Use of C99 extra long double math functions after r236148

[Peter Jeremy]

On 2012-Jul-20 21:27:41 -0500, Stephen Montgomery-Smith <stephen at missouri.edu> wrote:
>The way they handle the real part of clog(z), log(hypot(x,y)), when 
>hypot(x,y) is close to 1, is to do the calculation in double precision. 
>  So to do it properly, we need a double double precision arithmetic, 
>which we don't have.

Actually, we do.  r230363 includes both extended and quad long-double
emulation code in lib/libc/softfloat.  There's also ld128 code under
lib/libc/sparc64/fpu.

>The paper says that simulating double precision arithmetic using single 
>arithmetic is slow, so I would think the same is true for quad-precision.

It depends how much af the arithmetic needs to be in more-than-double
precision.  Possibly, careful choice of partitioning would allow double
precision to be used for most values with multi-precision only needed
some of the time.

I expect catanh() will run into similar problems evaluating
clog((1 + z)/(1 - z)).  Something like clog1p() would help when z 
is close to 1, as will using alternative expansions.

>Does the inexact flag also raise the SIGFPE signal?

It's under program control - see feenableexcept() and fedisableexcept(),

-- 
Peter Jeremy
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