linux-kernel - Re: [PATCH v10 1/3] lib: Add strongly typed 64bit int

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Date:   Fri, 22 Dec 2017 14:44:35 +0100
From:   Crt Mori <cmo@...exis.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ian Abbott <abbotti@....co.uk>,
        Larry Finger <Larry.Finger@...inger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@...natech.se>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt

>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@...lab.com> 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
>