  Over the past decade, neural network (NN)-based controllers have demonstrated
remarkable efficacy in a variety of decision-making tasks. However, their
black-box nature and the risk of unexpected behaviors and surprising results
pose a challenge to their deployment in real-world systems with strong
guarantees of correctness and safety. We address these limitations by
investigating the transformation of NN-based controllers into equivalent soft
decision tree (SDT)-based controllers and its impact on verifiability.
Differently from previous approaches, we focus on discrete-output NN
controllers including rectified linear unit (ReLU) activation functions as well
as argmax operations. We then devise an exact but cost-effective transformation
algorithm, in that it can automatically prune redundant branches. We evaluate
our approach using two benchmarks from the OpenAI Gym environment. Our results
indicate that the SDT transformation can benefit formal verification, showing
runtime improvements of up to 21x and 2x for MountainCar-v0 and CartPole-v0,
respectively.
