[Bug fortran/112873] F2023 degree trig functions

["sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112873

--- Comment #7 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Thu, Dec 07, 2023 at 07:59:25PM +0000, anlauf at gcc dot gnu.org wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112873
> 
> --- Comment #6 from anlauf at gcc dot gnu.org ---
> (In reply to Steve Kargl from comment #5)
> > On Wed, Dec 06, 2023 at 09:58:18PM +0000, anlauf at gcc dot gnu.org wrote:
> > > In your experience, how good (or bad) is a naive inline version
> > > like we do for the degree versions?
> > 
> > It would be possible to use a naive version, but it will
> > likely not approach the requirements placed on these functions
> > by at least the IEEE 754 and C23 standards.
> 
> I see.  The naive version would have done only range reduction and scaling,
> maybe good enough for F2023 ("SINPI (X) is approximately equal to SIN (X×π)")
> but not more.
> 
> Also, one needs mpfr-4.2.0 or higher for simple simplification, or do more
> work.

Correct. A newer version of MPFR contains these functions.  In fact,
the MPFR developer and I cross-checked each other.  We can probably
use a naive implementation with high precision if an older MPFR is
used.

> > I think gfortran
> > should test with configure for these functions in libm and
> > use those if available (e.g., HAVE_COSPI, etc).  In libgfortran,
> > we'll need fallbacks if not available.  To see the complexity
> > of the implementation details see
> > 
> > https://cgit.freebsd.org/src/tree/lib/msun/src/s_cospi.c
> > 
> > Header files defining computation kernels and these use
> > the computation kernels for sin() and cos() from fdlibm.
> > 
> > https://cgit.freebsd.org/src/tree/lib/msun/src/k_cospi.h
> > https://cgit.freebsd.org/src/tree/lib/msun/src/k_sinpi.h
> > 
> > Note, AFAIK, glibc does not implement these functions.
> 
> Right.
> 
> Would you like to open a separate PR for dealing with the half-cycle
> trigonometric functions?

Yes, I'll add a new PR for the half-cycle functions, and 
point to here for the initial discussion.