[Cython] checking for "None" in nogil function

mark florisson markflorisson88 at gmail.com
Mon May 7 19:18:08 CEST 2012

On 7 May 2012 17:52, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Dag Sverre Seljebotn, 07.05.2012 17:48:
>> On 05/07/2012 03:04 PM, Stefan Behnel wrote:
>>> Dag Sverre Seljebotn, 07.05.2012 13:48:
>>>>>> As far as I can remember (which might be biased towards my personal
>>>>>> view), the conclusion was that we left the current semantics in place,
>>>>>> relying on better control flow analysis to make None-checks cheaper, and
>>>>>> when those are cheap enough, make the nonecheck directive default to
>>>>>> True
>>>>> At least for buffer arguments, it silently corrupts data or segfaults in
>>>>> the current state of affairs, as you pointed out. Not exactly ideal.
>>>> No different than writing to a field in a cdef class...
>>> Hmm, aren't those None checked? At least cdef method calls are AFAIR.
>> Not at all. That's my whole point -- currently, the rule for None in Cython
>> is "it's your responsibility to never do a native operation on None".
>> I don't like that either, but that's just inherited from Pyrex (and many
>> projects would get speed regressions etc.).
>> I'm not against changing that to "we safely None-check", if done nicely --
>> it's just that that should be done everywhere at once.
> I think that gets both of us back on the same track then. :)
>> In current master (and as far back as I can remember), this code:
>> cdef class A:
>>     cdef int field
>>     cdef int method(self):
>>         print self.field
>> def f():
>>     cdef A a = None
>>     a.field = 3
>>     a.method()
>> Turns into:
>>   __pyx_v_a = ((struct __pyx_obj_5test2_A *)Py_None);
>>   __pyx_v_a->field = 3;
>>   ((struct __pyx_vtabstruct_5test2_A *)
>> __pyx_v_a->__pyx_vtab)->method(__pyx_v_a);
> Guess I've just been working on the builtins optimiser too long. There,
> it's obviously not allowed to inject unprotected code like this automatically.
> It would be fun if we could eventually get to the point where Cython
> replaces all of the code in f() with an AttributeError, as a combined
> effort of control flow analysis and dead code removal. A part of that is
> already there, i.e. Cython would know that 'a' "may be None" in the last
> two lines and would thus generate a None check with an AttributeError if we
> allowed it to do that. It wouldn't know that it's always going to be
> raised, though, so the dead code removal can't strike. I guess that case is
> just not important enough to implement.
> BTW, I recently tried to enable None checks in a couple of places and it
> broke memory views for some reason that I didn't want to investigate.

If you do want to implement it, don't hesitate to ask about any
memoryview shenanigans a certain person implemented.

> The
> main problems really seem to be unknown argument values and the lack of
> proper exception prediction, e.g. in this case:
>  def add_one_2d(int[:,:] buf):
>      for x in xrange(buf.shape[0]):
>          for y in xrange(buf.shape[1]):
>              buf[x,y] += 1
> it's statically obvious that only the first access to .shape (outside of
> all loops) needs a None check and will raise an AttributeError for None, so
> the check for the second loop can be eliminated as well as the None check
> on indexing.

Yes. This can be generalized to common subexpression elimination, for
bounds checking, for nonechecking, even for wraparound.

>>> I think we should really get back to the habit of making code safe first
>>> and fast afterwards.
>> Nobody has argued otherwise for some time (since the cdivision thread I
>> believe), this is all about Pyrex legacy. Guess part of the story is that
>> there's lots of performance-sensitive code in SAGE using cdef classes which
>> was written in Pyrex before Cython was around...
>> In fact, the nonecheck directive was written by yours truly! And I argued
>> for making it the default at the time!
> I've been working on the None checks (and on removing them) repeatedly,
> although I didn't remember the particular details of discussing the
> nonecheck directive.
> Stefan
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