[Numpy-discussion] Odd-looking long double on windows 32 bit

Charles R Harris charlesr.harris@gmail....
Sun Nov 13 10:21:59 CST 2011


On Sun, Nov 13, 2011 at 12:57 AM, Matthew Brett <matthew.brett@gmail.com>wrote:

> Hi,
>
> On Sat, Nov 12, 2011 at 11:35 PM, Matthew Brett <matthew.brett@gmail.com>
> wrote:
> > Hi,
> >
> > Sorry for my continued confusion here.  This is numpy 1.6.1 on windows
> > XP 32 bit.
> >
> > In [2]: np.finfo(np.float96).nmant
> > Out[2]: 52
> >
> > In [3]: np.finfo(np.float96).nexp
> > Out[3]: 15
> >
> > In [4]: np.finfo(np.float64).nmant
> > Out[4]: 52
> >
> > In [5]: np.finfo(np.float64).nexp
> > Out[5]: 11
> >
> > If there are 52 bits of precision, 2**53+1 should not be
> > representable, and sure enough:
> >
> > In [6]: np.float96(2**53)+1
> > Out[6]: 9007199254740992.0
> >
> > In [7]: np.float64(2**53)+1
> > Out[7]: 9007199254740992.0
> >
> > If the nexp is right, the max should be higher for the float96 type:
> >
> > In [9]: np.finfo(np.float64).max
> > Out[9]: 1.7976931348623157e+308
> >
> > In [10]: np.finfo(np.float96).max
> > Out[10]: 1.#INF
> >
> > I see that long double in C is 12 bytes wide, and double is the usual 8
> bytes.
>
> Sorry - sizeof(long double) is 12 using mingw.  I see that long double
> is the same as double in MS Visual C++.
>
> http://en.wikipedia.org/wiki/Long_double
>
> but, as expected from the name:
>
> In [11]: np.dtype(np.float96).itemsize
> Out[11]: 12
>
>
Hmm, good point. There should not be a float96 on Windows using the MSVC
compiler, and the longdouble types 'gG' should return float64 and
complex128 respectively. OTOH, I believe the mingw compiler has real
float96 types but I wonder about library support. This is really a build
issue and it would be good to have some feedback on what different
platforms are doing so that we know if we are doing things right.

Chuck
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