> What I want to know is what's wrong with __builtin_constant_p. 1. __builtin_constant_p might not return a constant--it might return a magic value that says "

> What I want to know is what's wrong with __builtin_constant_p. 1. __builtin_constant_p might not return a constant--it might return a magic value that says "I don't know yet, ask me again at a later optimization step". __builtin_choose_expr needs to be evaluated pretty early, so "ask again later" isn't good enough for it. https://lkml.org/lkml/2018/3/18/268 https://lkml.org/lkml/2018/3/18/268 2. It doesn't really care if it is constant in the __builtin_constant_p-sense, it cares if it has side-effects. > ({x++; 0;}) is constant in the eyes of __builtin_constant_p, but not side-effect free. https://lkml.org/lkml/2018/3/18/316 https://lkml.org/lkml/2018/3/18/316