An FPGA is an Impoverished Accelerator

November 25, 2014

Architects are prone to describing field-programmable gate arrays, or FGPAs, as a “light” version of hardware acceleration. While researchers have exploited FPGAs to great computational effect, especially recently, it is critical to remember that FPGAs are what we have, not what we want.

It is a misperception that FPGAs are a great way to bridge the gap between software flexibility and hardware efficiency. FPGAs are legacy baggage in the same way that GPGPUs are. Both have found incredible success as alternative computation models, but neither is what we would have designed if we were starting from scratch.

FPGAs were designed for prototyping ASICs and they do that well. But prototyping an ASIC is not tantamount to harnessing an ASIC’s acceleration benefits. A circuit design for an ASIC accelerator needs significant—often fundamental—changes to become a good FPGA design. An FPGA design worth its salt needs to exploit FPGA-specific resources, like hard arithmetic logic, and respect a specific FPGA part’s parameters.

It’s no shock, then, that people who work on FPGAs for computation end up concocting ways to extend them to make them better for the task.

As accelerators, FPGAs are hampered by an unintelligible programming interface that is a consequence of vendor lock-in due to a paucity of abstraction. To paper over the interface’s complexity, the two major vendors sell proprietary software stacks that are infeasible to replace. No one seems happy with these tools, but the duopoly means we’re stuck with them. Even more modern languages such as Chisel and Bluespec have to emit compatible HDLs on the backend and inherit the proprietary toolchains’ warts.

If the goal is accelerating applications, and it should be, then FPGAs are just data-flow accelerators with a terrible programming model. (Architects with EE backgrounds may be forgiven for thinking an HDL is a reasonable abstraction, but it’s not.) The community should work on ways to unlock the same fundamental efficiencies that FPGAs offer—spatial logic, fine-grained parallelism, explicit data movement, and bit-level flexibility—with an architecture designed from first principles. We should not be satisfied with abusing a legacy concept for acceleration; we should have the chutzpah to design an accelerator.