Reinforcement Learning for Syntax-Guided Synthesis

Program synthesis is the task of automatically generating code based on a specification. In Syntax-Guided Synthesis(SyGuS) this specification is a combination of a syntactic template and a logical formula, and any generated code is proven to satisfy both. Techniques like SyGuS are critical to guaranteeing correct synthesis results. Despite the proliferation of machine learning in other types of program synthesis, state-of-the-art techniques in SyGuS are still driven by automated reasoning tools and simple enumeration. We hypothesize this is for two reasons: first the complexity of the search problem, and second the relatively small data sets available. In this work, we tackle these challenges by framing general SyGuS problems as a tree-search, and present a reinforcement learning guided synthesis algorithm for SyGuS based on Monte-Carlo Tree Search (MCTS). Our algorithm incorporates learned policy and value functions combined with the upper confidence bound for trees to balance exploration and exploitation. We incorporate this search procedure in a reinforcement learning setup in order to iteratively improve our policy and value estimators which are based on boosted tree models. To address the scarcity of training data, we present a method for automatically generating training data for SyGuS based on \emph{anti-unification} of existing first-order satisfiability problems, which we use to train our MCTS policy. We implement and evaluate this setup and demonstrate that learned policy and value improve the synthesis performance over a baseline enumerator by over $26$ percentage points in the training and testing sets. With these results our tool outperforms state-of-the-art-tools such as CVC5 on the training set and performs comparably on the testing set. We make our data set publicly available, enabling further application of machine learning methods to the SyGuS problem.