[phobos] Fwd: Re: Ruling out arbitrary cost copy construction?

[Andrei Alexandrescu]

Thanks for the hard data! This is very useful.

The article about atomic reference counting is interesting but mostly 
compares eager copy with atomic reference counting (I attach one of the 
result files for convenience). We're also interested in the penalty 
exacted by atomicity of reference counting when compared with 
unprotected reference counting. (The trick of returning a smart proxy is 
clever but C++ specific.) Also, if I understand things correctly, the 
article does not induce contention; it only measures uncontested operations.

Anyhow, the analysis of the class constructors in Qt is extremely 
relevant. Thanks again!


Andrei

On 11/3/10 10:00 AM, Max Samukha wrote:
> These notes on copy-constructible Qt types may be useful for the discussion:
>
> 1. 1/5 (approx. 100 classes) of all classes in core, gui, network,
> webkit, svg and opengl packages define public copy-constructors.
>
> A half of those are in reference-counted COW types (approx. 50 classes).
> The remaining 50 classes are reference-counted types without COW, types
> with allocating copy-constructors and types with trivial non-allocating
> constructors.
>
> Most of the types with allocating copy-constructors I would probably
> implemented as classes in D. Polymorphic types like QListWidgetItem that
> provide the copy-constructor only for clone() reimplementation should
> definitely be classes in D.
>
> The conclusion I tend to draw is that expensive copy-constructors can be
> avoided in most applications.
>
> 2. Reference counters are atomic using interlocked integer operations.
> There have been a couple of complaints about performance
> [http://www.gotw.ca/publications/optimizations.htm] but those complaints
> seem ungrounded
> [http://labs.qt.nokia.com/2006/10/16/atomic-reference-counting-is-it-worth-it-2].
>
> 3. If a type exposes data by reference, proxy objects are used to avoid
> unnecessary copying. For example, QString::operator[](int) returns an
> instance of QСharRef, not QChar&. The shared data is copied only if the
> QCharRef instance is modified.
>
> On Sun, Oct 31, 2010 at 5:57 AM, Andrei Alexandrescu <andrei at erdani.com
> <mailto:andrei at erdani.com>> wrote:
>
>     I am highly interested in the opinion of Phobos contributors in the
>     matter of copy construction (just posted the message below).
>
>     Andrei
>
>     -------- Original Message --------
>     Subject: Re: Ruling out arbitrary cost copy construction?
>     Date: Sat, 30 Oct 2010 22:56:24 -0500
>     From: Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org
>     <mailto:SeeWebsiteForEmail at erdani.org>>
>     Newsgroups: digitalmars.D
>
>     On 10/30/2010 09:40 PM, Michel Fortin wrote:
>
>         On 2010-10-30 20:49:38 -0400, Andrei Alexandrescu
>         <SeeWebsiteForEmail at erdani.org
>         <mailto:SeeWebsiteForEmail at erdani.org>> said:
>
>             On 10/30/10 2:24 CDT, Don wrote:
>
>                 At the moment, I think it's impossible.
>                 Has anyone succesfully implemented refcounting in D? As
>                 long as bug 3516
>                 (Destructor not called on temporaries) remains open, it
>                 doesn't seem to
>                 be possible.
>                 Is that the only blocker, or are there others?
>
>
>             I managed to define and use RefCounted in Phobos. File also uses
>             hand-made reference counting. I think RefCounted is a pretty
>             good
>             abstraction (unless it hits the bug you mentioned.)
>
>
>         I like the idea of RefCounted as a way to automatically make things
>         reference counted.
>
>
>     Unfortunately it's only a semi-automated mechanism.
>
>         But like File and many similar ref-counted structs, it has this race
>         condition (bug 4624) when stored inside the GC heap. Currently,
>         most of
>         Phobos's ref-counted structs are race-free only when they reside
>         on the
>         stack or if your program has only one thread (because the GC doesn't
>         spawn threads if I'm correct).
>
>         It's a little sad that the language doesn't prevent races in
>         destructors
>         (bug 4621).
>
>
>     I hope we're able to solve these implementation issues that can be seen
>     as independent from the decision at hand.
>
>     Walter and I discussed the matter again today and we're on the brink of
>     deciding that cheap copy construction is to be assumed. This simplifies
>     the language and the library a great deal, and makes it perfectly good
>     for 95% of the cases. For a minority of types, code would need to go
>     through extra hoops (e.g. COW, refcounting) to be compliant.
>
>     I'm looking for more feedback from the larger D community. This is a
>     very important decision that marks one of the largest departures from
>     the C++ style. Taking the wrong turn here could alienate many
>     programmers coming from C++.
>
>     So, everybody - this is really the time to speak up or forever be
>     silent.
>
>
>     Andrei
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