[geos-devel] double double

[Greg Troxel]

Paul Ramsey <pramsey at cleverelephant.ca> writes:

> Mats, what ever happened to:
>
> https://github.com/libgeos/geos/pull/40
>
> I'm just coming up against some stuff in the JTS commit log that expects
> double double and I see you've done this work already some years ago, but
> it's not committed that I can see. JTS changed over some code to use DD
> exclusively about a year ago, so this is a case where porting has caught up
> to need.
>
> Were there any substantial problems w/ your PR? If not I'm "just" going to
> try and rebase it and use it as the basis for going ahead.
>
> (It seems like the state-of-the-art would eventually be to use the "long
> double" type, which has an implementation in GCC, but does not yet have one
> in clang, in hopes that by being close to the metal we'd eventually also
> start to reap performance gains as the silicon adds instructions to improve
> long double calculations. ???)

I don't follow this last paragraph.

As I understand it, long double is a type defined by C that can be more
precise than double, but doesn't have to be, and on x86 is typically 80
bits.  double double is a technique to use two doubles to get roughly
twice the precision.   I would expect that on many CPUs, long double is
the same as double.

So double double is far more precise than long double always.

https://en.wikipedia.org/wiki/Long_double
https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format#Double-double_arithmetic