아래 한글에서는 "그레인"이라고 말하는데 이 단어만 영어로 바꾼다면 어떤 철자를 써야하는가?

기존 학습 기반 APR 기법의 한계
프로그램 수리를 번역 작업 또는 클로즈 작업으로 처리
주로 Statement-level 또는 Token-level에서만 패치를 생성
다양한 그레인 수준의 버그를 효과적으로 처리하지 못함
실제 문제 상황
현실 세계의 버그는 Statement, Expression, 토큰 등의 다양한 그레인 수준에서 발생
단일 그레인에서만 패치를 생성하면 실제 버그 수리에 비효율적
우리의 관찰
버그 수정 데이터 분석 결과, 다양한 그레인 수준에서 수정이 이루어짐
특정 그레인 수준이 지배적이지 않음
Mulpor의 필요성
다중 그레인 패치 생성을 통한 효과적인 버그 수리 필요
Mulpor는 Statement, Expression, Token level에서 패치를 생성하여 다양한 버그를 처리


아래는 논문의 전문이다.

One Size Does Not Fit All: Multi-granularity Patch eneration for Better Automated Program Repair
Bo Lin∗
Shangwen Wang∗†
Ming Wen†
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
College of Computer Science,
National University of Defense
Technology
Changsha, China
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
College of Computer Science,
National University of Defense
Technology
Changsha, China
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
School of Cyber Science and
Engineering
Huazhong University of Science and
Technology
Wuhan, China
Liqian Chen∗
Xiaoguang Mao∗
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
College of Computer Science,
National University of Defense
Technology
Changsha, China
<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>
College of Computer Science,
National University of Defense
Technology
Changsha, China
Abstract
Automated program repair aims to automate bug correction and
alleviate the burden of manual debugging, which plays a crucial role
in software development and maintenance. Recent studies reveal
that learning-based approaches have outperformed conventional
APR techniques (e.g., search-based APR). Existing learning-based
APR techniques mainly center on treating program repair either as
a translation task or a cloze task. The former primarily emphasizes
statement-level repair, while the latter concentrates on token-level
repair, as per our observations. In practice, however, patches may
manifest at various repair granularity, including statement, expression, or token levels. Consequently, merely generating patches
from a single granularity would be ineffective to tackle real-world
defects. Motivated by this observation, we propose Mulpor, a multigranularity patch generation approach designed to address the
diverse nature of real-world bugs. Mulpor comprises three components: statement-level, expression-level, and token-level generator,
each is pre-trained to generate correct patches at its respective
granularity. The approach involves generating candidate patches
from various granularities, followed by a re-ranking process based
on a heuristic to prioritize patches. Experimental results on the
Defects4J dataset demonstrate that Mulpor correctly repair 92 bugs
on Defects4J-v1.2, which achieves 27.0% (20 bugs) and 12.2% (10
bugs) improvement over the previous state-of-the-art NMT-style
∗ Bo
Lin, Shangwen Wang, Liqian Chen and Xiaoguang Mao are also with the State
Key Laboratory of Complex & Critical Software Environment
† Shangwen Wang and Ming Wen are the corresponding authors.
Permission to make digital or hard copies of all or part of this work for personal or
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author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or
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© 2024 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ACM ISBN 979-8-4007-0612-7/24/09
https://doi.org/10.1145/3650212.3680381
Rap-Gen and Cloze-style GAMMA. We also investigated the generalizability of Mulpor in repairing vulnerabilities, revealing a notable
51% increase in the number of correctly-fixed patches compared
with state-of-the-art vulnerability repair approaches. This paper
underscores the importance of considering multiple granularities
in program repair techniques for a comprehensive strategy to address the diverse nature of real-world software defects. Mulpor, as
proposed herein, exhibits promising results in achieving effective
and diverse bug fixes across various program repair scenarios.
CCS Concepts
• Software and its engineering → Software maintenance tools;
Software defect analysis; Software testing and debugging.
Keywords
Automated Program Repair, Pre-Training, Deep Learning.
ACM Reference Format:
Bo Lin, Shangwen Wang, Ming Wen, Liqian Chen, and Xiaoguang Mao.
2024. One Size Does Not Fit All: Multi-granularity Patch Generation for
Better Automated Program Repair. In Proceedings of the 33rd ACM SIGSOFT
International Symposium on Software Testing and Analysis (ISSTA ’24), September 16–20, 2024, Vienna, Austria. ACM, New York, NY, USA, 13 pages.
https://doi.org/10.1145/3650212.3680381
1
Introduction
Modern software systems are witnessing an unprecedented surge
in complexity and scale, a phenomenon underscored by the escalating prevalence of software bugs [23, 43]. These bugs not only
impede the progress of software development but also yield a myriad of challenges, ranging from user dissatisfaction to financial
losses [17]. The manual identification and fixing of these bugs demand substantial investments of time and resources [2]. In response
to this challenge, Automated Program Repair (APR) has emerged
as a pivotal field in software engineering, aiming to automate the
correction of identified buggy code snippets and thereby alleviate
the arduous burden of manual debugging activities [17].
Over recent decades, traditional APR techniques have shown
promising results. They have evolved into various types, including search-based [26, 38, 45, 45, 54] and constraint-based techniques [39, 41, 44, 59]. Recently, the adoption of advanced deep
learning techniques for program repair has emerged as a notable
trend. Particularly, a number of studies have shown that leveraging
the power of deep learning can be more proficient at fixing bugs
than traditional template-based approaches [58, 64]. In the literature, there are mainly two ways applying deep learning models
on the program repair task. On the one hand, several techniques
treat program repair as a Neural Machine Translation (NMT) task
to transform the buggy program into the fixed version, with an
emphasis on translating the whole buggy statement into a correct
one [22, 40, 49, 61]. For instance, RewardRepair [61] uses a loss function based on compilation and execution information to construct
an NMT model, which takes a suspicious statement as the input
and outputs the fixed statement. Consequently, such techniques
generate patches at the statement level. On the other hand, some
latest studies utilize the weapon of LLMs and treat program repair
as a cloze-style task, in which the buggy token is replaced by the
predictions from LLMs based on the context around [58, 63]. Such
techniques achieve relatively high performances when there is only
one buggy token in the original program, probably because during
the pre-training, the masked language modeling task requires the
model to predict the content for a single masked token. To explore
this point, we manually analyzed the correct patches generated by
GAMMA [63], a state-of-the-art APR technique, and found that
76% of such patches focused on token-level change from the original buggy program. In this regard, such learning-based techniques
could be considered as generating patches at the token level.
Despite being effective, existing techniques ignore the fundamental property of bug-fixing activities, i.e., different bugs are required
to be fixed at different granularities [48]. Specifically, sometimes
the bug is required to be fixed at the statement level. In other cases,
simply modifying a code token in the faulty statement can correct
the behavior of the program. For the former cases, correct patches
would not be generated by approaches that produce patches at
the token level, since these approaches operate at a finer-grained
granularity (token vs. statement), and the search space of which
will not contain the correct patch. In contrast, for the latter cases,
completely rewriting the statement would reduce the probability of
generating correct patches. This is primarily due to the significantly
larger search space these approaches have to navigate (statement v.s.
token), which makes it challenging for them to accurately identify
the correct repair actions. To gain a comprehensive understanding towards such a multi-granularity property, we performed an
exploratory experiment (discussed in Section 3) on a dataset containing 787,178 bug-fixing instances collected from open-source
projects [49]. Results reveal that bugs from real-world are often
required to be repaired at various granularities, including the statement, expression, and token level. Moreover, patches generated
at different granularities share similar percentages in the dataset,
suggesting that no specific granularity dominates over the others.
These findings suggest that merely relying on the ability to produce
patches at a specific granularity may not be sufficient to effectively address real-world software defects. Therefore, it is crucial to consider
multiple granularities when devising program repair techniques.
Bo Lin, Shangwen Wang, Ming Wen, Liqian Chen, and Xiaoguang Mao
Motivated by our observations, in this paper, we propose Mulpor,
a Multi-granularity patch generator. It is simple yet effective that
can produce patches from various granularities, aiming at effectively handling the diverse nature of real-world bugs. Our key
idea is to generate patches at various granularities, increasing the
likelihood of producing the correct patches, and then re-rank the
candidate patches based on heuristics to ensure the correct patch
is recommended at the top positions. We foresee that an ideal resolution would be predicting the repair granularity first, followed
by selecting the corresponding generator. Nonetheless, predicting the repair granularity of a buggy program is non-trivial. For
instance, due to this challenge, existing template-based APR techniques [18, 30, 37] opt to traverse all the fix patterns at various
repair granularities when performing patch generation. To achieve
its target, Mulpor integrates three components, i.e., the statementlevel, expression-level, and token-level generator. Each component
is pre-trained to fill in masked contents at its respective granularity,
thus enabling them to generate correct programs across different
granularities. When provided with a buggy statement, Mulpor employs the three components individually. In particular, it invokes
the statement-level generator to regenerate the entire statement,
the expression-level generator to modify the expressions within
the statement, and the token-level generator to update the code
tokens in the statement. This process generates a set of candidate
patches, followed by a patch re-ranking process where the candidate patches are prioritized according to their similarity to the
original buggy program. The rationale of this step is that existing
studies have shown that correct patches often share certain similarities with the original buggy program and do not require significant
changes [31, 50]. We thus prioritize patches that show the highest
degree of similarity to the original program. To obtain the final
results, we check the ability of each candidate patch to pass the test
suite, and then manually examine all plausible patches to confirm
that the bugs are correctly fixed.
We evaluate the effectiveness of Mulpor on the widely-adopted
Defects4J-v1.2 benchmark. The results show that Mulpor outperforms all studied APR approaches, repairing 10 more bugs (82 → 92)
with perfect FL and 6 more bugs (50 → 56) without perfect FL than
the previous state-of-the-art models. Furthermore, we conducted
an in-depth exploration of Mulpor’s capabilities across a broader
spectrum of code repair scenarios, specifically focusing on vulnerability repair. The findings reveal that Mulpor produces a notable
51% more correct patches compared to the existing state-of-the-art
vulnerability repair approach VulRepair+ [24]. In summary, our
study makes the following contributions:
• New Dimension: Our work reveals the multi-granularity nature
of bug-fixing activities, and emphasizes that APR tools should
consider the design of various granularities, such as statement,
expression, and token levels.
• State-of-the-art APR tool: We propose a multi-granularity
patch generation approach, Mulpor. Mulpor initially acquires semantic and syntactic knowledge through intricate unsupervised
pre-training tasks at different granularities. Subsequently, it undergoes fine-tuning on real-world bug-fixing datasets. This enables Mulpor to generate patches at various granularities, thereby
increasing the likelihood of producing correct patches.

• Extensive Study: Mulpor is extensively evaluated on Defects4J,
showcasing superior bug repair performance. The study extends
to vulnerability repair, revealing significant advancements over
current state-of-the-art approaches. The results affirm the effectivenss of Mulpor across diverse program repair challenges.
statements or expressions [37]. In this context, we present Mulpor,
a simple yet effective learning-based APR technique. It augments
the model with the capability to generate patches at various levels
of granularity, therefore increasing the likelihood of producing the
correct patch compared to generating patches at a fixed granularity.
2 Background and Related Works
2.1 Automated Program Repair
2.2
Since the inception of Genprog [33], a wide array of APR techniques
have been proposed, contributing to the reduction of manual debugging efforts and the automatic generation of patches. According
to the study from Goues et al. [19], APR techniques can be divided
into three categories, namely search-based repair [26, 33, 38, 45,
54], constraint-based repair [39, 41, 44, 59], and learning-based repair [3, 8, 27, 34, 40, 64]. Search-based repair methods stand out as
conventional APR techniques, often relying on program modification through mutation with heuristic algorithms [45] or genetic
programming [33] to generate a multitude of candidate fixes for
validation via unit tests. To narrow down the search space in searchbased repair, search strategies have involved to incorporate fix patterns mined using redundancy-based techniques [26, 32, 38, 39, 55]
from history data, such as commits [32], existing codebase [26]
and even external Q&As from StackOverflow [38]. In contrast to
heuristic repair techniques, constraint-based methods proceed by
formulating repair constraints that the patched code should satisfy.
Constraint solving or other search techniques are then employed
to identify solutions to these repair constraints.
In recent years, the field of APR has witnessed significant advancements, primarily driven by the proliferation of deep learning
techniques. Most learning-based APR methods [3, 8, 27, 34, 40, 64]
aim to streamline the program repair process by framing it as a
NMT task, which typically translates a faulty statement into a correct one. These NMT-style APR techniques use an encoder-decoder
architecture [10]. The encoder extracts representations from the
buggy code, and the decoder generates the corrected code based on
these representations. As an illustration, Rap-Gen [51] adopts the
CodeT5 model along with a similar patch search strategy, achieving
state-of-the-art performance in NMT-style APR techniques. The
effectiveness of these techniques owes much to the remarkable
capacity from deep learning techniques to discern complex relationships within vast code corpora. Hence, these techniques have
consistently achieved state-of-the-art performance in recent years.
However, despite the promising performance, NMT-style APR
techniques still face certain limitations, primarily stemming from
the quality and quantity of available historical bug-fixing pairs used
for training [58]. Recognizing this, Xia et al. recently introduced
AlphaRepair [58], which formulates program repair as a cloze task.
This approach involves directly predicting the correct code based
on contextual information, using pre-trained models. While AlphaRepair capitalizes on existing pre-trained models, which were
not specifically designed for program repair, they are ill-suited for
handling complex code repair. For instance, CodeBERT [15], as
used in AlphaRepair, only supports filling in one token at a time.
Within this context, these techniques can be considered as generating patches at the token level. However, in practice, program repair
frequently requires changes at multi-granularity, such as modifying
Pre-Training Techniques
Training deep learning models from scratch often requires extensive labelled data, which can be resource-intensive and timeconsuming. In response, the self-supervised pre-training techniques
has emerged as a powerful alternative. Self-supervised objectives
are designed to endow models with common-sense knowledge
by leveraging large amount of unlabelled data. These pre-trained
models can subsequently be fine-tuned on specific downstream
tasks with a relative modest amount of labelled data. Initially pioneered in Natural Language Processing [11, 46], self-supervised
pre-training has evolved to address code-related tasks, with a number of pre-training tasks that are tailored to instill models with
domain-specific programming language knowledge being introduced [15, 21, 52]. One of the pioneering self-supervised objectives,
the Masked Language Modeling (MLM) [11], gained considerable
popularity within the research community. In MLM, a portion of
the training data is masked, and the primary training objective resolves around predicting or recovering the original data. MLM has
showcased its effectiveness in tasks related to language [11, 20, 21].
A promising avenue in pre-training technologies involves the
application of self-supervised techniques to program repair. Researchers have proposed leveraging pre-trained code models to
formulate program repair as a cloze task. This involves masking a
buggy code snippet and prompting the model to predict the correct code snippet within the given context. For instance, AlphaRepair [58] utilizes CodeBERT [15], and GAMMA [63] leverages UnixCoder [20] for donor code retrieval through a mask prediction task
without additional pre-training. However, pre-trained models used
in cloze-style APR have inherent limitations from their pre-training
objectives. They primarily focus on restoring one or several tokens
from masked locations. Operating at the individual token level
restricts their capacity to grasp broader contextual information, including relationships between groups of words [28]. This limitation
extends to their ability to address more complex bugs that require
to rewrite a complete statement. Consequently, these cloze-style
APR techniques are generally more effective for token-level repairs.
To better understand the limitation of cloze-style APR, we also
perform an investigation on the patches generated by GAMMA
for the defects from Defects4J-v1.2 benchmark. The results indicate that approximately 67% of the patches and 76% of the correct
patches generated by GAMMA focus on token-level modifications,
demonstrating its inability to perform complex repairs. However,
it is crucial to recognize that the repair granularity of buggy programs can vary [37, 57], encompassing levels such as statement
and expression granularity. This variability implies that pre-trained
models based on MLM may encounter limitations in addressing program repair tasks at a coarse-grained level, such as the statement
level. This observation underscores the necessity for specialized
pre-training models explicitly designed to accommodate the diverse
repair granularities encountered in program repair tasks.
3
Bo Lin, Shangwen Wang, Ming Wen, Liqian Chen, and Xiaoguang Mao
Statement-level repair from Chart-12:
MOTIVATING EXAMPLES
In existing literature, there are two primary ways to apply deep
learning models in the field of program repair. The first is to treat
program repair as an end-to-end NMT task, aiming to transform
a buggy program into its corrected version [27, 40, 58]. The second views program repair as a cloze-style task, wherein the faulty
token is replaced by predictions from LLMs based on the contextual information surrounding it. To elucidate the limitations of
current learning-based APR techniques, we present several illustrative cases, each operating at different repair granularities. We
also select the patches generated by state-of-the-art techniques in
the cloze-style APR (i.e., GAMMA [63]) and NMT-style APR (i.e.,
Rap-Gen [51]) to illustrate the limitations of existing approaches.
Repair granularity denotes the kinds of code entities that are
directly modified by different repair actions [37]. Following the templates derived from historical bug-fixing by experts [37], the repair
granularity can be categorized into three types: statement-level,
expression-level, and token-level. Statement-level repair refers to
the alteration of the statement-level node by actions such as addition, deletion, or modification. Expression and token-level repairs
mirror similar changes but at the respective node levels. Note that
token-level repairs may change more than one token in the buggy
program. Such as Chart-10 bug 1 from Defects4J, where an identifier
is updated to a method invocation. But this repair can be mapped
into the modification of leaf nodes in the buggy Abstract Syntax
Tree (AST), i.e., token-level repair.
Figure 1 illustrates real bugs from the widely-used Defects4J
benchmark [29] at different repair granularities. For instance, in addressing the Chart-12 bug, the developer replaces an old statement
with a new one, directly modifying the ExpressionStatement,
indicating a statement-level repair. However, when GAMMA utilizes CodeBERT as the base model, it struggles to generate a correct
patch due to the limited search space. Since GAMMA only masks
the identifier dataset guided by a fix pattern, its search space only
involves updating this identifier with other candidates, which is
inadequate for repairing this bug. Similarly, in case of the Closure123, repairs are associated with an expression node. The developer
updates a FieldAccess expression into a MethodInvocation expression. GAMMA tends to select a fix pattern that replaces the
identifier OTHER rather than the entire FieldAccess expression in
the buggy statement. In contrast, RAP-Gen aims to translate a buggy
statement into a fixed one, which leads to a complete rewrite of the
statement and thus introduces unnecessary changes. Token-level
repair in the Chart-24 bug exemplifies the limitation of Rap-Gen
further, where the correct patch updates the identifier value to v.
However, Rap-Gen rewrites the entire buggy statement, setting the
value to the minimum value between v and this.lowerBound.
In stark contrast to existing approaches that mainly focus on a
single repair granularity, various repair granularities exist in the
real world. In order to better understand this phenomenon, we first
perform an investigation into the repair distribution across different
granularities in historical data. This investigation was conducted
using GumTree [13] and computed on 787,178 bug-fixing pairs obtained from open-source projects provided by Tufano et al. [49].
Table 1 presents the proportion of repairs at different granularities,
1 https://program-repair.org/defects4j-dissection/#!/bug/Chart/10/
// Oracle Patch
- this.dataset = dataset;
+ setDataset(dataset);
// Patch generated by GAMMA
- this.dataset = dataset;
+ this.data = dataset;
Expression-level repair from Closure-123:
// Oracle Patch
- Context rhsContext = Context.OTHER;
+ Context rhsContext = getContextForNoInOperator(context);
// Patch generated by GAMMA
- Context rhsContext = Context.OTHER;
+ Context rhsContext = Context.STATEMENT;
// Patch generated by Rap-Gen
- Context rhsContext = Context.OTHER;
+ ContextFactory rhsContext = getContext(context, false);
Token-level repair from Chart-24:
// Oracle Patch
- int g = (int) (( value - this.lowerBound) /
(this.upperBound - this.lowerBound) * 255.0);
+ int g = (int) (( v - this.lowerBound) /
(this.upperBound - this.lowerBound) * 255.0);
// Patch generated by Rap-Gen
- int g = (int) ((value - this.lowerBound) /
(this.upperBound - this.lowerBound) * 255.0);
+ value = Math.min(v, this.lowerBound);
Figure 1: Program repair at different granularities.
along with the top three repair types in each category. Notably, statement, expression, and token-level repairs constitute 30.8%, 33.6%,
and 35.6% of the repairs, respectively. This distribution reveals that
existing approaches primarily operating at a single granularity level
(e.g., token-level) may limit their ability to support the patch generation at different granularities, thereby impacting their performance
in patch generation. Based on these observations, we hypothesize
that the effectiveness of APR approaches could be enhanced by
generating patches at different granularities, aiming at effectively
handling the diverse nature of real-world bugs. Our key idea is to
first generate patches from various granularities, which increases
the probability of generating the correct patch compared to a fixed
granularity. We then re-rank these candidate patches from various
granularities based on heuristics aims to maximize the likelihood
of recommending the correct patch at the top.
4
Mulpor
In this section, we present a detailed overview of Mulpor, the
workflow of which is depicted in Figure 2. During the pre-training
phase, we initially extract all functions provided by the CodeSearchNet [25] dataset, encompassing more than 2 million multi-linguistic
functions. Given that Mulpor comprises three integral components,
i.e., the statement-level generator, expression-level generator, and
token-level generator, all the functions are used as the pre-training
data at three distinct granularities, which facilitates the training
of the corresponding generators (Section 4.2). In the pre-training
phase, each component is pre-trained to fill in masked contents at
its respective granularity. This process enables the components to
accurately generate programs across diverse granularities.

Figure 2: The overall workflow of Mulpor.

Table 1: The proportion of repairs at different ranularities.
| Granularity | Type¹ | Proportion(in %) |
|---|---|---|
| Statement | Expression Statement | 17.3 |
|  | If Statement | 7.8 |
|  | Return Statement | 3.3 |
|  | Others | 2.4 |
|  | **Total** | **30.8** |
| Expression | Method Invocation | 20.1 |
|  | Variable Declaration | 5.7 |
|  | Assignment | 3.1 |
|  | Others | 4.7 |
|  | **Total** | **35.8** |
| Token | Identifier | 19.4 |
|  | Type Literal | 5.7 |
|  | Modifiers | 5.7 |
|  | Others | 4.8 |
|  | **Total** | **33.4** |
¹Types are from Eclipse JDT AST parser.

In the fine-tuning phase, we reuse the BFP [49] and VulRD [16]
dataset for bug repair and vulnerability repair, respectively. These
datasets contain both buggy and fixed code from real-world bugfixing commits. Subsequently, we generate data to fine-tune Mulpor
(Section 4.3). During the patch generation process, a buggy program and a set of test suites, inducing failures in the program, are
utilized to yield a list of suspicious code lines through fault localization approaches. We then use the fine-tuned Mulpor to employ
three generators and a beam search strategy to generate candidate
patches for these suspicious elements (Section 4.4). Afterwords, we
re-rank the candidate patches utilizing a heuristic to ensure that the
correct patch is prioritized at the top positions (Section 4.5). Finally,
Mulpor employs various strategies to validate the correctness of
candidate patches across different scenarios (Section 4.6).
4.1
Model Architecture
Mulpor consists of three generators at statement, expression and
token level. Following the T5 model [47], each generator uses an
encoder-decoder architecture. Both the encoder and decoder have
12 Transformer layers. In each layer, 12 attention heads are utilized for the multi-head attention computation, resulting in a total
parameter size of 220M. Such an architecture is widely-used by
state-of-the-art pre-trained models [6, 16, 36, 52]. As per existing
studies [16, 36, 51], we initialize the parameters of each generator
with the weights from CodeT5, with the aim to equip the model
with certain domain knowledge of programming languages.
4.2
Pre-Training Tasks
While existing pre-training techniques either focus on masked
token prediction [15, 21] or masked span prediction [28, 47, 52]
(typically less than 6 tokens), our study on the BFP dataset [49]
shows that bug fixing in real-world scenarios can happen at various
granularities. To effectively handle the diverse nature of real-world
bugs, our key idea is to generate patches from various granularities, thereby increasing the likelihood of producing the correct
patch compared to generating patches from a fixed granularity. To
that end, we design three pre-training tasks at the statement-level,
expression-level, and token-level to equip our approach with the
capacity to predict patches at the corresponding granularity. Note
that we opted to mask 15% of the elements in different granularities because it is a common practice adopted by previous studies [11, 15, 21, 52]. According to the study [56], this masking rate
provides sufficient context to learn satisfactory representations.
4.2.1 Statement-level mask prediction (SMP). In this pre-training
task, we delve into the realm of statement-level mask prediction,
a refinement of the traditional masked span prediction. Unlike its
predecessor, where code tokens or code spans were concealed (usually less than 6 tokens [47, 52]), statement-level mask prediction
operates at a coarser-grained level by obfuscating individual code
statements. To perform this pre-training task, we begin with the
input code that is augmented with statement-level masks. These
masks are strategically placed to target code statement rather than
randomly selected code span without integrity syntactic structure.
Specifically, to generate these masks, we utilize a well-regarded
AST parser, tree-sitter2 , to extract all statements from the input
code. Subsequently, a randomly selection process is implemented,
where 15% of these statements are masked, rendering them temporarily invisible to the model. The primary objective of this task
is to equip the model with a broader understanding of token distribution within masked statements while maintaining the contextual
coherence. Formally, the loss can be described as:
\mathcal{L}{SMP}(\theta)=\sum{i=1}^k-logP_\theta(s_i|s^{mask},s_{<i})
where 𝑠^{𝑚𝑎𝑠𝑘} is the masked statement, 𝑘 denotes the number of
code tokens in masked statement, and 𝑠 <𝑖 is the token sequence
predicted for the masked statement so far.
4.2.2 Expression-level mask prediction (EMP). In this pre-training
task, we extend our exploration to a finer granularity by introducing
EMP. This task builds upon the SMP by further refining the masking
process at the level of individual code expressions. Similar to SMP,
we leverage the tree-sitter parser to extract all expressions from the
input code. However, unlike SMP, EMP operates at the expressions
within statements and focuses on understanding and predicting
the masked expressions. Approximately 15% of these expressions
are randomly selected and masked, challenging the model to generate the correct expression based on the contextual information
provided. The objective of EMP is to enhance the model’s ability
to grasp the domain knowledge of expression-level syntax and
semantics. This finer granularity is expected to contribute to the
proficiency of model in generating accurate expression in program
repair. Formally, the loss can be described as:
L𝐸𝑀𝑃 (𝜃 ) =
𝑘
∑︁
−𝑙𝑜𝑔𝑃𝜃 (𝑒𝑖 |𝑒
𝑚𝑎𝑠𝑘
, 𝑒 <𝑖 )
𝑖=1
where 𝑒𝑚𝑎𝑠𝑘 is the masked expression, 𝑘 denotes the number of
code tokens in masked expression, and 𝑒 <𝑖 is the token sequence
predicted for the masked expression so far.
4.2.3 Token-level mask prediction (TMP). To generate patches at a
finer granularity, we introduce TMP as a pre-training task. While
SMP and EMP are at higher levels of abstraction, TMP focuses on
individual code tokens. This task aims to refine the model’s understanding of token-level syntax and semantics. To implement TMP,
we employ the tree-sitter to extract all tokens from the input code.
Subsequently, we randomly mask 15% of these tokens following
an existing study [15, 52]. This challenges the model to predict
the correct token in the context of code, fostering a more granular
understanding of code. By honing its ability to discern token-level
relationships, the model is better equipped to generate accurate
code during program repair. Formally, the loss can be described as:
L𝑇 𝑀𝑃 (𝜃 ) =
𝑘
∑︁
−𝑙𝑜𝑔𝑃𝜃 (𝑡𝑖 |𝑡 𝑚𝑎𝑠𝑘 , 𝑡 <𝑖 )
𝑖=1
where 𝑡 𝑚𝑎𝑠𝑘 is the masked input, 𝑘 denotes the number of code
tokens in masked input, and 𝑡 <𝑖 is the token sequence predicted
for the masked input so far.
4.3
Fine-Tuning
While the pre-training phase equips the model with the ability
to correct faulty code based on its context, it is noteworthy that
the pre-training dataset is not collected from real-world bug-fixing
commits. Also, the masked locations in pre-training tasks are chosen
randomly, not reflecting real-world fault locations. To bridge this
gap, we fine-tune the model on the dedicated dataset constructed
from real-world bug-fixing commits. This strategy aims to align
with the mask granularities employed in pre-training tasks, also
guiding the model to focus more on error-prone code snippets.
For instance, at the statement level, our pipeline mirrors the SMP
Buggy code
…… // context before
if (dataset == null) { // Buggy line
return result;
}
…… // context after
Statement level mask
…… // context before
<MASK>
…… // context after
…… // context before
<MASK>
if(dataset == null){
return result;
}
…… // context after
…… // context before
if(dataset == null){
<MASK>
return result;
}
…… // context after
Expression level mask
…… // context before
if(<MASK>){
return result;
}
…… // context after
…… // context before
if(dataset == null<MASK>){
return result;
}
…… // context after
…… // context before
if(<MASK> dataset == null){
return result;
}
…… // context after
…… // context before
if(dataset <MASK> null){
return result;
}
…… // context after
…… // context before
if(dataset == <MASK>){
return result;
}
…… // context after
Token level mask
…… // context before
if(<MASK> == null){
return result;
}
…… // context after
Figure 3: Mask generation at different granularities.
task, with a key distinction: instead of randomly selecting masked
statement, we exclusively mask the statements affected during the
code repair process. To achieve this, we employ GumTree [13] to
compare the buggy and corrected code, identifying the range of
code changes, and subsequently masking all statements within
this identified range. Thus Mulpor learns how to predict and fill in
the masked statements. This approach is extendable to expression
and token levels, where masking is applied to the expressions and
tokens within the identified range of code modifications.
4.4
Patch Generation
Given a buggy code program, fault localization techniques will
first return a list of suspicious code element, we then mask the
faulty line at three granularities with strategies. Considering that
fault localization is usually developed as an independent field and
existing APR techniques employ off-the-shelf fault localization tools
in the repair pipeline, we do not discuss the fault localization below
and reuse the GZoltar API [5] and Ochiai similarity coefficient [1],
which are commonly used in existing APR tools [26, 35, 55, 59, 64].
For perfect localization setting, we provide the location of buggy
lines directly following the previous studies [8, 37, 58, 63]. Figure 3
shows a mask generation example for Chart-1 bug in Defects4J at
three granularities: statement, expression and token-level.
Statement-level. At the statement level, we employ three strategies to generate the mask. The first is replacing the whole buggy
statement. We refer to this as statement replacement since we
query the Mulpor to generate a new statement directly to replace
the buggy statement. The rest two strategies is to generate mask
statement where we add <MASK> before/after the buggy line. These
represent bug fixes where a new statement is inserted before/after
the buggy location.
Expression-level. At the expression level, we employ the strategy of replacing all expressions in the faulty line with <MASK> for
model querying, a process referred to as expression update. Additionally, in the generation of the expression-level mask, we account
for insertions both before and after the erroneous expression by introducing <MASK> at the corresponding positions. It is noteworthy

that expression insertion is inherently synonymous with expression update. However, we emphasize the explicit specification of
the insertion location, as it provides the model with more guidance
(e.g., the insert location) and additional information (e.g., the old
expression). This approach is intended to enhance the likelihood of
generating the correct expressions.
Token-level. At the token level, we employ the strategy to restore
the correct token from a masked location. This strategy involves
replacing each token in the buggy line with <MASK>. The objective
is to address token-level repairs, wherein modifications can be
seamlessly mapped to the alteration of leaf nodes in the AST.
During inference, for each input generated by the aforementioned mask generation process, we employ beam search to generate
a ranked list of candidate patches, where the number of predictions
is determined by the beam size B (the setting of B is illustrated
in 4.7). At each decoding step, the beam search algorithm selects the
most B promising candidate patches with the highest probability
using a best-first search strategy until the terminator which notifying the end of sentence is emitted. For each input generated by
the corresponding strategies at three granularities, we generate B
patches, which are subsequently re-ranked and merged (as detailed
in Section 4.5) to form a final ranked list of candidate patches.
4.5
Patch Re-ranking
In the realm of APR, selecting and prioritizing candidate patches is
paramount to enhancing the efficiency of software maintenance. In
this pursuit, we introduce a patch ranking process to consolidate
the outcomes generated by three different generators and prioritizing the correct patch at the top of the list whenever possible. We are
inspired by prior research [31, 50], which establishes that more effective repairs often entail minimal code modifications. Additionally,
insights from previous learning-based APR approaches [34, 35, 42]
suggest a higher likelihood of correctness for patches positioned
closer to the front of the generated patch list. Based on such observations, our approach incorporates a mixed ranking strategy
to guide patch re-ranking. Specifically, each patch 𝑝 is assigned
a score tuple 𝑆𝑝 = (𝑅𝑝 , 𝐷𝑝 ), where 𝑅𝑝 denotes the rank of patch
within its respective generator, and 𝐷𝑝 represents the edit distance
between the original source code and the resulting patched code.
The final patch list is sorted in ascending order based on the scores
of the patches. In cases where multiple patches share the same 𝑅𝑝 ,
a secondary sorting is performed based on the second element 𝐷𝑝 .
4.6
Patch Validation
We validate patches by applying the corresponding changes to the
buggy code for each generated candidate patch. We then compile
each patched buggy code and filter out those that fail to compile.
We then run the test cases against each compiled patched buggy
code, seeking plausible patches that pass all the tests. We manually examine all plausible patches to ensure the bugs are fixed
correctly, confirming that the patches are semantically equivalent
to developer patches.
4.7
Implementation Details
Our model is implemented with PyTorch framework3 . All the experiments are performed on a server with 2 NVIDIA GeForce RTX 4090
GPUs. The learning rate and batch size in the pre-training stage are
set to 5e-5 and 32, respectively. When fine-tuning our Mulpor on
the downstream tasks, we use a batch size of 32 and a learning rate
of 2e-5, which are the same as we used during the pre-training stage.
Regarding the timeouts used for the repair, the learning-based APR
approaches [34, 58, 63, 64] typically set the running-time limit to
five hours for fixing each bug, while TBar [37] is set to three hours.
For a fair comparison, we set the running-time limit to the lowest
value used in the baselines, which is three hours. In the patch
inference phase, we configure the beam size to 15 to narrow the
search space at each granularity (discussed in Section 7.2). This
beam size is smaller than the values of 250 used in GAMMA [63]
and 1000 used in CURE [27] and CoCoNuT [40]. Note that unlike
existing learning-based approaches that predominantly focus on
a single repair granularity, Mulpor generates patches across multiple granularities. Consequently, Mulpor faces the challenge of
navigating a more extensive search space during patch generation,
potentially resulting in the generation of more invalid patches. To
mitigate this threat, we reduce the search space at each granularity
by decreasing the beam size during patch generation. Specifically,
while GAMMA generates an average of 475.8 patches for each bug
in the Defects4J-v1.2 dataset, our approach generates an average of
87.6 patches. This observation indicates that, although the overall
search space of Mulpor is larger than that of the tools operating
at a fixed granularity, Mulpor can more effectively locate correct
patches within its search space.
5 Study Design
5.1 Research Questions
In this paper, we seek to answer the following research questions:
RQ1: Compare with state-of-the-art APR approaches. How
does Mulpor perform in repairing general bugs in open source
projects compared with other APR approaches?
RQ2: Generalizability of Mulpor. How is the generalizability
of Mulpor in repairing software vulnerability?
RQ3: Ablation study. What are the contributions of the major
components of Mulpor?
5.2
Settings
5.2.1 Bug Repair. To answer RQ1, which evaluates the performance of Mulpor in general bug repair compared to other APR
approaches, we report the baselines and evaluation metrics.
Baselines. In our comparative analysis, we assess the performance of Mulpor compared to both traditional and learning-based
APR techniques. We choose nine recent Learning-based models including RAP-Gen [51], Repilot [53], AlphaRepair [58], Recoder [64],
CURE [27], CoCoNuT [40], RewardRepair [61], SelfAPR [60], and
GAMMA [63]. Additionally, to represent the traditional APR landscape, we incorporate two state-of-the-art template-based APR
tools, namely TBar [37] and PraPR [18].
3 https://pytorch.org/
Metrics. We compute how many bugs can be correctly fixed on
Defects4J based on unit testing and manually verification which
means the generated patch is semantically or syntactically equivalent to the developer patch by following the standard practice in
APR research. Specifically, we first run test suites to automatically
identify possible patches for each bug, and then manual checking
to completely verify its correctness. The all predictions of Mulpor
are included in our artifacts. We reuse the released results of baselines from the most recent work [51, 63] instead directly running
the APR tools following the common practice in the APR community [27, 40, 62, 63].
5.2.2 Vulnerability repair. To answer RQ2, which evaluates the generalizability of Mulpor in software vulnerability repair compared
to other vulnerability repair approaches, we report the selected
baselines and evaluation metrics.
Baselines. As baselines of our comparative analysis, we choose
four state-of-the-art learning-based vulnerability repair tools: VRepair [7], SeqTrans [9], VulRepair [16] and VulRepair+ [24]. VRepair
and SeqTrans are deep learning-based approaches that tackle insufficient training data issues in software vulnerability repair through
transfer learning. They undergo pre-training on a bug repair dataset
and subsequent fine-tuning on a vulnerability repair dataset. While
VRepair takes the code sequence as input, SeqTrans utilizes def-use
chains to construct code sequences, capturing syntax and structure
information around vulnerabilities with fewer noises. VulRepair,
on the other hand, is a T5-based automated software vulnerability
repair approach that employs a pre-trained model to address insufficient training data. It also utilizes the BPE algorithm to mitigate
the Out-Of-Vocabulary problem. Huang et al. [24] conducted an
empirical study, proposing modifications to the output format of
VulRepair and introducing the ensemble strategy by combining
multiple checkpoints. We have incorporated these enhancements
into our extended version and named it VulRepair+.
Metrics. In vulnerability repair, due to the lack of test cases in
the dataset, we evaluate the vulnerability repair performance of the
VRepair [7] and VulRepair [16] approaches using Perfect Prediction
(PP) accuracy following the previous studies [7, 9, 16]. PP measures
the percentage of vulnerable functions for which an approach can
generate vulnerability repairs that exactly match the ground-truth
data (i.e., syntactic equivalence), which is human-written repairs in
the VulRD dataset. Additionally, evaluating repair correctness is a
highly time-consuming process. Therefore, we aim to improve the
top-ranked repairs as much as possible. To measure the Perfect Prediction accuracy at different prediction levels, we utilize PP@Top1,
PP@Top-5, and PP@Top-10. These metrics represent the PP accuracy at the Top-1, Top-5, and Top-10 predictions respectively. For
all metrics, we present the results on a scale of 0-100 (%), where a
higher score indicates better performance.
5.3
Dataset
To assess the effectiveness of Mulpor, we first pre-train it using
multi-granularity pre-training tasks on CodeSearchNet [25] dataset,
and then fine-tune it on the training set of the BFP [49] dataset.
Defects4J is used to measure the effectiveness Mulpor in bug repair.
To evaluate it performance in vulnerability repair, we use the VulRD
Bo Lin, Shangwen Wang, Ming Wen, Liqian Chen, and Xiaoguang Mao
Table 2: Statistics of the datasets.
Dataset
CodeSearchNet
BFP
Defects4J-v1.2
Defects4J-v2.0
VulRD
Train
454,451
58,909
4,206
Valid
15,328
6,546
600
Test
395
430
1,202
dataset from VulRepair [16] for fine-tuning. We report their data
statistics in Table 2.
5.3.1 CodeSearchNet. CodeSearchNet [25] serves as a widely acknowledged pre-training dataset for code-related models [15, 52]
and stands as a pivotal resource in the realm of deep learning for
programming tasks. Comprising over 2 million data pairs from
six languages (Ruby, JavaScript, Go, Python, Java, and PHP), the
dataset is distributed in 80-10-10 proportions for training, validation, and testing. In our study, we exclusively utilize the training
set, meticulously excluding instances used in fine-tuning to prevent
data leakage and ensure the robustness of Mulpor training.
5.3.2 BFP. BFP [49] is a dataset for method-level bug repair, extracted from GitHub commits from 2011-2017 using GitHub Archive
and GumTree [13]. It contains 65,455 validated bug-fixes, split in a
90-10 ratio for training and validation as shown in Table 2.
5.3.3 Defects4J. Defects4J [29] is one of the most widely adopted
APR benchmarks, which contains 395 real bug-fix patches from 6
open source GitHub projects in version 1.2, and 835 real bug-fix
patches from 17 open source GitHub projects in version 2.0. Each
bug-fix example is accompanied with test cases to validate the fix.
5.3.4 VulRD. VulRD [16] merges the datasets Big-Vul [14] and
CVEfixes [4]. The Big-Vul dataset contains 3,754 C/C++ code vulnerabilities extracted from GitHub projects (2002-2019), while CVEfixes curates 5,495 vulnerabilities from the National Vulnerability
Database (2002-2021). Following the deduplication process, VulRD
comprises 6,008 unique vulnerabilities, distributed in a 70-10-20
ratio for training, validation, and testing respectively.
6 Study Result
6.1 Compare with state-of-the-art approaches
6.1.1 Experimental Design. In this section, we aim to evaluate the
performance of Mulpor on Defects4J [29]. Due to the previous
study [12] demonstrates that there exists a common benchmark
overfitting in APR evaluation, especially in Defects4J dataset some
tools perform better than other benchmarks. Therefore, we adopt
Defects4J-v1.2 and Defects4J-v2.0 as evaluation benchmark to mitigate the benchmark overfitting. We consider two settings with
the perfect fault localization (FL) and with the spectrum-based FL.
Following the prior works [18, 27, 37, 40, 58, 63], we use patch
correctness results gathered from previous papers [51, 58, 63] for
Defects4J-v1.2 and Defects4J-v2.0 evaluation. We only obtain repair
results under specific settings (either perfect or spectrum-based
fault localization) for a number of tools (i.e., PraPR, TBar, CURE,
Recoder, Repilot, AlphaRepair and GAMMA), and for such tools,
we obtained the replication packages or source codes from their
respective papers and then rerun them with the absent fault localization strategy. During the reproduction, we maintained the

Table 3: The performance of different approaches on the Defects4J v1.2 and v2.0 dataset (in number of correct repairs).
| Tool | Perfect FL v1.2 | v2.0 | Spectrum-based FL v1.2 | v2.0 |
|---|---|---|---|---| 
| PraPR† | - | - | 41 | 7 |
| TBar† | 68 | 8 | 43 | 6 |
| CoCoNuT | 43 | - | - | - |
| RewardRepair | 44 | 43 | 27 | 24 |
| CURE | 55 | 19 | 38 | 11 |
| SelfAPR | 63 | 45 | 39 | 28 |
| Recoder | 65 | 19 | 49 | 9 |
| Repilot | 66 | 50 | 47 | 30 |
| RAP-Gen | 72 | 53 | 48 | 26 |
| AlphaRepair | 74 | 36 | 50 | 20 |
| GAMMA | 82 | 45 | 47 | 20 |
| Mulpor | 92 | 59 | 56 | 31 |

Table 4: Comparison with state-of-the-art APR techniques with Perfect FL (in number of correct repairs).
Chart
11
7
5
10
9
10
6
9
9
11
13
Closure
16
9
12
14
19
21
22
22
23
24
26
Lang
13
7
7
9
10
11
15
12
13
16
19
Math
22
16
17
19
18
18
21
26
21
25
23
Mockito
3
4
3
4
5
2
0
2
5
3
7
Time
3
1
1
1
2
3
2
1
3
3
4
Total
68
44
45
57
63
65
66
72
74
82
92
same settings (e.g., the upper limit of the number of patches) as
in the original papers. Note that for CoCoNut, the authors did not
provide the trained model, and some training scripts are missing,
as reported by some researchers4 . In the case of PraPR, it currently
only supports spectrum-based FL, not perfect FL. To perform perfect FL with PraPR, we need to modify the source code. The author
provides the PraPR source code only for Maven projects, while
some projects in Defects4J-v1.2 are Gradle projects. Therefore, for
PraPR, we only replicate it under the spectrum-based FL setting in
Defects4J-v2.0. Unavailability values are indicated by “-” in Table 3.
6.1.2 Results. Table 3 represents the number of bugs that different
APR techniques successfully fix on the Defects4J-v1.2 and Defects4Jv2.0 with two FL settings. Overall, we find that Mulpor outperforms
compared APR techniques including both traditional and learningbased APR techniques in two FL settings. Specifically, it repairs 10
and 6 more bugs than best baselines in v1.2 and 2.0, respectively. For
the results with spectrum-based FL, Mulpor achieves the state-ofthe-art performance also, which repairs 6 and 3 more bugs compared
with AlphaRepair (50 v.s. 56) and SelfAPR (28 v.s. 31). Besides,
Mulpor fixes 13, 26, 19, 23, 7, 4 for Chart, Closure, Lang, Math,
Mockito, and Time projects, respectively. Five of these projects
exhibit the best performing, as shown in Table 4. Overall, both
results with or without perfect FL validate the superiority of our
Mulpor over both traditional and learning-based baselines.
4 https://github.com/lin-tan/CoCoNut-Artifact/issues/2
Table 5: The performance of different approaches on the VulRD
dataset (in %).
Approach
VRepair
SeqTran
VulRepair
VulRepair+
Mulpor
Spectrum-based FL
v1.2
v2.0
41
7
43
6
27
24
38
11
39
28
49
9
47
30
48
26
50
20
47
20
56
31
† PraPR and TBar are template-based tools.
“-” indicates data unavailability.
Tool
TBar
CoCoNuT
RewardRepair
CURE
SelfAPR
Recoder
Repilot
RAP-Gen
AlphaRepair
GAMMA
Mulpor
PP
12.3
20.9
18.1
22.5
34.2
PP@1
7.6
15.4
14.1
18.2
25.6
PP@5
11.2
19.4
17.0
21.7
31.7
PP@10
12.3
20.0
17.8
22.1
33.2
6.1.3 Overlap Analysis. To investigate to what extent Mulpor complements existing APR techniques, we further calculate the number
of overlapping bugs fixed by different techniques in Defects4J-v1.2.
We select one best-performing traditional technique (i.e., TBar) and
three best-performing learning-based techniques (i.e., GAMMA,
AlphaRepair, and RAP-Gen) including two cloze-style APR techniques(i.e., GAMMA and AlphaRepair). As shown in Fig. 4, we found
that Mulpor is able to fix the most
number of unique bugs of 13 that
other APR approaches fail to fix,
which is 9, 10, 6, and 4 more than
TBar, GAMMA, AlphaRepair, and
RAP-Gen, respectively. We analyzed the 13 unique bugs fixed by
Mulpor and found that the repair
granularity of 7 bugs is at the statement level, 4 bugs are at the expression level, and 2 bugs are at
the token level. This indicates that
Mulpor’s design can fix more bugs Figure 4: The overlaps of the bugs
through multi-granularity patch fixed by different approaches.
generation compared to the tools selected for overlap analysis.
Specifically, the selected tools, GAMMA, AlphaRepair, and RapGen,
are based on UniXcoder, CodeBERT, and CodeT5. These models are
pre-trained on a token-level mask language modeling task, which
may limit their ability to fix complex bugs. More importantly, compare with other two cloze-style APR techniques, i.e., GAMMA and
AlphaRepair, Mulpor can repair 25 and 40 unique bugs, respectively,
highlighting the benefits of pre-training model at different granularities. Overall, the result means that Mulpor can be integrated
with other techniques to further increase the number of correct
patches in Defects4J-v1.2 benchmark.
6.2
Generalizability of Mulpor
6.2.1 Experimental Design. We have demonstrated that Mulpor
achieves impressive performance to repair real-world bugs from
the widely-adopted Defects4J benchmark. In order to investigate
the Generalizability of Mulpor, we investigate the effectiveness of
Mulpor in a different scenario, i.e., vulnerability repair. We report
the comparison results with baselines on vulnerability repair in
Table 5. Due to there are duplicates in the original dataset and for
fair comparison, we rerun the baselines in the deduplicated VulRD
dataset. During the inference phase, we use beam search with a
beam size of 50 to maintain consistency with baselines.
6.2.2 Results. As shown in Table 5, Mulpor achieves state-of-theart performance under all metrics by repairing the largest set of
vulnerabilities. Specifically, compared with existing state-of-the-art
vulnerability repair approach (i.e., VulRepair+), Mulpor can repair
Bo Lin, Shangwen Wang, Ming Wen, Liqian Chen, and Xiaoguang Mao
more 52.0% vulnerabilities base on the generated repairs. We also
note that at the PP@1 metric, Mulpor achieves a score of 25.6%,
while VulRepair+ only achieve 18.2%. This indicates that Mulpor can
predict over 40.7% of correct patches at Top-1 prediction, making it
a more reliable tool for suggesting repairs to developers.
Table 7: The performance of Mulpor with different lengths of vulnerability (in PP).
Length
1-100
101-200
201-300
301-400
401-500
>501
Table 6: Mulpor’s effectiveness on the Top-10 most frequent CWEs.
CWE Type
CWE-119
CWE-125
CWE-20
CWE-264
CWE-476
CWE-200
CWE-416
CWE-190
CWE-787
CWE-399
Total
Name
Improper Restriction of Operations
within the Bounds of a Memory Buffer
Out-of-bounds Read
Improper Input Validation
Permissions, Privileges, and Access Controls
NULL Pointer Dereference
Exposure of Sensitive Information to
an Unauthorized Actor
Use After Free
Integer Overflow or Wraparound
Out-of-bounds Write
Resource Management Errors
PP
Proportion
29.9
92/308
25.0
26.7
46.0
40.9
28/112
28/105
23/50
18/44
37.2
16/43
35.7
34.2
34.4
41.9
31.9
15/42
15/38
11/32
11/31
257/805
6.2.3 Vulnerability Type Analysis. CWE (Common Weakness Enumeration) serves as a comprehensive catalog enumerating vulnerability weaknesses in software, highlighting potential security issues
with varying degrees of severity. This enumeration offers valuable
guidance to organizations and security analysts, aiding them in
fortifying their software systems against potential threats. To evaluate the practical implications of Mulpor in real-world scenarios,
we conducted a thorough investigation into the Top-10 most frequent CWEs, assessing its efficacy in repairing these commonly
occurring weaknesses. These vulnerabilities, representing the most
frequent occurrences in our dataset, pose significant risks due to
their inherent ease of discovery and exploitation. Exploitation of
these weaknesses could potentially empower adversaries to take
full control of a system, pilfer data, or disrupt an application’s
functionality. Table 6 presents an overview of the effectiveness
of Mulpor in addressing the Top-10 most frequent vulnerabilities.
Notably, Mulpor achieves Perfect Predictions at rates as high as
46.0% for CWE-264, 41.9% for CWE-399, and 40.9% for CWE-476.
However, its performance varies for other CWEs, such as CWE125 and CWE-20, where Mulpor can only repair 25.0% and 26.7%
of vulnerabilities, respectively. Further analysis reveals that the
median vulnerability lengths for CWE-125 and CWE-20 are 485
and 335, respectively. While Mulpor achieves a 46.0% repair rate for
CWE-264, it is important to note that the median length of vulnerabilities in this category is 211. This indicates a decrease in Mulpor’s
effectiveness as the length of the vulnerable program increases,
as discussed in Section 6.2.4, resulting in poorer performance for
CWE-20. It is noteworthy that Mulpor shows the ability to fix vulnerabilities perfectly at a rate of 31.9% for the Top-10 most frequent
CWEs, compared to 34.2% across the entire dataset. This highlights
the strong generalization capabilities of Mulpor, showcasing its
proficiency in addressing infrequent CWEs.
6.2.4 Vulnerability Length Analysis. While Mulpor demonstrates
proficiency in generating vulnerability repairs for a substantial
number of CWEs, it encounters challenges in accurately generating
repairs for a considerable number of vulnerable functions. To delve
deeper into this issue, we conducted an investigation to analyze
Mulpor’s performance concerning vulnerability length, including
†
Percentage
19.3
19.0
14.1
11.6
7.2
28.9
Avg.†
1.67
2.32
2.31
2.85
3.78
3.77
VulRepair+
41.3
25.3
25.8
20.3
23.9
11.7
Mulpor
47.2
38.2
40.8
31.7
21.8
20.2
Average number of buggy lines
Table 8: Results of the ablation study (in %).
Model
PP@1
PP
−SG
23.5
28.7
−EG
22.7
29.4
−TG
20.4
27.6
−RR
23.4
34.2
Mulpor
25.6
34.2
the number of buggy lines. Table 7 presents the performance of
Mulpor across different vulnerability lengths. Our findings reveal
that the effectiveness of Mulpor is contingent on the size and complexity of vulnerable functions. Specifically, for vulnerabilities with
fewer than 100 tokens, Mulpor achieves a PP of 41.3%. However, this
rate significantly decreases to 20.2% for vulnerabilities exceeding
500 tokens. Two primary reasons contribute to this decline: (1) As
vulnerability lengths increase, the average number of buggy lines
also rises, indicating an escalating difficulty in repair. (2) The window size of T5 architecture is limited to 512 tokens that constrains
the performance of Mulpor. For vulnerable functions surpassing
512 tokens, any extra tokens are truncated and left unprocessed,
resulting in a detrimental impact on the performance of Mulpor.
Additionally, we explore the performance of existing state-of-theart technique (i.e., VulRepair+) across varying vulnerability lengths.
Remarkably, vulnerability length exerted a more pronounced negative impact on VulRepair+ compared to Mulpor. Specifically, VulRepair+ can only repair 11.7% of vulnerabilities exceeding 500 tokens,
whereas Mulpor achieves a PP of 20.2%. This discrepancy highlights
the relative resilience of Mulpor in handling longer vulnerabilities
compared to existing techniques.
6.3
Ablation study
We have demonstrated that well-designed multi-granularity generator play a crucial role in enabling Mulpor to achieve state-of-the-art
performance across various APR scenarios. To further understand
the contribution of each component, we conducted an investigation
by creating four variants of Mulpor, each with one critical component removed, and subsequently evaluated their performance on
the similar downstream tasks. Note that we opted to conduct the
ablation study on vulnerability repair, as its dataset is larger than
that used for bug repair. This can reduce randomness and more
accurately reflect the contribution of each component.
Overall, the contributions of the statement-level generator (SG),
expression-level generator (EG), token-level generator (TG), and
patch re-ranking (RR) to the performance of Mulpor are evident,
as depicted in Table 8. Notably, the exclusion of the TG leads to
the most significant decrease in PP@1 and PP for Mulpor, by 20.3%
and 19.3%, respectively. This aligns with our expectations, as the
patch generation process involves sequential token generation. The
TMP task aids the token-level generator in comprehending token
distribution in code snippets, facilitating the generation of accurate

patches. The PP also experiences a noticeable decrease of 16.1%
when SG is excluded, highlighting that statement-level masking enhances the model’s understanding of code syntactics and semantics.
Furthermore, without the EG, the PP of Mulpor decreases by 14.0%.
This aligns with our expectations, as the granularity of expressions
lies between statements and tokens, allowing Mulpor to compensate for the absence of EG with contributions from other generators.
It is noteworthy that the decrease in PP@1 caused by SG is more
pronounced than that caused by EG. We attribute this to the larger
search space of SG compared to EG, making it more challenging
for SG to generate the correct patch at the top-1 compared to EG.
Additionally, the exclusion of the patch re-ranking process results
in an 8.6% decrease in PP@1, emphasizing the effectiveness of patch
re-ranking in enhancing the overall performance of Mulpor.
7 Discussion
7.1 Efficiency of Mulpor
Running on a single NVIDIA GeForce RTX 4090 GPU, Mulpor can
generate 15.5 patches per second. This means that Mulpor can
generate patches for a given snippet of buggy code in the Defects4Jv1.2 dataset in an average of 6 seconds. By comparison, the state-ofthe-art technique, GAMMA, can generate 20.7 patches per second,
but it produces more bugs than Mulpor and takes about 23 seconds
for each bug. Mulpor is faster since it introduces fewer patches
due to its more precise search space. As a result, the beam size of
Mulpor can be set to a value which is much smaller than that of
GAMMA (15 vs. 250).
7.2
Threats to Validity
Internal Threats. Our first internal threat comes from the manual validation of patch correctness. To mitigate the influence of
potential bias, we follow the previous studies [27, 58, 62, 63], first
three authors manually inspect all patches that pass all test cases.
A plausible patch is identified as a correct patch if first three authors identify it as semantically equivalent to a ground truth patch.
When evaluating the effectiveness of Mulpor for general repairs,
we only consider the first plausible patch after ranking, as done in
previous studies [37, 64]. To facilitate replication and verification
of our experiments, we have made the relevant materials publicly
available in our online repository.
The second internal threat involves potential data leakage from
pre-trained models. In our experiment, we initialize the parameters of Mulpor with weights from CodeT5, and then further train
it on three different pre-training tasks before evaluating it on a
downstream tasks. Considering the pre-training dataset includes
2.3M functions from CodeSearchNet, we first check for functions
in the pre-training dataset and then remove all duplicates before
the pre-training phase of Mulpor. After the pre-training phase of
Mulpor, we expect that the leaked knowledge from CodeT5 will be
somewhat forgotten. Besides, we manually perturb all 10 leaked
buggy codes (by changing variable names, adding dead code following the previous studies [58, 63]) in Defects4J-v1.2. We find
that Mulpor can still generate correct patches for all bugs. This
suggests that Mulpor is not merely overfitting to patches present
in the original CodeT5 training dataset. Thus, we are confident that
the data leakage is not a key point to our conclusion.
The third internal threat stems from the reuse of baseline results
from previous studies, specifically within the Defects4J benchmark.
This benchmark is a widely-used benchmark in the APR community, and many studies use it to assess the effectiveness of their
proposed approaches. Therefore, it is a common practice in the APR
community [27, 40, 51, 62, 63] by reusing the results of baselines on
Defects4J from previous studies. To ensure a fair comparison, we
thoroughly review the settings in the baselines and strive to maintain consistency. However, some settings do differ. For instance,
while most learning-based techniques [34, 58, 63, 64] have a 5-hour
time limit, TBar has a 3-hour limit. In such cases, we set the runningtime limit to the lowest value used in the baselines. Additionally,
the machines used in baselines also vary. In learning-based techniques, the performance of the machines only has limited impact on
the techniques’ performance. This is because the model weights in
these techniques are fixed, and only time efficiency may be affected.
For instance, when running on a single NVIDIA GeForce RTX 4090
GPU, the GAMMA takes about 23 seconds to generate patches for
each bug, while the running-time limit for each bug is five hours,
which means the inference procedure can be completely finished
within the time limit. On the other hand, different machines could
yield different results in template-based techniques due to the supported search space and time constraints. To mitigate potential bias,
we reran TBar in Defects4J-v2.0 and achieved the same results as
reported in previous studies [58, 60, 64]. Therefore, the impact of
different machines used in baselines is minimal.
External Threats. The primary external threats to validity arise
from the choice of evaluation benchmarks. The performance claims
made for Mulpor may not seamlessly extend to other datasets. To
mitigate this threat, we assess the generalizability of Mulpor by
conducting evaluations on Defects4J-v1.2 and Defects4J-v2.0. Furthermore, we evaluate our claims about the generalization to other
code repair scenarios by investigating the performance of Mulpor
in vulnerability repair. The results affirm that Mulpor exhibits generalizability across distinct datasets and scenarios.
8
CONCLUSION
This paper introduces Mulpor, which is a simple and effective multigranularity patch generation approach designed to address the
diverse nature of real-world software bugs. Mulpor initially acquires semantic and syntactic knowledge through unsupervised
pre-training tasks at different granularities. Subsequently, it undergoes fine-tuning on real-world bug-fixing datasets. This research
emphasizes the critical importance of generating patches from
multiple granularities for program repair techniques. By doing
so, Mulpor demonstrates promising results in achieving effective
bug-fixing performances across various program repair scenarios,
including bug repair and vulnerability repair. All data in this study
are publicly available at: https://zenodo.org/records/12660892.
Acknowledgments
This work is supported by the National Key R&D Program of China
(No. 2022YFA1005101), the National Natural Science Foundation
of China (No.62372193), and the Research Foundation from NUDT
(Grant No. ZK24-05).
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Received 2024-04-12; accepted 2024-07-03