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

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Message-ID: <CAKv63uscGw61r3Dezf_4-YXo-0JfuFpcuWa5gQa9Uo4J6b5+SA@mail.gmail.com>
Date:   Wed, 20 Dec 2017 16:41:46 +0100
From:   Crt Mori <cmo@...exis.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt

On 20 December 2017 at 15:39, David Laight <David.Laight@...lab.com> wrote:
> From: Crt Mori
>> Sent: 20 December 2017 14:20
>>
>> There is no option to perform 64bit integer sqrt on 32bit platform.
>> Added stronger typed int_sqrt64 enables the 64bit calculations to
>> be performed on 32bit platforms. Although int_sqrt() is a rough
>> approximation, the same algorithm is used in int_sqrt64() as good
>> enough on 32bit platform.
>
> The algorithm used gives an exact (rounded down) square root,
> not an approximation.
>

OK, noted. This is from new changed function indeed - will change in v11.

> With minor changes it ought to be possible to remove most of the
> 64bit arithmetic and shifts.
>
> If you care about performance then using 32 bit maths will be much faster.
>

I need precision not the performance. That is why I would like to
leave int_sqrt as one for good performance while my will be used when
you require a precision.

>         David
>