linux-kernel - Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int

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Date:   Tue, 19 Dec 2017 10:31:18 +0100
From:   Crt Mori <cmo@...exis.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Iio <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int_sqrt

On 18 December 2017 at 17:44, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Dec 18, 2017 at 04:05:44PM +0100, Crt Mori wrote:
>> 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.
>
> You clearly haven't read a recent version of the file you're patching.
> Please take a moment to do so now.
>

IIO kernel does not have the recent version in, so thanks for heads
up. It does not change much for my function.

>> +/**
>> + * int_sqrt64 - strongly typed int_sqrt function
>> + * @x: 64bit integer of which to calculate the sqrt
>> + */
>> +u64 int_sqrt64(u64 x)
>
> Please explain how the result of sqrt(u64) can be larger than u32.
>

My hand calculator tells me it could be.

ffffffff×ffffffff = FFFFFFFE00000001 which still has some margin which
will end up above the 32 bit number. Further more the __fls
optimization automatically casts the inputs to unsigned long (32 bit
on 32 bit machines), so that also makes it out of option.

> Also, I expect that this fact could be exploited to optimize this for
> 32bit archs if one were so inclined.
>
>> +{
>> +     u64 b, m, y = 0;
>> +
>> +     if (x <= 1)
>> +             return x;
>> +
>> +     m = 1ULL << (64 - 2);
>> +     while (m != 0) {
>> +             b = y + m;
>> +             y >>= 1;
>> +
>> +             if (x >= b) {
>> +                     x -= b;
>> +                     y += m;
>> +             }
>> +             m >>= 2;
>> +     }
>> +     return y;
>> +}
>
> so yeah, no, please try again after reading the current file.