RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
From: David Laight
Date: Thu Dec 21 2017 - 09:48:04 EST
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
> 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).