Re: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt

[Crt Mori]

>From simple strong typing of existing int_sqrt we came to something a
bit more complex or better. Can we decide now which we want in, or I
submit v12 and we decide then (although it is not a v12, but whole new
thing)?

On 21 December 2017 at 15:48, David Laight <David.Laight@xxxxxxxxxx> wrote:
> 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
>