[Development] Qt6 source changes

[André Pönitz]

On Fri, Nov 02, 2018 at 08:20:39AM +0000, Ulf Hermann wrote:
> > Depends on usage.
> > 
> > See e.g. 551efd91990e07902e5324f720cf5585865c323d
> > 
> >     QmlProfiler: Use QList for QmlRange container when loading .qtd
> >      
> >     As we are using this as a queue, with many calls to takeFirst(), a
> >     QVector is prohibitively expensive here.
> 
> I should have used QQueue there, and that will hopefully still be a 
> linked(-ish) list in Qt6. Or it might become a ring buffer, but please 
> not a plain vector.
> 
> If we don't want to keep QQueue, I will still have the option to use an 
> equivalent std container there (+/- implicit sharing).

I am not saying that QVector or QList is the best container here,
it was just an example for a (rare...) case where a substitution
introduces a significant performance regression in real life.

One concern here is also that the kind of regression can be hard to
discover as compilation succeeds, systematic performance testing
is often not done, and any manual testing before release usually
uses only smaller-than-normal sample sizes. So the first one to
notice the problem is the user of the code.

Andre'