Program slicing

Based on the original definition of Weiser, informally, a static program slice S consists of all statements in program P that may affect the value of variable v in a statement x. The slice is defined for a slicing criterion C=(x,v) where x is a statement in program P and v is variable in x. A static slice includes all the statements that can affect the value of variable v at statement x for any possible input. Static slices are computed by backtracking dependencies between statements. More specifically, to compute the static slice for (x,v), we first find all statements that can directly affect the value of v before statement x is encountered. Recursively, for each statement y which can affect the value of v in statement x, we compute the slices for all variables z in y that affect the value of v. The union of all those slices is the static slice for (x,v).

Example

For example, consider the C program below. Let's compute the slice for ( write(sum), sum ). The value of sum is directly affected by the statements "sum = sum + i + w" if N>1 and "int sum = 0" if N <= 1. So, slice( write(sum), sum) is the union of three slices and the "int sum = 0" statement which has no dependencies:

1. slice( sum = sum + i + w, sum),
2. slice( sum = sum + i + w, i),
3. slice( sum = sum + i + w, w), and
4. { int sum=0 }.

It is fairly easy to see that slice( sum = sum + i + w, sum) consists of "sum = sum + i + w" and "int sum = 0" because those are the only two prior statements that can affect the value of sum at "sum = sum + i + w". Similarly, slice( sum = sum + i + w, i) only contains "for(i = 1; i < N; ++i) {" and slice( sum = sum + i + w, w) only contains the statement "int w = 7".

When we union all of those statements, we do not have executable code, so to make the slice an executable slice we merely add the end brace for the for loop and the declaration of i. The resulting static executable slice is shown below the original code below.

The static executable slice for criteria (write(sum), sum) is the new program shown below.

In fact, most static slicing techniques, including Weiser's own technique, will also remove the write(sum) statement. Since, at the statement write(sum), the value of sum is not dependent on the statement itself. Often a slice for a particular statement x will include more than one variable. If V is a set of variables in a statement x, then the slice for (x, V) is the union of all slices with criteria (x, v) where v is a variable in the set V.

Dynamic slicing makes use of information about a particular execution of a program. A dynamic slice contains all statements that actually affect the value of a variable at a program point for a particular execution of the program rather than all statements that may have affected the value of a variable at a program point for any arbitrary execution of the program.

An example to clarify the difference between static and dynamic slicing: Consider a small piece of a program unit, in which there is an iteration block containing an if-else block. There are a few statements in both the if and else blocks that have an effect on a variable. In the case of static slicing, since the whole program unit is looked at irrespective of a particular execution of the program, the affected statements in both blocks would be included in the slice. But, in the case of dynamic slicing we consider a particular execution of the program, wherein the if block gets executed and the affected statements in the else block do not get executed. So, that is why in this particular execution case, the dynamic slice would contain only the statements in the if block.