question about long doubles on ppc64el

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question about long doubles on ppc64el

Thomas Caswell
Folks,

Over at h5py we are trying to get a release out and have discovered (via debian) that on ppc64el there is an apparent disagreement between the size of a native long double according to hdf5 and numpy.

For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .

In short, `np.longdouble` seems to be `np.float128` and according to the docs should map to the native 'long double'.  However, hdf5 provides a `H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double', but seems to be a 64 bit float.

Anyone on this list have a ppc64el machine (or experience with) that can provide some guidance here?

Tom 

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Re: question about long doubles on ppc64el

Jens Nielsen
According to https://docs.scipy.org/doc/numpy-dev/user/basics.types.html#extended-precision numpy long doubles are typically zero padded to 128 bits on 64 bit systems could that be the reason? 

On Mon, 16 Jan 2017 at 07:00 Thomas Caswell <[hidden email]> wrote:
Folks,

Over at h5py we are trying to get a release out and have discovered (via debian) that on ppc64el there is an apparent disagreement between the size of a native long double according to hdf5 and numpy.

For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .

In short, `np.longdouble` seems to be `np.float128` and according to the docs should map to the native 'long double'.  However, hdf5 provides a `H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double', but seems to be a 64 bit float.

Anyone on this list have a ppc64el machine (or experience with) that can provide some guidance here?

Tom 
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Re: question about long doubles on ppc64el

Charles R Harris
In reply to this post by Thomas Caswell


On Sun, Jan 15, 2017 at 11:00 PM, Thomas Caswell <[hidden email]> wrote:
Folks,

Over at h5py we are trying to get a release out and have discovered (via debian) that on ppc64el there is an apparent disagreement between the size of a native long double according to hdf5 and numpy.

For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .

In short, `np.longdouble` seems to be `np.float128` and according to the docs should map to the native 'long double'.  However, hdf5 provides a `H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double', but seems to be a 64 bit float.

Anyone on this list have a ppc64el machine (or experience with) that can provide some guidance here?

I believe the ppc64 long double is IBM double double, i.e., two doubles for 128 bits. It isn't IEEE compliant and probably not very portable. It is possible that different compilers could treat it differently or it may be flagged to be treated in some specific way.

Chuck


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Re: question about long doubles on ppc64el

Matthew Brett
In reply to this post by Thomas Caswell
Hi,

On Sun, Jan 15, 2017 at 10:00 PM, Thomas Caswell <[hidden email]> wrote:

> Folks,
>
> Over at h5py we are trying to get a release out and have discovered (via
> debian) that on ppc64el there is an apparent disagreement between the size
> of a native long double according to hdf5 and numpy.
>
> For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .
>
> In short, `np.longdouble` seems to be `np.float128` and according to the
> docs should map to the native 'long double'.  However, hdf5 provides a
> `H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double',
> but seems to be a 64 bit float.
>
> Anyone on this list have a ppc64el machine (or experience with) that can
> provide some guidance here?

I know that long double on numpy for the PPC on Mac G4 (power64 arch)
is the twin double, as expected, so I'd be surprised if that wasn't
true for numpy on ppc64el .

Do you want a login for the G4 running Jessie?   If so, send me your
public key off-list?

Cheers,

Matthew
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