Sorting is making a lot of QCollator objects, making it slow

[Mark Gaiser]

Hi,

I was doing some benchmarks for drag and drop for [1] and noticed that the
QCollator constructor was called _a_lot_ of times. The test was for 5000
files in a split view (so that would be 10.000 files) with QCollator being
called well over 81.000 times.

I do remember that we once had only one QCollator object prior to sorting
being parallel. Then parallel sorting came around and a trick was done by
introducing the KFileItemModelLessThan class which would manage the
QCollator lifetime for thread safety. This is all as far as i remember it
happening, i could be slightly off but it should be along those lines.

Now in benchmarks i see a lot of QCollator(QLocale) copy constructor calls.
That can't be right, can it?

Now i partly reverted the work that was done back then (just as an
experiment) to see if this stuff can be done more efficiently. I removed
the QCollator completely from KFileItemModelLessThan with the rationale
of KFileItemModelLessThan ultimatately just calling lessThan on the
provided model which is then being executed through QCollator::compare. The
Qt docs call QCollator reentrant (not thread-safe), but QCollator::compare
is a const function (aka, promising the compiler to not change class
internals) which in my mind makes that particular function thread-safe,
right?

You can find my patch here [2].

Now the fun stuff. It works 50% of the time...
The other 50% it crashes in NonMovableFileItem. Ouch.
I don't know why that happens, i'm hoping some of you can take a look with
this patch [2] applied? Or is my reasoning about QCollator::compare wrong
and it that causing the crash?

Note: the 50% that it does work does feel a lot faster (the sorting that
is). So this does matter.


Best regards,
Mark

[1] https://phabricator.kde.org/D10085
[2] https://p.sc2.nl/BkKVKXmIG
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