RFC: removing slow paths in various dbl-64 libm functions

[Joseph Myers]

On Tue, 1 Dec 2015, Szabolcs Nagy wrote:

> (low precision libm cannot solve this completely as the general
> bessel functions will take an enormous time depending on the n
> argument:
> 
>  double yn(int,double);
>  int main(){return yn(1e9,1e9);}
> 
> will dos gcc when it tries to eval yn at compile-time,
> but for other math functions than yn,jn a low time
> bound can be guaranteed for all inputs.)

A good implementation of cpow (i.e. following glibc's normal accuracy 
goals; see bug 14473; ISO C explicitly intends to allow less good 
implementations along the existing lines as cexp (y * clog (x))) will 
require multiple-precision computation of log and atan2 to over 16000 
places in some cases.  The time can be bounded, but the bound may not be 
that low.

-- 
Joseph S. Myers
joseph@codesourcery.com