[Interest] slower compilation with Qt6

[Hamish Moffatt]

On 24/7/23 17:01, Cristian Adam wrote:
> The compilation time degradation is being tracked at [QTBUG-97601] 
> Compilation speed decrease with Qt 6.2 compared to Qt 5.15.2 - Qt Bug 
> Tracker <https://bugreports.qt.io/browse/QTBUG-97601>
>
> For Qt Creator I use https://ccache.dev/ (yes, it works with MSVC) so 
> that even if it takes longer to compile, I won't be compiling the same 
> file in same situation twice.


Thanks, good to know this a known issue with work in progress. I do use 
ccache on my linux builds, though I'm finding it increasingly unreliable 
with PCH enabled so I will have to decide which is more useful.


My test compile results for 5.15.13 versus 6.5.2 are even worse than the 
figures you reported in that issue.


For a project with ~600 TUs, compiled on linux with clang 14, PCH on but 
no ccache, I see 40 seconds -> 96 seconds.


For another project with ~900 TUs, compiled with MSVC 2022 with jom, PCH 
on but no ccache, I see 178 seconds -> 332.



I compile with CONFIG += c++2a on both Qt versions.


Hamish
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