Energy-aware fetch mechanism: trace cache and BTB customization

A highly-efficient fetch unit is essential not only to obtain good performance but also to achieve energy efficiency. However, existing designs are inflexible and depending on program behavior, can be either insufficient or an overkill. We introduce a phase-based adaptive fetch mechanism that can be dynamically adjusted based on feedback information of the program behavior. This design adds very little hardware complexity and relegates complex tasks to the software components. It is also very effective: saving 26.8% and 34.1% fetch energy on average compared with a conventional and a trace cache-based fetch unit, respectively. At the same time, performance is improved by 5.7% and 0.6%, respectively.