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

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 02 November 2013 23:25:57 peter green wrote:
[snip] 
> Finally has there been any discussion with other linux distros. This is
> obviously something that impacts the ABI in a big way so ideally it's
> not something where we want different distros doing different things.

I have asked in Qt's dev ML, #fedora-arm, #fedora-kde and #opensuse-kde.

So far:

- sailfish will keep qreal = float (thay already shipped their SDK).
- One maintainer from fedora told me they will switch armhf (the only official 
arm port) without soname bump, but another one told me the contrary
- I have not received a reply from OpenSuse people.

I still have to ask an Ubuntu maintainer what are they planning to do.

Kinds regards, Lisandro.

-- 
 lo que conseguís != lo que buscás
 por regla general
 lo importante es que:
(lo que buscas - lo que conseguís)/lo que buscas ~= 1

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 09 November 2013 20:33:02 Steven Chamberlain wrote:
> On Saturday 02 November 2013 15:29:05 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*.

Hi Steven! As I said before, It's not in my power to remove qreal for proper 
double/float. I can only switch it to one of them for Qt5.

Really, I understand your frustration, but this really doesn't helps, except 
you want to switch qreal to double on armel too, in which case I want to read 
it explicitly. It might be slower but may be better for porters, but I'm not a 
porter nor an armel user, so that's why I'm asking you guys.

Kinds regards, Lisandro.

-- 
127.0.0.1 sweet 127.0.0.1

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Steven Chamberlain]

On Saturday 02 November 2013 15:29:05 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*.

Hi,

I just came across an example of this so I thought I would mention it
here;  FTBFS on armel/armhf which may be stalling the boost1.54 transition:
https://buildd.debian.org/status/logs.php?pkg=mcrl2&ver=201210.1-1

> /build/buildd-mcrl2_201210.1-1-armel-Kvmlhc/mcrl2-201210.1/tools/diagraphica/diagrameditor.cpp:528:51:
>  error: no matching function for call to 'qMin(double&, qreal)'
> /build/buildd-mcrl2_201210.1-1-armel-Kvmlhc/mcrl2-201210.1/tools/diagraphica/diagrameditor.cpp:528:51:
>  note: candidate is:
> /usr/include/qt4/QtCore/qglobal.h:1196:34: note: template const T& 
> qMin(const T&, const T&)

Regards,
-- 
Steven Chamberlain
ste...@pyro.eu.org



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

[Sune Vuorela]

On Saturday 09 November 2013 10:22:44 Philipp Kern wrote:
> What is your plan to support partial upgrades? BinNMUs can require new Qt
> versions to be installed, but Qt can be upgraded independently to the newer
> version, causing the rdepends to crash. This can potentially be solved by
> Breaks, but it still breaks assumptions of people using Debian in that such
> ABI breaks will be communicated through SONAME bumps. And the old lib will
> not even be coinstallable.

Qt5 is not in stable, and there is only like 5 rdeps in the archive so far, so 
I would consider it something that could be ignored.

/Sune
-- 
Man, how to remove a URL from AutoCAD and from the options inside Explorer NT?

First from the tools menu within Flash you can't save the forward, this way 
you neither need to ping to the FPU over a hardware on the utility of a tower, 
nor must cancel a cache to log on the MIDI LCD terminale.


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

[Philipp Kern]

On Thu, Nov 07, 2013 at 02:46:27PM -0300, Lisandro Damián Nicanor Pérez Meyer 
wrote:
> - 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 is your plan to support partial upgrades? BinNMUs can require new Qt
versions to be installed, but Qt can be upgraded independently to the newer
version, causing the rdepends to crash. This can potentially be solved by
Breaks, but it still breaks assumptions of people using Debian in that such
ABI breaks will be communicated through SONAME bumps. And the old lib will
not even be coinstallable.

(Of course a good time to do such changes are in fact SONAME bumps, but I
realize that this won't happen for Qt for quite some time.)

Kind regards
Philipp Kern


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

[Lisandro Damián Nicanor Pérez Meyer]

On Thursday 07 November 2013 18:55:31 Loïc Minier wrote:
> On Thu, Nov 07, 2013, Lisandro Damián Nicanor Pérez Meyer wrote:
> > - We don't know yet what other distros are going to do.
> 
> IMO we shouldn't have distro-specific patching for this kind of stuff;
> it seems to commonly impact apps (which should be using qreal instead of
> assuming qreal is double) way too commonly, and we want binaries to be
> compatible between distros, so diverging from upstream seems a really
> bad idea.

The problem is the can of worms it has been opened: now we have the *chance* 
to choose, so ideally we need maintainers from most distros cooperating to 
keep SONAME and ABI the same.

Let's hope maintainers from other distros stand up.

-- 

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Loïc Minier]

On Thu, Nov 07, 2013, Lisandro Damián Nicanor Pérez Meyer wrote:
> - We don't know yet what other distros are going to do.

IMO we shouldn't have distro-specific patching for this kind of stuff;
it seems to commonly impact apps (which should be using qreal instead of
assuming qreal is double) way too commonly, and we want binaries to be
compatible between distros, so diverging from upstream seems a really
bad idea.

> - 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.

Well, it's kind of ugly but we could take this hit once and go through a
binary transition with a package name change, or some kind of other
transition trick.  Not pretty and breaks compat between binaries built
against old lib and new lib, but things in Debian will be ok.

Also, as you note, not too many things are using Qt5 yet; might be a
good time to get this right.

Likely something the release team would be able to advise on, I would
guess.

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

Eh ok; if you get the chance to ask, that would be helpful to know when
porting apps; perhaps it's something to bring up upstream in "issues
faced by distros" kind of chatters  :-)

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

[Lisandro Damián Nicanor Pérez Meyer]

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 
here.

> * 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] 


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] 


So:

- 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
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/

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

[Loïc Minier]

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

[Lisandro Damián Nicanor Pérez Meyer]

On Wednesday 06 November 2013 17:40:11 Pino Toscano wrote:
> In data sabato 2 novembre 2013 15:29:05, Lisandro Damián Nicanor Pérez
> 
> Meyer ha scritto:
> > 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*.
> 
> Did upstream bump the SONAME of QtCore5 and any other qt5 library
> because of this?

No. And I also agree this should have been done from Qt5's first version, but 
that didn't happened.

> > 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?
> 
> This will basically break the ABI of basically all the Qt5 libraries on
> those architectures. Unless upstream already managed the SONAME bump
> already, it would imply that Debian has to maintain it on its own...
> are you sure these changes are worth the pain they cause?

Depends on how you see it. Only three apps curently B-D on Qt5 in the archive. 
Yes, this means that a binary incompatible change will need to occur without a 
soname bump in (so far) one arch. But I *think* this can be managed quite 
easily with three armhf-binNMUs.

Of course, I might be missing something else (as I did wrt LSB/binary 
compatibilty with other distros), that's why I asked here before doing 
anything.

> > On the other hand, if the above change is kept, symbols for Qt5 on
> > armhf managed with pkg-kde-tools' symbolshelper will need an explicit
> > double for armhf instead of using qreal's subst. This is because on
> > Qt4 qreal will be kept as float.
> 
> This is another issue, i.e. pkgkde-symbolshelper not being able to
> distinguish version of qt for substitutions.

I agree here.

> > 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.
> 
> My option goes on keeping the status quo of qreal as it was, on
> architectures that managed to build qtbase-opensource-src already.

Thanks for your opinion and time :)

I will ask in Qt's dev list to see if other distros are going to keep the ABI 
or not for this cases.


Kinds regards, Lisandro.

-- 

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Pino Toscano]

In data sabato 2 novembre 2013 15:29:05, Lisandro Damián Nicanor Pérez 
Meyer ha scritto:
> 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*.

Did upstream bump the SONAME of QtCore5 and any other qt5 library 
because of this?

> 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?

This will basically break the ABI of basically all the Qt5 libraries on 
those architectures. Unless upstream already managed the SONAME bump 
already, it would imply that Debian has to maintain it on its own...
are you sure these changes are worth the pain they cause?

> On the other hand, if the above change is kept, symbols for Qt5 on
> armhf managed with pkg-kde-tools' symbolshelper will need an explicit
> double for armhf instead of using qreal's subst. This is because on
> Qt4 qreal will be kept as float.

This is another issue, i.e. pkgkde-symbolshelper not being able to 
distinguish version of qt for substitutions.

> 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.

My option goes on keeping the status quo of qreal as it was, on 
architectures that managed to build qtbase-opensource-src already.

-- 
Pino Toscano

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

[Lisandro Damián Nicanor Pérez Meyer]

Note: readding p-k-t@ and debian-ports@...

On Tuesday 05 November 2013 19:22:30 peter green wrote:
> Lisandro Damián Nicanor Pérez Meyer wrote:
> > I really don't understand where Canonical gets in here.
> 
> If qreal is float on armhf and key software fails in that configuration
> then canonical have to fix it (ubuntu are usually either very close to
> or ahead of debian on key software)
> 
> Afaict neither armel or sh4 has anything like the level of "corporate
> support" that armhf gets from canonical.

Ah, you mean that armhf hardware has more support from the Ubuntu side. 
Anyway, we need to make this decision within Debian.

> > I also don't understand what you mean with "ports that stick with qreal".
> > qreal is a typedef which type is defined at compile time. Did you meant
> > float?
> Sorry I meant ports that stick with defining qreal as float.

I see, in this case only those ports will have to deal with that (as it has 
been with Qt4)

> > I have not participated in any way in upstream's decision nor I have the
> > power to overcome them. Anyway, we are giving the choice of a
> > compile-time parameter to better suit our needs on purpose.
> 
> The problem is this is going to have a massive affect on ABI which implies:
> 
> 1: changing the descision later would mean a soname change

The reason why I took the time to create this thread is because this is the 
time to take that decision, and we Qt maintainers will not change it later 
because that would mean a soname change.

> 2: if debian make a different descision from other distros we will be
> binary incompatible.

Sune just made me rmember LSB. Yes, indeed, we need to try and coordinate with 
other distros. How this is normally done?

Kinds regards, Lisandro.

-- 
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las papas fritas... ¡nunca sobran!

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

--  Forwarded Message  --

Subject: Re: Qt5 switching qreal from float to double on arm*
Date: Tuesday 05 November 2013, 19:22:30
From: peter green 
To: Lisandro Damián Nicanor Pérez Meyer , ARM 

Lisandro Damián Nicanor Pérez Meyer wrote:
> I really don't understand where Canonical gets in here. 
If qreal is float on armhf and key software fails in that configuration 
then canonical have to fix it (ubuntu are usually either very close to 
or ahead of debian on key software)

Afaict neither armel or sh4 has anything like the level of "corporate 
support" that armhf gets from canonical.
> I also don't understand what you mean with "ports that stick with qreal". 
> qreal is a typedef which type is defined at compile time. Did you meant 
float?
>   
Sorry I meant ports that stick with defining qreal as float.
>   
> I have not participated in any way in upstream's decision nor I have the 
power 
> to overcome them. Anyway, we are giving the choice of a compile-time 
parameter 
> to better suit our needs on purpose.
The problem is this is going to have a massive affect on ABI which implies:

1: changing the descision later would mean a soname change
2: if debian make a different descision from other distros we will be 
binary incompatible.


-
-- 
Background: des y lisandropm comparten el nombre Damián
 lisandropm: en qué año naciste? quiero saber si contás como arte
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 8 serptimbre mismo año
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* lisandropm llama desesperado a su abogado

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

On Monday 04 November 2013 17:57:35 Pau Garcia i Quiles wrote:
> Hello,
> 
> Simple question: what is the reason for the change?

This is all I know:



> Also, I seem to remember Thiago discussing about this on qt-interest but I
> don't have time to search just now.

Apart from the links Peter sent in his mail, I haven't seen the discussion at 
all (I would really love to be more involved upstream, but my needs for a 
paycheck don't allow it ;) )

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

[Pau Garcia i Quiles]

Hello,

Simple question: what is the reason for the change?

Also, I seem to remember Thiago discussing about this on qt-interest but I
don't have time to search just now.



On Sat, Nov 2, 2013 at 7:29 PM, Lisandro Damián Nicanor Pérez <
perezme...@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?
>
> On the other hand, if the above change is kept, symbols for Qt5 on armhf
> managed with pkg-kde-tools' symbolshelper will need an explicit double for
> armhf instead of using qreal's subst. This is because on Qt4 qreal will be
> kept as float.
>
> Any feedback will be kindly appreciated.
>
> Kinds regards, Lisandro.
>
> --
> Hi! I'm a .signature virus! Copy me into your ~/.signature, please!
>
> Lisandro Damián Nicanor Pérez Meyer
> http://perezmeyer.com.ar/
> http://perezmeyer.blogspot.com/
>
> --
> http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-kde-talk
>



-- 
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(Due to my workload, I may need 10 days to answer)
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Re: Qt5 switching qreal from float to double on arm*

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 02 November 2013 15:29:05 Lisandro Damián Nicanor Pérez Meyer 
wrote:
[snip]
> 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.

Just for the record, I din't considered the switch for armel and sh4 because I 
think that may slow down those archs a lot, but please understand that I do 
not have a solid background for stating this, so feel free to correct me here. 

-- 

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http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Konstantinos Margaritis]

On Sat, 02 Nov 2013 15:29:05 -0300
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?

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.

Regards

Konstantinos


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

[Lisandro Damián Nicanor Pérez Meyer]

Note: adding back debian-arm@..., please tell me if it's not necessary.

On Saturday 02 November 2013 23:25:57 peter green wrote:
> Lisandro Damián Nicanor Pérez Meyer wrote:
> > Any feedback will be kindly appreciated.
> 
> I've always thought there is something fundamentally wrong.
> 
> What is qreal supposed to be used for? If it's supposed to be used for
> things where float would be adequate then shouldn't it be float on all
> platforms? If it's supposed to be used for things where float is
> inadequate then why isn't it double on all platforms?
> 
> The current situation leads to people who develop on i386 and amd64
> (which is most developers) assuming that qreal is equivilent to double.
> Then we try and build it on arm* and float gets used. Sometimes this is
> ok because there was really no reason for the developer to be using
> double precision in the first place and there were no incorrect
> conversion assumptions. Sometimes this causes build failures which can
> be difficult to resolve* and I bet in some cases it causes more subtule
> bugs. It also goes against the principle of offering to the greatest
> extent possible the same functionality on all platforms.

Peter: I'm not in the position of changing qreal's existence, so I'm not here 
to argue if qreal is a good idea or not.

The important thing is that qreal has been in Qt since at least Qt3, and, as 
far as I remember, has always been defined to be double on archs that natively 
support it and float on those that didn't.

> Making qreal double on armhf but leaving it as float on armel will mean
> that canonical are no longer forced for fix bugs surrounding it. I'm
> sure canonical would like that but I also suspect it may cause severe
> degredation in the maintainance of QT software on those ports that stick
> with qreal.

I really don't understand where Canonical gets in here. I don't work for 
Canonical nor I am an Ubuntu maintainer .Qt5's change comes from Lars Knoll, 
the current main/lead architect of the project. And we are still left with the 
possibility to define what value will qreal take (see below).

I also don't understand what you mean with "ports that stick with qreal". 
qreal is a typedef which type is defined at compile time. Did you meant float?

> On the other hand presumablly there would be a performance hit from
> switching from float to double, especially on platforms which use
> software floating point (I presume that was why whoever controlled qt at
> the time made qreal float on arm in the first place).

That's why they added an option to bypass this at compile time (see my 
previous mail) and also the main reason that I left armel as float. Of course, 
it might be that some armhf's hardware isn't capable of doing double, which is 
not what I currently understand, but feel free to correct me.

But the most important thing here is that if the porters decide that we better 
stick to float for qreal, I have no problem in doing so.

> I found some older upstream discussion on what to do about float at
> https://groups.google.com/forum/#!topic/qt-project-list-development/dPcP3NAS
> Y1k
> 
> I also found some discussion at
> http://comments.gmane.org/gmane.comp.lib.qt.qt5-feedback/700 saying that
> on coretex A8 double was significantly slower than float.
> 
> Finally has there been any discussion with other linux distros. This is
> obviously something that impacts the ABI in a big way so ideally it's
> not something where we want different distros doing different things.

I have not participated in any way in upstream's decision nor I have the power 
to overcome them. Anyway, we are giving the choice of a compile-time parameter 
to better suit our needs on purpose.

Kinds regards, Lisandro.

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Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 02 November 2013 17:09:30 Scott Kitterman wrote:
[snip] 

> Doesn't this break binary compatibility?

Yes it does. Upstream agreed to do so for Qt5.

> Did you discuss this with upstream already?

If we find that it might be a good idea, yes, I would definitely get upstream 
ACK's before doing it.

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Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[peter green]

Lisandro Damián Nicanor Pérez Meyer wrote:

Any feedback will be kindly appreciated.
  

I've always thought there is something fundamentally wrong.

What is qreal supposed to be used for? If it's supposed to be used for 
things where float would be adequate then shouldn't it be float on all 
platforms? If it's supposed to be used for things where float is 
inadequate then why isn't it double on all platforms?


The current situation leads to people who develop on i386 and amd64 
(which is most developers) assuming that qreal is equivilent to double. 
Then we try and build it on arm* and float gets used. Sometimes this is 
ok because there was really no reason for the developer to be using 
double precision in the first place and there were no incorrect 
conversion assumptions. Sometimes this causes build failures which can 
be difficult to resolve* and I bet in some cases it causes more subtule 
bugs. It also goes against the principle of offering to the greatest 
extent possible the same functionality on all platforms.


Making qreal double on armhf but leaving it as float on armel will mean 
that canonical are no longer forced for fix bugs surrounding it. I'm 
sure canonical would like that but I also suspect it may cause severe 
degredation in the maintainance of QT software on those ports that stick 
with qreal.


On the other hand presumablly there would be a performance hit from 
switching from float to double, especially on platforms which use 
software floating point (I presume that was why whoever controlled qt at 
the time made qreal float on arm in the first place).


I found some older upstream discussion on what to do about float at 
https://groups.google.com/forum/#!topic/qt-project-list-development/dPcP3NASY1k 

I also found some discussion at 
http://comments.gmane.org/gmane.comp.lib.qt.qt5-feedback/700 saying that 
on coretex A8 double was significantly slower than float.


Finally has there been any discussion with other linux distros. This is 
obviously something that impacts the ABI in a big way so ideally it's 
not something where we want different distros doing different things.


* See scribus for an especially nasty example.

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

[Scott Kitterman]

"Lisandro Damián Nicanor Pérez Meyer"  wrote:
>Note: only for pkg-kde-talk.
>
>On Saturday 02 November 2013 15:29:05 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?
>> 
>> On the other hand, if the above change is kept, symbols for Qt5 on
>armhf
>> managed with pkg-kde-tools' symbolshelper will need an explicit
>double for
>> armhf instead of using qreal's subst. This is because on Qt4 qreal
>will be
>> kept as float.
>
>There's still the chance of doing **quite a bit** of work and
>backporting the 
>changes to Qt4 and make qreal a double on armhf too. But this means a
>**big** 
>transition for armhf would be needed, but on the other hand
>pkg-kde-tools' 
>symbolshelper could be changed to properly manage Qt4 and Qt5.
>
>I want to be clear: this last option could mean *lots* of work and I
>would 
>definitely *not* go ahead with it without a patch to Qt4 ACKed by
>upstream and 
>the rest of Qt/KDE-team.
>
>But I think at least should be considered and team approved/discarded.
>
>Kinds regards, Lisandro.

Doesn't this break binary compatibility? Did you discuss this with upstream 
already? 

Scott K


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

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 02 November 2013 15:29:05 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*.

I forgot to mention: please keep pkg-kde-talk at l.a.d.o CCed :)

-- 
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Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

On Saturday 02 November 2013 15:30:53 you wrote:
[snip] 
> There's still the chance of doing **quite a bit** of work and backporting
> the changes to Qt4 and make qreal a double on armhf too. But this means a
> **big** transition for armhf would be needed, but on the other hand
> pkg-kde-tools' symbolshelper could be changed to properly manage Qt4 and
> Qt5.
> 
> I want to be clear: this last option could mean *lots* of work and I would
> definitely *not* go ahead with it without a patch to Qt4 ACKed by upstream
> and the rest of Qt/KDE-team.

I forgot to say: yes, I would consider doing the patch myself and get it 
approved upstream if we decide this is viable.

-- 
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Ama como si nunca hubieses sido herido.
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Canta como si nadie escuchara.
Vive como si fuera el Cielo en la Tierra.
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Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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

[Lisandro Damián Nicanor Pérez Meyer]

Note: only for pkg-kde-talk.

On Saturday 02 November 2013 15:29:05 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?
> 
> On the other hand, if the above change is kept, symbols for Qt5 on armhf
> managed with pkg-kde-tools' symbolshelper will need an explicit double for
> armhf instead of using qreal's subst. This is because on Qt4 qreal will be
> kept as float.

There's still the chance of doing **quite a bit** of work and backporting the 
changes to Qt4 and make qreal a double on armhf too. But this means a **big** 
transition for armhf would be needed, but on the other hand pkg-kde-tools' 
symbolshelper could be changed to properly manage Qt4 and Qt5.

I want to be clear: this last option could mean *lots* of work and I would 
definitely *not* go ahead with it without a patch to Qt4 ACKed by upstream and 
the rest of Qt/KDE-team.

But I think at least should be considered and team approved/discarded.

Kinds regards, Lisandro.

-- 
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Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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