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

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Date:   Thu, 21 Dec 2017 14:17:43 +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

On 21 December 2017 at 12:43, David Laight <David.Laight@...lab.com> wrote:
> From: Crt Mori
>> Sent: 20 December 2017 17:30
>> I did a quick run through unit tests for the sensor and the results
>> are way off
>> ...
>
> Try this version instead:
> unsigned int sqrt64(unsigned long long x_in)
> {
>         unsigned int x = x_in >> 32;
>
>         unsigned int b = 0;
>         unsigned int y = 0;
>         unsigned int i;

i can be u8. And I will still use explicit typing.

>
>         i = 31;
>         if (!x) {
>                 x = x_in;
>                 i = 15;
>         }
>         if (!(x & 0xffff0000)) {
>                 x <<= 16;
>                 i -= 8;
>         }
>         if (!(x & 0xff000000)) {
>                 x <<= 8;
>                 i -= 4;
>         }
>         if (!(x & 0xf0000000)) {
>                 x <<= 4;
>                 i -= 2;
>         }
>

This part above looks like FLS

>         do {
>                 b <<= 2;
>                 b |= x >> 30;
>                 x <<= 2;
>                 if (i == 16)
>                         x = x_in;
>                 y <<= 1;
>                 if (b > y) {
>                         b -= ++y;
>                         y++;
>                 }
>         } while (--i);
>
>         /* 'b' becomes 33 bits if the input is greater than 2^62 */
>         b <<= 1;
>         b |= x >> 31;
>         if (b > y || (b == y && x & (1u << 30)))
>                 y |= 1;
>
>         return y;
> }
>
> I've tested that one with more values.
>
>         David
>

This one indeed works. I did some more testing this morning and I am
fine with either.

So question is: Do I make change as per David's suggestion with his
sign-off, or leave the version it was before the change?