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popcount() optimization for speed.

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We run the array by 32 bit words instead of processing it byte per byte.
If the code is compiled using GCC __builtin_popcount() builtin function
is used instead.
This commit is contained in:
antirez 2012-05-20 00:49:35 +02:00
parent dbbbe49ef5
commit 343d3bd287
1 changed files with 18 additions and 2 deletions
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20
src/bitop.c
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@ -30,10 +30,26 @@ static int getBitOffsetFromArgument(redisClient *c, robj *o, size_t *offset) {
* work with a input string length up to 512 MB. */
long popcount(void *s, long count) {
long bits = 0;
unsigned char *p = s;
unsigned char *p;
uint32_t *p4 = s;
static const unsigned char bitsinbyte[256] = {0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,4,5,5,6,5,6,6,7,5,6,6,7,6,7,7,8};
/* We can finally count bits. */
/* Count bits four bytes at a time */
while(count>=4) {
uint32_t aux = *p4++;
count -= 4;
#ifdef __GNUC__
/* Unsigned int is always >= 4 bytes if compiler is GCC */
bits += __builtin_popcount(aux);
#else
bits += bitsinbyte[aux&0xff] +
bitsinbyte[(aux>>8)&0xff] +
bitsinbyte[(aux>>16)&0xff] +
bitsinbyte[(aux>>24)&0xff];
#endif
}
/* Count the remaining bytes */
p = (unsigned char*)p4;
while(count--) bits += bitsinbyte[*p++];
return bits;
}