program slicing for refactoring
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Program Slicing for Refactoring
Advanced SW Tools Seminar
Jan 2005 Yossi Peery
Jan-05Yossi Peery Advanced SW Tools Seminar
Agenda Slicing Overview Slicing Algorithms Slicing with Inference Rules Refactoring Overview Slice Extraction Refactoring Example NATE – Slicing Based Refactoring Tool
Jan-05Yossi Peery Advanced SW Tools Seminar
Starter
Jan-05Yossi Peery Advanced SW Tools Seminar
Program Slicing History Mark Weiser, 1981
• Experimented with programmers to show that slices are:
“The mental abstraction people make when they are debugging a program” [Weiser]
• Used Data Flow Equations
Ottenstein & Ottenstein – PDG, 1984 Horowitz, Reps & Binkly – SDG, 1990
Jan-05Yossi Peery Advanced SW Tools Seminar
What is a Slice? All the statements of a program that may
affect the values of some variables in a set V at some point of interest p.
Slicing Criterion:C = (p , V)
Jan-05Yossi Peery Advanced SW Tools Seminar
Slice Example
A slice for the criterion (10 , {product})
Jan-05Yossi Peery Advanced SW Tools Seminar
What is it good for? Debugging Program Comprehension Reverse Engineering Program Testing Measuring Program Metrics
Coverage, Overlap, Clustering Refactoring
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Static Slicing
• Statically available information only• No assumptions made on input• Computed slice can never be accurate
(minimal slice)• Problem is undecidable – reduction to the
halting problem• Current static methods can only compute
approximations• Result may not be usefull
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Dynamic Slicing
• Computed on a given input• actual instead of might• Useful for applications that provide are input driven
(debugging, testing)Criterion: (n=-3, 5, {sign})
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Amorphous & Semantic Slicing
• Allows any semantic preserving transformations
• Used for program comprehension and reverse engineering
Instead of: We write if (n >= 0) if (n < 0)
; sign := -1else
sign := -1
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Backward Slicing
• Original Slicing Method• Backward Traversal of Program Flow
• Slicing starts from point p (C = (p , V))• Examines statements that are executed
before p (in run-time)• Keep statements that affect value of V at p,
or execution of p.• Not only statements that appear
before p
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Forward Slicing
• Forward Traversal of Program Flow• Slicing starts from p (C = (p , V))• Examine all statements that are executed after p• Keep statements that are affected by the values of
V at p or by the execution of p
• Shows downstream code that depend on a specific variable or statement
• Can show the code affected by a modification to a single statement
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Properties Intraprocedural Slicing
• Computes slice within one procedure• Assumes worse case for function calls
Interprocedural Slicing• Compute slice over an entire program• Two ways for crossing procedure boundary
• Up – going from sliced procedure into calling procedure
• Down – going from sliced procedure into called procedure
• Must Be Context Sensitive
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithm CFG – Control Flow Graph
• Each program statement is a node• A directed edge will connect between any 2 nodes
that represent statements with a possible control flow between them.
• Special nodes: Start, Stop• Definitions
- There is a directed path from I to j - Set of nodes that are influenced by i
- all of the variables that are defined (modified) at statement i. - all of the variables that are referenced (used)
at statement i.
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms Data Flow Equations (Weiser)
• Iterative Process (Over CFG)• Compute consecutive sets of “relevant” variables
for each node in the CFG using data dependencies• Control dependences are not computed explicitly• Variables of control predicates (if, while) are
“indirectly relevant” if any one of the statements in their body is relevant
• Start with slicing criterion: C = (p, V)• Continue until a fixed point is reached – last
iteration didn’t find new relevant statements
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing AlgorithmsIteration 0:
Iteration k+1:
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithm
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms Issues with algorithm
• Output statements are not included in sliceSolution: print(x) ≡ out = out + x , out V
• Interprocedural Slicing• Solution proposed by Weiser• Can go up or down procedure calls• Actual parameters of function call are changed
to call parameters (or the opposite)• Variables not in scope are removed• Is not “context sensitive” – too inaccurate
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms PDG – Program Dependance Graphs
• Each node represents a statement (like CFG)• Directed Edges represent:
• Control Dependence (Bold Lines) – between a predicate and the statements it controls
• Data Dependence (Regular Lines) – between statements modifying a variable and those that may reference it
• Special “Entry” node is connected to all nodes that are not control dependant
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithm Slicing with PDG
• Slicing criterion is less general:C = ( p, Def(p) ∩ Ref(p) )
• Graph is computed for a single procedure• Slicing becomes a reachability problem
A slice consits of all the nodes that have a directed path to the node in the slicing criterion are in the
• Linear in time, after graph is calculated Issues
• Method isn’t interprocedural
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms SDG – System Dependence Graph
• New nodes:Call Site, Procedure Entry, Actual-in-argument, Actual-out-argument, Formal-in-parameter, Formal-out-parameter
• New edges:• Call Edge – connect “call site” and “procedure
entry”• Parameter-In Edge – connect “Actual-in” with
“Formal-in”• Parameter-Out-Edge – connect “Actual-out” with
“Formal-out”
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithm
Jan-05Yossi Peery Advanced SW Tools Seminar
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithms Context Sensitivity
• Can not be solved by data flow equations• The <add> procedure, included through the
<multiply> procedure will include the call site <add(sum,i)> and consequently <sum:=0>
• Solved by SDG• New summary edges (dotted)
represent transitive dependences between “actual-in” and “actual-out” nodes.
• Slice is calculated in 2 passes (instead of 1):
1.Follow all edges except “parameter-out”
2.Follow all edges except “parameter-in”
Jan-05Yossi Peery Advanced SW Tools Seminar
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing Algorithm SDG - Issues
• Slicing remains a reachability problem• SDG of a program is complex and costly to
build (time, space)• After computation, many different slices can
be found using the same graph• Is not efficient for developing code• OO concepts and unstructured control flow
(jump statements, exceptions) further complicate the graph
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing with Inference Rules Concept
• Use inference rules when traversing backwards the flow of the program to determine relevant statements
• Rules are applied on a specific configuration of <S,Γ,R>
S – Statement or sequence of statements that have been analyized
Γ – Current slicing contextR – Set of statements that are relevant (so far)• Similar in nature to data flow equations method
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing with Inference Rules Context:
Inference Rule:
Initial Configuration:
• Our Example
Final Configuration:
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing with Inference Rules Rule Example
Inference rules are defined so that, at each step, there is at most one rule that matches the configuration
Jan-05Yossi Peery Advanced SW Tools Seminar
Slicing with Inference Rules Features
• Supports interprocedural slicing• Context Sensitive• Can be extended to support other language
features such as:• Complex expressions• Array access• Variable declarations• Structured Jumps (break, continue)• Object-oriented slicing (scoping, polymorphism)• Aliasing
Jan-05Yossi Peery Advanced SW Tools Seminar
Refactoring Overview Gradually improving design of existing code
• Source code transformations that,• Preserve behavior of original system• Manually or Automated
Introduced by William Opdyke, 1992• Formally defined the reasonable behavior preservation
degree expected from a refactoring tool Formally Disciplined by Martin Fowler 2000
• Formal description of a refactoring• Catalog of refactoring techniques
Jan-05Yossi Peery Advanced SW Tools Seminar
Refactoring Overview Over 70 refactoring techniques can be
found at:www.refactoring.com/catalog/index.html
Refactoring categories:Composing Methods, Moving features between Objects, Organizing Data, Making Method Calls Simpler
Some refactorings:Rename Method, Extract Method, Move Method, Replace Conditional with Polymorphism
Jan-05Yossi Peery Advanced SW Tools Seminar
Refactoring Overview
Jan-05Yossi Peery Advanced SW Tools Seminar
Slice Extraction Refactoring
Jan-05Yossi Peery Advanced SW Tools Seminar
Slice Extraction Refactoring
Jan-05Yossi Peery Advanced SW Tools Seminar
Slice Extraction Refactoring Idea introduced by K. Maruyama, 2001 Is not limited to consecutive statements
(like extract method) Allows the untangling of a single concern
from a complex method Extracted slice can be refactored to
• New Method• New Object• New Aspect
Jan-05Yossi Peery Advanced SW Tools Seminar
Slice Extraction Refactoring Slice Extraction Refactoring Concerns
• Not all of the statements in the slice can be deleted• Deleted statements are determined by re-slicing for
variables in statements that were not sliced:
Preconditions & Limitations• Clean compilation• Return statement• Global-scoped variables• Input/Output statements
Jan-05Yossi Peery Advanced SW Tools Seminar
Example – original code
Jan-05Yossi Peery Advanced SW Tools Seminar
Example – Extract as Method
Jan-05Yossi Peery Advanced SW Tools Seminar
Example – Extract as Object
Jan-05Yossi Peery Advanced SW Tools Seminar
Example – Extract as Aspect
Jan-05Yossi Peery Advanced SW Tools Seminar
Example – Extract as Aspect
Jan-05Yossi Peery Advanced SW Tools Seminar
NATE – Slicing based Refactoring Tool Oxford University – Programming Tools
Grouphttp://web.comlab.ox.ac.uk/oucl/research/areas/progtools/projects/nate/nate.html
Slicing based refactoring techniques for the Java programming language• Currently supports a small subset of Java
(March 2004)• Extract Slice as Method Refactoring
An Eclipse Plug-in
Jan-05Yossi Peery Advanced SW Tools Seminar
NATE – Slicing based Refactoring Tool How is it used?
• Programmer selects:• slicing criterion• Name for new extracted method
• Tool performs:• Compute Slice• Check refactoring preconditions• If extraction is possible – perform transformation
• Show original and transformed code in a preview dialog to the user for confirmation
• User can cancel any operation
Jan-05Yossi Peery Advanced SW Tools Seminar
Implementation• Uses JDT plug-in for access to the AST• Slicing is done with inference rules
• The AST node of a statement is associated with a related inference rule
• Intensive use of visitor pattern to visit the AST and its related rules
• Each AST node can be marked as relevant or not
NATE – Slicing based Refactoring Tool
Jan-05Yossi Peery Advanced SW Tools Seminar
Jan-05Yossi Peery Advanced SW Tools Seminar
Refernces “Untangling: A Slice Extraction Refactoring -
Ran Ettinger and Mathieu Verbaere (March 2004)
“Program Slicing for Refactoring” -Mathieu Verbaere (September 2003)
“Automated Tools for Refactoring” -Ran Ettinger (June 2003)
“Program Slicing” -Mark Weiser (1981)
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