GCC --gc-sections and finding symbol dependencies
c++, embedded, gcc, thumb
Solution
I wasn't able to find a command that showed the symbol dependencies. However, I was able to get the information I needed by using the following technique:
- Add the symbol in question to the `/DISCARD/` section of the linker script. This will output an error message revealing which symbol is using it. It looks something like this: `<symbol0>' referenced in section '<symbol1>' of <lib0.a file path>(<object0 file path>): defined in discarded section '<symbol0>' of <lib1.a file path>(object1 file path>)`
- Continue to add symbols from these messages up the call stack to the `/DISCARD/` section until you find the root of the problem.
The root of the problem for me was having a class that inherited from another. This created a virtual table, and the compiler cannot remove dead code that is referenced in a virtual table.
Lesson learned: if you want to reduce code size and still use C++, don't use inheritance. The GNU toolchain used to have a `-fvtable-gc` switch to help with this, but it was removed some time ago. I will refactor my code to address my specific problem.
Problem
I'm trying to reduce the size of my elf executable. I'm compiling with `-ffunction-sections` `-fdata-sections` and linking with `-gc-sections`, but it appears some of the symbols that I believe are unused are not being discarded. Is there some command in the GNU toolchain I can run to find out which symbols are being used and where? - Toolchain: GNU arm-none-eabi - Platform: Cortex-M4 - Language: C++ Here are my typical build flags: Compilation: `arm-none-eabi-g++.exe -Wall -O3 -mthumb -std=c++11 -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -fsingle-precision-constant -ffunction-sections -fdata-sections` Link: `arm-none-eabi-g++.exe -static -mthumb -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -Wl,-gc-sections -Wl,-T"LinkerScript.ld` Thanks for the help.