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Re: Qt5 switching qreal from float to double on arm*

On Thursday 07 November 2013 18:18:18 you wrote:
> On Sat, Nov 02, 2013, Lisandro Damián Nicanor Pérez Meyer wrote:
> > Hi! Starting from Qt 5.2.0 (most probably from rc1 and definitely not from
> > beta1 currently in experimental) Qt5 will switch qreal from float to
> > double on arm*.
> > 
> > We have the option to keep some archs in float by passing a compilation
> > parameter. I've done so for armel and sh4, so only armhf will switch to
> > double.
> > 
> > Of course we are still on time to discuss this, and this is the reason of
> > this mail. What do you think WRT the above changes?
> First, thanks a lot for the heads up on this.

Thank you too for replying :)

> qreal being float instead of double on ARM was the source of a bunch of
> work for ARM porters in the past; now I have these worries/questions:
> * switching it back might imply some new porting work (in the case where
>   the patches were something #if ARM use float #else use double); this
>   might be particularly painful if armel and armhf have different
>   definitions.  Maybe there's a nice define #QREAL_IS_FLOAT or something
>   to help with this.

Don't forget we are talking about *just* Qt5 here, *not* Qt4. We only have 3 
apps building against Qt5 right now. If apps switch to Qt5 they will surely 
find some bumps, so this can be managed.

> * what about arm64?  sounds like this one should be double from the
>   start; not sure what it is right now

I have not added any provisions to arm64, so with the next 5.2.0 [rc/final] 
upload it will switch to double. We are still on time if something needs a fix 

> * when you say you've changed armel and sh4 to keep using float, is this
>   Debian-specific?  Not sure we want a delta with upstream on this kind
>   of stuff.  Would it not work at all to use double, or would it just be
>   slow?  I'd rather have it slow for people using big software on slow
>   arches rather than keeping a delta; it sounds like we do a SONAME
>   transition no matter what anyway

Now this *will* be messy. I have asked upstream [0] and so far haven't got any 
explicit reply from other distro's maintainers.

[0] <http://lists.qt-project.org/pipermail/development/2013-November/014017.html>

But, according to [1]:

  "I should also point out that this option now allows selecting qreal to be
   float on other platforms, besides ARM."

That's why I'm still spamming debian-ports ;)

[1] <http://lists.qt-project.org/pipermail/development/2013-November/014017.html>


- We don't know yet what other distros are going to do.

- If we decide to do the change in Qt5, it will be *without* soname bump. Yes, 
I know many of you will think of this as **ugly**, but so far means 3 binNMUs 
per arch. Now if this is not acceptable, then no change will be made, because 
I won't change Qt5's SONAME.

> * what's the point in qreal anyway?  can't we just switch everything to
>   float or double?  sounds like software should know what kind of level
>   of precision it needs in the first place; e.g. if it's a scale in some
>   UI, then either float or double is enough, but it's not an arch
>   specific decision

I really don't know, it was already there when I started using Qt back in 
Qt3's final days ;-)

But I *do* know that if people want it gone, they will need to wait until Qt6 
and provide the necessary patches :-/

Hope that helps, Lisandro.


Lisandro Damián Nicanor Pérez Meyer

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