lkml.org 
[lkml]   [2017]   [Dec]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Date
From: Peter Zijlstra
> Sent: 21 December 2017 14:12
...
> > > This part above looks like FLS
> > It also does the rest of the required shifts.
>
> Still, fls() + shift is way faster on hardware that has an fls
> instruction.
>
> Writing out that binary search doesn't make sense.

If the hardware doesn't have an appropriate fls instruction
the soft fls()will be worse.

If you used fls() you'd still need quite a bit of code
to generate the correct shift and loop count adjustment.
Given the cost of the loop iterations the 3 tests are noise.
The open coded version is obviously correct...

I didn't add the 4th one because the code always does 2 iterations.

If you were really worried about performance there are faster
algorithms (even doing 2 or 4 bits a time is faster).

David

\
 
 \ /
  Last update: 2017-12-21 15:48    [W:0.076 / U:0.460 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site