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Micro-optimization in evaluate_space()

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Since &-ing with SpaceMask restricts the set to the home
half of the board, it is possible to use just one popcount
instead of 2 by shifting "safe" to the other half of the
board. This gives a small speedup especially on systems
where hardware popcount is not available.

Patch kindly sent by Richard Vida.

No functional change.
This commit is contained in:
Marco Costalba 2012-12-31 11:26:12 +01:00
parent e1d681458e
commit 009a0f88e0
1 changed files with 5 additions and 1 deletions
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6
src/evaluate.cpp
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@ -1146,7 +1146,11 @@ Value do_evaluate(const Position& pos, Value& margin) {
behind |= (Us == WHITE ? behind >> 8 : behind << 8);
behind |= (Us == WHITE ? behind >> 16 : behind << 16);
return popcount<Max15>(safe) + popcount<Max15>(behind & safe);
// Since SpaceMask[Us] is fully on our half of the board
assert(unsigned(safe >> (Us == WHITE ? 32 : 0)) == 0);
// Count safe + (behind & safe) with a single popcount
return popcount<Full>((Us == WHITE ? safe << 32 : safe >> 32) | (behind & safe));
}