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

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.

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.

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

* 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

* 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

Thanks again for raising this!

Loïc Minier

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