Dear Richard, Zan,
There are no target specifics present. The program will be flashed to the µC
via a programmer and that's it.
Therefore I want to have the output hex file as small as possible.
When I play around with my demo program and the keyword virtual, I can clearly
see that the keywork virtual is generating the additional memory usage (by the
way it's strange that RAM is used)
Following program will generate 160 bytes of flash and 8 bytes of RAM
class CObject
{
public: virtual void virtFunction() { }
};
CObject NotUsedObject;
int main()
{
return 0;
}
By removing the virtual keyword, the linker is able to find out it is no longer
needed and will be removed
class CObject
{
public: /*virtual*/ void virtFunction() { }
};
CObject NotUsedObject;
int main()
{
return 0;
}
--> 66 bytes of flash and 0 bytes of RAM
I added the complete make file steps here:
avr-g++.exe -c -Os -Wall -fdata-sections -ffunction-sections
-fvisibility=hidden -fvisibility-inlines-hidden -fno-rtti -flto -fwhole-program
-fuse-linker-plugin -mmcu=atmega8 -DF_CPU=16000000L -DABUILD_BATCH=1
-DARDUINO=158 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -fno-exceptions
-D__PLATFORM__=Atmel_AVR_ATmega8_gcc -D__Atmel_AVR_ATmega8_gcc
-D__RELEASE__=rel_1_3_2 -I. ./main.cpp -o obj/./main.obj
avr-gcc.exe -Wall -Os -Wl,-static -flto -fwhole-program -fuse-linker-plugin
-Wl,--strip-all -Wl,-s -Wl,--gc-sections -mmcu=atmega8
-Wl,-Map=out/Virtual_Atmega8.map,--cref -o out/Virtual_Atmega8.elf
obj/./main.obj -lm
avr-strip.exe -s -R .comment -R .gnu.version --strip-unneeded
out/Virtual_Atmega8.elf
avr-objcopy.exe -O srec -R .eeprom out/Virtual_Atmega8.elf
out/Virtual_Atmega8.hex
avr-objcopy.exe -O ihex -R .flash out/Virtual_Atmega8.elf
out/Virtual_Atmega8.hex
avr-size_atmel.exe -C --mcu=atmega8 out/Virtual_Atmega8.elf
AVR Memory Usage
----------------
Device: atmega8
Program: 66 bytes (0.8% Full)
(.text + .data + .bootloader)
Data: 0 bytes (0.0% Full)
(.data + .bss + .noinit)
I added the -fwhole-program flag (as mentioned by Zan Lynx) but without any
effect.
Is here something missing or too much?
Lg
Stefan
-----Ursprüngliche Nachricht-----
Von: Richard Biener [mailto:richard.guent...@gmail.com]
Gesendet: Dienstag, 07. April 2015 18:42
An: Stefan Ehrlich; gcc@gcc.gnu.org
Betreff: Re: g++keeps unused objects with virtual functions
On April 7, 2015 5:00:27 PM GMT+02:00, Stefan Ehrlich
<stefan.ehrl...@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
>
