[Cython] New early-binding concept [was: CEP1000]

[mark florisson]

On 19 April 2012 08:17, Dag Sverre Seljebotn <d.s.seljebotn at astro.uio.no> wrote:
> On 04/19/2012 08:41 AM, Stefan Behnel wrote:
>>
>> Dag Sverre Seljebotn, 18.04.2012 23:35:
>>>
>>> from numpy import sqrt, sin
>>>
>>> cdef double f(double x):
>>>     return sqrt(x * x) # or sin(x * x)
>>>
>>> Of course, here one could get the pointer in the module at import time.
>>
>>
>> That optimisation would actually be very worthwhile all by itself. I mean,
>> we know what signatures we need for globally imported functions throughout
>> the module, so we can reduce the call to a single jump through a function
>> pointer (although likely with a preceding NULL check, which the branch
>> prediction would be happy to give us for free). At least as long as sqrt
>> is
>> not being reassigned, but that should hit the 99% case.
>>
>>
>>> However, here:
>>>
>>> from numpy import sqrt
>
>
> Correction: "import numpy as np"
>
>>>
>>> cdef double f(double x):
>>>     return np.sqrt(x * x) # or np.sin(x * x)
>>>
>>> the __getattr__ on np sure is larger than any effect we discuss.
>>
>>
>> Yes, that would have to stay a .pxd case, I guess.
>
>
> How about this mini-CEP:
>
> Modules are allowed to specify __nomonkey__ (or __const__, or
> __notreassigned__), a list of strings naming module-level variables where
> "we don't hold you responsible if you assume no monkey-patching of these".
>
> When doing "import numpy as np", then (assuming "np" is never reassigned in
> the module), at import time we check all names looked up from it in
> __nomonkey__, and if so treat them as "from numpy import sqrt as 'np.sqrt'",
> i.e. the "np." is just a namespace mechanism.

I like the idea. I think this could be generalized to a 'final'
keyword, that could also enable optimizations for cdef class
attributes. So you'd say

cdef final object np
import numpy as np

For class attributes this would tell the compiler that it will not be
rebound, which means you could check if attributes are initialized in
the initializer, or just pull such checks (as wel as bounds checks),
at least for memoryviews, out of loops, without worrying whether it
will be reassigned in the meantime.

> Needs a bit more work, it ignores the possibility that others could
> monkey-patch "np" in the Cython module.
>
> Problem with .pxd is that currently you need to pick one overload (np.sqrt
> works for n-dimensional arrays too, or takes a list and returns an array).
> And even after adding 3-4 language features to Cython to make this work,
> you're stuck with having to reimplement parts of NumPy in the pxd files just
> so that you can early bind from Cython.
>
>
> Dag
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