Program understanding: What programmers really want

Program Understanding: What Programmers Really Want Einar W. Høst May 5th 2011

Description Introduction How Analysis can help Understanding Understanding Program Understanding Agenda Agenda Evaluation Examples ConsiderLimitationsofCurrentTools Look at some well-known Analyses

Program Analysis Program Understanding Program Analysis

Why is program understanding important?

“To program is to understand” - Kristen Nygaard

Why is program understanding hard?

The concept assignment problem.

“Few are the programmers who can explain their code well enough so that reading it is not incredibly frustrating.” - JefRaskin

How can program analysis help?

Program Understanding ? $s.=($f=$A RGV[$_%2] )x(substr$s ,$_,1or$f) for0..500;

Program Understanding ! $s.=($f=$A RGV[$_%2] )x(substr$s ,$_,1or$f) for0..500; Through themagic lens of a Tool

”Helpme in finding whatneeds to be changed...

Just-in-time program understanding.

Complete understanding is ,[object Object]

Understand the program well enough to fix a bug

Understand the program well enough to improveperformance

Without change, there is no need for understanding!

Program understanding ,[object Object]

Reasoning about designat the code level

Program understanding ,[object Object],at the task level

Comprehension strategies Reconstructknowledgeaboutthe program domain and mapping it to thesourcecode. Top-down

Comprehension strategies Readcodestatements and mentallygroupthesestatementsintohigherlevelabstractions. Bottom-up

Comprehension strategies Readcode in detail, followcontrolflow, gain global understanding. Systematic

Comprehension strategies Scancode, looking for cluesindicatingrelevance to thetask at hand. Opportunistic

Comprehension strategies conjecture question search read Inquiries

Comprehension strategies Understanding is formed by switchingbetweendifferentstrategies as needed. Integrated

Tools for program understanding

Analysis tools ,[object Object]

Software metrics,[object Object]

Context-awareness,[object Object]

print”This is not a program.”

print(”This is not a program.”)

The program ecosystem ,[object Object]

Execution environment (runtime)

Integrated development environment

External sources,[object Object]

Behavior-centric ,[object Object]

What happens if I change this?

Which parts of the program are affected by change?Requires program code

Evolution-centric ,[object Object]

Who can help me understand this?

Why was this feature implemented this way?

What are the most bug-prone parts of the program?

Where do changes happen most often?Requires program history

Interaction-centric ,[object Object]

Whichtasksare hard to accomplish?

Which parts of the program are hard to change? Requires coding-session history

Traditional program analysis All you need is code All you need is code All you need is code, code Code is all you need

Static All possible executions

Static Trades precision for soundness

Trades completeness for efficiency Dynamic

Program analysis for program understanding

Call graph construction Show calling relationshipsbetween parts of a program. Essential idea

Call graph construction f h main g Sample graph

Call graph construction The relationdescribingexactlythosecallsmade from oneentity to another in anypossibleexecutionofthe program. The ideal call graph

Call graph construction Make the program seem less fragmented by showingthecontrolflow links between program parts. Benefit

Call graph construction The graphbecomes too large and complex to be comprehensible. Limitation

Call graph construction ”The sightofgcc'scallgraph frightened my students so muchthattheyrequested a differentproject” - ArunLakhotia Limitation

Program slicing Find a reduced program exhibitingthe same behaviorofinterest as the full program. Essential idea

Program slicing The reduced program is called a program slice.

Program slicing C = < x, V > x : a statement in program P V : a subsetof variables in P Slicing criterion

Criterion: <12, z> 1begin 2read(x,y) 3total := 0.0 4sum := 0.0 5if x <= 1 6 then sum := y 7 else begin 8read(z) 9 total := x*y 10 end 11write(total, sum) 12 end. 1 begin 2read(x,y) 5 if x <= 1 6 then 7 else 8read(z) 12 end.

Criterion: <12, x> 1begin 2read(x,y) 3total := 0.0 4sum := 0.0 5if x <= 1 6 then sum := y 7 else begin 8read(z) 9 total := x*y 10 end 11write(total, sum) 12 end. 1 begin 2read(x,y) 12 end.

Program slicing Whatwouldthe program look like ifweassume an initial statesatisfyingC ? Forward conditioning

Program slicing Deletesstatementsthatwill not be executed given the initial state. Forward conditioning

Program slicing Whatwouldthe program look like ifweassume an eventual statesatisfyingC ? Backward conditioning

Program slicing Deletesstatementswhichcannot lead to the eventual state. Backward conditioning

Program slicing The reduced program is smaller. Benefit

Program slicing The reduced program looks foreign. Limitation

Program slicing Hard to integrateintotheprogrammer’sworkflow. Limitation

Concept analysis Identifygroupingsofobjectsthat have commonattributes. Essential idea

Concept analysis C = (O, A, R) O Set of objects A Set of attributes R Relation R⊆O ×A Formal context

Concept analysis σ(O) = {a∈A⎮∀o∈O : (o, a)∈R} O Set of objects A Set of attributes R Relation R⊆O ×A Common attributes

Concept analysis τ(A) = {o∈O⎮∀a∈A : (o, a)∈R} O Set of objects A Set of attributes R Relation R⊆O ×A Common objects

Concept analysis A pair (O, A) is a concept if A = σ(O) and O = τ(A) Definition of concept

Concept analysis O=> Subprograms A=> Features Feature location

Concept analysis (s, f) ⊆ R if subprogram s is invoked when feature f is invoked Feature location

Concept analysis Identify parts ofthe program relevant for a feature. Benefit

Concept analysis Recoveryofcomponents and generationofhigh-levelarchitectureviews. Benefit

Concept analysis Imperfecthigh-leveldescriptions have limited application for concretetasks. Limitation

Concept analysis Requireseffort from the programmer, withunclearbenefits. Limitation

Concept analysis Hard to integrateintotheprogrammer’sworkflow. Limitation

Change impact analysis Identifythepotentialconsequencesof a program change. Essential idea

Change impact analysis Estimatewhat must be modified to accomplish a changeofbehavior. Essential idea

Change impact analysis Subtyping and dynamicdispatchmeansthatchangeimpactcan be non-local and unexpected. Challenges in OO languages

Change impact analysis Atomic changes

Change impact analysis Provide a boundaryaroundtheeffectsof a program edit. Benefit

Change impact analysis Provideconfidencethat an editdoes not have unexpectedeffectsoutsidetheboundary. Benefit

Change impact analysis Limited by theprecisionofthechangeimpactanalysis. Limitation

Change impact analysis Still needassurancethatnounexpectedeffectsoccur in theimpacted part ofthe program. Limitation

Change impact analysis Hard to integrateintotheprogrammer’sworkflow. Limitation

Program metrics Quantifyaspectsofthe program presumed to be relevant. Essential idea

Program metrics ,[object Object]

Program metrics Answer questions regarding program quality. Benefit

Program metrics Identify potential problem areas in the program. Benefit

Program metrics Metrics are indirect indicators of quality. Limitation

Program metrics Task-generating, not task-solving. Limitation

Software visualisation Avoidoverwhelmingthe programmer by compressinginformation and usinggraphics. Goal

Program understanding: What programmers really want

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