Exploiting forwarding to improve data bandwidth of instruction-set extensions

Application-specific instruction-set extensions (custom instructions) help embedded processors achieve higher performance. Most custom instructions offering significant performance benefit require multiple input operands. Unfortunately, RISC-style embedded processors are designed to support at most two input operands per instruction. This data bandwidth problem is due to the limited number of read ports in the register file per instruction as well as the fixed-length instruction encoding. We propose to overcome this restriction by exploiting the data forwarding feature present in processor pipelines. With minimal modifications to the pipeline and the instruction encoding along with cooperation from the compiler, we can supply up to two additional input operands per custom instruction. Experimental results indicate that our approach achieves 87-100% of the ideal performance limit for standard benchmark programs. Additionally, our scheme saves 25% energy on an average by avoiding unnecessary accesses to the register file.