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;
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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