[fpc-pascal] Remove unused functions in a class

Jonas Maebe jonas.maebe at elis.ugent.be
Sun Apr 5 14:09:33 CEST 2009

On 05 Apr 2009, at 13:28, JoshyFun wrote:

> JM> If you use FPC 2.3.1 with whole-program optimization and the  
> compiler
> JM> can prove that it is never callable: yes. See
> JM> http://wiki.freepascal.org/Whole_Program_Optimization
> JM>   for more information. In case this is in relation to the  
> thread on
> JM> the Lazarus list about the big executables: note that it has  
> only a
> Yes and no :) The thread raises the question in my brain :) but not to
> reduce the LCL final size, it was only a conceptual question, how can
> the compiler determine that a virtual method is unused at all,

If you have a class hierarchy tbase->tderived1->tderived2 with a  
virtual method called "vmethod", and nowhere in the program there is a  
call to vmethod, then it is unused. Or if it is only called using  
tderived2 instances, then if the linker does not find any direct  
references to tderived1.vmethod or tbase.vmethod (e.g., via  
"inherited" calls from tderived2 methods), it knows that the VMT  
entries for vmethod in tderived1 and tbase can be set to nil.

> and
> even if it is removed the VMT entry should be present do not ?

The easiest is to keep it, but simply set it to nil or to  
FPC_ABSTRACTERROR. Then you don't have to start juggling with changing  
offsets everywhere.

> JM> The internal linker can also do it (only throwing away virtual  
> method
> [...]
> JM> that produces an error if you try to link a unit compiled with the
> JM> option using the external linker), but that hasn't been done yet.
> Just for curiosity which is the parameter set ?

Compile the compiler with -dvtentry. It's not been tested for a while  
though, as far as I know.

I also made a mistake in my previous explanation: currently, the WPO  
does not yet perform the optimization the -dvtentry does. -dvtentry  
looks at which VMT entries of a class hierarchy are never accessed,  
and replaces those by nil. As a result, if those methods are not  
referred directly, then they can be removed by the linker.

The WPO looks at which class types can be instantiated in the program  
and at the inheritance tree, and based on this determines which  
virtual method calls can be replaced by non-virtual method calls (and  
for those methods, it replaces the VMT entry with FPC_ABSTRACTERROR).  
It also replaces the VMT entries for (non-class) virtual methods in  
all classes that cannot be instantiated in the program with  
FPC_ABSTRACTERROR. As a result, again if such methods are not  
referenced directly, then they can be removed by the linker (and the  
ones that are referenced can be turned into normal function calls).


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