On Mon, 2013-04-08 at 08:42 +0200, Dag Sverre Seljebotn wrote: > On 04/06/2013 04:19 PM, Nathaniel Smith wrote: > > Hi all, > > > > If you build current numpy master with > > NPY_RELAXED_STRIDES_CHECKING=1 python setup.py install > > then Cython code using ndarrays starts blowing up, e.g.: > > > > # foo.pyx > > def add_one(array): > > cdef double[::1] a = array > > a[0] += 1. > > return array > > > >>>> foo.add_one(np.ascontiguousarray(np.arange(10.)[::100])) > > Traceback (most recent call last): > > File "<stdin>", line 1, in <module> > > File "foo.pyx", line 2, in foo.add_one (foo.c:1210) > > cdef double[::1] a = array > > ValueError: Buffer and memoryview are not contiguous in the same dimension. > > > > The problem (as discussed before) is that Cython has an unnecessarily > > strict definition of "contiguous", so NPY_RELAXED_STRIDES_CHECKING=1 > > pretty much breaks all existing compiled Cython modules. > > > > Our plan is to make NPY_RELAXED_STRIDES_CHECKING=1 into the default > > sooner or later, and Cython is a major blocker on this plan. It may > > become the default as soon as the 1.8 pre-releases (with the > > expectation that we'll probably have to switch back again before the > > actual release, but still). > > > > References: > > > > Previous thread: > > http://thread.gmane.org/gmane.comp.python.cython.devel/14634 > > Detailed discussion of the difference between numpy/cython's current > > definition of "contiguity", and the correct definition: > > http://thread.gmane.org/gmane.comp.python.cython.devel/14634/focus=14640 > > The PR implementing NPY_RELAXED_STRIDES_CHECKING: > > https://github.com/numpy/numpy/pull/3162 > > Another test case: > > https://github.com/numpy/numpy/issues/2956 > > > > We're hoping that Cython will also switch soon to the more accurate > > check for contiguity. This shouldn't cause any backwards compatibility > > problems -- it just means Cython code would make strictly fewer > > copies, and error out due to lack of contiguity strictly less often, > > even with older numpys. And it seems like a necessary step for getting > > this untangled and minimizing user pain. What do you think? > > I agree that we should follow NumPy here, but can't see myself having > time to do the change in near future. I don't know about Mark? > > I think it is a fairly simple change though if any NumPyers would like > to do it, look at in Cython/Utility/MemoryView_C.c in the function > > _pyx_memviewslice_is_contig > > looks like it should just be to add a check for shape too. > > I guess you have changed your implementation of PEP 3118 too slightly on > the NumPy side? Is this undefined in the PEP or are you now not in > strict adherence to it? >
Hi, maybe I will have a look at that, but not sure if I will manage. The PEP 3118 is a point, but it does not seem to cover this (probably we should try to get a clarification into 3118). I am still wondering whether for buffers that are requested contiguous numpy should set the strides again, since it cannot hurt. Would that make a difference for Cython? I expected Cython just got any buffer and then checked the strides instead of requesting the buffer contiguous and then double checking. Regards, Sebastian > Dag Sverre > _______________________________________________ > cython-devel mailing list > cython-devel@python.org > http://mail.python.org/mailman/listinfo/cython-devel > _______________________________________________ cython-devel mailing list cython-devel@python.org http://mail.python.org/mailman/listinfo/cython-devel
