Fast & accurate instruction fetch and branch prediction

Abstract

Accurate branch prediction is critical to performance; mispredicted branches mean that ten's of cycles may be wasted in superscalar architectures. Architectures combining very effective branch prediction mechanisms coupled with modified branch target buffers (BTB's) have been proposed for wide-issue processors. These mechanisms require considerable processor resources. Concurrently, the larger address space of 64-bit architectures introduce new obstacles and opportunities. A larger address space means branch target buffers become more expensive. In this paper, we show how a combination of less expensive mechanisms can achieve better performance than BTB's. This combination relies on a number of design choices described in the paper. We used trace-driven simulation to show that our proposed design, which uses fewer resources, offers better performance than previously proposed alternatives for most programs, and indicate how to further improve this design.