allow ffs & co. a binary search

[Andriy Gapon]

On 07/06/2015 16:54, Konstantin Belousov 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.

Out of curiosity, what are those (Bruce's) 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.

It seems that both clang and gcc are smart enough to replace ffs*() with
__builtin_ffs*() which expand to the corresponding instructions.
On the other hand, neither clang nor gcc has __builtin_fls*() and as far as I
can see neither does anything special for fls*() calls.

Funny that __builtin_clz is complemented by __builtin_ctz, but there is no
counterpart to __builtin_ffs.

Lastly, I see no reason to have have different implementations of these
functions for the kernel and userland.
-- 
Andriy Gapon