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Date: Sat, 5 Jan 2019 12:46:45 -0800 (PST)
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Subject: Re: class wide iterable (and indexable)
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:55203
Date: 2019-01-05T12:46:45-08:00
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On Saturday, January 5, 2019 at 2:21:55 AM UTC-7, Randy Brukardt wrote:
> "Dmitry A. Kazakov" wrote in message 
> ...
> > In real programs dispatch is rare, thus there is no overhead.
> 
> This is probably true, but then interfaces are even more useless (they 
> provide nothing that could possibly be of value other than dispatch!).
> 
> > I don't know which overhead Randy meant, though. Maybe, space overhead? 
> > That is not nearly close to what generics produce in GNAT...
> 
> Dispatching is very expensive for interfaces, in that some form of search is 
> needed. (Note: I have no idea how GNAT implements them exactly.) It might be 
> possible to do some sort of link-time assignment of interface identifiers 
> and link-time tag construction to avoid some of that cost (at the cost of a 
> lot of space), but that would be such a complex and unusual project 
> (conventional linkers don't support anything like that) that I've always 
> considered it impraactical.

There are some optimizations that can be done, these three papers show some interesting methods for resolving/optimizing dynamic-calls:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.53.1099&rep=rep1&type=pdf
http://web.cs.ucla.edu/~palsberg/tba/papers/dean-grove-chambers-ecoop95.pdf
https://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1008&context=cs_faculty_pubs

IIUC, the techniques also work in the presence of full-multi dispatch.