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Reachability Analysis

Reachability Analysis. Speaker : KJ Chang Advisor : Chun-Yao Wang Date : 2007.10.04. Outline:. SAT-Based Image Computation with Application in Reachability Analysis A. Gupta, Z. Yang, P. Ashar, and A. Gupta FMCAD 2000 Conclusion. Combining BDDs with SAT -based Methods:.

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Reachability Analysis

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  1. Reachability Analysis Speaker : KJ Chang Advisor : Chun-Yao Wang Date : 2007.10.04

  2. Outline: • SAT-Based Image Computation with Application in Reachability Analysis • A. Gupta, Z. Yang, P. Ashar, and A. Gupta • FMCAD 2000 • Conclusion

  3. Combining BDDs with SAT-based Methods: • Use BDD to represent state sets • Use CNF formula to represent the transition relation • All valid next state combinations are enumerated by SAT • Invoke BDD-based image computation within the SAT decision procedure

  4. CNF-BDDs image computation:

  5. Experiments: • #Reachable states > 1010 • Memory is not the problem • 274 MB is acceptable, but 6 million nodes is not

  6. Outline: • SAT-Based Image Computation with Application in Reachability Analysis • A. Gupta, Z. Yang, P. Ashar, and A. Gupta • FMCAD 2000 • Discussion

  7. Efficiency of reaching states: • VIS (BDD-based) : 1079437/sec • Simulation bits per second < 10000 • BDD is essential for reaching states

  8. Growing rate of states: • Reached states grows 3~4 time after each frame (s1423) • Only about ¼ simulation bits are valid • One state may reach hundreds of states, but most of them can be reached by other reached states

  9. Size & density: • For a sequential circuit with 100 latches • Reached states : • 252 = 4.5 E15 • Extremely big • State density : • 252/2100 = (2.8 E14)-1 • Extremely small

  10. Complexity: • A lot of previous states • A lot of next states • Search space can not be pruned easily • No easy way to check if one state is reachable or not • No easy way to simplify the transition relation

  11. Property of reachability: • A huge part of states are unreachable • A huge part of transition relation is unnecessary • A lot of computation is done, but only reach states repeatedly • Do we need to compute the exact image in each time frame?

  12. Future work: • Improve detection of conflict states • Try to eliminate useless states and transition relation • Try to improve the efficiency of reaching states

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