Liveness-Based Pointer Analysis

摘要

Precise flow- and context-sensitive pointer analysis (FCPA) is generally considered prohibitively expensive for large programs; most tools relax one or both of the requirements for scalability. We argue that precise FCPA has been over-harshly judged-the vast majority of points-to pairs calculated by existing algorithms are never used by any client analysis or transformation because they involve dead variables. We therefore formulate a FCPA in terms of a joint points-to and liveness analysis which we call L-FCPA. We implemented a naive L-FCPA in GCC-4.6.0 using linked lists. Evaluation on SPEC2006 showed significant increase in the precision of points-to pairs compared to GCC's analysis. Interestingly, our naive implementation turned out to be faster than GCC's analysis for all programs under 30kLoC. Further, L-FCPA showed that fewer than 4% of basic blocks had more than 8 points-to pairs. We conclude that the usable points-to information and the required context information is small and sparse and argue that approximations (e.g. weakening How or context sensitivity) are not only undesirable but also unnecessary for performance.