[phire]

I have been burned many times in the past by "the compiler is allowed to optimise something away".

You write your code assuming such an optimisation will happen, and for some reason, the compiler decides not to apply the optimisation. Perhaps the wind was blowing in the wrong direction, or it was in a bad mood, or you forgot to specify -fvisiblitly=hidden.

The exciting part here is that it happens by default, and the compiler is required to do it. I don't have to think about it, and the ABI automatically does the thing best for performance.

[jcelerier]

I don't think that fvisibility=hidden on its own is sufficient, it does not allow the compiler to break the call abi as the function could still be called from another .o (which will only know the mangled name of the original function). You need fvisibility=internal (or maybe fno-semantic-interposition but I'm not sure if it's enough).

[variadix]

I think you just need LTO if you want full optimization without regards for the ABI constraints