Write correct, readable code and let the compiler worry about efficiency.
If you have performance problems, and if when you run a profiler to measure the performance (which you MUST do before trying to optimize) this code shows up as a performance issue, then you can:
1) Examine the assembly language interpretation of the code generated by the compiler with full optimization enabled, or
2) Try it both ways, measure, and pick the faster one.
The results will be highly dependent on the compiler you are using and the flags you specify for that compiler.