On Wed, 2017-12-20 at 16:46 +0000, David Laight wrote:
> From: Crt Mori
> > Sent: 20 December 2017 16:17
> >
> > On 20 December 2017 at 17:00, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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;
>
Perhaps add:
if (x <= UINT_MAX)
return int_sqrt((unsigned long)x);
> 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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