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Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h

To: Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h
From: Edgar Toernig <froese@gmx.de>
Date: Thu, 26 Jan 2006 05:34:12 +0100
Cc: Akinobu Mita <mita@miraclelinux.com>, linux-kernel@vger.kernel.org, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Ian Molton <spyro@f2s.com>, dev-etrax@axis.com, David Howells <dhowells@redhat.com>, Yoshinori Sato <ysato@users.sourceforge.jp>, Linus Torvalds <torvalds@osdl.org>, linux-ia64@vger.kernel.org, Hirokazu Takata <takata@linux-m32r.org>, linux-m68k@vger.kernel.org, Greg Ungerer <gerg@uclinux.org>, linux-mips@linux-mips.org, parisc-linux@parisc-linux.org, linuxppc-dev@ozlabs.org, linux390@de.ibm.com, linuxsh-dev@lists.sourceforge.net, linuxsh-shmedia-dev@lists.sourceforge.net, sparclinux@vger.kernel.org, ultralinux@vger.kernel.org, Miles Bader <uclinux-v850@lsi.nec.co.jp>, Andi Kleen <ak@suse.de>, Chris Zankel <chris@zankel.net>
In-reply-to: <20060126000618.GA5592@twiddle.net>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20060125112625.GA18584@miraclelinux.com> <20060125113206.GD18584@miraclelinux.com> <20060125200250.GA26443@flint.arm.linux.org.uk> <20060126000618.GA5592@twiddle.net>
Sender: linux-mips-bounce@linux-mips.org
Richard Henderson wrote:
>
> On Wed, Jan 25, 2006 at 08:02:50PM +0000, Russell King wrote:
> > > + s = 16; if (word << 16 != 0) s = 0; b += s; word >>= s;
> > > + s =  8; if (word << 24 != 0) s = 0; b += s; word >>= s;
> > > + s =  4; if (word << 28 != 0) s = 0; b += s; word >>= s;
> ...
> > Basically, shifts which depend on a variable are more expensive than
> > constant-based shifts.
> 
> Actually, they're all constant shifts.  Just written stupidly.

Why shift at all?

int ffs(u32 word)
{
    int bit = 0;

    word &= -word; // only keep the lsb.

    if (word & 0xffff0000) bit |= 16;
    if (word & 0xff00ff00) bit |=  8;
    if (word & 0xf0f0f0f0) bit |=  4;
    if (word & 0xcccccccc) bit |=  2;
    if (word & 0xaaaaaaaa) bit |=  1;

    return bit;
}

Ciao, ET.

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