Automatically Extracting Configuration ConstraintsSarah Nadi *, Thorsten Berger *, Christian Kästner +, and Krzysztof Czarnecki *

Product Line Engineering Workshop, Univ. of Waterloo

Dec. 9th 2013*

+

2

Variability in real life

Sarah Nadi

• Integrated Garage Door Opener• Electronic Compass

Command navigation package

• Black Dacota Leather• Automatic Trunk

Premium Package

Comfort Seats, Front

Black Leatherette

Selection will result in:

+ Addition of premium package

- Removal of Black Leathertte

Handling variability

Build Independently

Sarah Nadi 3

Clone & Own Share Assets

Slide credits: T. Berger

Software Product Lines (SPL)

Product configurationVariability modelingComponentsDSLsGeneratorsPreprocessorsDesign patterns…

[Dubinski et al., CSMR ‚13]

4

But.. How can we build an SPL?

Sarah Nadi

XExpensive and not always possible

P1 P2 P3

Migrate

…

Build from scratch

5

What is involved in migrating?

Sarah Nadi

Refactor Code

Create Variability Model

Detect Variation Points

Identify Configurable Features

Identify Feature Dependencies

Refactor Architecture

[She et al., ICSE‘11]

6

Variability model constraints

Sarah Nadi

[Benavides et al., 2010]

Hierarchy Constraint:MP3 => Media

Cross-tree Constraint:Camera => High Res.

7

Can we automatically extract constraints?

To what extent?

Sarah Nadi

8

Scope• C based systems using conditional compilation• Focus on build-time variability

• Identify two sources of constraints

Sarah Nadi

9

1. Conditional build-time errors

Sarah Nadi

Specification 1: Every valid configuration of the system must

not contain build-time errors.

10

Pre-processor Error if ASH && NOMMU

Parser Error if ASH && EDITING && !MAX_LEN

Type Error if ASH && EDITING_VI && MAX_LEN && !

EDITING

Invalid Configuration

Constraint: ASH => !NOMMU

Sarah Nadi

11

2. Feature effect• Avoid meaningless configurations which do not

add/remove parts of the code• If we add/remove a feature, we want to get different

functionality• Determine under which configurations, a feature has an

effect on the code

Sarah Nadi

Specification 2: Every valid configuration should yield a lexically different program

12

Feature effect

Sarah Nadi

MAX_LEN && EDITING && ASH

MAX_LEN && EDITING_VI && ASH

MAX_LEN => ASH && (EDITING || EDITING_VI)

13

Extract constraints by brute force

Sarah Nadi

PnP3P2P1…

Build Individually

If every configuration with feature X compiles except when Y is also selected

X => !Y

Not scalable--- 2n combinations

If every configuration with feature Z does not change the selected code except if W is also selected

Z => W

14

Extracting constraints in a single pass• Make use of variability-aware parsing & type checking to

mimic build-time behaviour.

Sarah Nadi

[Kästner et al., OOPSLA ‘11]

15

Infrastructure• Developed tool

• FarCE – Feature Constraints Extraction https://bitbucket.org/tberger/farce

• Rely on previous work:• TypeChef – Type-Checking Ifdef Variability https://

github.com/ckaestne/TypeChef• KBuildMiner http://

code.google.com/p/variability/wiki/PresenceConditionsExtraction• To analyze variability models:

• LVAT https://code.google.com/p/linux-variability-analysis-tools• CDLTools https://bitbucket.org/tberger/cdltools

Sarah Nadi

16

Empirical study• Objectives:

• O1: Evaluate accuracy and scalability of extraction• O2: Quantitatively and qualitatively study kinds of (extractable)

constraints in real-world systems

• Used four systems with existing variability models

• Compare extracted constraints to existing hierarchy & crosstree edges in the model

Sarah Nadi

Linux Kernel7,691 C files

6,559 Features

BusyBox535 C files

844 Features

eCos579 C files

1,254 features

uClibc 1,628 C files367 features

17

Is the extraction accurate?

Sarah Nadi

Specification 1 is 95% accurate

Specification 2 is 76% accurate

18

Which constraints are recovered?

Sarah Nadi

Can automatically recover 23% of variability model constraints!5% by Specification 1

17% by Specification 2

Hierarchy edges

reflected in code

nesting

Crosstree edges prevent

build-time errors

19

• Qualitative analysis of 144 unrecovered constraints

What about constraints not found?

Sarah Nadi

21% additional analyses 19% limitation in comparison

19% configurator-related

29% unknown

9% domain knowledge 3% limitation in extraction

Manual analysis of constraints is hard!

Some constraints are non-technical & need expert knowledge

Qualitative analysis is subjective

20

Challenges• Presence conditions and constraints explode

• Limit complexity of constraints used• Use non-SAT based constraint combination techniques

• Different ways to compare constraints• Our comparison is limited to binary constraints• Other techniques which may be used?

• Understand the intent of different constraints without interviewing developers

Sarah Nadi

21Sarah Nadi

Automatically Done

Accurate

Extracts substantial parts of VM An expert may still be needed

Sarah Nadi (finishing soon … interested in a post doc)

snadi@uwaterloo.ca http://swag.uwaterloo.ca/~snadi

Extracting configuration constraints from code

(95% & 76% respectively)

Conditional Build-time errors & Feature Effect

(Avg. 23% & up to 65%)

Questions?

22

Pre-processor Error if ASH && NOMMU

Parser Error if ASH && EDITING && !MAX_LEN

Type Error if ASH && EDITING_VI && MAX_LEN && !

EDITING

Invalid Configuration

Constraint: ASH => !NOMMU

Sarah Nadi

Linker Error if ASH && EDITING && !INIT

Constraint formulas

Preprocessor, parser, and type-checking constraints

Linker constraints

Feature effect

23

conditional symbol table

Sarah Nadi

24

How are constraints used?• Hierarchy edges are mainly reflected in how features are

used/nested in the code (Spec 2: feature effect analysis)

• Cross tree edges are often used to prevent build-time errors (Spec 1: conditional build-time errors)

Sarah Nadi

Partial pre-processor (lexer)

25

slide credits: C. KästnerSarah Nadi

parser

26

slide credits: C. Kästner

Sarah Nadi

27

Is the analysis scalable?

Sarah Nadi

Can analyze Linux files in 12hr with parallelization