Cost-efficient branch target buffers

Abstract

Branch target buffers (BTBs) are caches in which branch information is stored that is used for branch prediction by the fetch stage of the instruction pipeline. A typical BTB requires a few kbyte of storage which makes it rather large and, because it is accessed every cycle, rather power consuming. Partial resolution has in the past been proposed to reduce the size of a BTB. A partial resolution BTB stores not all tag bits that would be required to do an exact lookup. The result is a smaller BTB at the price of slightly less accurate branch prediction. This paper proposes to make use of branch locality to reduce the size of a BTB. Short-distance branches need fewer BTB bits than long-distance branches that are less frequent. Two BTB organisations are presented that use branch locality. Simulation results are given that demonstrate the effectiveness of the described techniques.