Understanding Program Slicing: A Technique for Filtering Relevant Code in Software Systems, Study notes of Computer Science

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Program Slicing

Material drawn from [Weiser84,Gallagher91,DeMillo96]

What is a Program Slice?

• A program slice is a subset of a program.
• Program slicing enables programmers to view subsets of a program by filtering out code that is not relevant to the computation of interest.
• E.g., if a program computes many things, including the average of a set of numbers, slicing can be used to isolate the code that computes the average.

Definition of Program Slice

• Assume that:
  • P is a program.
  • V is the set of variables at a program location (line number) n.
• A slice S(V,n) produces the portions of the program that contribute to the value of V just before the statement at location n is executed.
• S(V,n) is called the slicing criteria.

A Program Slice Must Satisfy the Following Conditions:

• Slice S(V,n) must be derived from P by deleting statements from P.
• Slice S(V,n) must be syntactically correct.
• For all executions of P , the value of V in the execution of S(V,n) just before the location n must be the same value of V in the execution of the program P just before location n.

Slice S(num,26) main() {

2. int tmp, num;
3. tmp = readInt():
4. num = 1;
5. while(tmp >= 0)
6. {
7. ++num;
8. tmp = readInt();
9. }
10. printf(“\nNum=%d”, num); }

Slice S(sum, 25) main() {

2. int tmp, sum;
3. tmp = readInt():
4. sum = tmp;
5. while(tmp >= 0)
6. {
7. sum += tmp;
8. tmp = readInt();
9. }
10. printf(“\nSum=%d”, sum); }

Slice S(mn, 23) main() {

1. int mn;
2. int tmp;
3. tmp = readInt():
4. mn = tmp;
5. while(tmp >= 0)
6. {
7. if (mn > tmp)
8. mn = tmp;
9. tmp = readInt();
10. }
11. printf(“\nMin=%d”, mn); }

Slice S(mx, 22) main() {

1. int mx;
2. int tmp;
3. tmp = readInt():
4. mx = tmp;
5. while(tmp >= 0)
6. {
7. if (mx < tmp)
8. mx = tmp;
9. tmp = readInt();
10. }
11. printf(“\nMax=%d”, mx); }

Program Slicing Process

• Select the slicing criteria (i.e., a variable or a set of variables and a program location).
• Generate the program slice(s).
• Perform testing and debugging on the slice(s). During this step a sliced program may be modified.
• Merge the modified slice with the rest of the modified slices back into the original program.

Tools for Program Slicing

• Spyder
  • A debugging tool based on program slicing.
• Unravel
  • A program slicer for ANSI C.