On Tue, Feb 14, 2012 at 10:58:39PM +0000, Al Viro wrote:
> On Wed, Feb 15, 2012 at 01:45:27AM +0300, Alexey Dobriyan wrote:
> > 1) consistently use "unsigned int" for dentry name length,
> > 2) reuse subtraction result for return value, exact value doesn't matter
> > because function is only used in boolean context,
> > 3) use *p++ idiom for even better code.
> >
> > All of this results in performance speedup of "git diff"
> > which is way out of statistical error (0.4% vs 0.15% of 3 sigma):
>
> > - if (scount != tcount)
> > - return 1;
> > +
> > + ret = scount - tcount;
> > + if (ret)
> > + return ret;
> > do {
> > - ret = (*cs != *ct);
> > + ret = *cs++ - *ct++;
> > if (ret)
> > break;
> > - cs++;
> > - ct++;
> > tcount--;
> > } while (tcount);
> > return ret;
>
> I wonder what'll happen if you simply replace that loop with
> return memcmp(cs, ct, tcount);
> which is what it really is...
It was memcmp() at some point and Nick changed it and it became faster.
See 9d55c369bb5e695e629bc35cba2ef607755b3bee
"fs: implement faster dentry memcmp".
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