Thrifty BTB: A comprehensive solution for dynamic power reduction in branch target buffers

Abstract

We propose Thrifty BTB, a mechanism to reduce the dynamic power dissipated by the BTB. We studied two mechanisms that reduce dynamic power dissipation. The first one is a serial-BTB configuration. The second mechanism is the filter-BTB, a combination of a low power counting Bloom filter placed in front of a conventional BTB. We also studied the effect of placing a small 32 entry direct-mapped BTB, functioning as a bypass, in parallel with the first two mechanisms. The filter-BTB reduces the number of lookups relative to a conventional BTB and the dynamic power dissipated. The serial-BTB variant only accesses the data array of the BTB upon a hit, therefore for most of the accesses the actual power dissipated is only what is dissipated by accessing the tag array. The bypass is used in parallel to either the filter- or the serial-BTB and reduces the performance cost by providing a low latency response in case of a hit. By integrating these mechanisms into a BTB design we achieve an average reduction of 51% in the dynamic power dissipation of the BTB. These benefits come at a small performance cost that is on average slightly less than 1.2%. The energy delay product was reduced by an average of 50%.