svn commit: r290605 - in head/lib/msun: . man

[NGie Cooper]

Hi Bruce!

> On Nov 9, 2015, at 04:03, Bruce Evans <brde at optusnet.com.au> wrote:
> 
> On Mon, 9 Nov 2015, Garrett Cooper wrote:
> 
>> Log:
>> Document powl(3)
> 
> powl was garbage that was intentionally undocumented.  At least, I
> intentionally ignored its non-documentation together with it.

POSIX documents it and I noticed it was missing when porting the msun testcases from NetBSD. That’s the reason why I filed the bug.

> powl doesn't compute the value of .Ar x to the exponent .Ar y.  It computes
> the value of (double)(.Ar x) to the exponent (double)(.Ar y), converted to
> double.

Hmmm? The types look ok per the function signatures in lib/msun:

lib/msun/src/imprecise.c:imprecise_powl(long double x, long double y)
lib/msun/src/math.h:long double     powl(long double, long double);

From exp(3):

     double
     pow(double x, double y);

     float
     powf(float x, float y);

     long double
     powl(long double x, long double y);

If the implementation is bugged/questionable, it should be documented in CAVEATS.

>> @@ -122,9 +126,10 @@ Otherwise the error in these functions i
>> These functions will return the appropriate computation unless an error
>> occurs or an argument is out of range.
> 
> powl() almost never returns the appropriate computation.  Its bugs are
> most obvious when an argument is too large for double but not out of range.

These are bugs that need to be fixed then in the longterm, but in the short term should be documented under CAVEATS.

> I doubt that the rest of this section (about exception handling) is
> correct for powl().
> 
> The section on errors wasn't changed since it uses generic pow().  It
> claims that the error is generally less than 1 ulp.  But for powl(),
> the error is generally more that 4096 ulps.  Much more when the
> error exponentiates.  powf() and pow() do well to avoid exponentiation
> of roundoff errors.

I think the comment above applies here too about adding some notes to CAVEATS.

Thank you for the input!
-NGie