[Python-Dev] [numpy wishlist] Interpreter support for temporary elision in third-party classes

[Julian Taylor]

On 06.06.2014 04:26, Greg Ewing wrote:
> Nathaniel Smith wrote:
> 
>> I'd be a little nervous about whether anyone has implemented, say, an
>> iadd with side effects such that you can tell whether a copy was made,
>> even if the object being copied is immediately destroyed.
> 
> I can think of at least one plausible scenario where
> this could occur: the operand is a view object that
> wraps another object, and its __iadd__ method updates
> that other object.
> 
> In fact, now that I think about it, exactly this
> kind of thing happens in numpy when you slice an
> array!
> 
> So the opt-in indicator would need to be dynamic, on
> a per-object basis, rather than a type flag.
> 

yes an opt-in indicator would need to receive both operand objects so it
would need to be a slot in the object or number type object.
Would the addition of a tp_can_elide slot to the object types be
acceptable for this rather specialized case?


tp_can_elide receives two objects and returns one of three values:
* can work inplace, operation is associative
* can work inplace but not associative
* cannot work inplace

Implementation could e.g. look about like this:

TARGET(BINARY_SUBTRACT) {
   fl = left->obj_type->tp_can_elide
   fr = right->obj_type->tp_can_elide
   elide = 0
   if (unlikely(fl)) {
      elide = fl(left, right)
   }
   else if (unlikely(fr)) {
      elide = fr(left, right)
   }
   if (unlikely(elide == YES) && left->refcnt == 1) {
       PyNumber_InPlaceSubtract(left, right)
   }
   else if (unlikely(elide == SWAPPABLE) && right->refcnt == 1) {
       PyNumber_InPlaceSubtract(right, left)
   }
   else {
       PyNumber_Subtract(left, right)
   }
}