Firm-core virtual FPGA for just-in-time FPGA compilation

Just-in-time (JIT) compilation has been used in many applications to enable standard software binaries to execute on different underlying processor architectures, yielding software portability benefits. We previously introduced the concept of a standard hardware binary to achieve similar portability benefits for hardware, using a JIT compiler to compile the hardware binary to an FPGA. Our JIT compiler includes lean versions of technology mapping, placement, and routing algorithms that implement the standard hardware binary on a simple custom FPGA fabric designed specifically for JIT compilation. While directly implementing a custom FPGA fabric on silicon may be feasible for some applications, we investigated the option of implementing the simple FPGA fabric as a circuit mapped to a physical FPGA - a virtual FPGA. We described our simple fabric in structural VHDL, synthesized the fabric onto a Xilinx Spartan-IIE FPGA, and mapped 18 benchmark circuits onto the resulting virtual FPGA. Our results show a 6X decrease in performance and a 100X increase in hardware resource usage for the virtual FPGA approach compared to mapping the circuits directly to the physical FPGA. For applications in which hardware portability is essential, a designer could leverage the large capacity of current commercially available FPGAs to implement a virtual FPGA with tens of thousands of configurable gates, providing about the same amount of configurable logic as FPGAs produced in the mid 1990s. Nevertheless, the large overheads clearly indicate the need to develop a virtual FPGA approach tuned to physical fabrics in order to reduce the overhead.