[Bug target/105428] compilation never (?) finishes with __builtin_casinl() and __builtin_csqrtl() with -O -mlong-double-128

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105428

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
If you can identify specific arguments passed to mpc_asin for which it is 
excessively slow, that should be reported as an MPC bug.

Computing correctly rounded mpc_asin shouldn't need to be that slow - 
provided the algorithm used is appropriate to the input value.  See for 
example how glibc implements casin / casinh / cacos / cacosh.  Or 
https://dl.acm.org/doi/10.1145/275323.275324 (Hull et al, Implementing the 
complex arcsine and arccosine functions using exception handling, ACM TOMS 
vol. 23 no. 3 (Sep 1997) pp 299-335).  That may require several different 
algorithms to be implemented, but each such algorithm is straightforward.  
That's different from the case of Bessel functions of high order - for 
which there is some literature about computational techniques that 
shouldn't take time proportional to the order, but where the algorithms 
are certainly a lot more complicated.