[fpc-devel] Optimization breaks check for overridden function / Is there a better way to check?

[Pierre Muller]

Hi David,

this looks very similar to a problem that we are trying to
debug for a while, and that I recently submitted as a bug report:

https://gitlab.com/freepascal.org/fpc/source/-/issues/41439

The problem has been nicely simplified by alligator in

https://gitlab.com/freepascal.org/fpc/source/-/issues/41444

There is some problem related to PPU writing...

   I hope to be able using this simplified example t understand better this issue,
but I am currently still busy with the previous issue, also raised by alligator:

https://gitlab.com/freepascal.org/fpc/source/-/issues/41443

for which I hope to have found a fix.


Pierre



Le 17/10/2025 à 19:35, David Jenkins via fpc-devel a écrit :
> What is the best way to check if a virtual function in a base class has
> been overridden by a class that inherits the base class?
> 
> In LCL TPrinter.pas the following check is used:
> 
> 
> procedure TPrinter.NewPage;
> begin
>     Inc(fPageNumber);
>     if TMethod(@Self.DoNewPage).Code = Pointer(@TPrinter.DoNewPage) then
>       begin
>       ..
>       end
>    ..
> end
> 
> We have run across an optimization problem with the TCocoaPrinter class
> and that TPrinter code.
> 
> TCocoaPrinter = class(TPrinter)
> 
> with no override of DeNewPage
> 
> Sometime with Level2 or Level3 optimization,
> TMethod(@Self.DoNewPage).Code is changed to point to systemh.inc
> EmptyMethod instead of TPrinter.DoNewPage and the check no longer works
> appropriately.
> 
> Is there another way to make that check that is not affected by
> optimization?
> 
> I will eventually throw a fix to the LCL folks but we use a similar
> formulation in our code and are concerned that it might also be affected
> by optimization at some point.   Right now it is not because none of the
> base functions are completely empty, but they are trivial functions that
> match each other and could be optimized in the future by coalescing them.
> 
> Thanks
> David Jenkins
> Scooter Software.
> 
> 
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