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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
View file

@ -161,7 +161,7 @@ const int RShift[64] = {
21, 22, 22, 22, 22, 22, 22, 21, 20, 21, 21, 21, 21, 21, 21, 20 21, 22, 22, 22, 22, 22, 22, 21, 20, 21, 21, 21, 21, 21, 21, 20
}; };
#endif #endif // defined(IS_64BIT)
Bitboard RMask[64]; Bitboard RMask[64];
@ -245,16 +245,16 @@ void init_bitboards() {
/// pop_1st_bit() finds and clears the least significant nonzero bit in a /// pop_1st_bit() finds and clears the least significant nonzero bit in a
/// nonzero bitboard. /// 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); Bitboard bb = *b ^ (*b - 1);
uint32_t fold = int(bb) ^ int(bb >> 32); uint32_t fold = int(bb) ^ int(bb >> 32);
*b &= (*b - 1); *b &= (*b - 1);
return Square(BitTable[(fold * 0x783a9b23) >> 26]); return Square(BitTable[(fold * 0x783a9b23) >> 26]);
} }
#else #elif !defined(USE_BSFQ)
// Use type-punning // Use type-punning
union b_union { union b_union {
@ -267,7 +267,7 @@ union b_union {
}; };
// WARNING: Needs -fno-strict-aliasing compiler option // WARNING: Needs -fno-strict-aliasing compiler option
Square pop_1st_bit(Bitboard *bb) { Square pop_1st_bit(Bitboard* bb) {
b_union u; b_union u;
uint32_t b; uint32_t b;

26
src/bitboard.h
View file

@ -36,6 +36,11 @@
#define IS_64BIT #define IS_64BIT
#endif #endif
#if defined(IS_64BIT) && (defined(__GNUC__) || defined(__INTEL_COMPILER))
#define USE_BSFQ
#endif
//// ////
//// Includes //// Includes
//// ////
@ -383,14 +388,24 @@ inline Bitboard isolated_pawn_mask(Square s) {
/// first_1() finds the least significant nonzero bit in a nonzero bitboard. /// first_1() finds the least significant nonzero bit in a nonzero bitboard.
/// pop_1st_bit() finds and clears the least significant nonzero bit in a
/// nonzero bitboard.
#if defined(IS_64BIT) #if defined(USE_BSFQ) // Assembly code by Heinz van Saanen
inline Square first_1(Bitboard b) { inline Square __attribute__((always_inline)) first_1(Bitboard b) {
return Square(BitTable[((b & -b) * 0x218a392cd3d5dbfULL) >> 58]); Bitboard dummy;
__asm__("bsfq %1, %0": "=r"(dummy): "rm"(b) );
return (Square)(dummy);
} }
#else inline Square __attribute__((always_inline)) pop_1st_bit(Bitboard* b) {
const Square s = first_1(*b);
*b &= ~(1ULL<<s);
return s;
}
#else // if !defined(USE_BSFQ)
inline Square first_1(Bitboard b) { inline Square first_1(Bitboard b) {
b ^= (b - 1); b ^= (b - 1);
@ -398,6 +413,8 @@ inline Square first_1(Bitboard b) {
return Square(BitTable[(fold * 0x783a9b23) >> 26]); return Square(BitTable[(fold * 0x783a9b23) >> 26]);
} }
extern Square pop_1st_bit(Bitboard* b);
#endif #endif
@ -407,7 +424,6 @@ inline Square first_1(Bitboard b) {
extern void print_bitboard(Bitboard b); extern void print_bitboard(Bitboard b);
extern void init_bitboards(); extern void init_bitboards();
extern Square pop_1st_bit(Bitboard *b);
#endif // !defined(BITBOARD_H_INCLUDED) #endif // !defined(BITBOARD_H_INCLUDED)