Question

This question talks of an optimization of the sort function that cannot be readily achieved in C:
Performance of qsort vs std::sort?

Are there more examples of compiler optimizations which would be impossible or at least difficult to achieve in C when compared to C++?

Answer 1

As @sehe mentioned in a comment. It’s about the abstractions more than anything else. In other words, if the language allows the coder to express intent better, then it can emit code which implements that intent in a more optimal fashion.

A simple example is std::fill. Sure for basic types, you could use memset, but, let’s say it’s an array of 32-bit unsigned longs. std::fill knows that the array size is a multiple of 32-bits. And depending on the compiler, it might even be able to make the assumption that the array is properly aligned on a 32-bit boundary as well.

All of this combined may allow the compiler to emit code which sets the value 32-bit at a time, with no run-time checks to make sure that it is valid to do so. If we are lucky, the compiler will recognize this and replace it with a particularly efficient architecture specific version of the code.

(in reality gcc and probably the other mainstream compilers do in fact do this for just about anything that could be considered equivalent to a memset already, including std::fill).

often, memset is implemented in a way that has run-time checks for these types of things in order to choose the optimal code path. While this difference is probably negligible, the idea is that we have better expressed the intent of “filling” an array with a specific value, so the compiler is able to make slightly better choices.

Other more complicated language features do a good job of using the expression of intent to get larger gains, but this is the simplest example.

To be clear, my point is not that std::fill is “better” than memset, instead this is an example of how c++ allows better expression of intent to the compiler, allowing it to have more information during compile time, resulting in some optimizations being easier to implement.