CASE/Re-factoring and program slicing

CASE/RE-FACTORINGAND PROGRAM SLICING

© University of LiverpoolCOMP 319 slide 1

CASE tool construction

• File level - programming environment

• Language level - program representation, compiling, testing

• Work flow level - stages in the software engineering process itself: specification, design, development, verification, validation, management.

© University of LiverpoolCOMP319 slide 2

COMP319 Software Engineering II

Slide 3

Program Language Level

• Grammar – to capture the notion of text based instructions

• Graph – to deal with the concept of sequence

Tool construction at the language level exploits form – which is usually either:

Refactoring

• Why CASE?- not alter functionality (must be correct)- cover all instances (i.e. be complete)- Keep code tidy and to standard format- Be quick


Re-factor types

• Encapsulate field

public int getLength() { return(length);}

• Re-name method, field• String pw• - • String password


Generalisation of type

class Customer {}

class Person {}class Customer extends Person {}


Code breaking up re-factors• Extract methodvoid setLength(int length) { if (length<0) { throw (new BadArgumentException()); } this.length=length;}void validateLength(int length) { if (length<0) { throw (new BadArgumentException()); }}void setLength(int length) { validateLength(length); this.length=length;}


Graphs• A diagram depicting a network• Points at the end of arcs• Nodes at the junction of arcs• Regions enclosed by arcs• Used for representing:- Solid figures (vertices, edges, faces)- Electrical circuits- Relationships between entities

• Graph or network theory


Dependence graphs• All computing systems have

dependencies• Control dependence- 1 method calling another

• Data dependence- 1 expression effecting another- A=B*2- C=A*4

• Control/data dependence- If (age<=18) { println(“Age invalid”);


Program dependency graphs

• Term appears in a paper by Kuck, Muraoka & Chen (1981) although the idea is in Turing’s early description of “algorithms” in 1936

• Captures sequence/time between entities (compare connection/distance)

• Control Dependence• Data Dependence


Example Program and its Dependence Graph


Example Program and its Dependence Graph


Dependency graph usage• Optimisation-Multiple independent statements can run in parallel- Code that never runs can be removed- Boolean skip=true;- If (!skip) then

- Loop invariance- For (k=1;k<max_items;k++) {- sum=sum+a*b;- }


Program slicing• Interactive method for-Debugging- Program understanding- Program maintenance

• Program reduction technique (highlighter)

• A demonstration of SDG allowing:- Control flow analysis-Data flow analysis


How does it work

• Choose v a variable or set of variables

• Choose n a point of interest • Using the dependence graph the

slice v at n is constructed • The slice v at n can be compiled

and studied separately• Slices may be forward or backward

from n


Backward Slicing

x = 1;y = 2; z = y-2; r = x; z = x+y; /* the slice point is the end of the program */.

x = 1;y = 2;z = x+y;

Original program

Backward Slice

Debugging with a slice

Pass = 0 ; Fail = 0 ; Count = 0 ;while (!eof()){ TotalMarks=0; scanf("%d",Marks); if (Marks >= 40) Pass = Pass + 1; if (Marks < 40) Fail = Fail + 1; Count = Count + 1; TotalMarks = TotalMarks+Marks ;}printf("Out of %d, %d passed and %d failed\n", Count, Pass, Fail) ;average = TotalMarks/Count; /* point of interest */printf("The average was %d\n",average) ;PassRate = Pass/Count*100 ;printf("This is a pass rate of %d\n",PassRate) ;


Bug location with backward slicingwhile (!eof()) { TotalMarks = 0; scanf("%d",Marks); Count = Count + 1; TotalMarks = TotalMarks+Marks;}average = TotalMarks/Count;printf("The average was %d\n",average) ;



Forward Slicing

x = 1; /* considering changing this line */y = 3;p = x + y ;z = y -2 ;if(p==0)r++ ;

Original program

Forward Slice

/* Change to first line will affect */p = x + y ;if(p==0)r++ ;

Maintenance - Example


n = 0; product = 1; sum = 1;scanf("%d",&x) ;while (x >= 0){ sum = sum + x; product = product * x ; n = n + 1; scanf("%d",&x);}average = (sum - 1) / n ;printf("The total is %d\n",sum) ;printf("The product is %d\n",product) ;printf("The average is %d\n",average) ;

Maintenance – backward slice


sum = 1;scanf("%d",&x) ;while (x >= 0) { sum = sum + x; scanf("%d",&x);}printf("The total is %d\n",sum) ;

Maintenance – forward slicen = 0; product = 1; sum = 0;scanf("%d",&x) ;while (x >= 0) { sum = sum + x; /* AFFECTED */ product = product * x ; n = n + 1; scanf("%d",&x);}Average = (sum - 1) / n ; /* AFFECTED */printf("The total is %d\n",sum) ; /* AFFECTED */printf("The product is %d\n",product) ;printf("The average is %d\n",average) ; /* AFFECTED */


Types of slicing

• Static – described above. Slices are constructed at compile time

• Dynamic slicing where slices are constructed once the input is known

• Conditional slicing done at breakpoints during execution

• Inter-modular slicing - complex systems


The Horowitz, Prins, & Rep (HPR) algorithm (merging)

• Step 1. Determine changed and preserved slices e.g. adding a diameter calculation

• Step 2. Form the merged graph. Using the idea of ‘graph union’

• Step 3. Test for interference i.e the merged graph preserves all the slices of all the variants.

• Step 4. Construct source from the merged graph


• Why richer constructs?-More useful slicing

• SDG and inter-module slicing- SDG from parse trees-Other methods of generating an SDG- Calls and variable scope handling

• Pointers, aliases, classes


JSlice

• Java slicing software developed by National University of Singapore

• Performs dynamic slicing• Uses a compressed trace which records- Flow control instructions- Data manipulation

• JVM- Kaffe- Clean room implementation of Java


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CASE/Re-factoring and program slicing