[Numpy-discussion] Disabling Extended Precision in NumPy (like -ffloat-store)

Adrien Guillon aj.guillon@gmail....
Wed Apr 21 22:04:07 CDT 2010

Thank you for your questions... I'll answer them now.

The motivation behind using Python and NumPy is to be able to "double
check" that the numerical algorithms work okay in an
engineer/scientist friendly language.  We're basically prototyping a
bunch of algorithms in Python, validating that they work right... so
that when we move to the GPU we (hopefully) know that the algorithms
are okay sequentially... any problems we come against therefore would
have to be something else with the implementation (i.e. we don't want
to be debugging too much at once).  We are only targeting GPUs that
support IEEE floating point... in theory the results should be similar
on the GPU and CPU under certain assumptions (that the floating point
is limited to single precision throughout... which Intel processors
are a bit funny about).

The main concern I have, and thus my motivation for wanting a
restricted mode, is that parts of the algorithm may not work properly
if done in extended precision.  We are trying to ensure that
theoretically there should be no problems, but realistically it's nice
to have an extra layer of protection where we say "oh, wait, that's
not right" when we look at the results.

The idea here, is that if I can ensure there is never extended
precision in the Python code... it should closely emulate the GPU
which in fact has no extended precision in the hardware.  Ordinarily,
it's probably a silly thing to want to do (who would complain about
extended precision for free)?  But in this case I think it's the right

Hope this clarifies the reasons and intentions.


On Wed, Apr 21, 2010 at 1:20 PM, Christopher Barker
<Chris.Barker@noaa.gov> wrote:
> Adrien Guillon wrote:
>>  I use single-precision floating point operations.
>> My understanding, however, is that Intel processors may use extended
>> precision for some operations anyways unless this is explicitly
>> disabled, which is done with gcc via the -ffloat-store operation.
> IIUC, that forces the FPU to not use the 80 bits in temps, but I think
> the fpu still uses 64 bits -- or does that flag actually force singles
> to stay single (32 bit) through the whole process as well?
> -CHB
> --
> Christopher Barker, Ph.D.
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