Program Slicing

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Program Slicing

• Binoy Ketan Dash

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Outline

• What is Program Slicing?
• Why Program Slicing?
• How to Slice Program?
• Slicing Classifications
• Applications
• Program Slicing Tools
• Conclusion

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What is Program Slicing?

• More descriptively, it is a decomposition
  technique that extracts statements relevant to a
  particular computation from a program.
• Slicing Criterion lts, vgt
• Program Slices as Originally introduced by
  Weiser1 are known as executable backward static
  slices

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Why Program Slicing?

• Program Debugging thats how slicing was
  discovered!
• Testing reduce cost of regression testing after
  modifications (only run those tests that needed)
• Parallelization
• Integration merging two programs A and B that
  both resulted from modifications to BASE

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Why Program Slicing?

• Program Understanding
• Reverse Engineering comprehending the design by
  abstracting out of the source code the design
  decisions
• Software Maintenance changing source code
  without unwanted side effects
• Software Quality Assurance validate interactions
  between safety-critical components

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How?

• A slice is constructed by deleting those parts of
  a program which is irrelevant to the program with
  respect to the chosen set of variables at the
  chosen point.
• A slice is taken from a program with the slicing
  criterion lts , vgt, which specifies a location
  (Statement s) and a variable (v).

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How? Contd...

• Intermediate Representation of programs for
  slicing
• Control Flow Graph (CFG)
• Data Flow equations are solved
• Program Dependence Graph (PDG)
• Slice is computed as graph reachability problem

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Slicing Example

• 1 main( )
• 2
• 3 int i, sum
• 4 sum 0
• 5 i 1
• 6 while(i lt 10)
• 7
• 8 sum sum 1
• 9 i
• 10
• 11 Coutltlt sum
• 12 Coutltlt i
• 13
• An Example Program its slice w.r.t. lt12, igt

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PDG of the Example Program
Control Dep. Edge Data Dep. Edge
1
3
12
4
5
6
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Slice Point
8
9
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Slicing classifications

• Types of slices
• Static
• Dynamic
• Direction of slicing
• Backward
• Forward
• Executabiliy of slice
• Executable
• Closure
• Levels of slices
• Intraprocedural
• Interprocedural

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Slicing Properties

• Static Slicing
• Statically available information only
• No assumptions made on input
• Computed slice can never be accurate (minimal
  slice)
• Problem is undecidable reduction to the halting
  problem
• Current static methods can only compute
  approximations
• Result may not be usefull

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Slicing Properties

• Dynamic Slicing
• Computed on a given input
• Deterministic instead of Probabilistic
• Useful for applications that are input driven
  (debugging, testing)
• Slicing Criterion lti, p, vgt

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Example of Dynamic Slicing

• read (n)
• for I 1 to n do
• a 2
• if c11 then
• if c21 then
• a 4
• else
• a 6
• z a
• write (z)

Input n is 1 c1, c2 both true Execution history
is 11, 21, 31, 41, 51, 61, 91, 22,
101 Criterionlt1, 101, zgt
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Backward Static Slicing

• A backward slice of a program with respect to a
  program point p and set of program variables V
  consists of all statements and predicates in the
  program that may affect the value of variables in
  V at p

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Example of Backward Slicing

• 1 main( )
• 2
• 3 int i, sum
• 4 sum 0
• 5 i 1
• 6 while(i lt 10)
• 7
• 8 Sum sum 1
• 9 i
• 10
• 11 Coutltlt sum
• 12 Coutltlt i
• 13
• An Example Program its slice w.r.t. lt12, igt

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Forward Static Slicing

• A forward slice of a program with respect to a
  program point p and set of program variables V
  consists of all statements and predicates in the
  program that may be affected by the value of
  variables in V at p

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Example of Forward Slicing

• 1 main( )
• 2
• 3 int i, sum
• 4 sum 0
• 5 i 1
• 6 while(i lt 10)
• 7
• 8 sum sum 1
• 9 i
• 10
• 11 Coutltlt sum
• 12 Coutltlt i
• 13
• An Example Program its slice w.r.t. lt3, sumgt

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Slicing Properties

• Intraprocedural Slicing
• Computes slice within one procedure
• Assumes worse case for function calls
• Interprocedural Slicing
• Compute slice over an entire program
• Two ways for crossing procedure boundary
• Up going from sliced procedure into calling
  procedure
• Down going from sliced procedure into called
  procedure
• Must Be Context Sensitive

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Applications of Slicing

• Differencing
• Debugging
• Testing
• Program understanding
• Software maintenance
• Complexity measurement / Functional Cohesion
• Program integration
• Reverse engineering
• Software Quality Assurance

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Program Slicing Tools

• CodeSurfer
• Commercial product by GammaTech Inc.
• GUI Based
• Scripting language-Tk
• Unravel
• Static Program Slicer Developed at NIST
• Slices ANSI C programs
• Limitations are in the treatment of Unions, Forks
  and pointers to functions

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Conclusion

• No Universal algorithm
• Improving performance
• New algorithms

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References

• 1 M. Weiser. Program Slicing. In Proceedings of
  the Fifth International Conference on Software
  Engineering, pages 439-449, May 1981.
• 2 D. Binkley, K. Gallagher. Program Slicing. In
  Proceedings of In Advances in Computers, Volume
  43, 1996.
• 3 H.Agrawal, J. Horgan. Dynamic Program
  Slicing. In Proceedings of the ACM SIGPLAN90
• 4 A. DeLucia. Program Slicing Methods and
  Applications.
• IEEE workshop on Source Code Analysis and
  Manipulation (SCAM 2001)
• 5 K. B. Gallagher and J. R. Lyle. Using Program
  Slicing in Software Maintenance. IEEE
  Transactions on Software Engineering, 17(8),
  pages 751-761, August 1991.

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