[Numpy-discussion] float128 in fact float80

Matthew Brett matthew.brett@gmail....
Sun Oct 16 13:41:48 CDT 2011


Hi,

On Sun, Oct 16, 2011 at 11:40 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
> Hi,
>
> On Sun, Oct 16, 2011 at 1:18 AM, David Cournapeau <cournape@gmail.com> wrote:
>> On Sun, Oct 16, 2011 at 8:33 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
>>> Hi,
>>>
>>> On Sun, Oct 16, 2011 at 12:28 AM, David Cournapeau <cournape@gmail.com> wrote:
>>>> On Sun, Oct 16, 2011 at 8:04 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On Sat, Oct 15, 2011 at 11:04 PM, Nadav Horesh <nadavh@visionsense.com> wrote:
>>>>>> On 32 bit systems it consumes 96 bits (3 x 32). and hence float96
>>>>>> On 64 bit machines it consumes 128 bits (2x64).
>>>>>> The variable size is set for an efficient addressing, while the calculation in hardware is carried in the 80 bits FPU (x87) registers.
>>>>>
>>>>> Right - but the problem here is that it is very confusing.  There is
>>>>> something called binary128 in the IEEE standard, and what numpy has is
>>>>> not that.  float16, float32 and float64 are all IEEE standards called
>>>>> binary16, binary32 and binary64.
>>>>
>>>> This one is easy: few CPU support the 128 bits float specified in IEEE
>>>> standard (the only ones I know are the expensive IBM ones). Then there
>>>> are the cases where it is implemented in software (SPARC use the
>>>> double-pair IIRC).
>>>>
>>>> So you would need binar80, binary96, binary128, binary128_double_pair,
>>>> etc... That would be a nightmare to support, and also not portable:
>>>> what does binary80 become on ppc ? What does binary96 become on 32
>>>> bits Intel ? Or on windows (where long double is the same as double
>>>> for visual studio) ?
>>>>
>>>> binary128 should only be thought as a (bad) synonym to np.longdouble.
>>>
>>> What would be the nightmare to support - the different names for the
>>> different precisions?
>>
>> Well, if you have an array of np.float80, what does it do on ppc, or
>> windows, or solaris ? You will have a myriad of incompatible formats,
>> and the only thing you gained by naming them differently is that you
>> lose the ability of using the code on different platforms. The
>> alternative is to implement in software a quadruple precision number.
>
> The thing you gain by naming them correctly is the person using the
> format knows what it is.
>
> If we use float64 we know what that is.  If we are using float128,
> we've got no idea what it is.
>
> I had actually guessed that numpy had some software emulation for IEEE
> float128.   I don't know how I would have known otherwise.
>
> Obviously what I'm proposing is that the names follow the precisions
> of the numbers, not the itemsize.
>
> If what we actually have is something that is sometimes called
> float128, sometimes float96, that is always what C thinks of as long
> double, then surely the best option  would be:
>
> float80
> floatLD
>
> for intel 32 and 64 bit, and then
>
> floatPPC
> floatLD

Sorry - I missed out:

Where floatLD is float80, floatPPC is floatLD

Matthew


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