TST: Skip f2py TestSharedMemory for LONGDOUBLE on macos/arm64

[ogrisel]

Those test fail on macos/arm64 as discussed here:

#18143 (comment)

Honestly I do not really understand what these tests are checking but LONGDOUBLE is an alias for DOUBLE (float64) on macos/arm64 while it is equivalent to float128 on x86_64. See the linked discussion for more details.

[ogrisel]

Alternatively one could check:

np.longdouble.dtype is not np.double.dtype

instead of platform info.

[seberg]

This is a bit strange, I guess there is some place here were the alignment for the clongdouble is taken as the itemsize instead of the size of a double? (I am also surprised a 64bit systems malloc will only guarantee an 8-byte alignment even then).

But if the failure is alignment related, then this seems OK considering the current comment. (I am not sure its strictly correct, but it seems fine)

[ogrisel]

The failures are here:

https://gist.github.com/ogrisel/851992e4c555af67881c9e0f2281c5df

But I do not understand the tests themselves. I have never use f2py.

[seberg]

Yeah, hopefully @pearu may have some insight.

[ogrisel]

You might also want to have a look at #18143 (comment) to compare that to what we get on other platforms.

[charris]

Thanks @ogrisel . The @if decorator could possibly be simplified. What does finfo('longdouble') say on M1?

EDIT: And on Windows.

[ogrisel]

@charris

What does finfo('longdouble') say on M1?

>>> np.finfo('longdouble')
finfo(resolution=1e-15, min=-1.7976931348623157e+308, max=1.7976931348623157e+308, dtype=float64)