Overhead of calling tiny functions from a tight inner loop? [C++]
- by John
Say you see a loop like this one:
for(int i=0;
i<thing.getParent().getObjectModel().getElements(SOME_TYPE).count();
++i)
{
thing.getData().insert(
thing.GetData().Count(),
thing.getParent().getObjectModel().getElements(SOME_TYPE)[i].getName()
);
}
if this was Java I'd probably not think twice. But in performance-critical sections of C++, it makes me want to tinker with it... however I don't know if the compiler is smart enough to make it futile.
This is a made up example but all it's doing is inserting strings into a container. Please don't assume any of these are STL types, think in general terms about the following:
Is having a messy condition in the for loop going to get evaluated each time, or only once?
If those get methods are simply returning references to member variables on the objects, will they be inlined away?
Would you expect custom [] operators to get optimized at all?
In other words is it worth the time (in performance only, not readability) to convert it to something like:
ElementContainer &source =
thing.getParent().getObjectModel().getElements(SOME_TYPE);
int num = source.count();
Store &destination = thing.getData();
for(int i=0;i<num;++i)
{
destination.insert(thing.GetData().Count(), source[i].getName();
}
Remember, this is a tight loop, called millions of times a second. What I wonder is if all this will shave a couple of cycles per loop or something more substantial?
Yes I know the quote about "premature optimisation". And I know that profiling is important. But this is a more general question about modern compilers, Visual Studio in particular.
