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

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Date:   Thu, 21 Dec 2017 11:43:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Crt Mori' <cmo@...exis.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: 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 = 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;
        }

        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