High-Performance Instruction Scheduling Circuits for Superscalar Out-of-Order Soft Processors.

Soft processors have a role to play in simplifying field-programmable gate array (FPGA) application design as they can be deployed only when needed, and it is easier to write and debug single-threaded software code than create hardware. The breadth of this second role increases when the performance of the soft processor increases, yet the sophisticated out-of-order superscalar approaches that arrived in the mid-1990s are not employed, despite their area cost now being easily tolerable. In this article, we take an important step toward out-of-order execution in soft processors by exploring instruction scheduling in an FPGA substrate. This differs from the hard-processor design problem because the logic substrate is restricted to LUTs, whereas hard processor scheduling circuits employ CAM and wired-OR structures to great benefit. We discuss both circuit and microarchitectural trade-offs and compare three circuit structures for the scheduler, including a new structure called a fused-logic matrix scheduler. Using our optimized circuits, we show that four-issue distributed schedulers with up to 54 entries can be built with the same cycle time as the commercial Nios II/f soft processor (240MHz). This careful design has the potential to significantly increase both the IPC and raw compute performance of a soft processor, compared to current commercial soft processors.