Semantic comparison of Visual Dataflow Programs Anh Dang Phil Cox, Dalhousie University.

1. Semantic comparison of Visual Dataflow Programs Anh Dang Phil Cox, Dalhousie University. M 10 C I1 I2 I3 M1 M2 C 1 C 2 C 3 Introduction Semantic equivalence • A set of subgraph isomorphisms between two directed graphs is computed, and for each function in this set, local differences of an isomorphism is calculated, as follows. • Local differences due to an isomorphism is the sum of the differences between matched operations plus differences due to mismatched connections and extra program elements, as shown in Figure 4. • To compare two methods, the algorithm traverses a search tree in which a node is either a pair of methods (M), a pair of cases (C) or an isomorphism (I), as shown in figure 5. • The value of a method or an isomorphism node is the sum of the values of its children plus the local difference. The value of a case node is the smallest value among its child nodes. • The algorithm uses depth-first search to traverse the tree, guided by heuristics based on estimates of the numbers of differences between the items being compared at each node. Alpha-beta pruning • Despite 25 years of continuing research, visual programming languages (VPLs) have made few inroads into the world of industrial software development. • Visual representations are used to specify the architecture of software systems, but have not replaced textual programming languages (TPLs) for representing algorithms. • One reason is that VPLs lack source-code analysis tools, such as differencing tools. • We present a semantic differencing algorithm for controlled dataflow VPLs such as Prograph. • Prograph • In Prograph, a program is a set of methods together with a set of persistents, which are globally accessible storage locations. • A method in Prograph consists of a sequence of cases, each of which is a data flow diagram of operations connected by datalinks. For example, Figure 1 shows a Prograph method. • Two program elements in Prograph are semantically equivalent iff with the same inputs they will produce the same outputs. • The semantics of controlled data flow programs, like the semantics of functional programs and unlike those of imperative languages, is closely aligned with the syntax. • Existing differencing tools for VPLs are quite primitive compared with those that are available for TPLs and cannot detect semantic differences. Cut-off Mismatched datalinks 10 Figure 6. Difference search tree. • The value of a case node can only get smaller as search proceeds. Hence a modified version of alpha-beta search algorithm is used to reduce the number of nodes explored in the search tree. • If at any time, the difference count of a methodor isomorphism node is larger the alpha value, the algorithm does not need to explore further (cut-off). • If at any time, the difference count of a casenode is equal to zero, the algorithm does not need to explore further (cut-off). • For a method node, its children nodes are sorted increasingly by case local differences to significantly reduce the total number of nodes searched. • For an isomorphism node, its children nodes are sorted decreasingly by method local differences. The comparison algorithm • Comparing operations: • In Figure 2, the number of differences between two operations is 5 • Comparing cases and methods: • Method local difference is the different number of cases between two methods. • Each case is considered as a directed acyclic graph where the vertices are the operations, and there is an edge from operation A to operation B iff there is a datalink or a synchro from A to B. Extra nodes and datalinks Extra synchro The first terminals have different types, as do their second terminals, count 2 Figure 4. Counting differences between cases A method consists a sequence of cases M The numbers of roots is different, count 1 C C Two operations have different controls and types, count 2 Figure 2. Counting differences between operations I I I I M M M M M M C C C C C Figure 1. A method with two cases implementing the quicksort algorithm. Concluding remarks • The algorithm: • Determines if two programs are semantically equivalent, even if they are syntactically different. • Finds semantic differences rather than syntactic differences • Can be adapted to any controlled dataflow VPL • Future work • Replace subgraph isomorphism with maximal common subgraph search • Investigate domain-specific matching algorithm for cases • Apply semantic differences to program integration and regression testing. Figure 5. The search tree. Input and output bars must be matched Input Input 1 1 <= <= -1 -1 -1 FactA FactA * * Output Output Figure 3. Directed acyclic graphs for two diagrams in Figure 4.