PRECIS: Inferring Invariants Using Program Path Guided Clustering

1. PRECIS: Inferring Invariants Using Program Path Guided Clustering ParthSagdeo, VirajAthavale, SumantKowshik, ShobhaVasudevanCoordinated Sciences Laboratory, University of Illinois at Urbana-Champaign Test cases Clusters with output functions Trace data Data generation Predicate Clustering Invariant generation Program source Tool Flow Invariants • Introduction • PRECIS(PREdicate Clustering for Invariant Synthesis) is an invariant generation technique that uses program path information to guide the statistical analysis of dynamic data. In contrast to existing approaches, PRECIS: • Gains context lacking in other dynamic tools such as DAIKON by using statically generated program path information • Resolves pointer aliasing, which is not possible through purely symbolic approaches. • Remains more scalable than static approaches by not performing expensive tasks such symbolic execution or theorem proving. • The PRECIS Algorithm • The major steps in the PRECIS flow are to: • Instrument the program to capture inputs, target outputs and path conditions. • Use a regression strategy to infer a linear relationship from the inputs to the function to each output. • Cluster program paths to generate succinct invariants. • Paths are examined as candidates for clustering with neighboring groups. • Among the neighboring groups, those that represent a common input-output behavior are clustered. • At the end of this process, each cluster represents a single input-output invariant common to all paths in the cluster. Results Summary of benchmark programs and PRECIS invariant coverage. Example inta, b, c; intmin = a, max = a; if (min > b) //p0 min = b; else if(max < b) //p1 max = b; if(min > c) //p2 min = c; else if(max < c) //p3 max = c; Example Trace Data: The result after running a number of test vectors. The tuples are grouped by their predicate word (1X01, 1X00, and 0100). A graph of the dependence of invariant quality (path coverage) on the test suite size. PRECIS converges to 100% path coverage quickly – within 100 to 2000 test executions Example Source Code: Computes the min and max of three ints. All inputs (a,b,c), predicates (p0,p1,p2,p3), and outputs (min, max) are instrumented. Predicate Groups After Regression: The result after a linear regression is performed on the predicate groups with enough support: 1X01 and 0100. DAIKON PRECIS Rank 0: True for the given trace data but do not hold for the program in general. Rank 1: Provide useful information about outputs, but do not express outputs as exact function of other variables. Rank 2: Express an output as an exact function of inputs, but only holds for a subset of program paths Rank 3: Express the output function completely. • Applications • The invariants generated by PRECIS can be used for a variety of purposes: • Program Correctness • Developer Understanding • Documentation • Experiments • Invariants generated for the Siemens benchmarks: • replace.c: A string replacement and regex library • schedule.c: A scheduling program • BST.java: A binary search tree implementation • Each demonstrates PRECIS as a powerful approach to inferring a large number of useful invariants. Predicate Clusters after Merging: The two predicate clusters (in blue and purple) after the merging of predicate groups. Each represents a single invariant. Resulting Invariants: The invariants generated for the two predicate clusters above. Each is of the form: (predicate word) (output = lin. comb. of inputs)