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SwE 455 Program Slicing

SwE 455 Program Slicing. Our Goals. Debug your thousands lines of code easily by reducing the complexity of the program Write a robust program before testing your code Save your regression testing time by limiting the tests to only those that exercise the changed code. Approach.

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SwE 455 Program Slicing

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  1. SwE 455Program Slicing

  2. Our Goals • Debug your thousands lines of code easily by reducing the complexity of the program • Write a robust program before testing your code • Save your regression testing time by limiting the tests to only those that exercise the changed code

  3. Approach “Break your code into smaller pieces”

  4. Program Slicing • What is it? - A well-known program analysis and transformation technique that uses program statement dependence information to identify parts of a program that influence or are influenced by an initial set of program points of interest which is called the slice criteria - Introduced by Mark Weiser in his Ph.D. thesis (1979)

  5. indirectly relevant Slice Resulting slice Program Slicing (cont’d) • slicing criterion is provided by the user Slicing Criterion Source program

  6. Variants of Program Slicing • Many different variants of program slicing exist • Static Slicing • Backward Slicing • Forward Slicing • Dynamic Slicing • Conditional Slicing • Chopping • Also Many Different tools, however • Most program slicing tools are written for C but there are also some for C++ and Java • Most of these have problems with dynamic binding, inheritance, polymorphism and performance

  7. Example of backward slicing Static • Slice criterion <12,i> • 1 main( ) • 2 { • 3 int i, sum; • 4 sum = 0; • 5 i = 1; • 6 while(i <= 10) • 7 { • 8 Sum = sum + 1; • 9 ++ i; • 10 } • 11 Cout<< sum; • 12 Cout<< i; • 13 }

  8. Backward Slice Example public class SimpleExample { static int add(int a, int b){ return(a+b); } public static void main(final String[] arg){ inti = 1; int sum = 0; while (i < 11) { sum = add(sum, i); i = add(i, 1); } System.out.println("sum = " + sum); System.out.println("i = " + i); } } Slicing Criterion

  9. Example of forward static slicing • Slice criterion <3,sum> • 1 main( ) • 2 { • 3 int i, sum; • 4 sum = 0; • 5 i = 1; • 6 while(i <= 10) • 7 { • 8 sum = sum + 1; • 9 ++ i; • 10 } • 11 Cout<< sum; • 12 Cout<< i; • 13}

  10. Forward Slice Example public class SimpleExample { static int add(int a, int b){ return(a+b); } public static void main(final String[] arg){ inti = 1; int sum = 0; while (i < 11) { sum = add(sum, i); i = add(i, 1); } System.out.println("sum = " + sum); System.out.println("i = " + i); } } Slicing Criterion

  11. What is program slicing? • 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

  12. Example of program slicing • Original program: • 1 begin • 2 read(x,y) • 3 total := 0.0 • 4 sum := 0.0 • 5 if x <= 1 • then sum := y • else begin • read(z) • total := x*y • end • 11 write(total, sum) • 12 end. Slice criterion: <9, x> begin read(x,y) end. Slice criterion: <12, z> begin read(x,y) if x <= 1 then else read(z) end. Slice criterion: <12, total> begin read(x,y) total := 0 if x <= 1 then else total := x*y end.

  13. Variants of program slicingStatic slices • Slice criterion <p, V> • Where p is a program point and V is a subset of program variables • Program slice on the slicing criterion <p, V> is a subset of program statements that preserves the behavior of the original program at the program point p with respect to the program variables in V

  14. Variants of program slicingStatic slices • Slices derived from the source code for all possible input values • No assumptions about input values • May lead to relatively big slices • Contains all statements that may affect a variable for every possible execution • Current static methods can only compute approximations

  15. Static slices example • Slice criterion (12,i) • 1 main( ) • 2 { • 3 int i, sum; • 4 sum = 0; • 5 i = 1; • 6 while(i <= 10) • 7 { • 8 sum = sum + 1; • 9 ++ i; • 10 } • 11 Cout<< sum; • 12 Cout<< i; • 13 }

  16. Example of dynamic slices • read (n) • for I := 1 to n do • a := 2 • if c1==1 then • if c2==1 then • a := 4 • else • a := 6 • z := a • write (z) • Assumptions • Input n is 1 • C1, c2 both true • Execution history is 11, 21, 31, 41, 51, 61, 91, 22, 101 • Slice criterion<1, 101, z>

  17. Applications of program slices • Program debugging • Was introduced by Mark Weiser as debugging aid • Slicing visualizes control and data dependencies • It highlights statements influencing the slice • Testing: reduce cost of regression testing after modifications (only run those tests that needed) • Integration : merging two programs A and B that both resulted from modifications to BASE

  18. Applications of program slices • 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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