Partitioning Composite Code Changes to Facilitate Code Review

Yida Tao and Sunghun KimThe Hong Kong University of Science and Technology

Atomic Code Change

Fixed bug #123

Atomic Code Change

Fixed bug #12, #34, #56Removed duplicate codeAdd a featureJavadoc updated

Composite Code Change

Fixed bug #123

Atomic Code Change

Fixed bug #12, #34, #56Removed duplicate codeAdd a featureJavadoc updated

Composite Code Change

Fixed bug #123

Difficult to reviewLikely be rejected

Research Questions

• RQ1: Are composite code changes prevalent?

• RQ2: Can we propose an approach to improve the semantic atomicity of composite code changes?

• RQ3: Can our approach help developers better review composite code changes?

6

RQ1: Occurrence of composite code changes • Data source• 4 open-source Java projects• Revisions that changed >= 2 lines of code• Commit logs and source code were manually inspected

Time period Revisions

Avg. cLOC

Avg. files

Ant 2010/04/27 -- 2012/03/05

137 26.1 2.0

Commons Math

2011/11/28 -- 2012/04/12

107 84.7 3.5

Xerces 2008/11/03 -- 2012/03/13

116 63.6 3.0

JFreeChart 2008/07/02 -- 2010/03/30

93 144.9 4.1

Total revisions

453

7

82%

13%5%

Xerces

8% - 29% revisions address multiple issues

RQ1: Occurrence of composite code changes

92%

7%

1%

Ant

82%

12%6%

Commons Math

71%

18%

11%

JFreeChart

1 issue

2 issues

> 2 issues

8

Approach

A set of changed statements

Partition of the change(A subset is a change-slice)

A composite code change

Identify related stateme

nts

9

Approach Formatting

Dependency

SimilarityA set of

changed statements

A composite code change

Partition of the change(A subset is a change-slice)

10

Approach

Partition of the patch

Unix diff(text differencing)

ChangeDistiller*(AST differencing)

*http://www.ifi.uzh.ch/seal/research/tools/changeDistiller.html

Formatting changes

Formatting

Dependency

SimilarityA set of

changed statements

A composite code change

11

Approach

Partition of the patch

IBM T.J. Watson Libraries for Analysis(WALA)*

Inter-proceduralBackward static slicing*http://wala.sourceforge.net/wiki/index.php/Main_Page

Formatting

Dependency

SimilarityA set of

changed statements

A composite code change

12

Approach

Partition of the patch

“protect array entries against corruption by returning a clone”

Same change typeSimilar delta

Formatting

Dependency

SimilarityA set of

changed statements

A composite code change

P

P’

Evaluation

• 78 composite code changes from the previously inspected data• 3 human evaluators establish manual partitions for

these changes• Automatic partition results are compared to manual

partitions• Considered acceptable if it exactly matched the manual

partition

82%

13%5%

Xerces

92%

7%

1%

Ant

82%

12%6%

Commons Math

71%

18%

11%

JFreeChart

1 issue

2 issues

> 2 issues

Evaluation

• 78 composite code changes from the previously inspected data• 3 human evaluators establish manual partitions for

these changes• Automatic partition results are compared to manual

partitions• Considered acceptable if it exactly matched the manual

partition

82%

13%5%

Xerces

92%

7%

1%

Ant

82%

12%6%

Commons Math

71%

18%

11%

JFreeChart

1 issue

2 issues

> 2 issues

Evaluation

• 78 composite code changes from the previously inspected data• 3 human evaluators establish manual partitions for

these changes• Automatic partition results are compared to manual

partitions• Considered acceptable if it exactly matched the manual

partition

Acceptable # / Total #

Ant 8 / 11

Commons Math

10 / 19

Xerces 16 / 21

JFreeChart 20 / 27

54 / 78 (69%)

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Ant revision 943068 (24 changed LOC)

“Wrong assignment after I renamed the parameter. Unfortunately there doesn’t seem to be a testcase that catches the error.”

Later fixed in revision 943070

One of the two change-slices after partitioning

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Preliminary User Study• RQ3• Can our automatic partition help developers better review

composite changes?

• Participants• 18 CS graduate students

• Task• Participants review 12 composite code changes• Answer a series of code review questions [1], e.g.,

• “What is the consequence of removing the schemaType field?”• “What do changes in these files have in common?”

[1]“Questions programmers ask during software evolution tasks” Sillito et al. FSE 2006

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Experimental Settings

•Treatments• Control group: review code changes by file• Experimental group: review code changes by partition

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Results

By file

By partition

By file

By partition

p = 0.01

p = 0.81

Formatting

Dependency

SimilarityComposite

Code Changes

Partition of the change

8% - 29%

69%

By fileBy partition

Discussion

• Impact of unsatisfactory change partitions

•Balancing between partition costs and benefits

Related Work

• Helping developers help themselves: Automatic decomposition of code review changesets. Barnett et al. ICSE 2015• The industrial perspective of composite code changes

• The impact of tangled code changes. Kim Herzig and Andreas Zeller. MSR 2013• Filtering Noise in Mixed-Purpose Fixing Commits to

Improve Defect Prediction and Localization. Nguyen et al. ISSRE 2013• How composite code changes affect defect prediction

Acknowledgment

• We thank all the students that participated in our user study.

• We especially thank Tao He and Hai Wan for their kind help of arranging the user study.

• We also thank Ananya Kanjilal for her comments on the paper draft.