> Function calls also have that negative property that the compiler doesn’t know what happens after calling them so it needs to assume the worst happens. And by the worst, it has to assume that the function can change any memory location and optimize for such a case. So it omits many useful compiler optimizations.

This is not the case in C. It might be technically possible for a function to modify any memory, but it wouldn't be legal, and compilers don't need to optimise for the illegal cases.