Casting function result into a void speed comparison

I wonder how casting function into a void will affect speed. By that i mean :

int a,b;

int main (){
  (void)my_func();
}

int my_func(){
  return a+b;
}

What would compiler do with this.

int a,b;

int main (){
  my_func();
}

int my_func(){
  return a+b;
}

Which of this blocks of code would be faster. If noone of them, then what is the purpose of writing (void) before function call

Solution

In C language, programmer should not care too much for low level optimizations^(*). The mantra says the compiler is smarter than you. That means that such low level details are normally handled by the optimizing compilers.

What should matter are:

using efficient algorithms. A more efficient algorithm can lead to better performances in orders of magnitude
readability. If a co-worker (or even you) has to fix or improve something later in a code you have written, it is essential that all intends are clear.

Here the (void) cast mainly says that the programmer willingly discarded the return value and that they did not forget it by mistake.

^(*) As for any rule, there are of course exceptions. When you are writing code where execution time is critical, you should carefully profile it and care for low-level optimizations in the portions where most time is spent. But you must be aware that those optimizations only make sense on a specific architecture and a specific compiler. It was even common to use assembly language in this use case.