[Development] Unity Build on CI for Qt Base for 3 Platforms

[Friedrich W. H. Kossebau]

Hi,

Am Freitag, 11. August 2023, 22:14:13 CEST schrieb Marc Mutz via Development:
> All my -unity-build-batch-size 100'000 patches except for one are now
> merged: https://codereview.qt-project.org/q/topic:unity-build
> 
> I'm currently testing without the unmerged one, because it won't apply
> to 6.5, anyway, and the goal is to have unity-builds from 6.5 onwards.
> Maybe a few more TUs will have to be excluded, but you should be able to
> see the pattern. So far it looks like it's not needed.
> 
> Compile with -unity-build -unity-build-batch-size 100000 and add all TUs
> that create problems with a small comment as to what's breaking to
> NO_UNITY_BUILD_SOURCES.

Just a quick note/question given 100000 seems to be used to denote "all the 
sources!":

CMake actually supports "0" as value for that purpose of "all!", had that been 
considered in the design, to be consistent with CMake's approach?

>From https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD_BATCH_SIZE.html
--- 8< ---
Although strongly discouraged, the batch size may be set to a value of 0 to 
combine all the sources for the target into a single unity file, regardless of 
how many sources are involved. This runs the risk of creating an excessively 
large unity source file and negatively impacting the build performance, so a 
value of 0 is not generally recommended.
--- 8< ---

Curiousity:

While at it, I am curious what purpose unity builds of Qt are supposed to 
serve, i.e. in which cases would exclusive use of unity builds be recommended 
to be used? Are there some docs/notes somewhere, when to use and when not?
Personally had been very impressed by the speed up when I first learned about 
the concept decades ago (KDE's --enable-final flag with unsermake IIRC) and 
the compiler at that time having more options to optimize the generated code. 
But with LTO these days, by what I understood, that advantage/purpose is gone.
And while I made some own project recently work with cmake unity builds for 
curiousity (thus knew about that special value "0" for batch sizes :) ) I am 
still wondering when actually unity builds make sense to be used, especially 
in parallel with supporting non-unity builds.
So, for which use cases are Qt's unity builds done? Whom will the docs 
recommend to use that build option?

Cheers
Friedrich