Register File Caching for Energy Efficiency

With the use of faster clocks and larger instruction windows in high-end superscalar processors, the physical register files (RFs) can no longer be accessed in a single cycle. To combat the consequential performance penalty, the RFs employ multiple levels of bypassing. Register file caching, which caches a small subset of the registers in a faster, smaller structure called the register file cache (RFC) has also been proposed as a remedy for this problem. We introduce a relatively simple RFC design that partitions the RFC into two separate components: a FIFO queue for holding register values that are used over a short duration following their writeback and another small set-associative cache holding values that are likely to be used over a longer duration. Results written to the RFC are easily classified into these categories and the classification bit is also used to predict the nature of the result for the next execution of the same instruction. We show that significant energy savings - about 38% on the average - occurs in accessing register operands when a 28-entry RFC is used, together with a 96-entry RF with no additional bypassing when compared with a base case design that has 128 registers with a 2 cycle access time and having one additional level of bypassing. The performance drop compared against the base case is also negligible (0.3% drop).