allow ffs & co. a binary search

[Davide Italiano]

On Sun, Jun 7, 2015 at 6:54 AM, Konstantin Belousov <kostikbel at gmail.com> wrote:
> On Sun, Jun 07, 2015 at 07:52:45PM +0800, Erich Dollansky wrote:
>> What I saw is that all CPUs except ARM uses the software version [of ffs].
>
> Without quantifiers, this statement is not true. i386 libc function ffs(3)
> uses bsfl instruction to do the job.  Compilers know about ffs(3) and friends
> as well, so e.g. gcc 5.1.0 generates the following code for the given
> fragment:
>         return (ffs(x) + 1);
> is translated to
>    0:   0f bc c7                bsf    %edi,%eax
>    3:   ba ff ff ff ff          mov    $0xffffffff,%edx
>    8:   0f 44 c2                cmove  %edx,%eax
>    b:   83 c0 02                add    $0x2,%eax
> (arg in %edi, result in %eax).
>
> I wrote a patch for amd64 libc long time ago to convert ffs/fls etc to use
> of the bitstring instruction, but Bruce Evans argued that this would be
> excessive.  Your patch is excessive for the similar reasons.
>
> My guess is that significantly clever compiler would recognize a pattern
> used by native ffs implementation and automatically use bitstring
> instructions. E.g., this already happens with popcnt and recent
> gcc/clang, I am just lazy to verify ffs.
> _______________________________________________
> freebsd-hackers at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe at freebsd.org"

Clang trunk to the best of my knowledgde hasn't a way to recognize
ffs() pattern.
http://llvm.org/docs/doxygen/html/LoopIdiomRecognize_8cpp_source.html
I can't comment about gcc as long as I'm not familiar with the implementation.

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare