Parallelism in the front-end

Abstract

As processor back-ends get more aggressive, front-ends will have to scale as well. Although the back-ends of superscalar processors have continued to become more parallel, the front-ends remain sequential. This paper describes techniques for fetching and renaming multiple non-contiguous portions of the dynamic instruction stream in parallel using multiple fetch and rename units. It demonstrates that parallel front-ends are a viable alternative to high-performance sequential front-ends.Compared with an equivalently-sized trace cache, our technique increases cache bandwidth utilization by 17%, front-end throughput by 20%, and performance by 5%. Parallelism also enhances latency tolerance: a parallel front-end loses only 6% performance as the cache size is decreased from 128 KB to 8 KB, compared with a 50-65% performance loss for sequential fetch mechanisms.