[Development] Let's get rid of qDebug/qWarning/qCritical!

[Poenitz Andre]

Koehne Kai wrote:
> > -----Original Message-----
> > From: development-bounces+kai.koehne=digia.com at qt-project.org
> > [...]
> > As a blueprint I've started  a patch for a 'qt.core.io' category:
> >
> > https://codereview.qt-project.org/#change,74862,patchset=1
> 
> Andre raised concerns about the (runtime/space) overhead of the 
> QDebug streaming style, compared to the printf style. 
> Here are some conclusions (gcc 4.8, release build, CONFIG+=c++11) ...
> 
> RUNTIME OVERHEAD
> 
> I did a quick benchmark where I did following variants in a 
> QBENCHMARK (category was enabled, to output was generated 
> to a file. I had to patch qtestlib to not intercept the qWarning output...):
> 
> void CatLogBench::qWarningPrintf1() { QBENCHMARK { qWarning("Hi there"); } }
> void CatLogBench::qWarningPrintf2() { QBENCHMARK { qWarning("%s", "Hi there"); } }
> void CatLogBench::qWarningStream() { QBENCHMARK { qWarning() << "Hi there"; } }
> void CatLogBench::qCWarningStream() { QBENCHMARK { qCWarning(cat) << "Hi there"; } }
> void CatLogBench::qCWarningPrintf() { QBENCHMARK { qCWarning(cat, "Hi there"); } }
> 
> (the last one is a new variant enabled by https://codereview.qt-project.org/#change,75029 .)
> 
> And here are the results:
> 
> PASS   : CatLogBench::qWarningPrintf1()
> RESULT : CatLogBench::qWarningPrintf1():
>      0.0010 msecs per iteration (total: 67, iterations: 65536)
> PASS   : CatLogBench::qWarningPrintf2()
> RESULT : CatLogBench::qWarningPrintf2():
>      0.0013 msecs per iteration (total: 90, iterations: 65536)
> PASS   : CatLogBench::qWarningStream()
> RESULT : CatLogBench::qWarningStream():
>      0.0016 msecs per iteration (total: 54, iterations: 32768)
> PASS   : CatLogBench::qCWarningStream()
> RESULT : CatLogBench::qCWarningStream():
>      0.0018 msecs per iteration (total: 59, iterations: 32768)
> PASS   : CatLogBench::qCWarningPrintf()
> RESULT : CatLogBench::qCWarningPrintf():
>      0.0010 msecs per iteration (total: 69, iterations: 65536)
> PASS   : CatLogBench::cleanupTestCase()
> Totals: 7 passed, 0 failed, 0 skipped
> 
> So, there is of course some measurable overhead, but I wouldn't say it disqualifies the streaming variant per se :)


BINARY SIZE

On to the binary size. I removed the QBENCHMARK macro and disassembled the gcc output. Here it shows that QDebug is completely inlined, the streaming operator variants do indeed generate a hell lot of instructions ! But since this is quite a bogus metric here are the size increase of Qt5Core with, and without the core.io patch included (again a release non-developer build with gcc 4.8.2):


-rwxr-xr-x 1 kkoehne users 5311213 Jan 10 12:55 libQt5Core.so.5.3.0.original
-rwxr-xr-x 1 kkoehne users 5357104 Jan 10 12:58 libQt5Core.so.5.3.0_patch


So the overhead is measurable.




Given these concerns, I'd like to propose adding a printf style overload to qCDebug:

https://codereview.qt-project.org/#change,75029


Note the patch is fairly ugly because we're gracefully handling the case where the toolchain doesn't respect Q_COMPILER_VARIADIC_MACROS, I'll post a second mail about this soon ...

Regards

Kai


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