The Uniqueness of Changes

Microsoft Research. Technical Report MSR-TR-2014-149.
The Uniqueness of Changes:
Characteristics and Applications
Baishakhi Ray
Meiyappan Nagappan
Christian Bird, Nachiappan Nagappan, Thomas Zimmermann,
Univ. of California, Davis Rochester Institute of Technology
Microsoft Research, Redmond
Abstract—Changes in software development come in many
forms. Some changes are frequent, idiomatic, or repetitive (e.g.
adding checks for nulls or logging important values) while others
are unique. We hypothesize that unique changes are different
from the more common similar (or non-unique) changes in
important ways; they may require more expertise or represent
code that is more complex or prone to mistakes. As such, these
changes are worthy of study. In this paper, we present a definition
of unique changes and provide a method for identifying them
in software project history. Based on the results of applying
our technique on the Linux kernel and two large projects at
Microsoft, we present an empirical study of unique changes.
We explore how prevalent unique changes are and investigate
where they occur along the architecture of the project. We
further investigate developers’ contribution towards uniqueness
of changes. We also describe potential applications of leveraging
the uniqueness of change and implement two such applications,
evaluating the risk of changes based on uniqueness and providing
change recommendations for non-unique changes.
Software is a lot like constructing buildings. When we make
changes to buildings, some changes are more repetitive than
others. For example, a typical kitchen remodeling project might
introduce the same marble tops and same colors found in many
kitchens while keeping other elements such as table lamps and
chairs distinct. Note that, a "typical change" may also evolve
over time: the 1950s saw colorful kitchens (sometimes pink)
while in the 1970s colors got more serious, and the 1980s
introduced more bright colors. Appliances are another example
of a typical renovation: they often get replaced with the latest
models in a remodeling project. However not all changes to
buildings are similar or repetitive. A billionaire might have
expensive taste that requires many unique changes to a kitchen.
The concept of uniqueness is not new to software engineering: Gabel and Su [8] and Hindle et al. [11] showed that
source code is in general repetitive and predictable in nature.
In this paper, we wanted to see whether the same theory can be
applied for software changes as well. In particular, we check
when developers modify an existing piece of code, whether they
change it in a unique way or they follow some repetitive (or nonunique) pattern. To do that, we first introduce a methodology
to identify unique/non-unique changes to a software based
on lexical and syntactic matching of changes. Then, using
two Microsoft projects and the Linux Kernel 3.0, we ask the
following questions:
RQ1. What is the extent of unique changes? On
average, 75%, 83%, and 87% changes are unique in the
two Microsoft projects and Linux respectively.
© 2014 Microsoft Corporation. All rights reserved.
RQ2. Who introduces unique changes? In general, all
developers commit unique changes; on average, 57% to
94% of the total contribution of a developer is unique in
Microsoft and Linux. Each developer has her own set of
change templates. While introducing non-unique changes,
developers often reuse these templates.
RQ3. Where do unique changes take place? Certain
subsystems of a project are more prone to non-unique
changes. For example, in the module sound/drivers/
in Linux, 94% of total changes are non-unique, while
in the module fs/jbd, the Linux journal base filesystem module, only 3% changes are non-unique. Also,
developers introduce non-unique changes to the same
file—66% of the non-unique changes take place in the
same file.
Knowing which changes are unique and which changes
are non-unique has several possible applications in software
Risk analysis: One would expect that changes that are
unique are more error prone than changes that developers repeatedly make (non-unique changes). We provide
empirical evidence to support this statement in our paper.
Code reviews: If non-unique changes are recognized in
a code review, then the developers who introduced the
same change earlier can be involved in the code review
process. Conversely, unique changes could be highlighted
to guarantee that they are carefully reviewed.
Recommendation systems: Non-unique changes can be
used as input for recommendation systems: for example,
recommend how a line would typically be changed or
after some change has been made, recommend other nonunique changes that are typically made with the initial
change based on past co-occurrence (change completion).
Automated program repair: We expect that non-unique
changes in bug fixes are better candidates for automated
program repair operations than unique changes. Nguyen
et al. [23] provided initial empirical evidence for this hypothesis. They found that non-unique bug fixes are usually
smaller changes and therefore automated patching tools
could start with small changes and gradually compose
To demonstrate the usefulness of change uniqueness, we
implement a risk analyzer and two recommendation systems.
Based on bug history, our risk analyzer can evaluate how risky
a unique/non-unique change is. An evaluation on our data
shows that non-unique changes are in general less risky. By
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learning from past non-unique changes, we also implement two
For the Microsoft projects, we use CODEMINE [5]—a
types of recommendation systems: one for suggesting relevant framework for collecting and analyzing software development
changes and other for change completion. On average, our history. In particular, we retrieve all the committed versions of
recommendation systems can suggest changes with 52.11% to each source file and their associated change information. For
59.91% precision, and recommend change completion correctly each file version, a patch is computed by comparing it with
its previous version using the widely known gnu-diff utility.
with 38.48% to 42.95% precision.
We represent the patches in unified diff format with 5 lines of
We make the following contributionsin this paper:
unchanged code as context and also ignore white spaces while
• An approach to identify and measure the uniqueness of
the two versions.
changes to a software (Section II).
git as its version control system. We use
• Characterization of unique vs. non-unique changes along
log -w -unified=5 to retrieve all
developer and spatial dimensions of an evolving project
along with change-specific meta(Section III).
outputs the associated
• Provide evidence that unique changes can be more risky
than non-unique changes by implementing a risk analyzer commit patch in a unified diff format with 5 lines of unchanged
context, as shown in Table I. Option -w ignores white spaces.
(Section IV-A).
• Implement and evaluate two types of recommendation sysB. Identifying Unique Changes
tems based on change suggestion and change completion
In this step, we identify the program statements in a project
(Section IV-B).
that are uniquely/non-uniquely changed, by analyzing all the
changed lines retrieved from previous step. This takes place in
two steps:
This section describes the methodology we used to study
1. Identify change hunks: Input to this step is a set of
the uniqueness of changes. First, we identify the program
program patches (which can be defined as the code that is
statements in a project that are non-uniquely changed in its
committed in a single commit to the source code repository).
development history. The rest of the program statements in
Each patch typically contains multiple change regions. Each
the development history of the project are then considered as
such change region with a contiguous list of deleted and added
unique changes.
lines is called a change hunk. Thus, a hunk is defined as a list
Consider the example in Table I. Developer Johannes
of program statements deleted or added contiguously, separated
Berg made some modification to Linux source file
by at least one line of unchanged context.
rtl8192ce/hw.c on 7th February, 2013 as shown in
In Figure I, line A7 to A11 of Commit A is a hunk. A1 to
Commit A. The added and deleted lines have prefixes ‘+’
A6 and A12 to A15 are unchanged context. We identify all
and ‘–’ symbols respectively. Three months later, develthe hunks that are committed to the source code repository
oper Larry Finger made similar modifications to source file
within the studied period of time, by parsing the committed
rtl8192cu/hw.c in Commit B. The green lines A9 to
A11 and B9 to B11 show the non-uniquely added code in the
2. Identify unique & non-unique changes: In this step,
corresponding commits. The rest of the changes are considered
we first identify pairs of non-uniquely edited lines across
as unique changes—A7, A8 in commit A and B6 to B8, and
all the hunks of a project. An edited line r of hunk Hi is
B12 in commit B.
considered to be non-unique, if there is at least one program
In the rest of this section, we first describe our data collection statement t in another hunk H such that r and t have similar
process in Section II-A. Then we talk about our methodology content (identical lexical and syntactic content) and undergo
to categorize unique and non-unique changes. An overview identical edit operation. For example, we consider edits “+
of the methodology is shown in Figure 1. This involves three a = a ∗ b" and “+ x = y ∗ z" are non-unique since they are
steps: Given a set of changes as input, Step 1 identifies program syntactically equivalent i.e. both represent similar multiplication
statements that are added or deleted non-uniquely. The rest of operations, and also have identical edit operation (both are
the changes are marked as unique (see Section II-B). Step 2 added statements). However, edits “- a = a ∗ b" and “+
further categorizes non-unique changes to non-unique addition, x = y ∗ z" are unique even though they have similar content,
deletion, and modification (see Section II-C). Finally, Step 3 because they are changed in a different manner—previous
extracts non-unique change patterns that repeat multiple times statement is deleted and the later one is added.
during the project’s evolution (see Section II-D).
Pair (r, t) of hunk Hi and Hj thus forms a non-unique edit
(NEPij ) between the hunks Hi and Hj . All such nonA. Data Collection
unique edit pairs are then aggregated by pair-wise comparison
First step is to extract all the source code changes from the of all the studied hunks and form a global set of NEP (see
version control repository of a project. For each source file Equation 1)
commit in the project evolution, we retrieve the associated
code changes—deleted lines corresponding to the old version
NEPij = {(r, t)|r ∈ Hi ∧ t ∈ Hj ∧ clone(r, t)}
and added lines corresponding to the new version. We also
extract some change-specific meta-information including author,
commit message, and commit date.
© 2014 Microsoft Corporation. All rights reserved.
Page 2
Commit A: e1a0c6b3a4b27ed5f21291d0bbee2167ec201ef5
src file: /drivers/net/wireless/rtlwifi/rtl8192ce/hw.c
developer: Johannes Berg
commit date: 2013-02-07
Log: mac80211: stop toggling IEEE80211_HT_CAP_SUP_WIDTH_20_40
Commit B: 5b8df24e22e0b00b599cb9ae63dbb96e1959be30
src file: drivers/net/wireless/rtlwifi/rtl8192cu/hw.c
developer: Larry Finger
commit date: 2013-05-30
Log: rtlwifi: rtl8192cu: Fix problem in connecting to WEP or WPA(1) networks
A1 . v o i d r t l 9 2 c e _ u p d a t e _ h a l _ r a t e _ m a s k ( . . . ) {
A2 .
A3 .
s t r u c t r t l _ h a l ∗ r t l h a l = r t l _ h a l ( r t l _ p r i v ( hw ) ) ;
A4 .
s t r u c t r t l _ s t a _ i n f o ∗ s t a _ e n t r y = NULL ;
A5 .
A6 . u8 r a t r _ i n d e x ;
A7.− u8 ci_40mhz = ( c a p & 8 0 2 1 1 ) ? 1 : 0 ;
A8.− u8 ci_20mhz = ( c a p & 8 0 2 1 1 ) ? 1 : 0 ;
A9 . + u8 cbw_40mhz = ( s b a n d w i d t h >= 8 0 2 1 1 ) ? 1 : 0 ;
A10 . + u8 cgi_40mhz = cbw_40mhz ? 1 : 0 ;
A11 . + u8 cgi_20mhz = c a p & 80211 ? 1 : 0 ;
A12 . enum w i r e l e s s _ m o d e w i r e l e s s m o d e = 0 ;
A13 . b o o l s h o r t g i = f a l s e ;
A14 .
A15 . }
B1 . v o i d r t l 9 2 c u _ u p d a t e _ h a l _ r a t e _ m a s k ( . . . ) {
B2 .
B3 .
s t r u c t r t l _ p h y ∗ r t l p h y = &( r t l p r i v −>phy ) ;
B4 .
s t r u c t r t l _ m a c ∗mac = r t l _ m a c ( r t l _ p r i v ( hw ) ) ;
B5 .
B6.− u8 ci_40mhz = mac−>s g i _ 4 0 ;
B7.− u8 ci_20mhz = mac−>s g i _ 2 0 ;
B8.− enum w i r e l e s s _ m o d e w i r e l e s s m o d e = mac−>mode ;
B9 . + u8 cbw_40mhz = ( b a n d w i d t h >= 8 0 2 1 1 ) ? 1 : 0 ;
B10 . + u8 cgi_40mhz = curtxbw_40mhz ? 1 : 0 ;
B11 . + u8 cgi_20mhz = c a p & 80211 ? 1 : 0 ;
B14 . . . .
B15 . }
TABLE I: Example of non-unique changes adapted from Linux. The deleted and added statements start with ‘–’ and ‘+’ respectively;
non-unique changes are marked in green. Lines A9, A10, and A11 are non-uniquely added w.r.t. B9, B10, B11 respectively. However, A7 and
A8 are unique deletions since it does not resemblance any of the corresponding deleted lines B6, B7 or B8.
Non-unique Changes (NUC ) are a set of edited lines that as discussed in RQ1 in Section III. This ensures at least 50
are present in NEP . The rest of the changes in a project are contiguous tokens (around 7-8 lines) are non-unique in the
unique changes. Thus, if C represents all the changed lines in detected cloned region.
a project, Unique Change (UC ) is a set of edited lines that are
(3) For each identified cloned line pair, Repertoire matches
present in C but not included in NUC , i.e., UC = C − NUC . their edit operations. The clone pairs without identical edit
In Equation 1, similarity (or non-uniqueness) between the operations are eliminated. Repertoire also disregards the clone
edited statements is determined by a function clone. Although pairs that has unmodified contexts. The final output is NEP —a
there is no precise definition of clone in the literature, it set of edit pairs with similar edit content and edit operations.
mostly relies on the computation of individual clone detectors. Note that, since CCFInderX outputs clones that have at least
The most common one is textual similarity of two edits. It 7-8 lines of contiguous non-uniqueness, Repertoire marks only
can also be modeled as n-gram similarity [11], AST-based those changes as a non-unique edit pair that either belong to a
similarity [23], [26], etc. In this work, we consider two edits larger non-unique change, or a small non-unique change that
are non-unique, if their contents have identical lexical and takes place in between two large unchanged contexts, with at
syntactic content [14] and they are also edited similarly (either least 7-8 lines of similarity. Such a large threshold helps us to
both are added or both are deleted).
focus only on significant non-unique changes and thus avoids
Step 1 of Figure 1 summarizes the above stages. It takes unintended clones.
five hunks as input. Edits -a2 and +a3 of Hunk_a are nonIn Table I, Repertoire identifies (A9, B9), (A10, B10), and
unique to -b2 and +b3 of Hunk_b. Hence, NEPab = {(-a2,-b2), (A11, B11) are non-uniquely added. The other edits (A7 and
(+a3,+b3)}. Likewise, NEPbc = {(-b2,-c2), (+b3,+c4)}, NEPcd A8 in Commit A and B6 to B8 and B12 in Commit B are
= {(-c1,-d1)}, and NEPad = {(+a3,+d2)}. Thus, Non-unique marked as unique changes.
Change set (NUC) = {-a2,+a3,-b2,+b3,-c2,+c4,-d1,+d2}. The
rest of the changes of the input hunks are unique (UC) = {-a1, C. Categorizing Change Uniqueness
+a4, -b1, -c3, -e1, +e2}.
In this step, we further categorize the non-unique changes
Implementation: To detect the NEP , we adapt Repertoire [25], a lexical based change analysis tool that identify to non-unique addition, deletion, and modification. Since it is
difficult to establish one-to-one correspondence between an
non-unique changes. The basic steps are as follows:
(1) Repertoire pre-processes the hunks to eliminate diff- added and deleted line, we focus on the code region i.e. hunk
specific meta-information such as edit operations and commit instead.
dates. Th meta-information is stored in a database for future ND: non-unique Deletion. Between hunks (hi ,hj ), if there
exists a non-unique edit pair (of 50 tokens in our implemenuse.
(2) Using CCFinderX [14], a lexical token based clone tation) where program statements are deleted (possibly with
detection technique, Repertoire determines non-unique code some unchanged context of program statements) in both hi
content (clone) between the processed hunks. The output of and hj non-uniquely, but there is no addition of program
CCFinderX is a set of line pairs having identical syntax. statements. For example between Hunk_c and Hunk_d in
CCFinderX takes a token threshold as input, that ensures a Figure 1, only c1 is deleted non-uniquely to d1. Thus, Hunk_c
minimum number of contiguous tokens that has to be non- and Hunk_d is categorized as ND. ND indicates that the code
unique between the cloned regions. In our experiment, we region corresponding to the hunk pair was non-unique before
set this token threshold to 50, based on experimental analysis the change. However, after the modification, the non-unique
© 2014 Microsoft Corporation. All rights reserved.
Page 3
Hunk_a (-­‐a1, -­‐a2, +a3, +a4) Hunk_c (-­‐c1, -­‐c2, -­‐c3, +c4) Hunk_e (-­‐e1, +e2) Input
Hunk_b (-­‐b1, -­‐b2, +b3) Hunk_d (-­‐d1, +d2) Non-­‐unique Edit Pair (NEP): (-­‐a2, -­‐b2), (+a3, +b3), (-­‐b2, -­‐c2), (+b3, +c4), (-­‐c1, -­‐d1), (+a3, +d2) Non-­‐unique Change (NUC): {-­‐a2, +a3, -­‐b2, +b3, -­‐c1, -­‐c2, +c4, -­‐d1, +d2} Unique Change (UC): {-­‐a1, a4, -­‐b1, -­‐c3, -­‐e1, +e2} Hunk_a NM Hunk_b NM Hunk_c (-­‐a2, +a3) (-­‐b2, +b3) (-­‐c2, +c4) Hunk_a (-­‐a2, +a3) Hunk_b (-­‐b2, +b3) Hunk_c ND Hunk_d (-­‐c1) (-­‐d1) Hunk_a NA Hunk_d Step 1
Identifying Unique Changes
(+a3) Unique Hunk_e Hunk_c (-­‐c2, +c4) (+d2) Step 2
Categorizing Change Uniqueness
Step 3
Extracting Non-unique Pattern
Fig. 1: Overview of Methodology. Hunk_a, Hunk_b, Hunk_c, Hunk_d, and Hunk_e are initial input. The deleted and added lines in each of
the hunks are represented by ‘–’ and ‘+’ . Here we assume that, between Hunk_a and Hunk_b, line a2 is deleted similarly to b2, and a3 is
added similarly to b3. Between Hunk_b and Hunk_c, line b2 is deleted similarly to c2, and b3 is added similarly to c4. Likewise, c1 and d1
are similarly deleted, and a3 and d2 are similarly added.
lines were deleted, and the region became unique with respect
to each other, since unique program statements were added.
NA: non-unique Addition. Similar to ND, but there is nonunique addition of program statements, but not non-unique
deletion. For example hunk pair (Hunk_a, Hunk_d) in Figure 1
shows NA non-uniqueness since only a3 is added non-uniquely
to d2. NA indicates two non-identical code region became
non-unique after the modifications.
NM: non-unique Modification. Since a modification can be
represented as a deletion in the old version and an addition
in the new version, NM between two hunks indicates at least
one non-unique edit pair between the two hunks is added
and at least one non-unique edit pair is deleted. Consider
the hunk pair (Hunk_a, Hunk_b) in Figure 1: a2 and b2 are
non-unique deletion while a3 and b3 are non-unique addition.
Thus, (Hunk_a, Hunk_b) belongs to NM. Likewise, (Hunk_b,
Hunk_c) is NM. NM signifies the corresponding code region
of the hunk pair was non-unique before, and even after the
modification they remain non-unique.
A hunk is Unique, if all of its changes belong to the unique
changed set (UC ), i.e., none of its edits resemble other edits
across all the studied hunks. In Figure 1, Hunk_e is unique
since its edits -e1, +e2 are not similar to any of the changes.
Such fine grained categorization of hunk uniqueness shows
how uniquely similar code evolve over time, similar to
tracking clone geneology [16]. For example, the code regions
corresponding to Commit A and Commit B in Figure I were
unique initially, but after the addition they become non-unique
(NA). In this case, with time unique code becomes non-unique.
D. Extracting Non-unique Patterns
As shown in Figure I, program statements are often changed
non-uniquely. Some of these non-unique changes always occur
together to form a non-unique pattern. For example, all the
three edits A9, A10, A11 of Commit A in Figure I repeat in
Commit B as B9, B10, B11; thus showing a repeated change
pattern. In this step, we extract such non-unique patterns from
the non-unique hunks. Later, to build recommendation system
in Section IV-B, we use these patterns as common change
If a list of edited lines Ei of hunk hi is non-unique to a list
of edits Ej of hunk hj , a Non-unique Pattern (NPij ) exists
© 2014 Microsoft Corporation. All rights reserved.
between hunks hi and hj . Ei and Ej represent the signature
of RPij corresponding to hunks hi and hj respectively. For
example, in Step 3 of Figure 1, edits [-a2, +a3] of Hunk_a are
similar to [-b2, +b3] of Hunk_b; Thus, they form a repetitive
pattern NPab = {[-a2, +a3], [-b2, +b3]}, where [-a2, +a3]
is the signature of NPab for Hunk_a, and [-b2, +b3] is the
signature of NPab for Hunk_b.
A change pattern may be repeated across multiple hunks. If
hunk hi shares a non-unique pattern with hunk hj and hunk
hk with identical signature, they are merged to form a single
non-unique pattern NPijk . NPabc = {[-a2, +a3], [-b2, +b3],
[-c2,+c4]} is a non-unique pattern extracted from Hunk_a,
Hunk_b, Hunk_c, as shown in Step 3 of Figure 1.
This reduces to a maximal clique problem for a graph formed
by non-unique hunk pairs. We adapted Carraghan et al.’s
algorithm of maximal clique solving problem [2] to detect
the non-unique patterns.
Study Subject
In this study, we analyzed uniqueness of changes in both
open and closed source software development. We studied the
evolution of proprietary projects A and B from Microsoft, and
a large scale open source software, Linux 3.0 (see Table II).
Project A and Linux are written in C, and Project B is written
in C++. We analyze the changes made in the source files (.c,
.cpp etc.), ignoring the interface declarations in the header
files (.h), documentation etc.
First, from the version control system of a project we retrieve
all the file commits that are made within the studied period
(2011-05-19 to 2013-08-29). Next, we classify the changes
associated with the commits into two categories: unique and
non-unique. In total, we studied more than 17 million lines
of changes in all the three projects in their two plus years
of parallel evolution history. Around six thousand developers
contributed those changes.
Prior to analyzing properties of unique and non-unique
changes, we begin with a straightforward question that checks
the proportion of unique and non-unique changes in a project,
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Project A
Total number of changed files
Number of File Commits
Number of Changed Lines
Number of Hunks
Development Period
Number of Developers
2010-12-03 to 2013-06-03
Project B
Total number of changed files
Number of File Commits
Number of Changed Lines
Number of Hunks
Development Period
Number of Developers
2010-11-30 to 2013-06-11
Linux 3.0
Total number of changed files
Number of File Commits
Number of Changed Lines
Number of Hunks
Development Period
Number of Developers
2011-05-19 to 2013-08-29
at token threshold 50. Project B shows maximum non-unique
changes (three million lines) over its entire evolution period.
Changed Lines (LOC)
Project A
Project B
TABLE III: Extent of Uniquely Changed Lines (with a token
threshold of 50)
Now that we have seen the extent of non-unique changed
lines in a project - 12% to 25%, we would like to shed light
on their evolutionary pattern, i.e. whether they are added,
deleted, or modified non-uniquely. Since it is difficult to
identify mapping between individual added and deleted lines,
we focus on added and deleted region (i.e. hunk), as discussed
in Section II-C. Table IV shows the distribution of hunk
uniqueness across projects. In projects A and B, non-unique
addition dominates significantly (82% and 71%), while nonunique deletion and modification share similar proportion. In
Linux, all the three categories are in the same range (30% to
34%) .
TABLE II: Study Subject
RQ1. What is the extent of unique changes?
100.00% uniquely changed lines 364,737
90.00% 80.00% 70.00% Project A
Project B
60.00% 10 20 30 40 50 60 70 80 90 100 non-unique
token size unique add unique delete Fig. 2: Extent of unique changes over different token size in Linux
Figure 2 shows the extent of uniquely added and deleted
lines with a varying token threshold (10 to 100), in Linux. Note
that a threshold represents minimum number of contiguous
tokens that need to be similar between two cloned regions. At
threshold 10 (around 1-2 lines), 70.81% of changed lines is
unique, while at threshold 100 (around 16-20 lines), 93.61%
changed lines are unique. The median and average of uniquely
changed lines are 88.38% and 85.61% respectively, which is
also achieved at threshold 50. Thus, uniqueness increases with
the increase of token threshold for both added and deleted
Since, source code in general lacks uniqueness [8], nonunique changes detected at a very low threshold like 10
may simply detect changes caused by program construct,
for example an addition of a for loop. In contrast, if the
threshold size is set to a very high value, we might ignore
some important non-unique changes that developers introduce
in purpose. This leads us to choose a middle ground—threshold
50 for the rest of our experiment. A non-unique change at
threshold 50 means that there are at least 7 to 8 non-unique
lines. These 7 - 8 lines can be either a large non-unique change
or a small non-unique change with unchanged context above
and below the change also being non-unique. Table III shows
the extent of uniquely changed lines for all the studied projects
© 2014 Microsoft Corporation. All rights reserved.
TABLE IV: Distribution of Non-Uniquely Changed Hunks. Note
that these categories are not disjoint, because a hunk can share only
non-unique addition with one hunk, while sharing non-unique deletion
or modification with another.
Finally, we check how many non-unique patterns are formed
from the observed non-unique changes (see Section II-D).
Following table shows the result. Three million non-unique
changed lines in Microsoft codebase come from only 300K
distinct non-unique patterns. In Linux, 582K non-unique
changed lines come from 142K patterns. On average, these
patterns occur 3.4 and 3.3 times in Microsoft projects and
Linux respectively. They are often short-lived—average lifetime
(last commit date - first commit date) is 63 and 67 days
in Microsoft projects and Linux respectively. These results
indicate developers often introduce non-unique change patterns,
use them few times at quick succession, and then stop using
(A + B)
change (KLOC)
Result 1: Unique changes are more common than
non-unique changes. non-unique changes form distinct
patterns that often repeat multiple times in the code base
within a short span of time.
Page 5
300 linux 80 (Project A + Project B) 60 40 Frequency (in 1000 scale) developer count (%) 100 250 Linux 200 Project A + Project B 150 100 50 20 0 0 10 20 30 40 50 60 70 80 non-­‐unique changes (%) 90 100 (a) Frequency of developers performing non-unique changes
1 3 5 7 9 11 13 15 17 19 21 23 Number of developers per nun-­‐unique pa;ern (developer diversity) (b) Developer diversity of non-unique patterns
Fig. 3: Characteristics of non-unique changes along the dimension of developers.
Since extent of non-unique changes is non-trivial, we wonder
who commits such non-unique changes. Since developers
usually have individual coding style, it may be possible that
some developers introduce more non-unique changes than
others. It is also important to know whose changes they borrow.
Especially, if we find that developers follow their own nonunique pattern often, this broadens the scope of personalized
recommendation system [13]. All these lead us to question:
RQ2. Who introduces unique changes?
Result 2: Developers have their own set of change
patterns that they use repeatedly.
Since we have seen that developers repeatedly use nonunique patterns, we wonder where they are used. Knowing the
answer not only serves researcher’s curiosity, it has several
advantages. For example, if two software components often
change in non-unique manner, the developers of one component
may help in reviewing the code of the other. Also, same group
of developers may be assigned to maintain two such closely
related modules. Thus, we ask the following question:
First, we measure the extent of non-unique changes over
total changes that a developer has committed over time.
Figure 3(a) shows the frequency distribution of the proportion
of non-unique changes per developer. Almost all developers
commit non-unique changes, although some commit more RQ3. Where do unique changes take pace?
than others. For example, we found 10 and 20 developers in
Unique Changes (%)
Non-Unique Changes (%)
Linux and Microsoft respectively, who only committed non1.
unique changes. A closer look reveals that these developers
contributions in respective projects are considerably low—only
1 to 16 lines of changes during the studied period. On average,
42.67% changes of a Microsoft developer is non-unique, and
Linux developers commit more unique changes—only 5.83%
of a developer’s commit is non-unique in Linux.
We further check whose changes developers borrow to
introduce non-uniqueness. We measure that by computing
TABLE V: Top 10 development directories containing unique
developer diversity—the number of developers using a nonand non-unique changes
unique pattern. Figure 3(b) shows the frequency distribution of
First we measure extent of non-unique changes per file. For
developer diversity. A large number of patterns (105,208 and
278,509 in Linux and Microsoft Projects) are actually owned each file, we take the ratio of unique and total changes across
by a single developer. The curve falls sharply as developer all the commits. Thus, if a file f is committed n times within
diversity increases. Only 0.39% and 0.55% of total non-unique the studied period, ratio of unique changes for file f =
patterns are introduced by more than 4 developers in the
i uniquely changed lines
Microsoft projects and Linux respectively. Such less diverse
i total changed lines
changes suggest that developers have their own set of patterns
Table V shows top 10 sub-directories in Linux (up to 2 levels
that they repeatedly use. A highly diverse change pattern
often suggests a system wide pervasive change. For example from the root of the source code) that contain most unique and
we find a change pattern with developer diversity of 10 in non-unique changes. While the journaling block-device module
Linux that modified an existing logging feature. In another fs/jbd contains 97.52% of unique changes, the sound driver
instance, multiple developers repeatedly change identifier type module sound/drivers has 94.34% non-unique changes.
proc_inode to type proc_ns and the associated code over Non-unique changes are mostly restricted within the same file.
In 23.67% cases, non-unique changes are introduced across
a long period of time.
different commits of the same file, while in 42.24% cases even
© 2014 Microsoft Corporation. All rights reserved.
Page 6
Changed Lines
TABLE VI: Top 5 file couplings with non-unique changes
pre-release period
within the same commit (but across hunks). The rest 34.07%
of non-unique changes are made across different files.
Also, some files often change in a non-unique fashion.
Table VI shows top 5 file pairs in Linux sharing non-unique
changes. Note that, in most cases name of the file pairs are also
very similar and relates to similar functionality. This shows
that similar software artifacts often change non-uniquely.
Result 3: Unique & non-unique changes are localized
in certain modules.
lookup time
post-release period
File f
Fig. 4: Workflow of the Risk Analyzer per File Commit. The
timeline shows the evolution of a file f . Each marker (triangle
or star) is a commit, where red stars indicate bug-fix commits
(c3, c4, and c6). c3 and c4 are pre-release fixes, c6 is a postrelease fix as c6’s commit date is after the release date, marked
in red line.
Distinguishing non-unique changes from unique ones can
facilitate many software engineering applications. We demonWe measure risk of a commit by its bug potential—the number
strate this concretely by implementing a risk analysis sysof bugs that are fixed within t months of the commit. The bug
tem (Section IV-A) and two recommendation systems (Secpotential starts from 0, indicating zero risk.
tion IV-B).
We treat pre-release and post-release bugs differently. As
A. Risk Analysis
the name suggests, the pre-release bugs are detected and fixed
There has been decades of research on software risk before the release of a software. Usually they are detected
analysis [10]. Using sophisticated statistical or machine learning through continuous testing, often parallel to development
models [24], [22], risk of a software component is predicted, process. Hence, we assume that these bugs should be detected
primarily based on its evolutionary history. Different types and fixed within few months from their date of introduction.
of software metrics including product metrics (lines of code, To detect pre-release bugs, we look forward in the evolution
source code complexity), process metrics (pre-release bugs, history of the file up-to a predefined lookup time t, and check
software churn), and social metrics (number of developers) whether the file has undergone any bug-fixes in the future.
are typically used for risk prediction models [24]. Nagappan The bug potential of a commit is equal to the number of preet al. found that changed code is also a good indicator of release bug-fixes found within that lookup time. For example,
bug-proneness [22]. However, all changes are not necessarily in Figure 4, for a lookup time t = T1 , commit c1 sees only
buggy. In this section, we show that categorizing changes as one pre-release bug-fix c3. Hence, c1’s bug potential become
unique and non-unique can further facilitate the risk assessment 1. Similarly, c2 has bug potential 2 as it sees 2 pre-release
fixes c3 and c4 within lookup time t = T1 .
of a file commit.
Post-release bugs are reported by customers facing realMethodology. Our risk analyzer works on the assumption
that if a bug is introduced to the codebase, the bug will be fixed world problems, only after the software is released. Since
within few months. For example, if a commit c introduces a these bugs are noticed only after real-world deployment, they
bug to file f , soon there will be a bug-fix commit cb to the are in general serious in nature. The post-release bugs were
file (within t months from the commit date of c). Here, we not detected during the active development period. Thus, we
build a prediction model that assesses c’s risk of introducing assume every commit in the pre-release can potentially cause
a bug. We start with analyzing the evolution history of file f . the post-release bug irrespective of its time frame; i.e. if a
post-release bug is fixed to a file f , any change throughout f ’s
Figure 4 illustrates how the risk analyzer works.
First, we identify all the bug-fix commits (cb ) that fix errors evolution can potentially introduce the bug. Thus for a postor bugs in the codebase. For Microsoft projects, we identify release bug, we increment the bug potential of each commit of
such commits from bug fix database. For Linux, we identify f prior to the fix, similar to Tarvo et al. [29]. . For instance, for
the bug fix commits whose commit messages contain at least the post release fix c6, we assume all the previous commits (c1
one of the key words: ‘bug’, ‘fix’, and ‘error’. Then for each to c5) have equal potential to introduce the error and increment
file commit we analyze its risk of introducing a bug w.r.t. pre their bug potential by one. Thus, c1’s bug potential become
and post-release bugs. For a file f , if a bug-fix commit cb is 1 + 1 = 2, c2’s bug potential become 2 + 1 = 3 and so on.
found within t months of a commit c, we consider that c may
To check whether unique file commits are more risky than
have introduced that bug, hence c’s bug-potential is non-zero. non-unique file commits, we compare the bug potential of
© 2014 Microsoft Corporation. All rights reserved.
Page 7
the two using non-parametric Mann-Whitney-Wilcoxon test
(MWW) [28]. First, we calculate the non-uniqueness of a file
commit as the ratio of number of non-unique changed lines (S)
to the total changed lines (T) associated in the commit 1 . Next,
we categorize the file commits into unique and non-unique
groups based on their non-uniqueness—a file commit is nonunique, if its non-uniqueness is more than a chosen threshold
and vice versa. We then measure the risk of introducing bugs
of non-unique commits over the unique commits.
average bug potential of non-unique group
risk =
average bug potential of unique group
Note that risk is computed as a ratio of the average bug
potentials. Therefore, the number of unique or non-unique
changes will not impact the risk value.
Microsoft (proj A + B)
< 0.0001
< 0.0001
TABLE VII: Wilcoxon-Mann-Whitney test of unique vs. nonunique commits in assessing risk of introducing bugs
Result. We repeat the experiments for lookup time 1 to
6 months with varying uniqueness threshold (0.5 and 0.6).
The result of WMW test on the potential bug count of
unique and non-unique groups is shown in Table VII. For
all lookup periods, p-values of WMW is significant indicating
the bug-potential of unique and non-unique changes differ with
statistical significance.
Also, risk is less than 1 for all lookup periods and nonuniqueness thresholds, except the one marked in red. This
means, on average, the bug potential of non-unique changes is
less than bug potential of unique changes and the difference
is statistically significant. In fact, for Microsoft projects nonunique changes are 50% less risky than the unique changes.
That means in Microsoft code developers may introduce nonunique changes with more confidence.
However, for Linux projects the risk ratio is close to one.
To further understand this, we measure CohenD’s effect size
between unique and non-unique commit groups [3]. In all the
cases we observe a low effect size, varying from 0.12 to 0.19.
This shows, non-unique changes in Linux may not differ much
from the unique changes in terms of their bug proneness. In
fact, we found 836 non-unique patterns in Linux that have
average bug potential beyond 50. However, we also notice that
there are 180K non-unique patterns that do not introduce any
errors to the codebase.
1 Since
our goal is to assess risk for each file commit, the risk analysis
model is based on changed lines associated with each commit, instead of hunk
© 2014 Microsoft Corporation. All rights reserved.
B. Recommendation System
There exists a wide variety of recommendation systems that
suggest possible changes or change locations to developers to
facilitate software development process [27]. Learning from
the non-unique changes in the history of a project evolution,
here we build two different recommendation systems:
• REC-I. When developers select a code fragment to modify,
it recommends possible changes that similar code has
experienced previously.
• REC-II. When developers make a non-unique change,
REC-II recommends other change patterns that cooccurred with that committed change in the past.
Recommendation System I (REC-I): REC-I suggests relevant
changes to the developer using the history of non-unique
changes. When developers modify code, it shows up in the
commit as a set of program statements that are deleted and a
set of program statements that are added. Therefore, when a
developer selects some program statements to modify, REC-I
searches for a similar set of deleted program statements from
the commit history of the project. If a non-unique match is
found, REC-I recommends the set of corresponding program
statements that were added in the commit history to the
developer. In case of multiple matches (i.e., different set of
program statements that are added for a similar set of program
statements that were deleted in different parts of the code),
REC-I suggests all of them along with their frequency counts.
For example, consider Table VIII. If a developer selects line
B1 to delete, REC-I searches from the previous change history
and find a match A1 that is a non-unique deletion. REC-I then
suggests the corresponding line A2 as possible addition.
To measure the accuracy of REC-I, we need a training data
set and a test data set consisting of non-unique changes. We
split the commit history of a project at a given point of time,
and all the commit history data before this point is considered
as training data and the data over the next three months from
this point in time is considered as test data. For each change
in the test data, we query REC-I that searches the training
data for a recommendation. Thus, for a query q if Rq denotes
REC-I output, and Eq denotes actual usage (obtained from the
test data),
Precision (P ): Percentage of REC-I recommendation that
|Eq ∩ Rq |
appears in expected usage from the test data, i.e.,
|Rq |
Recall (R): Percentage of expected usage (as appeared in
|Rq ∩ Eq |
the test data) that is recommended by REC-I, i.e.,
|Eq |
Note that we evaluate the precision and recall only for those
changes in the test data for which a recommendation was made
by REC-I.
The accuracy of REC-I is measured at each month (the point
in time that separates the training and testing data) for the
entire study period (see Figure 5(a)). The overall performance
of REC-I is measured by computing the mean of all the
precision and recall values over the entire study period, similar
to Zimmermann et al. [32].
P =
N i=i
N i=i
Page 8
100 Recall Precision recall precision 80% Accuracy (%) 80 60 40 60% 40% 20% ay
3 -­‐1
3 M
2 month month (a) Accuracy of REC-I
2 Ja
1 M
1 Ju
M ar
2 Ju
M ar
3 10
1 0% 20 Ja
Accuracy (%) 100% (b) Accuracy of REC-II
Fig. 5: Accuracy of Recommendation Systems for Project B
also appear in different method bodies. Leveraging such coTable IX shows the average precision and recall of REC-I. occurrences, we build Recommendation System II (REC-II) to
For project A, B, and Linux precision are 59.91%, 57.41%, suggest relevant non-unique changes. If a developer introduces
and 52.11% respectively. This means when REC-I is returning a non-unique change in the code-base, REC-II searches for
a query with suggestive changes, there is on average 52.11% other change patterns that are committed together in the past
to 59.91% chances that developers will accept that recom- along with the introduced change. For each match, REC-II
mendation. REC-I’s recall values are 67.36%, 65.44%, and displays frequency, i.e., number of times the recommended
59.02% respectively i.e., REC-I successfully returned 59% to changes were committed together.
67% of expected correct suggestion. Such low value of recall is
Similar to REC-I accuracy, we measure REC-II accuracy
partially due to our choice of high token threshold; we could not over a continuous time period. Figure 5 shows the rolling
suggest non-unique chnages that have smaller change size. For precision and recall for REC-II for project B. The other two
the same reason, we perform reasonably well in precision w.r.t. projects show similar trend. We notice that REC-II’s precision
state of the art recommendation system by Nguyen et al. [23], and recall shows similar periodic nature as those of REC-I.
based on similar change uniqueness philosophy. They reported The average precision for projects A, B, and Linux are 38.48%,
a precision of 30% with top-3 and top-1 suggestion. Though 41.16%, and 42.95% respectively. Similarly, the recall values
a direct comparison with the top-3 and top-1 precision is not are 50.79%, 46.95%, and 37.53% respectively (see Table IX).
possible (though in most of the cases REC-I suggests fewer
than five suggestion), our recommendation system performs
reasonably well in predicting non-unique changes.
Uniqueness of Code. Prior researches show a general lack
Interestingly, from Figure 5(a) we find that the precision and
of uniqueness in source code. Gable and Su study source code
recall vary periodically over time. We believe this behavior
uniqueness across 6000 projects including 420 million lines of
is related to the short lifespan of the non-unique patterns
code and find that code fragments are similar up to seven lines
(See RQ1 of Section III). When developers introduce a non(6 to 40 tokens) [8]. Using n-gram model, Hindle et al. show
unique pattern, they use it frequently for a short amount of
that source code is repetitive in nature and has high degree
time and then stop using it. Once REC-I learns the pattern,
of predictability [11]. Kamiya et al. find 10% to 30% of code
it continues suggesting it. As long as the developers use that
similarity in large scale projects (e.g., gcc-8.7%, JDK-29%,
pattern, the accuracy of REC-1 remains high. However, when
Linux-22.7% etc) [14]. James et al. find evidences of adopted
the developers stop using the pattern, the accuracy falls off
code in device driver modules between Linux and FreeBSD [4].
until new non-unique changes are introduced. All the three
In this work, instead of looking at non-unique code, we look
projects show this periodic trend.
at how non-uniquely code evolves with time.
There are also research on repetitiveness of code change
well. In a recent study, Nguyen et al. [23] inspect a large
of changes for 2,841 open source java projects, and find
Project A
Project B
that up to 70-100% of source code is non-uniquely changed
within and across projects. Barr et al. also find evidence that
TABLE IX: Overall Performance of the Recommendation Systems change lines reuse existing code. However, these prior works
look at changes at small granularity—one or two lines of nonRecommendation System II (REC-II): We observe that unique changes. In contrast, we purposefully focus on either
developers often use multiple non-unique changes together, larger non-unique changes (or smaller non-unique changes
in the same commit. For example, changes of Table VIII are but made in similar code context) to avoid unintentional noncommitted together 4 times in Linux in different files. They uniqueness. Thus, our work does not take into account smaller
© 2014 Microsoft Corporation. All rights reserved.
Page 9
non-unique pattern 1
method: at91_add_device_mmc()
A1. A2. +
Location: /drivers/i2c/busses/i2c-pxa.c;
Developer: Doug Anderson
Commit Date: 02-28-2013
non-unique pattern 2
method: at91_add_device_nand
/* ready/busy pin */
if (data->wp_pin)
if (gpio_is_valid(data->wp_pin))
at91_set_gpio_input(data->wp_pin, 1);
B1. B2. +
/* ready/busy pin */
if (data->rdy_pin)
if (gpio_is_valid(data->rdy_pin))
at91_set_gpio_input(data->rdy_pin, 1);
TABLE VIII: Example of non-unique changes committed together in Linux. These changes co-occur 4 times in Linux in different files.
non-unique changes that may be introduced due to program
In terms of internal validity, for risk analysis in Section IV-A,
construct, for example addition of while loop etc.. We further we assume that a file commit is error-prone if a bug is fixed in
characterize change uniqueness w.r.t.before-after relation (non- the same file in a later period. However, it is possible that the
unique deletion, non-unique addition, non-unique modification) error occurred due to other factors, such as other error-inducing
and analyze them in developer and architectural dimensions.
changes in the same or different files.
Also, while measuring accuracy of the recommendation
Risk Analysis. Fenton and Niel [7], summarize some of the
earlier work, while Hall et. al. [10] summarize the more recent system in Section IV-B, we measure how accurately we suggest
contributions in this area of research. Lessmann et. al. [18] change templates as opposed to actual changes. A template may
and D’Ambros et. al. [6] compare the various risk analysis differ from the actual change by identifier names and types.
approaches using benchmark datasets, to identify the strengths However, we believe existing auto program generation tools
and weaknesses of each approach. More recently, Menzies et. like GenProg [9], Sydit [19], LASE [20], and PAR [15] can
al. [21] and Bettenburg et. al. [1] have shown that splitting the adapt our suggested templates to the relevant change context
data in smaller subsets before building fault prediction models and produce more accurate program patches.
External validity concerns generalization of our findings.
is very useful. In this paper we propose a new software metric
for risk analysis based on the uniqueness of change in order In this work, we analyzed C and C++ projects. It is possible
that other languages show different characteristics of changes.
to predict faults in software.
Recommendation System. Recommending possible Also, we only studied intra-project uniqueness. In interchanges to programmers to guide software development is project settings, the uniqueness characteristics may be different.
a well researched area [27]. While, some recommendation However, Nguyen et al. [23] studied repetitive changes within
system suggest API usages [12], [30] and bug-fixes [17] and across different java projects and reported similar findings.
based on the previous development history, others recommend This indicates our results may still hold in other projects written
possible code location (file, class, method) that developers in different languages.
should check associated with a given software task. For
example, by mining evolutionary data of a project, the ROSE
The source code in software is constantly changed by
tool [32], [33] and the tool from Ying et. al. [31] identify
software artifacts that often modify together. When developers developers. In this paper we empirically examined how unique
edit code, these tools recommend other possible code these changes are, by studying a large number of commits from
locations that developers should change as well, leveraging both open source and proprietary domain. We find that the
extent of unique changes is a lot more than that of non-unique
the co-evolution in the history of the project.
We further extend the change recommendation system (to changes; although developers frequently commit non-trivial
REC-II) by learning from the co-evolution in the history of amount of non-unique changes.
We believe that because there is a considerable number
the project. Similar to Zimmermann et al. [32], [33], when a
non-unique changes to a software, the developers can
developer is about to commit a code change, REC-II suggests
in many ways to build better software including
other non-unique changes that were committed together in the
past. REC-II recommends with 38% to 43% precision and 37% risk analysis, code reviews, recommendation systems, and
automated program repair. To demonstrate the usefulness of
to 51% recall, on average.
non-unique changes we build two recommendation systems and
a risk analysis system. We intend to examine other scenarios
Threats to construct validity concern the relation between in the future.
theory and observation. To detect non-unique changes, we
rely on the effectiveness of widely used clone detection tool
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