Exploiting value locality in physical register files

Abstract

The physical register file is an important component of a dynamically-scheduled processor. Increasing the amount of parallelism places increasing demands on the physical register file, calling for alternative file organization and management strategies. This paper considers the use of value locality to optimize the operation of physical register files. We present empirical data showing that: (i) the value produced by an instruction is often the same as the value produced by another recently executed instruction, resulting in multiple physical registers containing the same value, and (ii) the values 0 and 1 account for a considerable fraction of the values written to and read from physical registers. The paper then presents three schemes to exploit the above observations. The first scheme extends a previously-proposed scheme to use only a single physical register for each unique value. The second scheme is a special case for the values 0 and 1. By restricting optimization to these values, the second scheme eliminated many of the drawbacks of the first scheme. The third scheme further improves on the second, resulting in an optimization that reduces physical register requirements with simple micro-architectural extensions. A performance evaluation of the three schemes is also presented.