Bug#942359: Please provide libfftw3-long3 on all platforms

[Ole Streicher]

Hi Julian,

On 15.10.19 19:30, Julian Taylor wrote:
> This would require disabling the testsuite on these architectures as
> with no real long doubles they will likely fail (which I think is the
> original reason the package is not built, but I'd have to look it up).

I think this would be a minor problem.

> Imo it is also dangerous to provide a long double numerical library on
> platforms that do not really support it. You likely will just get wrong
> results.

>From my C understanding, "long double" is always implementation
dependent and is not guaranteed to be longer than "double". So, with a
"long double" library one gets only the accuracy that is expected for
the number of bits on that platform; this seems obvious to me.

> I think it would be better to just not build cubature on these platforms
> depending on how significant the long double support is.
> Have you engaged with upstream to check if they want to make long double
> support optional?

The nominal precision (printed with 18 digits) will be a bit better.
However, when limiting to 15-16 digits (as they are finally converted to
double), almost all numbers are equal (a few deviate in the last digit
by 1). This looks acceptable for me.

Best

Ole