C++: Structs slower to access than basic variables?

Question

I found some code that had \"optimization\" like this:

void somefunc(SomeStruct param){
    float x = param.x; // param.x and x are both floats. supposedly this          


        

Answer 1

There are good and valid reasons to do that kind of optimization when pointers are used, because consuming all inputs first frees the compiler from possible aliasing issues which prevent it from producing optimal code (there's restrict nowadays too, though).

For non-pointer types, there is in theory an overhead because every member is accessed via the struct's this pointer. This may in theory be noticeable within an inner loop and will in theory be a diminuitive overhead otherwise.
In practice, however, a modern compiler will almost always (unless there is a complex inheritance hierarchy) produce the exact same binary code.

I had asked myself the exact same question as you did about two years ago and did a very extensive test case using gcc 4.4. My findings were that unless you really try to throw sticks between the compiler's legs on purpose, there is absolutely no difference in the generated code.