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

Charles R Harris charlesr.harris@gmail....
Sun Nov 13 15:34:19 CST 2011


On Sun, Nov 13, 2011 at 2:25 PM, Matthew Brett <matthew.brett@gmail.com>wrote:

> Hi,
>
> On Sun, Nov 13, 2011 at 8:21 AM, Charles R Harris
> <charlesr.harris@gmail.com> wrote:
> >
> >
> > 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.
>
> Is it possible that numpy is getting confused by being compiled with
> mingw on top of a visual studio python?
>
> Some further forensics seem to suggest that, despite the fact the math
> suggests float96 is float64, the storage format it in fact 80-bit
> extended precision:
>
>
Yes, extended precision is the type on Intel hardware with gcc, the 96/128
bits comes from alignment on 4 or 8 byte boundaries. With MSVC, double and
long double are both ieee double, and on SPARC, long double is ieee quad
precision.

<snip>

Chuck
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