GCC --gc-sections and finding symbol dependencies

Question

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.

Answer 1

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.

Answer 2

You can try the linker option -Wl,--trace-symbol=<symbol_name>. It will display in the output where the symbol is defined, and where the symbol is used.

Answer 3

Create a linker map file with cross-reference output:

-Wl,-Map=output.map -Wl,--cref