C++ AVR big code size & excess unused symbols in build file

Question

I'm developing the program for the AVR microcontroller in C++. I have created a class that contains 8 methods(with constructors) but used only 3. Class code:

class Pin {
    Pin();
    Pin(PinId pin_id);
    Pin(Port port, uint8_t pin);

    void SetDirection(Direction direction);
    Direction GetDirection() const;

    void Write(Signal signal);
    Signal Read() const;

    void Toggle();
};

I expect that in the build file unused methods won't be, but it will. I use the Ghidra disassembler to disassemble the build file. Pin(Port port, uint8_t pin), SetDirection and Toggle are used only (simple LED flasher). But there are all of these methods in the build file and there are no references to these methods. So, why compiler/optimizer/linker doesn't remove these methods from the build file? I compile the program with -Os optimization parameter and Release CMake configuration. For example, the Write method in the Ghidra: But the Write method isn't used in my code (and in other class' methods too). How can I optimize it? Thanks in advance!

Answer 1

If using GCC (AVR-GCC), the following options can be helpful

Compiler:

-fdata-sections -ffunction-sections

Linker:

-Wl,--gc-sections

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Additional explanation:

The above options work together as follows

-ffunction-sections and -fdata-sections -- these put each function or data item into its own section in the output file (i.e. object file coming out of compiler). At this stage it's too soon to say whether we can garbage collect anything, only when linking will there be enough information to make that decision, hence the linker option below

-Wl,--gc-sections -- figures out if there are "sections" that are never used and removes them ("garbage-collection")...

Reference: the documentation

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Also, examining the .map file is a way to deduce how much individual functions/methods or static data items are contributing to the total size