No, GP's proposal is ugly in the majority case. If you're going to make signed overflow defined behaviour then every time you write:

    int c = a + b;

You have to assume it will overflow and give an incorrect result. So now you need to check everything, everywhere, and you don't get any optimizations unless you explicitly ask for them with those ugly #push_optimize annotations. I completely fail to see how this is an advantage.

The way C works right now, the assumption is that you want optimization by default and safety is opt-in. The GP's proposal takes away the optimization by default. It then makes incorrect results the default, but it does not make safety the default. To make safety the default you would have to force people to write conditionals all over the place to check for the overflows with ckd_add, ckd_mul etc. Merely writing:

    int c = a + b;

Does not give you any assurances that your answer will be correct.