better expose NullPointerAnalysis and InterprocNullPointerAnalysis

[msridhar]

@mohrm talked about pruning certain imprecise exceptional edges from control-flow graphs based on a null-pointer analysis in the Workshop on WALA. I found the NullPointerAnalysis and InterprocNullPointerAnalysis classes in WALA, but I didn't see any uses of them. Maybe we can provide a standard API for computing pruned CFGs based on these analyses and make it more visible?

[mohrm]

There is the class NullPointerAnalysis which provides several utility methods to accomplish something like that: Through the method computeInterprocAnalysis(...) you can get an InterprocAnalysisResult instance from which you can get the pruned control flow graph for each call graph node (@jgf, correct me if I'm wrong). Maybe we can add another facade to simplify the usage.

[jgf]

@msridhar I support the idea of making it more visible, but I'm unsure about which form the standard API should have. Do you already have something in mind?

[msridhar]

We have the class PrunedCFG which gives a standard way to prune edges from a ControlFlowGraph. ExceptionPrunedCFG shows a use of the class, to prune all exceptional edges from a CFG. We could maybe create a class or method that creates a PrunedCFG with just infeasible exceptional edges pruned.

For the inter-procedural case, perhaps we could change InterproceduralCFG to optionally make use of the pruned control-flow graphs as constructed above. This way, any analysis operating on an InterproceduralCFG (e.g., any IFDS analysis using an ICFGSupergraph) could take advantage of having more precise control-flow graphs.

Just my initial thoughts, I'm open to other suggestions.