Why isn't this unused variable optimised away?

Question

I played around with Godbolt\'s CompilerExplorer. I wanted to see how good certain optimizations are. My minimum working example is:

#include 

int         


        

Answer 1

As the comments note, operator new can be replaced. This can happen in any Translation Unit. Optimizing a program for the case it's not replaced therefore requires Whole-Program Analysis. And if it is replaced, you have to call it of course.

Whether the default operator new is a library I/O call is unspecified. That matters, because library I/O calls are observable and therefore they can't be optimized out either.