[Cython] problems with C numeric types

[Robert Bradshaw]

On Feb 7, 2009, at 3:09 PM, Lisandro Dalcin wrote:

> On Sat, Feb 7, 2009 at 6:13 PM, Stefan Behnel <stefan_ml at behnel.de>  
> wrote:
>> Hi,
>>
>> Lisandro Dalcin wrote:
>>> Could we assume that PY_LONG_LONG is the widest integral?? Should we
>>> assume that 'Py_ssize_t' and 'size_t' are between 'long' and
>>> 'PY_LONG_LONG'?
>>
>> For starters, Py_ssize_t is defined as a signed size_t according  
>> to PEP
>> 353. However, it does not mention a relation to PY_LONG_LONG.
>>
>> I would at least expect Py_ssize_t to be <= PY_LONG_LONG (the  
>> latter being
>> the largest available type), and my (limited) knowledge about the  
>> "long"
>> type makes me assume that it should be <= Py_ssize_t. So, yes, I  
>> would expect
>>
>>        long <= Py_ssize_t <= PY_LONG_LONG
>>
>> and
>>
>>        unsigned long <= size_t <= unsigned PY_LONG_LONG
>>
>
> I think the same... Then I'll review this and push fixes.

Excellent.

>
>>
>>> However, I'm not sure how to we will handle code like the one below
>>> with the current implementation,
>>>
>>> cdef f():
>>>     cdef Py_ssize_t a = 5
>>>     cdef size_t b = 7
>>>     return a + b
>>>
>>> I believe the value of 'a+b' should be promoted to 'signed long  
>>> long',
>>> but it cannot see the way to implent such behavior in the current
>>> Cython codebase.
>>
>
> Well, I was partially wrong in the above comments...
>
>> Wouldn't Py_ssize_t be a suitable target type? Admittedly, it's <<  
>> size_t
>> for non-negative types, but the missing bit shouldn't matter in  
>> almost all
>> cases.
>
> It will not matter until you get a segfault or strange results... ;-)
>
>
>> And if users really need to make use of it, I'm fine with requiring
>> them to write
>>
>>        return <PY_LONG_LONG>a + <PY_LONG_LONG>b
>>
>
> Yes, you are right, but then better for users to not forget the  
> casts...
>
> In short, If we can do it better, go for it and push testcases 
> +fixes...


This is like asking if someone does

cdef unsigned int a
cdef int b
return a + b

I don't think a + b should be "upgraded" to a long (which may not  
even be bigger), just because it might overflow.

- Robert