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Date:   Thu, 21 Dec 2017 10:59:19 +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
> >> OK, is there any more easy optimizations you see?
> >
> > I think this version works.
> > It doesn't have the optimisation for small values.
> >
> > unsigned int sqrt64(unsigned long long x)
> > {
> >         unsigned int x_hi = x >> 32;
> >
> >         unsigned int b = 0;
> >         unsigned int y = 0;
> >         unsigned int i;
> >
> >         for (i = 0; i < 32; i++) {
> >                 b <<= 2;
> >                 b |= x_hi >> 30;
> >                 x_hi <<= 2;
> >                 if (i == 15)
> >                         x_hi = x;
> >                 y <<= 1;
> >                 if (b > y)
> >                         b -= ++y;
> >         }
> >         return y;
> > }
..
> 
> I did a quick run through unit tests for the sensor and the results
> are way off. On the sensor I had to convert double calculations to
> integer calculations and target was to get end result within 0.02 degC
> (with previous approximate sqrt implementation) in sensor working
> range. This now gets into 3 degC delta at least and some are way off.
> It might be off because of some scaling on the other hand during the
> equation (not exactly comparing sqrt implementations here).

I didn't get it quite right...
The last few lines need to be:
		if (b > y) {	
			b -= ++y;
			y++;
		}
	}
	return y >> 1;
}

Although that then fails for inputs larger than 2^62.

	David

	

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