Static Slicing of Reactive Programs

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Static Slicing of Reactive Programs

• Aditya Kulkarni (KReSIT, I.I.T Bombay)
• Prof. S. Ramesh (Dept. of CSE, I.I.T Bombay)

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Outline

• What is Slicing ?
• Slicing intra-procedural sequential programs
• Slicing intra-procedural concurrent programs
• Esterel constructs and compilation scheme
• Dependencies in Esterel
• Slicing Algorithm

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Introduction

• Motivation
• Formal verification
• Debugging
• Slicing criterion ltline no, variablegt
• Slicing typically involves
• Computing dependencies and constructing PDG
• Backward traversal of the PDG

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Slicing concurrent programs

• Data, control, interference dependence
  represented in TPDG
• Simple backward traversal -gt imprecise slice
• Need to capture proper execution paths
• Threaded witness
• Trace witness
• Data, control, loop-carried data dependence ?
  trace witness exists
• Interference dependence does not ? trace witness
  exist

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Features of Esterel

• Imperative paradigm, synchronous concurrency
• Used to model control behaviour of reactive
  systems
• Some statements conceptually instantaneous
• Explicit delay pause
• Communication via broadcast signals emit S,
  present S
• Rich in control constructs
• Preemption abort p when S
• Suspension suspend p when S
• Exception handling trap and exit

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Trap and Exit
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Pause
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Abort
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Parallel
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Dependencies in Esterel

• Data dependencies cant exist across threads
• Signal dependencies
• Simple exist in non-concurrent threads
• Interference in concurrent threads
• Control dependencies
• Induced in non-concurrent threads
• Induced in concurrent threads due to preemption
• Time dependency captured as control dependency

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Interference control dependency

• Control dependence j control dependent on i iff
• j doesnt post-dominate i
• ? k along path i to j, j post-dominates i

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Slicing Algorithm

• Slicing criterion ltline no, signalgt
• State of execution recorded in tuple t0,,tn
• Perform backward traversal of STPDG
• For nodes y reached via edge y?x
• Data, control, simple signal, time dependency
  add y to slice, update tuple
• Interference signal or control dependency
• t last node visited in ys thread
• If trace witness exists for lty,tgt, add y. Update
  tuple.

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Summary

• Complex control constructs of Esterel make
  slicing non-trivial
• New way of modeling control constructs
• New dependencies time and interference control
  dependencies