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

Edgar Toernig <[email protected]> Thu, 26 Jan 2006 04:34:12 +0000

Newsgroups	org.kernel.vger.ultralinux,org.kernel.vger.linux-ia64,org.kernel.vger.linux-kernel,org.kernel.vger.linux-mips,org.kernel.vger.sparclinux,org.ozlabs.lists.linuxppc-dev
Message-ID	<[email protected]>

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.