Compared to other low-level languages, C is a difficult language to optimise because of the aliasing rules and the incredibly vague/flexible machine model that

Compared to other low-level languages, C is a difficult language to optimise because of the aliasing rules and the incredibly vague/flexible machine model that allows it to be implemented for nearly any CPU and allows the programmer to break many assumptions that compiler writers would like to make. All the undefined behaviour in the C spec enables optimisations, yes, but it would be so much better if those optimisations didn't require a lot of mental labour by both compiler writers and programmers.