Safe reference counting cannot be implemented as a library

[Andrei Alexandrescu via Digitalmars-d]

On 11/01/2015 03:51 PM, Martin Nowak wrote:
> On 10/27/2015 01:27 PM, Andrei Alexandrescu wrote:
>> Unrelated, and a foreshadowing of the discussion on the lifetime mailing
>> list: the compiler has ample opportunity to fuse incs/decs together, so
>> the signatures of these functions is:
>>
>> void opInc(uint delta);
>> void opDec(uint delta);
>
> Any hint/numbers showing that this is actually useful?

Would be great to collect some, and generally get rigorous about the 
whole approach.

> Implementing such a cross statement optimization is quite some work. If
> this occurs often enough (in particular for shared classes with atomic
> ref counting) it might be worth the effort.

Most reference counting techniques revolve around reducing mutation of 
the reference count. See e.g. 
https://users.cecs.anu.edu.au/~steveb/downloads/pdf/rc-ismm-2012.pdf.

So we need to show that many refcount updates take it from 1 to larger 
than 1 and back. According to 
https://users.cecs.anu.edu.au/~steveb/downloads/pdf/rc-ismm-2012.pdf, 
many objects have a reference count of just one; the "eclipse" benchmark 
has 31.8% objects with a refcount greater than 1.


Andrei