Contrasting branch characteristics and branch predictor performance of C++ and C programs

Abstract

In today's superscalar and superpipelined computers, branch instructions are the main culprits that limit the processor from issuing instructions at a peak rate. Although C++, which is an object-oriented programming language, provides great flexibility to help solve complex programming problems, the cost is a penalty in performance when compared to C. This paper compares the intrinsic branch behavior and basic branch characteristics of C++ and C benchmarks and correlates these characteristics to the performance of branch predictors. This study produced interesting insights into the differences between C++ and C benchmarks with respect to branch behavior. The basic branch characteristics showed that C++ programs have more inconsistency for the targets of branch instructions than C benchmarks. Various branch prediction schemes were tested, and the results show that although it is possible to predict whether a branch is taken or not taken with similar prediction accuracies in C++ and C, when branch target buffers (BTB) are considered, target inconsistency makes target prediction more difficult in C++. Improving the BTB to better accommodate the inconsistent Mature of C++ targets can reduce the penalty due to specific programming styles and practices of C++.