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

On Sat, 02 Nov 2013 15:29:05 -0300
Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com> 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?

FWIW, I was a bit sceptical about switching qreal to double. True this
would minimise the patches some packages would need on armhf, but OTOH,
I don't know what would happen to packages that use both GL graphics
and Qt at the same time. All armhf platforms support only OpenGLES and
not full OpenGL stack which supports *only* 32-bit floats.

However Lissandro just told me on IRC that GL-stuff on Qt5 switched to
float for exactly that reason). So apart from speed I don't see a
reason for not going that route. If anything, FPU in recent armv7-a
systems has become increasingly better so this will be better in a
couple of years (it will still suck on a Cortex-A8, but will be less
apparent on a Cortex-A15 or better).

FTR, I don't think many apps would mind that much. Most apps that
would actually care for speed/accuracy would use float/double directly
and not qreal, for most it would save us the burden of patching (eg.
scribus, qgis).

So unless, we find some particular strong cases for *not* switching to
double, I'd vote in favour of that.



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