Speculative execution and reducing branch penalty in a parallel issue machine

Parallel instruction issue is essential for performance improvement in current microprocessor designs. Just extra function units are, however, little beneficial in non-numerical applications since control dependence severely limits exploitation of instruction-level parallelism (ILP) and frequent branches consume ILP due to its long latency. Boosting is an interesting technique to reduce control dependence. It allows general speculative execution with little cycle time penalty. From the cost/performance point of view, we propose the efficient implementation of boosting, which requires the small support hardware and maximizes performance gain from boosting in the limited hardware. We also propose a new branch scheme to reduce the branch penalty which has a particularly big performance impact in a parallel issue machine. Our scheme fetches from both directions of the branch with small hardware cost through integration of a code movement and hardware support. We evaluate our schemes and find that they significantly contribute to performance improvement