A Creature so Fierce…
I’ve been wrestling with control flow graphs like this. I eliminated those pesky “empty” nodes found in the previous incarnations, improved branching to track conditions and realized that what I’m really doing is developing a tool to bite off the excessive necks and heads (otherwise known as edges and basic blocks) of the Hydra monster that is represented by the bloody graph. Thus, say hello to DeHydra which was once known as dos.
In the near term I really need some value-awareness through abstract interpretation. For example, this graph of a simple function demonstrates several things that dehydra will need to notice:
- There is a path from hydra-unfeasable.c:6 to hydra-unfeasable.c:10 and that those blocks are part of only a single control flow, not 4.
- hydra-unfeasable.c:12 requires a contradiction in the condition in order to have flow through it and should be dropped or even reported as dead code.
I pondered a number of approaches ranging from value analysis, using a theorem prover and reversing conditions via prolog code. A simple C++ symbolic expression interpreter in C++ seems like the most productive thing at this point. So I will focus on that in the next short while.
While I was reviewing my options I found a paper on null pointer analysis for Java to be a useful stating point for references to other relevant literature.
Longer Term DeHydra Plans
There is literature on how source-to-source transformation (like squash but also like the future C++ to ES4 tool) is assisted by flow sensitive static analysis tools (like dehydra) which I will have to absorb.
DeHydra will gain whole-program analysis capability by:
- Inlining control flow graphs into the main() function
- Supporting user-defined function attributes and using some sort of attribute inference (like in ML) to propagate them. This method would have higher performance, but less precision.
There is also Whaley’s paper which uses prolog as a concise and expressive query engine for the source code. I would really like to see dehydra evolve such capabilities.
All this will take more than a little time on my own, but this stuff is too exciting for other impatient people to idly observe.