[Numpy-discussion] supporting quad precision

[David Cournapeau]

On Sun, Jun 9, 2013 at 8:35 AM, Henry Gomersall <heng at cantab.net> wrote:
> On Sat, 2013-06-08 at 14:35 +0200, Anne Archibald wrote:
>> Looking at the rational module, I think you're right: it really
>> shouldn't be too hard to get quads working as a user type using gcc's
>> __float128 type, which will provide hardware arithmetic in the
>> unlikely case that the user has hardware quads. Alternatively,
>> probably more work, one could use a package like qd to provide
>> portable quad precision (and quad-double).
>
> In this vague area, and further to a question I asked a while ago on
> StackOverflow (http://stackoverflow.com/q/9062562/709852), is there some
> deep reason why on some platforms longdouble is float128 and on other
> it's float96?

Long double is not standardized (like single/double are), so it is CPU
dependent. On Intel CPU, long double is generally translated into the
extended precision 80 bits. On 32 bits, it is aligned to 12 bytes (the
next multiple of 4 bytes), on 64 bits 16 bytes (the next multiple 8
bytes). MS compilers are a notable exception where long double ==
double.

So it depends on the CPU, the OS and the compiler. Using long double
for anything else than compatibility (e.g. binary files) is often a
mistake IMO, and highly unportable.

David