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

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Date:   Wed, 20 Dec 2017 16:46:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Crt Mori' <cmo@...exis.com>, 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-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 16:17
> 
> On 20 December 2017 at 17:00, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Dec 20, 2017 at 02:39:26PM +0000, David Laight wrote:
> >
> >> 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.
> >
> > Some, u64 add/sub/shift isn't exactly expensive, but yes, I also
> > indicated that improvement is possible. At the very least y can be made
> > a u32 I suppose.
> 
> 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;
}

Put it through cc -O3 -m32 -c -o sqrt64.o sqrt64.c and then objdump sqrt64.o
and compare to that of your version.

	David