[Development] CMake UNITY_BUILD ( QTBUG-109394 )

[Edward Welbourne]

On Monday, 16 January 2023 04:49:23 PST Friedemann Kleint via Development wrote:
>>>>> 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?

On 17.01.23 05:09, Thiago Macieira wrote:
>>>> I was going to object to it being in COIN, but considering it may
>>>> improve build times in the CI, especially for constrained platforms
>>>> (like macOS), I withdraw my objection before I even make it. I
>>>> think this effort is worth it.

Christian Tismer-Sperling (17 January 2023 11:55) asked:
>>> What was the reason that you first considered to object to it?

On 17 Jan 2023, at 13:35, Edward Welbourne via Development <development at qt-project.org> wrote:
>> While I can't be sure what Thiago was thinking of, one obvious
>> objection to doing this in Coin would be that: if someone changes a
>> source file in a way that needs a #include addition to the changed
>> source file, but Coin's Unity build combines that file with one that
>> has the #include, Coin won't notice, the change will get integrated
>> and those doing a non-Unity builds, or for whom the Unity build
>> splits files up differently, may find themselves with broken builds.
>> I imagine there are other mistakes that can likewise be concealed by
>> a Unity build.

Volker Hilsheimer (17 January 2023 16:26) replied:
> I would actually turn Thiago’s objection on its head and ask that we
> have at least one configuration in COIN that verifies that
> UNITY_BUILDs work, if we agree that this is a good idea.

That's more or less the conclusion I understood Thaigo to have reached,
too.

> In the short term I don’t expect that this will improve effective CI
> performance considerably, at least not until every CI configuration
> uses UNITY_BUILDs. The spread of build times in
> e.g. https://testresults.qt.io/coin/integration/qt/qtbase/tasks/1681530493
> is wild (from a few minutes to almost 1.5hours). No idea why
> windows-11_22h2-mingw11-developer-build is so slow, but in the end,
> the slowest platform for building and testing defines the time we have
> to wait until we know that our changes were good (failing build or
> test shortcuts that, of course).

That rather suggests making the slower builders use Unity while keeping
the fastest build of each platform a traditional build.  Then we test
that both types work while improving Coin's build speeds.

	Eddy.