You really need to test if you get any advantage from it.

1 - having a variable or parameter that isn't modified configured const should not make the code larger or slower. It is more a hint in the other direction - that the compiler can be more aggresive with the optimization.

2 - optimizing compilers should be quite able to figure out themselves that the parameter is never modified, meaning they can produce the same code with or without the const keywored.

3 - if making a parameter const, then you prototype must also use the const keyword.

4 - the biggest advantage with const, is that you get a compilation error if you later try to modify the function so that the compiler can't anymore take advantage of the parameter being const.