I've just pushed a feature[1] to moc that makes it process multiple headers at
the same time, producing only one output file, and a second feature[2] that
"precompiles" a header. Since moc is single-threaded, this often means more
wall-clock time than the previous case, but it's always less CPU time:
CPU time (s)
Moc – before 11,56
Moc – after 4,33
If you add that to the time to compile the moc output and to link, there's a
definite gain.
Debug Release Release + LTO
Compile – before 30,05 23,71 22,35
Compile – after 5,85 6,28 5,36
Link – before 31,27 24,15 110,57
Link – after 7,04 6,52 92,21
For the full build, the change in wall-clock (seconds, 2-core * 2 HT CPU) is
small, but visible:
Release Release + LTO
Before 68,813 77,736
After 66,426 61,084
(note: the "before" numbers are *after* I removed the #includes, so they're
slighty higher than the QtCore before all my changes)
This feature is optional. You can enable it for your module by doing:
CONFIG += moc_combine
BUT (and of course there's a "but")
You can only enable it if you rid your module of Q_PRIVATE_SLOT first. Since
this feature produces a single moc output for all headers, you can't have
#include at the end of your files [exception: if you have only one such
#include].
To get rid of the Q_PRIVATE_SLOTs, you can use the QObjectPrivate::connect
overload that takes a QObjectPrivate pointer as third parameter. We can divide
the porting in three groups:
1) easy/trivial:
- simple connect() statements (even supports Qt::UniqueConnection, provided
that you're connecting PMFs [note: there might be bugs!!])
- simple disconnect() statements where the four parameters match the same four
of the connect()
- QMetaObject::invokeMethod becoming QTimer::singleShot of 0 seconds
examples: [3] and [4]
2) doable:
- when there are disconnect()s that aren't trivial, in which case you need to
retain the QMetaObject::Connection result of the connect() call
example: [5]
3) hard:
- when you need to do a major refactoring of the internals because it depended
on QMetaMethod or something similar (see [6])
- when the front-end API requires modification too
[1] https://codereview.qt-project.org/160755
[2] https://codereview.qt-project.org/160756
[3] https://codereview.qt-project.org/160752
[4] https://codereview.qt-project.org/160753
[5] https://codereview.qt-project.org/#/c/160751/1/src/corelib/animation/
qanimationgroup_p.h
[6] https://codereview.qt-project.org/160758
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
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