Shachar Itzhaky, Anindya Banerjee, Neil Immerman, Aleks Nanevski, Mooly Sagiv Computer Aided Verification, 2013, St. Petersburg, Russia, July 2013.
This paper proposes a novel method of harnessing existing SAT solvers to verify reachability properties of programs that manipulate linked-list data structures. Such properties are essential for proving program termination, correctness of data structure invariants, and other safety properties. Our solution is complete, i.e., a SAT solver produces a counterexample whenever a program does not satisfy its specification. This result is surprising since even first-order theorem provers usually cannot deal with reachability in a complete way, because doing so requires reasoning about transitive closure.
Our result is based on the following ideas:
- Programmers must write assertions in a restricted logic without quantifier alternation or function symbols.
- The correctness of many programs can be expressed in such restricted logics, although we explain the tradeoffs.
- Recent results in descriptive complexity can be utilized to show that every program that manipulates potentially cyclic, singly- and doubly-linked lists and that is annotated with assertions written in this restricted logic, can be verified with a SAT solver.
We implemented a tool atop Z3 and used it to show the correctness of several linked list programs.
Shachar Itzhaky, Sumit Gulwani, Neil Immerman, Mooly Sagiv In the Proceedings of the ACM SPLASH Conference (SPLASH 2010), Reno, NV, United States, October 2010.
Given a high-level specification and a low-level programming language, our goal is to automatically synthesize an efficient program that meets the specification. In this paper, we present a new algorithmic methodology for inductive synthesis that allows us to do this.
We use Second Order logic as our generic high level specification logic. For our low-level languages we choose small application-specific logics that can be immediately translated into code that runs in expected linear time in the worst case.
We explain our methodology and provide examples of the synthesis of several graph classifiers, e.g, linear-time tests of whether the input graph is connected, acyclic, etc. In another set of applications we automatically derive many finite differencing expressions equivalent to ones that Paige built by hand in his thesis. Finally we describe directions for automatically combining such automatically generated building blocks to synthesize efficient code implementing more complicated specifications.
The methods in this paper have been implemented in Python using the SMT solver Z3.
Shachar Itzhaky, Anindya Banerjee, Neil Immerman, Aleks Nanevski, Mooly Sagiv
We describe a framework that combines deductive, numeric, and inductive reasoning to solve geometric problems. Applications include the generation of geometric models and animations, as well as problem solving in the context of intelligent tutoring systems.
Our novel methodology uses (i) deductive reasoning to generate a partial program from logical constraints, (ii) numerical methods to evaluate the partial program, thus creating geometric models which are solutions to the original problem, and (iii) inductive synthesis to read off new constraints that are then applied to one more round of deductive reasoning leading to the desired deterministic program. By the combination of methods we were able to solve problems that each of the methods was not able to solve by itself.
The number of nondeterministic choices in a partial program provides a measure of how close a problem is to being solved and can thus be used in the educational context for grading and providing hints.
We have successfully evaluated our methodology on 18 Scholastic Aptitude Test geometry problems, and 11 ruler/compass-based geometry construction problems. Our tool solved these problems using an average of a few seconds per problem.