Using GCC to find unreachable functions ("dead code")

Dead code optimization is typically done by the linker - the compiler doesn't have the overview. However, the compiler might have eliminated unused static functions (as they have internal linkage).

Therefore, you shouldn't be looking at GCC options, but at ld options. It seems you want --print-gc-sections. However, note that you probably want GCC to put each function in its own section, -ffunction-sections. By default GCC will put all functions in the same section, and ld isn't smart enough to eliminate unused functions - it can only eliminate unused sections.