Branch Target Buffer Organizations

Abstract

To accommodate very large instruction footprints, modern high-performance processors rely on fetch directed instruction prefetching through huge branch predictors and a hierarchy of Branch Target Buffers (BTBs). Recently, significant effort has been undertaken to reduce the footprint of each branch in the BTB, in order to either minimize the storage occupied by the BTB on die, or to increase the number of tracked branches at iso-storage. However, designing for branch density, while necessary, is only one dimension of BTB efficacy. In particular, BTB entry organization plays a significant role in improving instruction fetch throughput, which is a necessary step towards increased performance. In this paper, we first revisit the advantages and drawbacks of three classical BTB organizations in the context of multi-level BTB hierarchies. We then consider three possible improvements to increase the fetch PC throughput of the Region BTB and Block BTB organizations, bridging most of the performance gap with the impractical but highly storage-efficient Instruction BTB organization, thus paving the way for future very high fetch throughput machines.