[Numpy-discussion] float128 in fact float80
Sun Oct 16 13:41:48 CDT 2011
On Sun, Oct 16, 2011 at 11:40 AM, Matthew Brett <email@example.com> wrote:
> On Sun, Oct 16, 2011 at 1:18 AM, David Cournapeau <firstname.lastname@example.org> wrote:
>> On Sun, Oct 16, 2011 at 8:33 AM, Matthew Brett <email@example.com> wrote:
>>> On Sun, Oct 16, 2011 at 12:28 AM, David Cournapeau <firstname.lastname@example.org> wrote:
>>>> On Sun, Oct 16, 2011 at 8:04 AM, Matthew Brett <email@example.com> wrote:
>>>>> On Sat, Oct 15, 2011 at 11:04 PM, Nadav Horesh <firstname.lastname@example.org> 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:
> for intel 32 and 64 bit, and then
Sorry - I missed out:
Where floatLD is float80, floatPPC is floatLD
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