Efficient Memory via Formal Verification

My new PhD student Jianyi Cheng is presenting a very exciting paper at the ACM International Symposium on FPGAs (FPGA 2019). This is work he did for his Masters degree, and is a collaboration with Joy Chen and Jason Anderson at the University of Toronto, as well as Shane Fleming and myself at Imperial. In this blog post, I aim to summarise the main idea.

Multi-threaded programming is now a fairly mainstream activity, and has found its way into high-level synthesis tools, both through OpenCL and also LegUp pthreads support. We focus here on the latter.

At FPL 2017, Joy and Jason had a paper that automatically decided how to partition shared arrays for multi-threaded code, aiming to reduce the amount of arbitration required between hardware units and chunks of memory. Their approach used a simulation trace to identify candidate partitions, and designed the arbiters so that, for example, if accesses to partition P were only observed in that trace to come from thread T, then there is very low latency access to P from T at execution time. In this way, they were able to significantly speed up synthesised multi-threaded code making use of shared memories.

However, the arbiters were still there. They were necessary because while no access by some other thread T’ was observed during simulation, there was no guarantee that such an access might not occur at run-time. So the arbiters sat there, taking up FPGA area and – for large enough numbers of ports – hitting the critical path of the design.

Enter our work.

In our paper, we show – building on the excellent PhD thesis by Nathan Chong that I examined a few years back – how the original multi-threaded code can be translated into  single-threaded code in a verification language developed by Microsoft Research called Boogie. We then show how to automatically construct assertions in Boogie that, if passed, correspond to a formal proof that a particular thread can never access a particular partition. This lets us strip out the arbiters, gaining back the area and significantly boosting the clock frequency.

I think it’s a really neat approach. Please come and hear Jianyi give his talk and/or read the paper!

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