Hans Petter Selasky wrote this message on Thu, Dec 04, 2014 at 08:21 +0100:
> On 12/04/14 01:54, John-Mark Gurney wrote:
> >Hans Petter Selasky wrote this message on Wed, Dec 03, 2014 at 21:55 +0000:
> >>Author: hselasky
> >>Date: Wed Dec 3 21:55:44 2014
> >>New Revision: 275468
> >>URL: https://svnweb.freebsd.org/changeset/base/275468
> >>
> >>Log:
> >> Optimise the bit searching loops, by quickly skipping the 16 first set
> >> bits if all the 16 first bits are set. This way the worst case
> >> searching time is reduced from 32 to 16 cycles.
> >
> >You could use ffs instead:
> > x = ffs(~map);
> > if (x) {
> > x--;
> > /* normal code */
> > }
> >
> >This has the benefit of using a single instruction on platforms that
> >support it (bsfl on i386), though apparently, we haven't optimized this
> >for all platforms... arm has a version for int, but their ffsl does
> >the same linear search instead of just calling ffs, or at least
> >detecting if sizeof(long) == sizeof(int) and calling ffs...
> >
>
> Yes, I'm aware about that, but like you say it is not optimised for all
> platforms yet. So I'm not sure if it will give any benefit for the
> platform the driver is running on ....
>
> I'll see if I can change the logic inside the C-version of ffs() and the
> do the swap like you suggest.
Did you see the comment about usig __builtin_ffs{,l,ll} instead? this
seems to be a better route...
I was going to say we should leave the code for compatibility w/ other
compilers, now that we are getting close to having an external tool
chain, but I don't see us supporting non-gcc and non-clang compilers
anytime soon, and it isn't hard to bring back the code...
So, how about changing libkern.h to define the various ones to the
built in, and dropping all the special versions? This should be an
even better improvement as it'll eliminate some function call overhead..
Probably the same should be done in strings.h in userland too...
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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