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Use bsfq asm instruction to count bits

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On 64 bit systems we can use bsfq instruction to count
set bits in a bitboard.

This is a patch for GCC and Intel compilers to take advantage
of that and get a 2% speed up.

Original patch from Heinz van Saanen, adapted to current tree
by me.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba
2009-07-03 08:28:13 +01:00
parent 063e2441b1
commit a87ea9846d
2 changed files with 26 additions and 10 deletions
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10
src/bitboard.cpp
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@@ -161,7 +161,7 @@ const int RShift[64] = {
21, 22, 22, 22, 22, 22, 22, 21, 20, 21, 21, 21, 21, 21, 21, 20
};
#endif
#endif // defined(IS_64BIT)
Bitboard RMask[64];
@@ -245,16 +245,16 @@ void init_bitboards() {
/// pop_1st_bit() finds and clears the least significant nonzero bit in a
/// nonzero bitboard.
#if defined(IS_64BIT)
#if defined(IS_64BIT) && !defined(USE_BSFQ)
Square pop_1st_bit(Bitboard *b) {
Square pop_1st_bit(Bitboard* b) {
Bitboard bb = *b ^ (*b - 1);
uint32_t fold = int(bb) ^ int(bb >> 32);
*b &= (*b - 1);
return Square(BitTable[(fold * 0x783a9b23) >> 26]);
}
#else
#elif !defined(USE_BSFQ)
// Use type-punning
union b_union {
@@ -267,7 +267,7 @@ union b_union {
};
// WARNING: Needs -fno-strict-aliasing compiler option
Square pop_1st_bit(Bitboard *bb) {
Square pop_1st_bit(Bitboard* bb) {
b_union u;
uint32_t b;