The hexatope and octatope abstract domains for neural network verification

Efficient verification algorithms for neural networks often depend on various abstract domains such as intervals, zonotopes, and linear star sets. The choice of the abstract domain presents an expressiveness vs. scalability trade-off: simpler domains are less precise but yield faster algorithms. This paper investigates the hexatope and octatope abstract domains in the context of neural net verification. Hexatopes are affine transformations of higher-dimensional hexagons, defined by difference constraint systems, and octatopes are affine transformations of higher-dimensional octagons, defined by unit-two-variable-per-inequality constraint systems. These domains generalize the idea of zonotopes which can be viewed as affine transformations of hypercubes. On the other hand, they can be considered as a restriction of linear star sets, which are affine transformations of arbitrary \(\mathcal {H}\)-Polytopes. This distinction places hexatopes and octatopes firmly between zonotopes and linear star sets in their expressive power, but what about the efficiency of decision procedures? An important analysis problem for neural networks is the exact range computation problem that asks to compute the exact set of possible outputs given a set of possible inputs. For this, three computational procedures are needed: (1) optimization of a linear cost function; (2) affine mapping; and (3) over-approximating the intersection with a half-space. While zonotopes allow an efficient solution for these approaches, star sets solves these procedures via linear programming. We show that these operations are faster for hexatopes and octatopes than they are for the more expressive linear star sets by reducing the linear optimization problem over these domains to the minimum cost network flow, which can be solved in strongly polynomial time using the Out-of-Kilter algorithm. Evaluating exact range computation on several ACAS Xu neural network benchmarks, we find that hexatopes and octatopes show promise as a practical abstract domain for neural network verification.