Low-power data forwarding for VLIW embedded architectures
In this paper, we propose a low-power approach to the design of embedded very long instruction word (VLIW) processor architectures based on the forwarding (or bypassing) hardware, which provides operands from interstage pipeline registers directly to the inputs of the function units. The power optimization technique exploits the forwarding paths to avoid the power cost of writing/reading short-lived variables to/from the register file (RF). Such optimization is justified by the fact that, in application-specific embedded systems, a significant number of variables are short-lived, that is', their liveness (from first definition to last use) spans only few instructions. Values of short-lived variables can thus be accessed directly through the forwarding registers avoiding writeback to the RF by the producer instruction and successive read from the RF by the consumer instruction. The decision concerning the enabling of the RF writeback phase is taken at compile time by the compiler static scheduling algorithm. This approach implies a minimal overhead on the complexity of the processor control logic and, thus, no critical path increase. The application of the proposed solution to a VLIW embedded core has shown an average RF power saving of 7.8% with respect to the unoptimized approach on the given set of target benchmarks (patent pending owned by ST Microelectronics).
low-power design microprocessors vlsi design