From mboxrd@z Thu Jan  1 00:00:00 1970
From: mita@miraclelinux.com (Akinobu Mita)
Date: Mon, 06 Feb 2006 11:52:57 +0000
Subject: Re: [patch 14/44] generic hweight{64,32,16,8}()
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References: <20060201090224.536581000@localhost.localdomain> <20060201090325.905071000@localhost.localdomain> <20060202012637.GA25093@iram.es>
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To: Gabriel Paubert <paubert@iram.es>
Cc: linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, Ian Molton <spyro@f2s.com>, David Howells <dhowells@redhat.com>, linuxppc-dev@ozlabs.org, Greg Ungerer <gerg@uclinux.org>, sparclinux@vger.kernel.org, Miles Bader <uclinux-v850@lsi.nec.co.jp>, Linus Torvalds <torvalds@osdl.org>, Yoshinori Sato <ysato@users.sourceforge.jp>, Hirokazu Takata <takata@linux-m32r.org>, linuxsh-shmedia-dev@lists.sourceforge.net, linux-m68k@vger.kernel.org, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Richard Henderson <rth@twiddle.net>, Chris Zankel <chris@zankel.net>, dev-etrax@axis.com, ultralinux@vger.kernel.org, Andi Kleen <ak@suse.de>, linuxsh-dev@lists.sourceforge.net, linux390@de.ibm.com, Russell King <rmk@arm.linux.org.uk>, parisc-linux@parisc-linux.org, Balbir Singh <bsingharora@gmail.com>, linux@horizon.com

On Thu, Feb 02, 2006 at 02:26:38AM +0100, Gabriel Paubert wrote:
> 
> The first step can be implemented slightly better:
> 
> unsigned int res = w-((w>>1)&0x55555555);
> 

Yes. I've got many advices about hweight speedup.


static unsigned int hweight32(unsigned int w)
{
        unsigned int res = w - ((w >> 1) & 0x55555555);
        res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
        res = (res + (res >> 4)) & 0x0F0F0F0F;
        res = res + (res >> 8);
        return (res + (res >> 16)) & 0x000000FF;
}

static unsigned int hweight16(unsigned int w)
{
        unsigned int res = w - ((w >> 1) & 0x5555);
        res = (res & 0x3333) + ((res >> 2) & 0x3333);
        res = (res + (res >> 4)) & 0x0F0F;
        return (res + (res >> 8)) & 0x00FF;
}

static unsigned int hweight8(unsigned int w)
{
        unsigned int res = w - ((w >> 1) & 0x55);
        res = (res & 0x33) + ((res >> 2) & 0x33);
        return (res + (res >> 4)) & 0x0F;
}

static unsigned long hweight64(__u64 w)
{
#if BITS_PER_LONG < 64
	return hweight32((unsigned int)(w >> 32)) +
				hweight32((unsigned int)w);
#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;
#endif
}