Program Slicing

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Program Slicing

• Neelam Gupta
• The University of Arizona
• Tucson, Arizona, USA

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Program Slice

• A program slice contains the statements in a
  program that affect the value of a variable
  computed at some designated point (statement) of
  interest in the program.
• Static Slice is computed using statically
  available information.
• Dynamic Slice assumes a specific execution of the
  program.

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• Slicing criterion (program point, set of
  variables).
• A static slice S(Si,v) of variable v at statement
  number Si is defined as the transitive closure of
  data and control dependences of v at Si
• set of statements on which the value of v at Si
  is directly or indirectly data or control
  dependent.

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• Data dependence refers to value dependence.
• For example, in the following program value of
  X in statement 3 is data dependent on statement
  1.
• 1. Y 2 W
• 2. U 2 W
• 3. X 2 Y

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• Control dependence statement S is directly
  control dependent upon predicate P if the output
  of P (true or false) determines whether or not S
  will be executed.
• Example
• S0
• while P1 do
• S1
• if P2 then S2 else S3 endif
• S4
• endwhile
• S0 and P1 are not directly control dependent upon
  any other predicate.
• S1, P2, and S4 are directly control dependent
  upon P1
• S2 and S3 are directly control dependent upon P2
• Note we have considered only direct(immediate)
  control dependence above.

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• The indirect dependences are taken into account
  when transitive closure is taken while computing
  the slice.
• Note that so far we are referring to only
  backward program slice, i.e. the statements on
  which value of v at Si is directly or indirectly,
  control or data dependent is computed backwards
  starting from statement Si.

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Example

• Read(n)
• I 1
• Sum 0
• Product 1
• While Iltn do
• sum sum I
• product product I
• I I 1
• Endwhile
• Write(sum)
• Write(product)

Read(n) I 1 Product 1 While Iltn do
product product I I I
1 Endwhile Write(product)
Example Program
Backward Static Slice for variable product at
last statement
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• A dynamic slice D(Si,v,I) of variable v at
  statement instance Si along the execution trace
  for a program input I is defined as the
  transitive closure of data and control
  dependences of v at Si over the set of statements
  that are executed with input I.

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Example

• Read(n)
• I 1
• Sum 0
• Product 1
• While Iltn do
• sum sum I
• product product I
• I I 1
• Endwhile
• Write(sum)
• Write(product)

Read(n) I 1 Product 1 While Iltn do
product product I I I
1 Endwhile Write(product)
Product 1 Write(product)
Example Program
Static Slice for variable product at last
statement
Dynamic Slice for variable product for n0
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Dynamic slice of res1 at statement 18 for input
val -12.1
Dynamic slice of res1 at statement 18 for input
val 9.5
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Dynamic slice of res2 at statement 18 for input
val 9.5
Dynamic slice of res2 at statement 18 for input
val -12.1
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• A Static slice contains union of the statements
  in all the dynamic slices of the given variable
  at the point of interest in the program. However,
  it may also contain some statements that may
  never belong to any dynamic slice for the given
  slicing criterion.
• Therefore static slices are conservative in
  estimating the statements that may affect the
  value of a variable at a point in a program.

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• In a forward slice we are interested in
  considering only those statements in which the
  value of a variable v at a statement Si can be
  used directly or indirectly (we are considering
  only reachable uses of value of v at Si).
• Backward slice is useful in debugging
• Forward slice is useful in determining which all
  statements in a program can be effected by
  change in value of v at statement Si.