1. SwE 455 Program 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. “Break your code into smaller pieces”
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)
What it essentially means is we first pick a statement to be our slice criteria.
Next, we generally are interested in two kinds of situations:
All other statements in the program that are affecting the criteria. (Help us to find a bug, criteria is the statement that produce incorrect output)
All other statements that are affected by the criteria. (If now the criteria is the fixed statements that used to be buggy)

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

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) {
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){
int i = 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);
A backward slice consists of all statements that affect a given point in the program.
All other statements that influence the output of variable 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; 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){
int i = 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
A forward slice consists of all statements that are affected by a given point in the program.
In this example, the forward slice indicates exactly which statements are influenced by the initialization of variable sum

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 slicing Static 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 slicing Static 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) {
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