Use of C99 extra long double math functions after r236148

[Warner Losh]

On Jul 8, 2012, at 6:40 AM, David Schultz wrote:

> On Tue, May 29, 2012, Peter Jeremy wrote:
>> On 2012-May-28 15:54:06 -0700, Steve Kargl <sgk at troutmask.apl.washington.edu> wrote:
>>> Given that cephes was written years before C99 was even
>>> conceived, I suspect all functions are sub-standard.
>> 
>> Well, most of cephes was written before C99.  The C99 parts of
>> cephes were written to turn it into a complete C99 implementation.
> 
> I'm a bit late to the party, but I thought I'd chime in with some
> context.  We did consider using Cephes years ago, and even got
> permission from the author to release it under an acceptable license.
> We later decided not to use it for technical reasons.
> 
> By the way, virtually none of the people who have complained about the
> missing functions actually need them.  Mostly they just want to
> compile some software that was written by a naive programmer who
> thought it would be cool to use the most precise type available.  The
> complex functions are even less commonly needed, and the truth is that
> they have no business being part of the C standard anyway.
> 
> The question remains of what to do about the missing functions.  Bruce
> and Steve have been working on expl and logl for years.  If those ever
> get in the tree, the remaining long double functions are easy.  Those
> functions are basically done, modulo a bunch of cleanup and testing,
> and I encourage any mathematically inclined folks who are interested
> in pushing things along to get in touch with them.  I'm not going to
> have any time myself for a few months at least.

Where can I find these?

> Lastly, there's the question of mediocre alternatives, such as
> solutions that get the boundary cases wrong or don't handle 128-bit
> floating point.  For the exponential and logarithmic functions, Bruce
> and Steve have already written good implementations, so there's no
> reason to settle for less.  As for the other long double functions,
> bringing in some Cephes code in a separate directory as a temporary
> fix might be the way to go.  I don't like that solution, and Steve
> raises some good technical points about why it isn't ideal; however, a
> better solution is more than a decade overdue, and people are
> justified in finding that unacceptable.

Don't let the perfect be the enemy of the good.  It is better to have OK implementations of these functions than none at all.  We originally had so-so double support, but bruce spent many years optimizing them to make them very good.  If we were just starting out, and hadn't let 10 years get behind us, I'd give the substandard argument some weight.  But now that we're 13 years down the line from c99's publication I think we need to relax our standards a bit.  I'd even argue that these functions being a little bad could easily spur people to make them better.  Their absence makes people just #define llexp(x) lexp(x), etc. :(

Warner

Warner