A speculative control scheme for an energy-efficient banked register file

Multiported register files are critical components of modern superscalar and simultaneously multithreaded (SMT) processors, but conventional designs consume considerable die area and power as register counts and issue widths grow. Banked multiported register files consisting of multiple interleaved banks of lesser ported cells can be used to reduce area, power, and access time and previous work has shown that such designs can provide sufficient bandwidth for a superscalar machine. These previous banked designs, however, have complex control structures to avoid bank conflicts or to buffer conflicting requests, which add to design complexity and would likely limit cycle time. This paper presents a much simpler and faster control scheme that speculatively issues potentially conflicting instructions, and then quickly repairs the pipeline if conflicts occur. We show that, once optimizations to avoid regfile reads are employed, the remaining read accesses observed in detailed simulations are close to randomly distributed and this contributes to the effectiveness of our speculative control scheme. For a four-issue superscalar processor with 64 physical registers, we show that we can reduce area by a factor of three, access time by 25 percent, and energy by 40 percent, while decreasing IPC by less than 5 percent. For an eight-issue SMT processor with 512 physical registers, area is reduced by a factor of seven, access time by 30 percent, and energy by 60 percent, while decreasing IPC by less than 2 percent.