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MINOR: tools: implement functions to look up the nth bit set in a mask

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Function mask_find_rank_bit() returns the bit position in mask <m> of
the nth bit set of rank <r>, between 0 and LONGBITS-1 included, starting
from the left. For example ranks 0,1,2,3 for mask 0x55 will be 6, 4, 2
and 0 respectively. This algorithm is based on a popcount variant and
is described here : https://graphics.stanford.edu/~seander/bithacks.html.
This commit is contained in:
Willy Tarreau 2019-02-26 09:56:22 +01:00
parent 9e85318417
commit f3241115e7
2 changed files with 101 additions and 0 deletions
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14
include/common/standard.h
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@ -938,6 +938,20 @@ static inline unsigned int __full_hash(unsigned int a)
return a * 3221225473U;
}
/* Return the bit position in mask <m> of the nth bit set of rank <r>, between
* 0 and LONGBITS-1 included, starting from the left. For example ranks 0,1,2,3
* for mask 0x55 will be 6, 4, 2 and 0 respectively. This algorithm is based on
* a popcount variant and is described here :
* https://graphics.stanford.edu/~seander/bithacks.html
*/
unsigned int mask_find_rank_bit(unsigned int r, unsigned long m);
unsigned int mask_find_rank_bit_fast(unsigned int r, unsigned long m,
unsigned long a, unsigned long b,
unsigned long c, unsigned long d);
void mask_prep_rank_map(unsigned long m,
unsigned long *a, unsigned long *b,
unsigned long *c, unsigned long *d);
/* sets the address family to AF_UNSPEC so that is_addr() does not match */
static inline void clear_addr(struct sockaddr_storage *addr)
{

87
src/standard.c
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@ -2650,6 +2650,93 @@ unsigned int full_hash(unsigned int a)
return __full_hash(a);
}
/* Return the bit position in mask <m> of the nth bit set of rank <r>, between
* 0 and LONGBITS-1 included, starting from the left. For example ranks 0,1,2,3
* for mask 0x55 will be 6, 4, 2 and 0 respectively. This algorithm is based on
* a popcount variant and is described here :
* https://graphics.stanford.edu/~seander/bithacks.html
*/
unsigned int mask_find_rank_bit(unsigned int r, unsigned long m)
{
unsigned long a, b, c, d;
unsigned int s;
unsigned int t;
a = m - ((m >> 1) & ~0UL/3);
b = (a & ~0UL/5) + ((a >> 2) & ~0UL/5);
c = (b + (b >> 4)) & ~0UL/0x11;
d = (c + (c >> 8)) & ~0UL/0x101;
r++; // make r be 1..64
t = 0;
s = LONGBITS;
if (s > 32) {
t = (d >> 32) + (d >> 48);
s -= ((t - r) & 256) >> 3; r -= (t & ((t - r) >> 8));
}
t = (d >> (s - 16)) & 0xff;
s -= ((t - r) & 256) >> 4; r -= (t & ((t - r) >> 8));
t = (c >> (s - 8)) & 0xf;
s -= ((t - r) & 256) >> 5; r -= (t & ((t - r) >> 8));
t = (b >> (s - 4)) & 0x7;
s -= ((t - r) & 256) >> 6; r -= (t & ((t - r) >> 8));
t = (a >> (s - 2)) & 0x3;
s -= ((t - r) & 256) >> 7; r -= (t & ((t - r) >> 8));
t = (m >> (s - 1)) & 0x1;
s -= ((t - r) & 256) >> 8;
return s - 1;
}
/* Same as mask_find_rank_bit() above but makes use of pre-computed bitmaps
* based on <m>, in <a..d>. These ones must be updated whenever <m> changes
* using mask_prep_rank_map() below.
*/
unsigned int mask_find_rank_bit_fast(unsigned int r, unsigned long m,
unsigned long a, unsigned long b,
unsigned long c, unsigned long d)
{
unsigned int s;
unsigned int t;
r++; // make r be 1..64
t = 0;
s = LONGBITS;
if (s > 32) {
t = (d >> 32) + (d >> 48);
s -= ((t - r) & 256) >> 3; r -= (t & ((t - r) >> 8));
}
t = (d >> (s - 16)) & 0xff;
s -= ((t - r) & 256) >> 4; r -= (t & ((t - r) >> 8));
t = (c >> (s - 8)) & 0xf;
s -= ((t - r) & 256) >> 5; r -= (t & ((t - r) >> 8));
t = (b >> (s - 4)) & 0x7;
s -= ((t - r) & 256) >> 6; r -= (t & ((t - r) >> 8));
t = (a >> (s - 2)) & 0x3;
s -= ((t - r) & 256) >> 7; r -= (t & ((t - r) >> 8));
t = (m >> (s - 1)) & 0x1;
s -= ((t - r) & 256) >> 8;
return s - 1;
}
/* Prepare the bitmaps used by the fast implementation of the find_rank_bit()
* above.
*/
void mask_prep_rank_map(unsigned long m,
unsigned long *a, unsigned long *b,
unsigned long *c, unsigned long *d)
{
*a = m - ((m >> 1) & ~0UL/3);
*b = (*a & ~0UL/5) + ((*a >> 2) & ~0UL/5);
*c = (*b + (*b >> 4)) & ~0UL/0x11;
*d = (*c + (*c >> 8)) & ~0UL/0x101;
}
/* Return non-zero if IPv4 address is part of the network,
* otherwise zero. Note that <addr> may not necessarily be aligned
* while the two other ones must.