g++keeps unused objects with virtual functions

[Richard Biener]

On April 7, 2015 5:00:27 PM GMT+02:00, Stefan Ehrlich <stefan.ehrlich@ehrlich.eu> wrote:
>Hello GCC developer team,
>I hope I am right here to address my problem with memory usage and g++:
> 
>I am writing C++ software for several very small embedded systems (8k
>and smaller) and a feature with the virtual tables and the linker does
>not make my life easy :-) I have a lot of objects with virtual
>functions, where not all of them are used in each application, but they
>remain existing in the source code.
>Until now I could not find a way to get rid of them in the output
>elf/hex file automatically (they are not removed by the linker).
> 
>For better understanding an example:
> 
>The program:
>int main()
>{
>    for(;;)
>    {
>     // Nothing to do
>    }
>    // unreachable code
>    //return 0;
>}
>
>uses 62 bytes of flash and 0 bytes of RAM on an atmega8 µC (compiled
>with gcc 4.9.2)
> 
>When I add a not used object with virtual functions (in the below
>listed example named as Derived0):
> 
>class CBase
>{
>public:
> virtual void virtFunction() = 0;
>};
>
>class CDerived : public CBase
>{
>public:
>    virtual void virtFunction() { }
>};
> 
>CDerived Derived0;
> 
>int main()
>{
>    for(;;)
>    {
>     // Nothing to do
>    }
>    // unreachable code
>    //return 0;
>}
> 
>the memory usage jumps up to 156 bytes flash and 8 bytes RAM usage
>(same compiler 4.9.2)
> 
>compiler and linker options are:
>avr-g++.exe  -c -Os -Wall -fdata-sections -ffunction-sections
>-fvisibility=hidden -fvisibility-inlines-hidden -fno-rtti -flto
>-fuse-linker-plugin -mmcu=atmega8 ...
>avr-gcc.exe  -Wall -Os -Wl,-static -Wl,-flto -fuse-linker-plugin
>-Wl,--strip-all -Wl,-s -Wl,--gc-sections -mmcu=atmega8 ...
> 
>The more not used objects I use the worse the problem gets.
> 
>Is there any possibility to remove unused virtual functions or at least
>the objects, which are not used? I have not find any solution so far.
>If not, is there a plan to add this feature to the linker?

I think it should already work with LTO.  Maybe there are some target specifics which make the vtables referenced?

Richard.

>greetings from Austria
> 
>Stefan
>