Re: Slaying the HYPOTamus

Tom Lane wrote:

> Greg Stark <gsstark(at)mit(dot)edu> writes:
>
>> If there's a performance advantage then we could add a configure test
>> and define the macro to call hypot(). You said it existed before C99
>> though, how widespread was it? If it's in all the platforms we support
>> it might be reasonable to just go with it.
>>
>
> For one data point, I see hypot() in HPUX 10.20, released circa 1996.
> I suspect we would want a configure test and a substitute function
> anyway. Personally I wouldn't have a problem with the substitute being
> the naive sqrt(x*x+y*y), particularly if it's replacing existing code
> that overflows in the same places.
>
> regards, tom lane
>
>

A hypot() substitute might look something like this psudo-code, this is
how Python does it if the real hypot() is missing.

double hypot( double dx, double dy )
{
double yx;

if( isinf(dx) || ifinf(dy) ) {
return INFINITY;
}

dx = fabs(dx);
dy = fabs(dy);
if (dx < dy) {
double temp = dx;
dx = dy;
dy = temp;
}
if (x == 0.)
return 0.;
else {
yx = dy/dx;
return dx*sqrt(1.0+yx*yx);
}
}

As the following link shows, a lot of care could be put into getting a
substitute hypot() correct.
http://gforge.inria.fr/plugins/scmsvn/viewcvs.php/trunk/hypot.c?rev=5677&root=mpfr&view=markup