Dual-IS: Instruction Set Modality for Efficient Instruction Level Parallelism
Exploiting instruction level parallelism (ILP) is a widely used method for increasing performance of processors. While traditional very long instruction word (VLIW) processors can exploit ILP energy-efficiently thanks to static instruction scheduling, they suffer from bad code density with serial parts that cannot utilize the multi-issue capabilities. Transport triggered architecture (TTA) is a variation of the VLIW paradigm with an exposed datapath that improves scaling of VLIW processors with the cost of even wider instructions by exposing the datapath interconnection network to the programmer.
To this end, we propose Dual-IS, an architecture that implements a TTA multi-issue and RISC-V compatible single-issue instruction-set modes by means of a microcoded control path. By utilizing the instruction set modality, the TTA mode can be used in codes that benefit from ILP, while a single-issue RISC-V ISA mode reduces instruction stream energy and code size for sequential programs. Thanks to the TTA programming model, the static multi-issue mode can be implemented without additional register file ports.
The processor was synthesized on a 28 nm ASIC technology. For this design point, when instruction-set mode was selected based on energy-delay product at the program granularity, with CHStone benchmarks, Dual-IS had on average 14% lower energy-delay product compared to a single-mode TTA processor with a similar datapath, while only adding a 3% overhead in the core area. Dual-IS achieved on average 15% and in the best case 33% smaller run times than a single-issue RISC-V implementation by running programs in the TTA mode only when it was beneficial in terms of performance.
A pre-print accepted to ARCS 2022 https://arcs-conference.org