[Development] qt_add/removeObject hooks

[Volker Krause]

On Tuesday 15 April 2014 08:02:56 Thiago Macieira wrote:
> Em ter 15 abr 2014, às 09:49:53, Matthew Woehlke escreveu:
> > If you add the rule that callbacks can't be removed once registered, you
> > could do an atomic exchange of the old callback for the new, and require
> > that any attached callback calls the previously-registered callback
> > also. (This would cost a compare per callback though in case the old is
> > null.) This takes a small amount of cleverness at the call site to
> > ensure that the pointer to old is written at the same time as the new
> > callback is registered, but should be doable. (Depending on available
> > intrinsics, you might need to do a CAS loop to first set your old, then
> > register your own callback if no one else has done so in the mean time.
> > Retry as needed. Under the circumstances, it's pretty unlikely you'd
> > ever get stuck in an infinite loop doing this.)
> 
> In other words: use QBasicAtomicPointer.

As mentioned on https://codereview.qt-project.org/#change,83272 Kevin Funk 
measured the difference between the current approach and using atomics using 
Olivier's initial benchmark, it's about 4-5x slower for checking if there's a 
callback set. Since this is affecting every Qt application on a not uncommon 
hot path, I'd rather trade in theoretical tool stability for general 
performance.

regards,
Volker

-- 
Volker Krause | volker.krause at kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel. Germany +49-30-521325470, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions
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