[Development] MSVC not inlining QRect inline functions (anymore?)

[Philippe]

My final findings are:

/Ob3 causes inlining of much more code, like QPoint or QMargins, etc.
My release binary size increases by +11% when using /Ob3. 
I can understand why Microsoft doesn't promote that option much. At the
same time, it's unfortunate that /Ob3 is needed to inline tiny classes
such as QPoint.

As for QPolygon, there's no inlining (I only checked setPoint).

>> It's not worth my time to work around even more shortcomings of MSVC

Totally understandable. If improvement comes, it should be from
Microsoft's compiler team. 

Philippe


On Mon, 08 Jun 2026 16:48:42 -0700
Thiago Macieira <thiago.macieira at intel.com> wrote:

> On Monday, 8 June 2026 14:06:34 Pacific Daylight Time Philippe wrote:
> > > Can you try /Ob3 ?
> > 
> > Yes it works!
> > But only with both /Ob3 and Mårten's patch
> > (https://codereview.qt-project.org/c/qt/qtbase/+/742796)
> 
> I'm not sure how safe that is. I raise you QPolygon.
> 
> See Qt 5's qvector.h:
> https://github.com/qt/qtbase/blob/v5.15.0/src/corelib/tools/qvector.h#L1119-L1132
> 
> This problem is a reason to follow Marc's advice and not to export the whole 
> class. But the fix was in QPolygon, not QVector. So even if we don't export the 
> class, someone deriving from it and exporting their own class may cause our 
> class to get exported in their DLL's ABI.
> 
> I'm personally going to spend mental cycles trying to figure this out. It's not 
> worth my time to work around even more shortcomings of MSVC, especially when 
> it comes to performance (we've had to forbid LTO/LTCG mode for it, because it 
> appears to be broken). The ball is on Microsoft's court. And while they're at 
> it, they could fix data dllimports and add IFUNC support.
> 
> Other people are welcome to spend time figuring this out. But a patch to QtCore 
> needs to convince me beyond doubt, because I'm the one who ends up having to 
> maintain these hacks when they break.
> 
> -- 
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Principal Engineer - Intel DCG - Platform & Sys. Eng.