1. Partitioning Composite Code Changes
to Facilitate Code Review
YidaTaoand Sunghun Kim
TheHongKongUniversityof ScienceandTechnology

2. Atomic Code Change
Fixed bug #123

3. Atomic Code Change
Fixed bug #12, #34, #56
Removed duplicate code
Add a feature
Javadoc updated
Composite Code Change
Fixed bug #123

4. Atomic Code Change
Fixed bug #12, #34, #56
Removed duplicate code
Add a feature
Javadoc updated
Composite Code Change
Fixed bug #123
Difficult to review
Likely be rejected

5. 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
6
Timeperiod Revisions Avg.cLOC Avg.files
Ant 2010/04/27-- 2012/03/05 137 26.1 2.0
CommonsMath 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
Totalrevisions 453

7. 82%
13%
5% Xerces
7
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
statements
8

9. Approach Formatting
Dependency
Similarity
9
A set of changed
statements
A composite
code change
Partition of the change
(A subset is a change-slice)

10. Approach
Partition of the patch
Unixdiff
(textdifferencing)
ChangeDistiller*
(ASTdifferencing)
*http://www.ifi.uzh.ch/seal/research
/tools/changeDistiller.html
Formatting
changes
10
Formatting
Dependency
Similarity
A set of changed
statements
A composite
code change

11. Approach
Partition of the patch
IBM T.J.Watson
Librariesfor Analysis
(WALA)*
Inter-procedural
Backwardstatic slicing
*http://wala.sourceforge.net/wiki/in
dex.php/Main_Page
11
Formatting
Dependency
Similarity
A set of changed
statements
A composite
code change

12. Approach
Partition of the patch“protect array entries against
corruption by returning a clone”
Same changetype
Similar delta
12
Formatting
Dependency
Similarity
A set of changed
statements
A composite
code change
P
P’

14. 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 acceptableif 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

15. 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 acceptableif 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

16. 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 acceptableif it exactly matched the manual partition
Acceptable # / Total #
Ant 8 / 11
Commons Math 10 / 19
Xerces 16 / 21
JFreeChart 20 / 27
54 / 78 (69%)

17. 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.”
Laterfixedin revision 943070
17
One of the two change-slices after partitioning

18. Preliminary User Study
• RQ3
• Can our automatic partition help developers betterreview 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?”
18
[1]“Questions programmers ask during softwareevolution tasks” Sillitoet al. FSE 2006

19. Experimental Settings
•Treatments
• Control group: review code changes by file
• Experimental group: review code changes by partition
19

20. Results
By file By partition By file By partition
20
p = 0.01 p = 0.81

21. Formatting
Dependency
Similarity
Composite
Code Changes
Partition of
the change
8% - 29% 69%
By file By partition

22. Discussion
•Impact of unsatisfactory change partitions
•Balancing between partition costs and benefits

23. 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 affectdefect prediction

24. 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.