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[PATCH] optimize hweight64 for x86_64

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Based on patch from David Rientjes <rientjes@google.com>, but
changed by AK.

Optimizes the 64-bit hamming weight for x86_64 processors assuming they
have fast multiplication.  Uses five fewer bitops than the generic
hweight64.  Benchmark on one EMT64 showed ~25% speedup with 2^24
consecutive calls.

Define a new ARCH_HAS_FAST_MULTIPLIER that can be set by other
architectures that can also multiply fast.

Signed-off-by: Andi Kleen <ak@suse.de>
This commit is contained in:
Andi Kleen 2006-09-26 10:52:38 +02:00 committed by Andi Kleen
parent 8380aabb99
commit 0136611c62
2 changed files with 10 additions and 2 deletions
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2
include/asm-x86_64/bitops.h
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@ -399,6 +399,8 @@ static __inline__ int fls(int x)
return r+1;
}
#define ARCH_HAS_FAST_MULTIPLIER 1
#include <asm-generic/bitops/hweight.h>
#endif /* __KERNEL__ */

10
lib/hweight.c
View file

@ -1,5 +1,6 @@
#include <linux/module.h>
#include <asm/types.h>
#include <asm/bitops.h>
/**
* hweightN - returns the hamming weight of a N-bit word
@ -40,14 +41,19 @@ unsigned long hweight64(__u64 w)
#if BITS_PER_LONG == 32
return hweight32((unsigned int)(w >> 32)) + hweight32((unsigned int)w);
#elif BITS_PER_LONG == 64
#ifdef ARCH_HAS_FAST_MULTIPLIER
w -= (w >> 1) & 0x5555555555555555ul;
w = (w & 0x3333333333333333ul) + ((w >> 2) & 0x3333333333333333ul);
w = (w + (w >> 4)) & 0x0f0f0f0f0f0f0f0ful;
return (w * 0x0101010101010101ul) >> 56;
#else
__u64 res = w - ((w >> 1) & 0x5555555555555555ul);
res = (res & 0x3333333333333333ul) + ((res >> 2) & 0x3333333333333333ul);
res = (res + (res >> 4)) & 0x0F0F0F0F0F0F0F0Ful;
res = res + (res >> 8);
res = res + (res >> 16);
return (res + (res >> 32)) & 0x00000000000000FFul;
#else
#error BITS_PER_LONG not defined
#endif
#endif
}
EXPORT_SYMBOL(hweight64);