Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h
Edgar Toernig <[email protected]>
| Newsgroups | gmane.linux.ports.ultrapenguin,gmane.linux.kernel,gmane.linux.ports.ia64,gmane.linux.ports.hppa,gmane.linux.ports.ppc.devel,gmane.linux.ports.sh.devel,gmane.linux.ports.sparc |
|---|---|
| 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.
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