C or C++ : for loop variable

Question

My question is a very basic one. In C or C++:

Let's say the for loop is as follows,

for(int i=0; i<someArray[a+b]; i++) {
 ....
 do operations;
}

My question is whether the calculation a+b, is performed for each for loop or it is computed only once at the beginning of the loop?

For my requirements, the value a+b is constant. If a+b is computed and the value someArray[a+b]is accessed each time in the loop, I would use a temporary variable for someArray[a+b]to get better performance.

Answer 1

You can find out, when you look at the generated code

g++ -S file.cpp

and

g++ -O2 -S file.cpp

Look at the output file.s and compare the two versions. If someArray[a+b] can be reduced to a constant value for all loop cycles, the optimizer will usually do so and pull it out into a temporary variable or register.