program understanding: what programmers really want

Program Understanding:What Programmers Really Want

Einar W. HøstMay 5th 2011

Evaluation Examples

DescriptionIntroduction

Consider Limitations of Current Tools

Look at somewell-known

Analyses

How Analysis can help

Understanding

Understanding Program

Understanding

AgendaAgenda

Program Analysis

Program Analysis

Program Understanding

Why is program understanding important?

It’s what programmers do!

“To program is to understand” - Kristen Nygaard

Why is program understanding hard?

Size.

Complexity.

Heterogenity.

Side-effects.

The concept assignment problem.

Intangibility.

=>

Disorientation.

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

How can program analysis help?

$s.=($f=$ARGV[$_%2])x(substr$s,$_,1or$f)for0..500;

Program Understanding

?

Program Understanding

!

Through the magic lens of a Tool

$s.=($f=$ARGV[$_%2])x(substr$s,$_,1or$f)for0..500;

What do programmers want?

”Help me in finding what needs to be changed...

...make the change...

...and get the !$&?/% out!”

Just-in-time program understanding.

Complete understanding is

- Not realistic- Not cost-effective- Not necessary

Understand the program well enough to

implement a new feature

Understand the program well enough to

fix a bug

Understand the program well enough to

improve performance

Without change, there is no need for understanding!

Program understanding

- Understanding execution behavior- Understanding control flow- Understanding data flow- Understanding dependencies- Understanding side-effects- Understanding change impact- Reasoning about design

at the code level

Program understanding

- Understanding what is relevant

at the task level

Comprehension strategies

Comprehension strategies

Top-down

Reconstruct knowledge about the program domain and mapping it to the source code.

Comprehension strategies

Bottom-up

Read code statements and mentally group these statements into higher level abstractions.

Comprehension strategies

Systematic

Read code in detail,follow control flow,gain global understanding.

Comprehension strategies

Opportunistic

Scan code, looking for clues indicating relevance to the task at hand.

Comprehension strategies

Inquiries

conjecture

read

searchquestion

Comprehension strategies

Integrated

Understanding is formed by switching between different strategies as needed.

Tools for program understanding

Presentation

Analysis

Analysis tools

- Call graphs- Program slicing- Feature location- Effects of change- Software metrics

Presentation tools

- Visualisations- Context-awareness

What is a program?

print ”This is not a program.”

print(”This is not a program.”)

The program as

text

The program as

binary

The program as

versioned

The program as

process

The program ecosystem

- Source code / Compiled binary- Execution environment (runtime)- Test suite - Version control system- Issue tracking system- Integrated development environment- The programmer- External sources

What questions can we ask about a program?

Behavior-centric

- What does this piece of code do? - What happens if I change this?- Where is this code used?- Which parts of the program are affected by change?

Requires program code

Evolution-centric

- Who wrote this piece of code?- 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

- How was this piece of code written?- Which tasks are hard to accomplish?- Which parts of the program are hard to change?

Requires coding-session history

What is program analysis?

Traditional program analysis

All you need is codeAll you need is code

All you need is code, codeCode is all you need

Static

Dynamic

Static

All possible executions

Static

Sound+

Conservative

Static

Trades precision

for soundness

Sample executions

Dynamic

Efficient+

Precise

Dynamic

Trades completeness

forefficiency

Dynamic

Static

Dynamic

Synergy

Program analysis for program understanding

Call graph construction

Call graph construction

Essential idea

Show calling relationships between parts of a program.

Call graph construction

Sample graph

main

f

g

h

Call graph construction

The ideal call graph

The relation describing exactly those calls made from one entity to another in any possible execution of the program.

Call graph construction

Benefit

Make the program seem less fragmented by showing the control flow links between program parts.

Call graph construction

Limitation

The graph becomes too large and complex to be comprehensible.

Call graph construction

”The sight of gcc's call graph frightened my students so much that they requested a different project”

- Arun Lakhotia

Limitation

Program slicing

Program slicing

Essential idea

Find a reduced program exhibiting the same behavior of interest as the full program.

Program slicing

The reduced program is called a program slice.

Program slicing

x : a statement in program PV : a subset of variables in P

C = < x, V >

Slicing criterion

1 begin

2 read(x,y)

3 total := 0.0

4 sum := 0.0

5 if x <= 1

6 then sum := y

7 else begin

8 read(z)

9 total := x*y

10 end

11 write(total, sum)

12 end.

1 begin

2 read(x,y)

5 if x <= 1

6 then

7 else

8 read(z)

12 end.

Criterion: <12, z>

1 begin

2 read(x,y)

12 end.

Criterion: <12, x> 1 begin

2 read(x,y)

3 total := 0.0

4 sum := 0.0

5 if x <= 1

6 then sum := y

7 else begin

8 read(z)

9 total := x*y

10 end

11 write(total, sum)

12 end.

Program slicing

Forward conditioning

What would the program look like if we assume an initial state satisfying C ?

Program slicing

Forward conditioning

Deletes statements that will not be executed given the initial state.

Program slicing

Backward conditioning

What would the program look like if we assume an eventual state satisfying C ?

Program slicing

Backward conditioning

Deletes statements which cannot lead to the eventual state.

Program slicing

Benefit

The reduced programis smaller.

Program slicing

Limitation

The reduced programlooks foreign.

Program slicing

Limitation

Hard to integrate into the programmer’s work flow.

Concept analysis

Concept analysis

Essential idea

Identify groupings of objects that have common attributes.

Concept analysis

Formal context

O Set of objectsA Set of attributesR Relation R⊆O ×A

C = (O, A, R )

Concept analysis

σ(O) = {a∈A⎮∀o∈O : (o, a)∈R}

Common attributes

O Set of objectsA Set of attributesR Relation R⊆O ×A

Concept analysis

τ(A) = {o∈O⎮∀a∈A : (o, a)∈R}

Common objects

O Set of objectsA Set of attributesR Relation R⊆O ×A

Concept analysis

A pair (O, A) is a conceptif A = σ(O) and O = τ(A)

Definition of concept

Concept analysis

O => SubprogramsA => Features

Feature location

Concept analysis

if subprogram s is invoked when feature f is invoked

Feature location

(s, f) ⊆ R

Concept analysis

Benefit

Identify parts of the program relevant for a feature.

Concept analysis

Benefit

Recovery of components and generation of high-level architecture views.

Concept analysis

Limitation

Imperfect high-level descriptions have limited application for concrete tasks.

Concept analysis

Limitation

Requires effort from the programmer, with unclear benefits.

Concept analysis

Limitation

Hard to integrate into the programmer’s work flow.

Change impact analysis

Change impact analysis

Essential idea

Identify the potential consequences of a program change.

Change impact analysis

Essential idea

Estimate what must be modified to accomplish a change of behavior.

Change impact analysis

Challenges in OO languages

Subtyping and dynamic dispatch means that change impact can be non-local and unexpected.

Add empty classDelete empty class

Add fieldDelete field

Add empty methodDelete empty method

Change method bodyChange method lookup

Change impact analysis

Atomic changes

Change impact analysis

Benefit

Provide a boundary around the effects of a program edit.

Change impact analysis

Benefit

Provide confidence that an edit does not have unexpected effects outside the boundary.

Change impact analysis

Limitation

Limited by the precision of the change impact analysis.

Change impact analysis

Limitation

Still need assurance that no unexpected effects occur in the impacted part of the program.

Change impact analysis

Limitation

Hard to integrate into the programmer’s work flow.

Program metrics

Program metrics

Essential idea

Quantify aspects of the program presumed to be relevant.

Program metrics

Code-centric metrics

- Lines of code- Depth of inheritance tree- Internal cohesion- Coupling to other elements- Cyclomatic complexity- Halstead complexity- ...

Program metrics

Other metrics

- Test coverage- Bug density- Change rate- ...

Program metrics

Benefit

Answer questions regardingprogram quality.

Program metrics

Benefit

Identify potential problemareas in the program.

Program metrics

Limitation

Metrics are indirectindicators of quality.

Program metrics

Limitation

Task-generating, not task-solving.

Software visualisation

Software visualisation

Goal

Avoid overwhelming the programmer by compressing information and using graphics.

Table lens

Table lens

Idea

Zoom out the table layout and display cells as pixel bars scaled and colored by data values

Table lens

Example

Table lens

Application

Present a compact view of software metrics for program elements.

Treemap

Treemap

Idea

Display hierarchical data as nested rectangles.

Treemap

Example

Treemap

Application

Compare metric values for various program elements.

Polymetric view

Polymetric view

Idea

Combine several metrics for an entity into a single view, by relating a visual aspect to each metric.

Polymetric view

Example

Polymetric view

Application

Present a compact view of software metrics for program elements.

Hierarchical edge bundles

Hierarchical edge bundles

Idea

Bundle association data with structure data in a radial view.

Hierarchical edge bundles

Example

Hierarchical edge bundles

Application

Compact view of associations between program elements.

What about task-orientedprogram understanding?

The paradox of software visualisation

Programmers use visualisation all the time when discussing programs informally.

Computers are good at presenting information graphically.

...and yet...

Software visualisation tools are rarely used for everyday software development and understanding.

Why?

Hypothesis

Research is out of touch with reality!

Claim

Software visualisationis a generic solution looking for problems.

Claim

Programmers need specific solutions to specific problems.

Remedy

An easy-to-use tool that lets the programmer define the problem that needs visualisation.

Task-aware environments

Task-aware environments

Idea

Adapt interface to reflect the task the programmer is currently working on.

Task-aware environments

Details

Introduces the notion of development sessions.

Task-aware environments

Details

Links development sessions to tasks.

Task-aware environments

Details

Sessions can be thrown away or saved for later.

Code bubbles

Evaluation Examples

DescriptionIntroduction

Consider Limitations of Current Tools

Look at somewell-known

Analyses

How Analysis can help

Understanding

Understanding Program

Understanding

AgendaSummary