) const {
}
/// Position::see() is a static exchange evaluator: It tries to estimate the
/// material gain or loss resulting from a move.
/// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
/// SEE value of move is greater or equal to the given value. We'll use an
/// algorithm similar to alpha-beta pruning with a null window.
Value Position::see_sign(Move m) const {
bool Position::see_ge(Move m, Value v) const {
assert(is_ok(m));
// Early return if SEE cannot be negative because captured piece value
// is not less then capturing one. Note that king moves always return
// here because king midgame value is set to 0.
if (PieceValue[MG][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
return VALUE_KNOWN_WIN;
return see(m);
}
Value Position::see(Move m) const {
Square from, to;
Bitboard occupied, attackers, stmAttackers;
Value swapList[32];
int slIndex = 1;
PieceType nextVictim;
Color stm;
assert(is_ok(m));
from = from_sq(m);
to = to_sq(m);
swapList[0] = PieceValue[MG][piece_on(to)];
stm = color_of(piece_on(from));
occupied = pieces() ^ from;
// Castling moves are implemented as king capturing the rook so cannot
// be handled correctly. Simply return VALUE_ZERO that is always correct
// unless in the rare case the rook ends up under attack.
// Castling moves are implemented as king capturing the rook so cannot be
// handled correctly. Simply assume the SEE value is VALUE_ZERO that is always
// correct unless in the rare case the rook ends up under attack.
if (type_of(m) == CASTLING)
return VALUE_ZERO;
return VALUE_ZERO >= v;
Square from = from_sq(m), to = to_sq(m);
PieceType nextVictim = type_of(piece_on(from));
Color stm = ~color_of(piece_on(from)); // First consider opponent's move
Value balance; // Values of the pieces taken by us minus opponent's ones
Bitboard occupied, stmAttackers;
if (type_of(m) == ENPASSANT)
{
occupied ^= to - pawn_push(stm); // Remove the captured pawn
swapList[0] = PieceValue[MG][PAWN];
occupied = SquareBB[to - pawn_push(~stm)]; // Remove the captured pawn
balance = PieceValue[MG][PAWN];
}
else
{
balance = PieceValue[MG][piece_on(to)];
occupied = 0;
}
// Find all attackers to the destination square, with the moving piece
// removed, but possibly an X-ray attacker added behind it.
attackers = attackers_to(to, occupied) & occupied;
if (balance < v)
return false;
// If the opponent has no attackers we are finished
stm = ~stm;
stmAttackers = attackers & pieces(stm);
occupied ^= to; // For the case when captured piece is a pinner
if (nextVictim == KING)
return true;
// Don't allow pinned pieces to attack pieces except the king as long all
// pinners are on their original square.
if (!(st->pinnersForKing[stm] & ~occupied))
stmAttackers &= ~st->blockersForKing[stm];
balance -= PieceValue[MG][nextVictim];
if (!stmAttackers)
return swapList[0];
if (balance >= v)
return true;
// The destination square is defended, which makes things rather more
// difficult to compute. We proceed by building up a "swap list" containing
// the material gain or loss at each stop in a sequence of captures to the
// destination square, where the sides alternately capture, and always
// capture with the least valuable piece. After each capture, we look for
// new X-ray attacks from behind the capturing piece.
nextVictim = type_of(piece_on(from));
bool relativeStm = true; // True if the opponent is to move
occupied ^= pieces() ^ from ^ to;
do {
assert(slIndex < 32);
// Find all attackers to the destination square, with the moving piece removed,
// but possibly an X-ray attacker added behind it.
Bitboard attackers = attackers_to(to, occupied) & occupied;
// Add the new entry to the swap list
swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][nextVictim];
while (true)
{
stmAttackers = attackers & pieces(stm);
// Don't allow pinned pieces to attack pieces except the king as long all
// pinners are on their original square.
if (!(st->pinnersForKing[stm] & ~occupied))
stmAttackers &= ~st->blockersForKing[stm];
if (!stmAttackers)
return relativeStm;
// Locate and remove the next least valuable attacker
nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
if (nextVictim == KING)
return relativeStm == bool(attackers & pieces(~stm));
balance += relativeStm ? PieceValue[MG][nextVictim]
: -PieceValue[MG][nextVictim];
relativeStm = !relativeStm;
if (relativeStm == (balance >= v))
return relativeStm;
stm = ~stm;
stmAttackers = attackers & pieces(stm);
// Don't allow pinned pieces to attack pieces except the king
if ( nextVictim != KING
&& !(st->pinnersForKing[stm] & ~occupied))
stmAttackers &= ~st->blockersForKing[stm];
++slIndex;
} while (stmAttackers && (nextVictim != KING || (--slIndex, false))); // Stop before a king capture
// Having built the swap list, we negamax through it to find the best
// achievable score from the point of view of the side to move.
while (--slIndex)
swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
return swapList[0];
}
}