From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,3a7c118fd2cc64f9
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news3.google.com!feeder.news-service.com!kanaga.switch.ch!news-zh.switch.ch!switch.ch!news.belwue.de!LF.net!news.enyo.de!not-for-mail
From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: A hole in Ada type safety
Date: Sat, 07 May 2011 11:57:07 +0200
Message-ID: <87oc3en898.fsf@mid.deneb.enyo.de>
References: <87oc3odtci.fsf@mid.deneb.enyo.de>
	<wccoc3oq7ev.fsf@shell01.TheWorld.com>
	<87tydfbtp3.fsf@mid.deneb.enyo.de> <ipi6kl$cbd$1@munin.nbi.dk>
	<87d3k2u36e.fsf@mid.deneb.enyo.de> <ipnmee$1l1$1@munin.nbi.dk>
	<877ha2op0n.fsf@mid.deneb.enyo.de>
	<hirpsfv33v9e$.1gzrgdlok5kja$.dlg@40tude.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Trace: ruchba.enyo.de 1304762228 11784 172.17.135.6 (7 May 2011 09:57:08
 GMT)
X-Complaints-To: news@enyo.de
Cancel-Lock: sha1:YLscLBd7WqEZFceP22FEwz6aIPg=
Xref: g2news2.google.com comp.lang.ada:20156
Date: 2011-05-07T11:57:07+02:00
List-Id: <comp.lang.ada>

* Dmitry A. Kazakov:

> On Sat, 07 May 2011 11:09:44 +0200, Florian Weimer wrote:
>
>> Run-time complexity for reference counting is less than for full
>> garbage collection.  Making it a language feature, rather than a
>> library-provided functionality, makes it easier for implementations to
>> optimize updating the counts.
>
> Hmm, what about tasking? I gather that a built-in counter must be atomic.
> Whether implemented lock-free or using a spin lock, that would be a
> considerable overhead.

Modern architectures eliminate most of the overhead in the non-shared
case.  In the M and E states of the MESI protocol, the operation can
be performed locally.

There are techniques to emulate this in software, but they require
safepointing support, which in turn needs extensive run-time
facilities.  (A safepoint allows you to carry out in a safe way an
operation which is clearly unsafe in the presence of multiple tasks.)
Hotspot performs a similar optimization, called biased locking.

>> As usual, there is a trade-off between flexibility and overhead: if
>> the reference counters are kept separate, it is possible to
>> implement weak pointers and safe references to sub-components.
>
> Interesting. Do you have a certain schema in mind, how to this?

The shared pointers in C++ and Boost implement this.  Andrei
Alexandrescu's Modern C++ Design contains a short overview of
implementation choices (but does not cover weak pointers).

GCC's implementation of std::share_ptr uses separate pointers to
objects and their reference counts.  The reference counts contain two
counters, for strong and weak references.  The actual object is
dealloated when the strong count reaches zero.  The reference counts
are deallocated when the strong and weak counts reach zero.