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

Matthew Brett matthew.brett@gmail....
Sun Nov 13 16:56:50 CST 2011


Hi,

On Sun, Nov 13, 2011 at 1:34 PM, Charles R Harris
<charlesr.harris@gmail.com> wrote:
>
>
> 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.

Right - but I think my researches are showing that the longdouble
numbers are being _stored_ as 80 bit, but the math on those numbers is
64 bit.

Is there a reason than numpy can't do 80-bit math on these guys?  If
there is, is there any point in having a float96 on windows?

See you,

Matthew


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