Hi,
I would like to propose considering making Qt suitable for CMake
UNITY_BUILDs ( https://bugreports.qt.io/browse/QTBUG-109394 ,
https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html ) .
This has originally been proposed for Qt for Python (
https://bugreports.qt.io/browse/PYSIDE-2155 ). It has greatly benefited
from it; builds are much faster; particuarly for us in home-office where
no distributed build environment exists.
It is basically what webengine has been using for a long time: Source
files are concatenated to jumbo files which are then compiled.
For Qt for Python with its lots of generated files, the speed-up is
drastic (~10x); for an average module (qttools), less so, but still:
Initial build of qttools:
Standard (675 files): real 3m26,016s / user 19m4,352s / sys 1m48,346s
Unity (203 files) : real 2m2,557s / user 9m40,147s / sys 0m47,144s
(CMAKE_UNITY_BUILD_BATCH_SIZE=16)
with CCACHE/warmed up:
Standard (675 files): real 1m50,605s / user 8m9,512s / sys 1m9,187s
Unity (203 files) : real 1m30,990s / user 5m11,773s / sys 0m39,687s
Increasing CMAKE_UNITY_BUILD_BATCH_SIZE might also give better results.
Some modifications of the source code are required, though. The majority
is actually improvements; things like missing header guards and macro
re-definitions immediately show up; also Q_DECLARE_METATYPE() can be
removed (see
https://lists.qt-project.org/pipermail/development/2022-July/042744.html ).
But there is also a part that is more questionable; static functions
need to be disambiguated; "using namespace" can cause clashes; class
member fields from anonymous namespaces cause errors (
-Werror=subobject-linkage producing qerrormessage.cpp:38:7: error:
‘QErrorMessagePrivate’ has a field ‘QErrorMessagePrivate::pending’ whose
type uses the anonymous namespace ) and similar.
Some things are not fixable; exclusions can be done on a source file or
project level. Alternatively, one can group suitable source files
together; but this requires more work.
In development work, if COIN uses UNITY_BUILD, there is also a small
risk of changes with missing header files being accepted.
Summmarising: we stand to gain a speed-up of compilation; particularly
for clean builds like in COIN; but it requires some work. We might do a
step-by step approach process enabling modules one by one. What do you
think?
Regards, Friedemann
--
Friedemann Kleint
The Qt Company GmbH
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