[Development] Proposing CMake as build tool for Qt 6

[Mark De Wit]

> -----Original Message-----
> From: Development <development-bounces at qt-project.org> On Behalf Of
> Konstantin Tokarev
> Sent: 06 June 2019 14:30
> To: Simon Hausmann <simon.hausmann at qt.io>; Bogdan Vatra
> <bogdan.vatra at kdab.com>; development at qt-project.org
> Subject: Re: [Development] Proposing CMake as build tool for Qt 6
> 
> 
> 
> 06.06.2019, 16:25, "Simon Hausmann" <simon.hausmann at qt.io>:
> > Hi,
> >
> > Regarding PCH, it seems that right now it would be easiest to include
> something like https://github.com/sakra/cotire . Patches are welcome to
> integrate this or alternatively work with upstream CMake for a built-in
> solution.
> 
> Yet another alternative is https://github.com/larsch/cmake-precompiled-
> header/blob/master/PrecompiledHeader.cmake
> which is not so monstrous as cotire

I have used Cotire until recently and would caution against it.  The library at its core is a clever idea but its implementation caused a lot of build instability for us.  Its current version doesn't work on MSVC with CMake > 3.12, also fails on Mac with AppleClang 8.0 (bad command line parameter).  It appears to be unmaintained as my bug reports have gone completely unanswered / not even acknowledged.

In the end I wrote my own pch script that works ok for our scenario.  MSVC support is pretty trivial, gcc/clang more awkward because their pch compilation is a separate build step.  My script did include some ideas from larsch github repo linked above, but I don't like his approach of injecting build flags into the clang/gcc pch stage (it didn't work for me, I ended up missing critical flags).

I used some ideas from cotire to generate a list of includes to seed each module PCH header file, but maintain updates manually (like MSVC stdafx.h/pch.h approach).  I'm sure pch header generation could be automated too...

Mark