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SubjectRE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Date
From: Joe Perches
> Sent: 20 December 2017 17:20
...
> > 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;
> >
>
> Perhaps add:
>
> if (x <= UINT_MAX)
> return int_sqrt((unsigned long)x);

Actually something like:
i = 32;
if (!x_hi) {
x_hi = x;
i = 16;
}

if (!(x_hi & 0xffff0000)) {
x_hi <<= 16;
i -= 8;
}
Repeat for 0xff000000, 0xf0000000 and 0xc0000000 and adjust loop to count down.

David

>
> > 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
> >

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