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bitcount32: replace lengthy comment with a reference to SWAR

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MFC after:	5 days
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avg 2011-05-10 15:08:13 +00:00
parent f59b74bdc1
commit 2d672b24ae
1 changed files with 2 additions and 38 deletions
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40
sys/sys/systm.h
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@ -374,44 +374,8 @@ int alloc_unrl(struct unrhdr *uh);
void free_unr(struct unrhdr *uh, u_int item);
/*
* This is about as magic as it gets. fortune(1) has got similar code
* for reversing bits in a word. Who thinks up this stuff??
*
* Yes, it does appear to be consistently faster than:
* while (i = ffs(m)) {
* m >>= i;
* bits++;
* }
* and
* while (lsb = (m & -m)) { // This is magic too
* m &= ~lsb; // or: m ^= lsb
* bits++;
* }
* Both of these latter forms do some very strange things on gcc-3.1 with
* -mcpu=pentiumpro and/or -march=pentiumpro and/or -O or -O2.
* There is probably an SSE or MMX popcnt instruction.
*
* I wonder if this should be in libkern?
*
* XXX Stop the presses! Another one:
* static __inline u_int32_t
* popcnt1(u_int32_t v)
* {
* v -= ((v >> 1) & 0x55555555);
* v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
* v = (v + (v >> 4)) & 0x0F0F0F0F;
* return (v * 0x01010101) >> 24;
* }
* The downside is that it has a multiply. With a pentium3 with
* -mcpu=pentiumpro and -march=pentiumpro then gcc-3.1 will use
* an imull, and in that case it is faster. In most other cases
* it appears slightly slower.
*
* Another variant (also from fortune):
* #define BITCOUNT(x) (((BX_(x)+(BX_(x)>>4)) & 0x0F0F0F0F) % 255)
* #define BX_(x) ((x) - (((x)>>1)&0x77777777) \
* - (((x)>>2)&0x33333333) \
* - (((x)>>3)&0x11111111))
* Population count algorithm using SWAR approach
* - "SIMD Within A Register".
*/
static __inline uint32_t
bitcount32(uint32_t x)