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From: David Laight <David.Laight@ACULAB.COM>
To: "'Crt Mori'" <cmo@melexis.com>, Peter Zijlstra <peterz@infradead.org>
CC: Jonathan Cameron <jic23@kernel.org>, Ingo Molnar <mingo@kernel.org>,
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        Joe Perches <joe@perches.com>
Subject: RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Thread-Topic: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
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Date: Wed, 20 Dec 2017 16:46:25 +0000
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From: Crt Mori
> Sent: 20 December 2017 16:17
> 
> On 20 December 2017 at 17:00, Peter Zijlstra <peterz@infradead.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