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## Python Symbolic Execution with LLM-powered Code Generation WENHAN WANG ∗ , University of Alberta, Canada AN RAN CHEN, University of Alberta, Canada KAIBO LIU ∗ , Peking University, China GE LI, Peking University, China GANG HUANG, Peking University, China ZHI JIN, Peking University, China LEI MA, The University of Tokyo, Japan and University of Alberta, Canada Symbolic execution is a key technology in software testing, which generates test cases by collecting symbolic path constraints and then solving constraints with SMT solvers. Symbolic execution has been proven helpful in generating high-coverage test cases, but its limitations, e.g., the difficulties in solving path constraints, prevent it from broader usage in software testing. Moreover, symbolic execution has encountered many difficulties when applied to dynamically typed languages like Python, because it is extremely challenging to translate the flexible Python grammar into rigid solvers. To overcome the main challenges of applying symbolic execution in Python, we proposed an LLMempowered agent, LLM-Sym, that automatically calls an SMT solver, Z3, to solve execution path constraints. Based on an introductory-level symbolic execution engine, our LLM agent can extend it to supporting programs with complex data type 'list'. The core contribution of LLM-Sym is translating complex Python path constraints into Z3 code. To enable accurate path-to-Z3 translation, we design a multiple-step code generation pipeline including type inference, retrieval and self-refine. Our experiments demonstrate that LLM-Sym is capable of solving path constraints on Leetcode problems with complicated control flows and list data structures, which is impossible for the backbone symbolic execution engine. Our approach paves the way for the combination of the generation ability of LLMs with the reasoning ability of symbolic solvers, and opens up new opportunities in LLM-augmented test case generation. CCS Concepts: · Software and its engineering → Empirical software validation . Additional Key Words and Phrases: Software testing, Large language models, Symbolic execution ## ACMReference Format: Wenhan Wang, Kaibo Liu, An Ran Chen, Ge Li, Zhi Jin, Gang Huang, and Lei Ma. 2018. Python Symbolic Execution with LLM-powered Code Generation. In Proceedings of Make sure to enter the correct conference title from your rights confirmation emai (Conference acronym 'XX). ACM, New York, NY, USA, 20 pages. https://doi.org/XXXXXXX.XXXXXXX ∗ Both authors contributed equally to this research. Authors' Contact Information: Wenhan Wang, wenhan12@ualberta.ca, University of Alberta, Canada; Kaibo Liu, Peking University, China, liukb@pku.edu.cn; An Ran Chen, University of Alberta, Canada, anran6@ualberta.ca; Ge Li, Peking University, China, lige@pku.edu.cn; Zhi Jin, Peking University, China, zhijin@pku.edu.cn; Gang Huang, Peking University, China, hg@pku.edu.cn; Lei Ma, The University of Tokyo, Japan and University of Alberta, Canada, ma.lei@acm.org. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. Conference acronym 'XX, June 03-05, 2018, Woodstock, NY © 2018 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 978-1-4503-XXXX-X/18/06 https://doi.org/XXXXXXX.XXXXXXX ## 1 Introduction Symbolic execution [1, 10] is a widely used program analysis technique which explores program execution paths with symbolic values. Together with constraint solving (with an SMT solver), symbolic execution can generate real variable values to satisfy a certain execution path. The key application of symbolic execution is software testing. Because symbolic execution can solve constraints for certain execution paths, we can use it to generate test cases that cover a specific execution path or expose hidden bugs. Symbolic execution has been successfully applied to various program languages, including Java [9, 13], LLVM IR [5] and binary code [26]. To address the above difficulties, we took the first step to perform a large language modelaugmented symbolic execution engine, LLM-Sym, for Python. LLM-Sym is an LLM agent that can automatically call the Z3 [8] SMT solver to solve path constraints in symbolic execution. On the basis of our LLMSym lies an introductory-level symbolic execution engine, The Fuzzing Book [38], which only supports variables with explicit type hints in int/float types. The basis engine consists of two components. The first one is a control flow graph (CFG) explorer for extracting execution paths. The second one is a program for translating the collected execution path constraints into Z3 code using a pre-defined set of rules. In our approach, we only adopt the CFG explorer in the engine for execution path collection. Instead of writing a program to translate Python constraints to Z3, we leverage an LLM to generate code for calling the Z3 solver to solve path constraints. This gives us more flexibility in solving some constraints that cannot be easily translated into Z3. During the generation of Z3 code, we propose a workflow with multiple-step generation and self-refinement to improve the correctness of the generated code. However, symbolic execution faces significant difficulties when applied to dynamically typed languages such as Python [4]. The major difficulty lies in mapping program execution paths to SMT solver constraints. First, without explicit type annotations, it can be difficult to define symbolic variables in the solver, where all symbolic variables must have their own data type. Second, existing symbolic execution engines often use rules to convert Python statements into SMT constraints [2, 38], but Python data structures are highly flexible. For example, a Python list can have an unfixed length, which is different from similar data structures in most other languages (e.g., array in C, Java). An obvious challenge this feature brings is that in Python programs, there are condition constraints on list lengths, and we may need to write extra SMT constraints to solve the length variable. To deal with these unique Python features, tremendous efforts may be required to design Python-to-SMT rules to support them. These unique difficulties result in Python not having a widely adopted symbolic execution engine. Most existing symbolic execution engines only support simple data types, such as integers and floats. Some developers tried to overcome these obstacles with concolic execution [25], which combines symbolic execution with concrete execution on real values. However, concolic execution is a dynamic method that requires a runnable code environment. However, in many practical applications, such a code environment is often difficult to configure. In general, LLM-Sym solves a CFG path constraint with the following steps: (1) Given a program under test, LLM-Sym first performs type inference to predict the type of variables inside the program. (2) For the execution path, we decompose it into chunks and generate the Z3 code with multiple steps. Before each generation step, LLM-Sym queries a path-to-Z3 knowledge base to acquire code generation templates. We manually build this knowledge base to enable the generation of Z3 code to solve constraints on Python lists. (3) After generating code for a path chunk, we execute the generated code to check whether it contains bugs. If the generated code is buggy, LLM-Sym tries to fix the code given the error message. If all fixes are unsuccessful, we regard this path as unsolvable and directly use an LLM solver to solve the constraints. (4) After LLM-Sym finishes the code generation for an execution path, the Z3 solver will try to solve the generated constraints, and LLM-Sym generates a final test case based on the solving results. All the code generation steps in LLM-Sym are fulfilled with LLMs, which minimizes the effort of writing complicated logic for Z3 code generation. To evaluate the LLMSym framework, we conduct experiments on a set of LeetCode problem solutions for unit test case generation. LeetCode programs contain plenty of control flows and list operations, which make them suitable for evaluating our symbolic execution engine. We build a dataset of 111 execution traces from 50 programs, and ask LLM-Sym to solve these traces and generate test cases that can satisfy them. In the experiments, most components in LLM-Sym is based on an efficient LLM GPT-4o-mini. In our experiments, we aim to answer the following research questions: - RQ1 : Can LLM-Sym generates correct Z3Py code to solve Python path constraints? - RQ3 : What are the strengths/weaknesses of the Z3 code generator and the LLM solver in solving path constraints? - RQ2 : How do each module and different settings of the Z3 code generator work when solving a path constraint? - RQ4 : How is the time and money cost of LLM-Sym? The results show that LLM-Sym can correctly solve around 60% of the given execution traces and outperforms pure LLM-base approaches, proving the potential of using LLM to further guide symbolic execution by generating code that calls SMT solvers. Moreover, our approach is also cost-effective: as an LLM agent, its cost is less than one-third of a stronger model, GPT-4o, while achieving similar results. To summarize, the main contributions of our paper are: - We propose LLM-Sym, the first LLM agent-based system that augments an existing symbolic execution engine to solve more complex constraints. In LLM-Sym, we put forward the idea of using LLM to call the Z3 solver for solving constraints in symbolic execution. This allows us to build LLM-Sym on a simple introductory-level symbolic execution engine without the laborious effort of writing lengthy code to translate path constraints into Z3. - We create a dataset from LeetCode programs execution paths and evaluate LLM-Sym using this dataset. We find that LLM-Sym is capable of generating correct Z3 code and test cases for long execution paths. - As far as we know, LLM-Sym is the first Python symbolic execution engine that supports Python 'list' and dynamic data types without concolic execution. It is also the first attempt to integrate LLMs with SMT solvers for test case generation. The remainder of this paper is organized as follows. Section 2 describes a motivating example. Section 3 describe the methodology of LLM-Sym in details. Section 4 presents the experimental evaluation and answer to the RQs. Section 5 is threats to validity. The related works in introduced in Section 6, and Section 7 concludes our work. ## 2 Motivating Example Figure 1 shows a motivating example for LLM-empowered symbolic execution. Figure 1 (a) (top) depicts a simple Python execution path with function definition, comparison operators, and variable assignments. In this execution path, all variables are explicitly annotated with a basic data type (int), and all operators are simple computational operators. In this case, we can easily generate the code which invokes Z3 to solve this path (Figure 1 (a) (bottom)). In this example, we follow the format defined in [38]: variables are transformed into the single static assignment (SSA) form to keep variable values before assignments. We can see that the translation from the execution path to Z3 is simple: we convert variables to SSA and then add every statement to the solver. This can Fig. 1. A motivating example of using LLMs to generate Z3 code for symbolic execution. LLMs are capable of generating complex code with non-trivial patterns (c). <details> <summary>Image 1 Details</summary>  ### Visual Description ## Code Snippets and Solvability ### Overview The image presents three code snippets, each labeled with its solvability status: "Solvable, trivial," "Unsolvable," and "Solvable, non-trivial." Each snippet consists of a short Python-like function definition followed by a series of operations, presumably for a solver. The "Unsolvable" snippet is incomplete, indicated by question marks. ### Components/Axes * **(a): Solvable, trivial** - Contains a function `gcd` and subsequent solver operations. * **(b): Unsolvable** - Contains a function `xxx` and an indication that the subsequent steps are unknown. * **(c): Solvable, non-trivial** - Contains a function `func` and subsequent solver operations. ### Detailed Analysis **(a): Solvable, trivial** * **Function Definition:** ```python 1. def gcd(a: int, b: int): 2. a < b 3. c: int = a 4. a = b ``` * **Solver Operations:** ```python _a_0 = Int('_a_0') _b_0 = Int('_b_0') solver.add(_a_0 < _b_0) _c_0 = Int('_c_0') solver.add(_c_0 == _a_0) _a_1 = Int('_a_1') solver.add(_a_1 == _b_0) ``` **(b): Unsolvable** * **Function Definition:** ```python 1. def xxx(obj1, obj2): 2. x = obj1.a 3. y, z = unk_func(obj2) 4. y + z == x ``` * **Solver Operations:** ``` ??? ``` **(c): Solvable, non-trivial** * **Function Definition:** ```python 1. def func(nums, x): 2. len(nums) > x 3. nums[1] = 1 4. nums[0] == abs(x) ``` * **Solver Operations:** ```python _nums_0 = Array('_nums_0', IntSort(), IntSort()) _nums_0_len = Int('_nums_0_len') solver.add(_nums_0_len >= 0) _x_0 = Int('_x_0') solver.add(_nums_0_len > _x_0) _nums_1 = Store(_nums_0, 1, 1) _nums_1_len = _nums_0_len solver.add(_nums_1[0] == If(_x_0 > 0, _x_0, -_x_0)) ``` ### Key Observations * Snippet (a) defines a function `gcd` (likely intended to be greatest common divisor, but the code is incorrect) and sets up constraints for a solver based on the function's logic. * Snippet (b) defines a function `xxx` but lacks the solver operations, indicating it's unsolvable in its current state. The function uses an unknown function `unk_func`. * Snippet (c) defines a function `func` that operates on a list `nums` and a variable `x`, and sets up constraints for a solver based on the function's logic, including array operations and conditional logic. ### Interpretation The image illustrates different scenarios of code solvability. The "Solvable, trivial" example shows a simple function with corresponding solver constraints. The "Unsolvable" example highlights a case where the solver operations are missing, making it impossible to determine a solution. The "Solvable, non-trivial" example demonstrates a more complex function with array operations and conditional logic, requiring more sophisticated solver techniques. The image suggests that the presence and complexity of solver operations are crucial factors in determining the solvability of a code snippet. </details> be implemented without extensive coding effort: The Fuzzing Book [38] used 76 lines of code to translate one line of execution path into Z3 code. It first uses abstract syntax tree (AST) rewriting to generate Python statements in SSA form, then translates the SSA statements into Z3-compatible form with simple rules. In addition to the two types of path constraints above, there is another type that lies in between: the constraints are theoretically solvable by a solver, but require non-trivial reasoning by domain experts. Figure 1 (c) (top) demonstrates an example execution path that contains list indexing, list assignments, and a simple built-in function (abs()). For these operations, designing rules to map them to Z3 expressions can be difficult and time-consuming. However, human developers with knowledge of Z3 can still write a program to solve these constraints in only a few lines of code (Figure 1 (c) (bottom)). Based on this observation, we propose utilizing LLMs to automatically generate Z3 code for such non-trivial constraints, which is one of the key contributions of LLM-Sym. However, traditional rule-based symbolic execution has its limitations. The major limitations include difficulties in handling complex data structures and external function calls. For instance, in Figure 1 (b) (top), there are user-defined data types and functions with external functions that cannot be interpreted as SMT-compatible constraints. As a result, it becomes infeasible to generate a Z3 code snippet to accurately describe the constraints for this execution path. Traditionally, researchers tend to overcome these situations by concolic execution [25], where test inputs are directly executed with concrete values. However, concolic execution also has its limitations. First, relying on concrete inputs might cause concolic execution to have difficulty reaching certain paths. Second, directly executing unverified test inputs may introduce undesired security risks [12]. To address these complex, non-linear constraints, a natural approach is to leverage large language models (LLMs) for solving. LLMs have demonstrated considerable capability in logical reasoning, including tasks such as solving mathematical problems [7] or logic puzzles [27]. Similarly, we can harness the reasoning ability of LLMs to solve path constraints in symbolic execution. Previous works have shown that LLMs are capable of calling the Z3 solver [19, 28, 36], but they mainly focus on solving simple logical/arithmetic problems with only a few lines of code. On the other hand, symbolic execution brings new challenges to Z3 code generation as it requires us to generate longer, more complicated Z3 code to solve lengthy execution paths with diverse data types and execution behaviors. Fig. 2. The overview of LLMSym workflow. <details> <summary>Image 2 Details</summary>  ### Visual Description ## System Diagram: Test Case Generation Workflow ### Overview The image is a system diagram illustrating a workflow for test case generation. It depicts the flow of information and processes involved, starting from a code snippet and culminating in the generation of test cases. The diagram includes components like type inference, Z3 code generation, and an LLM solver, showing how they interact to achieve the final output. ### Components/Axes * **Code snippet:** Represents the input code that needs to be tested. * **CFG (Control Flow Graph):** A graphical representation of the code's execution paths. * **Execution path:** The actual sequence of steps executed by the code. * **Type inference:** A process to determine the data types of variables in the code. * **Z3 code generator:** A component that generates code for the Z3 theorem prover. * **Test case generator:** A module responsible for creating test cases based on the generated Z3 code. * **LLM solver:** A Large Language Model used to solve constraints and generate test cases. * **Path-to-Z3 Knowledge base:** A database storing information for generating Z3 code. * **Test cases:** The final output, representing the generated test cases. ### Detailed Analysis or Content Details 1. **Code Snippet to CFG:** The process begins with a "Code snippet" which is then converted into a "CFG" (Control Flow Graph). A downward-pointing blue arrow indicates the flow. 2. **CFG to Execution Path:** The CFG is further processed into an "Execution path," again indicated by a downward-pointing blue arrow. 3. **Code Snippet to Type Inference:** The "Code snippet" also feeds into "Type inference," which determines "Variable types." 4. **Type Inference and CFG to Z3 Code Generator:** Both "Type inference" (specifically "Variable types") and the "CFG" are inputs to the "Z3 code generator." 5. **Z3 Code Generator Self-Refinement:** The "Z3 code generator" has a "Self-refine" loop, indicating an iterative process. 6. **Z3 Code Generator to Test Case Generator:** If "Generation succeed," the "Z3 code generator" feeds into the "Test case generator." The input is labeled "Z3 code". 7. **Test Case Generator to Test Cases:** The "Test case generator" produces "Test cases." 8. **Z3 Code Generator to LLM Solver:** If "Generation failed," the "Z3 code generator" feeds into the "LLM solver." 9. **LLM Solver to Test Cases:** The "LLM solver" also produces "Test cases." 10. **Path-to-Z3 Knowledge Base to Z3 Code Generator:** The "Path-to-Z3 Knowledge base" provides input to the "Z3 code generator" via "Retrieval." 11. **Test Case Generator to Test Case:** The "Test case generator" output loops back to the "Test cases" box. ### Key Observations * The diagram illustrates a multi-faceted approach to test case generation, utilizing both Z3 code generation and an LLM solver. * The Z3 code generator has a self-refinement loop, suggesting an iterative process to improve code generation. * The system uses a knowledge base to aid in Z3 code generation. * The LLM solver is used when Z3 code generation fails, providing an alternative approach. ### Interpretation The diagram outlines a sophisticated system for automated test case generation. It leverages static analysis (CFG, Type Inference), symbolic execution (Z3 code generator), and machine learning (LLM solver) to create test cases. The system attempts to generate test cases using the Z3 code generator first. If this fails, it falls back to an LLM solver, indicating a robustness strategy. The Path-to-Z3 Knowledge base suggests that the system learns from previous attempts to improve future test case generation. The self-refinement loop in the Z3 code generator indicates an optimization process to enhance the quality of the generated code. </details> ## 3 Approach Figure 2 depicts the overall workflow of our LLM-Sym. LLM-Sym is a system with two components: an execution path extractor, and an LLM agent which generates test cases based on execution paths. As demonstrated, for a given Python program under test, we first generate its control flow graph (CFG), then traverse the CFG to extract execution paths. After we extract an execution path, an LLM-powered Z3 code generator will translate this path into Z3 constraints. To enable the correct generation of Z3 code, we build a knowledge base with path-to-Z3 pairs and retrieve templates from this knowledge base to guide the code generator. If the initially generated Z3 code is buggy, the code generator will try to fix it using a self-refine mechanism. If the Z3 code for the complete execution path is generated successfully, then a test case generator LLM takes the solver's results as input and interprets it as a complete test case. If the Z3 code generation is unsuccessful, we directly use LLM to solve the path constraints and produce test cases. The main components of the LLMSys agent are based on a sub-optimal and cost-efficient LLM GPT-4o-mini, which helps in reducing the overall cost. ## 3.1 Execution path extractor The main body of the path extractor is built on the implementation in The Fuzzing Book 1 [38]. Basically, it first builds the program CFG from its abstract syntax tree (AST), then uses breadth-first search to extract all paths from the CFG. In order to successfully create execution paths that LLMs can comprehend, we made the following improvements to the original implementation: - In the CFG parsing module, we add support to class definitions so that We can build CFGs for Python functions that are located inside a class. - In the CFG, each node corresponds to a Python statement. We categorize all statements into 3 types. The first type is 'enter', where we encounter a function definition statement, and enter this function during execution. The second type is 'condition', where we encounter a - For 'for' loop statements, we add a number to the execution path node to denote which iteration of the loop the path has reached. 1 The code for path extraction can be found at https://www.fuzzingbook.org/html/SymbolicFuzzer.html branch/loop condition in a 'if'/'while'/'for' statement. The third type is 'expression': all other CFG nodes belong to this type, most of which are assignment statements. ``` ``` Fig. 3. An example of an extracted execution path (b) for the program (a). An example of an extracted execution path is shown in Figure 3. Every execution step consists of a line number, a statement type, and the statement itself. After the execution path is extracted, we split it into chunks before providing it to the Z3 code generator. This is because we find that the Z3 code generator is more likely to err when the input execution path sequence is long. In our default setting, we split the path by lines: each chunk only contains one statement in the execution path. We also adopt an alternative approach that splits the path by condition statement, and we will discuss this further in the follow-up sections. We need to be aware that our improvements do not mitigate the path extractor from its intrinsic weakness: the path explosion problem, as optimizing path extraction is not the main goal of our work. ## 3.2 LLM agent for constraint solving 3.2.1 Type predictor. One of the critical features of Python, different from many other program languages, is its dynamically typed variables. In Python, when we declare a new variable, we do not need to annotate its type explicitly. On the other hand, in Z3, symbolic variables must be assigned with a fixed type when initialized. In order to translate Python expressions to Z3 expressions, we first need to know the variable types in the execution path. So, before we start generating Z3 code, we use an LLM to predict the variable types in the program under test. Specifically, we ask the LLM to gather all variables and input arguments in the program, and predict their types. We use few-shot examples to provide relevant knowledge for type inference, and regulate the output format of the type predictor. Here, we made an approximate assumption: the type of a variable does not change during the execution, which can be applied to the majority of variables. Figure 4 shows the prompt template for our type predictor. You are given a Python code snippet, and you need to predict the type of all the variables in the code. If the type of a variable cannot be determined, you should choose the most likely type. If the type of a variable is a sequence/mapping type (e.g., list, dict), you should specify the type of its elements/keys and values. ``` ``` Fig. 4. The prompt template for the type predictor. 3.2.2 Z3 code generator. The core of our LLM agent in LLM-Sym is to translate extracted path constraints into Z3 code. For a given execution path, the Z3 code generator generates the Z3 constraints for the path in multiple steps. In each step, the code generator takes three pieces of information as input: an execution path chunk (ends with a condition statement), the variable types (generated by the type predictor), and the previously generated Z3 code (generated by the Z3 code generator in previous steps). A more difficult reasoning step is to accurately translate the constraints semantics in the Python statement into the Z3Py form. During practice, we find that LLMs often make syntax errors when generating Z3Py code, which is understandable since Z3 code rarely occurs in the training corpus of LLMs. Moreover, when we encounter complex data structures (e.g., Python 'list'), it is non-trivial to translate its operations and constraints into Z3Py. To improve the syntactical and semantical correctness of generated Z3 code, we leverage retrieval augmented generation techniques to provide expert knowledge to the code generator LLM. Specifically, we build a Python-to-Z3 knowledge base with execution paths and Z3Py code templates. In each template, we implement a Python operation that cannot be directly interpreted to Z3Py by LLM. One of the main targets of the knowledge base is to support Python operations on 'list'. In LLM-Sym, we approximate Python lists using 'Z3.Array', because the 'Array' data structure has an infinite length, which is suitable for Python lists which has an unfixed length. To represent the length of a Python list, we additionally define a 'Z3.Int' symbolic length variable for each list so that the Z3 solver can reason about Python list lengths. Currently, we support 4 types of list operations in our knowledge base: list initialization, list indexing (read list values), list assignments (write list values), and list append. Table 1 shows how we implemented these functionalities in Z3Py. Apart from list operations, we also include several other operations in our knowledge base, including general assignments, numerical computations, etc. In total, we design 14 Python-to-Z3 templates in the knowledge base for retrieving relevant information. Each template contains a full input-respond example of the Z3 code generator, which includes an execution path chunk (with one or several statements), an input SSA variable environment, with the target Z3Py code, and an updated variable environment. A complete example of a retrieval template is demonstrated in Figure 5. When the Z3 code generator tries to generate Z3 code for an execution path chunk, it first searches the Python-to-Z3 knowledge base, using the path chunk as the query. In the knowledge base, the execution path for each template is used as the key. All keys and queries are encoded to vectors using the Sentence-BERT [21] model, and we use the L2-norm as the distance metric for retrieval. For each retrieval query, we return the most similar templates, and these templates are used as the few-shot examples for Z3Py code generation. The complete prompt template for the Z3 code generator is shown in Figure 6. To translate a Python expression in the execution into the Z3 Python API (Z3Py) 2 , the Z3 code generator is required to perform two steps of reasoning: first, similar to [38], all variables should be transformed into SSA. This is not a difficult step since the knowledge of SSA is already processed by LLMs. We designed a few examples to demonstrate the variable naming specifications (e.g., the 2nd re-assignment of variable 'x' should be written as '\_x\_2' in our SSA setting). To further enhance the memory of SSA for the LLM, we introduce a variable environment to store the SSA indices for each variable. In each generation with the Z3 code generator, we give the current SSA variable environment to the LLM, and ask it to return the updated variable environment along with the generated Z3Py code. Even if we integrate multiple-step generation and retrieval templates, the Z3 code generator may still generate buggy code. When the Z3Py code for a chunk is generated, we first concatenate it with the previously generated code and then execute it. If the execution fails with an error, the code 2 https://z3prover.github.io/api/html/namespacez3py.html Table 1. Examples of translating Python list operations into Z3Py constraints for LLM-Sym. | Operation | Python execution path | Z3Py code example | |---------------------|----------------------------|--------------------------------------------------------------------------------------------------------------------------------------------| | List initialize | enter func(n1: List[int]): | _n1_0 = Array('_n1_0', IntSort(), IntSort()) _n1_0_len=Int('_n1_0_len') solver.add(_n1_0_len >= 0) | | List length | len(n)>5 | solver.add(_n_0_len) > 5 | | List indexing | lst[i] == j | solver.add(_lst_1[_i_0] == _j_0) | | List assignment | lst[i] = 2 | _lst_2 = Store(_lst_1, _i_0, 2) _lst_2_len = _lst_1_len # list length is not changed | | List append | n.append(x) | _n_1 = Store(_n_0, _n_0_len, _x_0) # n.append(x) _n_1_len = _n_0_len + 1 #increase length by 1 | | List pop | n.pop() | solver.add(_n_0_len > 0) #before pop(), check length _n_1 = _n_0 #Array values do not change _n_1_len = _n_0_len - 1 #decrease length by 1 | | List negative index | lst[-2] == z | solver.add(_lst_0[_lst_0_len - 2] == _z_2) solver.add(-2 >= -_lst_0_len) | ``` ``` Fig. 5. An example for the path-to-Z3 template in the retrieval knowledge base. generator tries to fix it with a self-relection mechanism. We input the buggy code and the error message into an LLM, and ask it to fix the code. In our experiments, we keep tying this self-repair process 3 times for each chunk. If all fixing attempts fail, we terminate the Z3 code generation and directly ask an LLM (the LLM solver in Figure 2) to solve the execution path constraints. - 3.2.3 Test case generator. After the Z3 code generator successfully generates the Z3Py code for a complete path, the test case generator module will generate the final test case based on the solving ``` ``` Fig. 6. The prompt template for the Z3 code generator. results of the generated Z3 constraints. The Z3 solver will output the values of all solved SSA variables, and the test case generator needs to analyze these values, find variables that are related to the test input, and build a test case from the value of these variables. Among all types of variables returned by the Z3 solver, the 'Z3.Array' type needs special consideration. The value of Z3 Array variables cannot be easily interpreted as a Python list: it is represented by first defining a constant array (e.g., K(Int, 1)), then applying 'Store' operations (e.g., Store(x, 0, 1) means set x[0] to 1) on it. For example, an Array 'Store(K(Int, 0), 2, 3)' can be transferred to a Python list as [0, 0, 3, 0, ...]. In LLM-Sym, with the Array value and the length value, the test case generator should be able to restore them as a Python list in a test input. For this module, we choose GPT-4o as the backend model because it shows significantly better performances in interpreting Z3 Arrays than GPT-4o-mini. Figure 7 demonstrates the prompt template used in the test case generator. ``` ``` Fig. 7. The prompt template for the test case generator. 3.2.4 LLM solver. When the Z3 code generator is incapable of generating Z3 code to solve path constraints, we directly seek the LLM to accomplish this task. In the LLM solver, the LLM is given the program under test and the execution path. The LLM solver is asked to solve these constraints directly, produce possible values for input arguments, and then generate a test case from the produced values. Similar to the Z3 code generator, when the LLM solver is asked to solve the constraints of an execution path, it also needs to determine whether this path can be satisfied. If the LLM solver predicts that a path cannot be satisfied, it will terminate generating the answer, allows us to explore other paths. As this module is not the key contribution of our LLM-Sym, we only design a simple prompting mechanism in our current implementation and leave the improvement of the LLM solver as future works. Figure 8 shows the prompt template of the LLM solver. Given an execution path for a Python program under test, you need to find a group of input arguments that can satisfy all the constraints in the execution path. Think step by step. After you find a group of valid input arguments, generate a test case for the Python program under test. I will provide some guidance for you to understand the execution path and generate the test case accordingly: 1. The execution path contains a list of statement that are executed in order. Each element in the execution path is a dictionary with the following keys/values: a line number, a statement type ('enter' for entering a function, 'condition' for a branch/loop condition, and 'expr' for other statements (such as assignments)), and an expression (the expression in the statement). 2. After you reason about the constraints in the execution path, you should explicitly generate a group of valid input arguments that satisfy the execution path. If all the constraints cannot be satisfied simultaneously, you should immediatly print "UNSAT" and terminate further reasoning. 3. Your generated test case should be a Python function formatted as: {Format template} You should generate the complete test function without any code outside the function. Now please solve the given execution path and generate the test case accordingly. Program under test: {code} Execution path: {path} Fig. 8. The prompt template for the LLM solver. Table 2. The number (rate) of passed paths solved by LLM-Sym in our LeetCode dataset. | SAT | Execution pass | Path correct | |------------|------------------|----------------| | 99 (89.2%) | 97 (87.4%) | 70 (63.1%) | ## 4 Experiments ## 4.1 Experiment settings In LLM-Sym, apart from the test case generator module, all other modules are powered by the cost-efficient LLM GPT-4o-mini. The test case generator is built upon GPT-4o. We implement the LLM-Sym agent using the AutoGen [33] library. ## 4.2 Dataset To evaluate the capability of LLM-Sym, we build a small-scale dataset from the online programming platform LeetCode. We select 50 problems from LeetCode, whose Python solutions 3 with up to 25 lines of code. These programs contain statements with multiple data types and control flows. For each problem, we run the example test cases in their problem description and collect the execution traces. This results in a total of 111 paths. We further truncate the traces with a max length of 20, and the average trace length in our dataset is 12.3. The collected execution traces do not contain information on branch selection, so we use the CFG path extractor in LLM-Sym to find the complete execution path corresponding to the execution trace. Our main task is to let LLM-Sym solve the constraints for these execution paths and generate test cases. After we obtain the generated test cases, we execute them to find whether the generated test case can cover the same path as the example test case. Due to the lack of explicit variable type annotations and the presence of Python list data structures, our backbone symbolic execution engine [38] can solve None of these traces. ## 4.3 RQ1: Capability of LLM-Sym Table 2 shows the results of LLM-Sym on our LeetCode dataset. We measure the number of generated results (test case or 'UNSAT') that can solve path constraints ('SAT"), produce runnable test cases, and correctly fulfill the given execution trace. From all 111 results generated from LLM-Sym, over 63% percent of them can produce the exact same execution trace as the given input. This shows the potential of using LLMs to generate code for solving symbolic path constraints. 3 The Python solutions are collected from a public GitHub repository https://github.com/walkccc/LeetCode. , Vol. 1, No. 1, Article . Publication date: September 2018. Fig. 9. An example of the Z3Py code and test case correctly generated by LLM-Sym. Due to the path length, we only display a part of it. <details> <summary>Image 3 Details</summary>  ### Visual Description ## Program Analysis Diagram: Sort Colors ### Overview The image presents a program analysis of a `sortColors` function within a `Solution` class. It shows the code, type inference results, execution paths with associated expressions, generated Z3 code (likely for symbolic execution or verification), final execution results, and a generated test case. The analysis traces the execution flow and variable assignments within the function. ### Components/Axes * **Program Under Test:** Python code snippet defining the `Solution` class and the `sortColors` method. * **Type Inference Result:** Specifies the data types of variables `nums` (List[int]), `l` (int), `r` (int), and `i` (int). * **Path:** A series of dictionaries, each representing a line of code executed, its type (e.g., 'enter', 'expr', 'condition'), and the expression evaluated. * **Retrieved Examples:** Lists of examples related to the code path, such as 'list_pop', 'assign', 'list_negid', etc. * **Generated z3 code:** Code generated for symbolic execution, including variable declarations and constraints. * **Updated Env:** The updated environment showing the current values of variables. * **Final Execution Result:** The final values of variables after execution. * **Generated Test Case:** A test case to verify the correctness of the `sortColors` function. * **Arrows:** Arrows indicate the flow of execution and data transformation. ### Detailed Analysis **1. Program Under Test:** ```python class Solution: def sortColors(self, nums: list[int]) -> None: # ... (implementation details not fully shown) ``` **2. Type Inference Result:** * `nums`: `List[int]` * `l`: `int` * `r`: `int` * `i`: `int` **3. Execution Paths and Z3 Code Generation:** * **Path 1 (Line 2):** Entering the `sortColors` function. * `Path`: `[{'line': 2, 'type': 'enter', 'expression': 'enter: sortColors(nums)'}]` * `Retrieved examples`: `['list_pop', 'assign']` * `Generated z3 code`: ``` _nums_0 = Array('_nums_0', IntSort(), IntSort()) _nums_0_len = Int('_nums_0_len') solver.add(_nums_0_len >= 0) ``` * `Updated env`: `{nums: _nums_0}` * **Path 2 (Line 3):** Assignment `l = 0`. * `Path`: `[{'line': 3, 'type': 'expr', 'expression': 'l = 0'}]` * `Retrieved examples`: `['assign', 'list_negid']` * `Generated z3 code`: Not explicitly shown in the image. * **Path 3 (Line 4):** Assignment `r = len(nums) - 1`. * `Path`: `[{'line': 4, 'type': 'expr', 'expression': 'r = (len(nums) - 1)'}]` * `Retrieved examples`: `['list_append', 'list_initconst']` * `Generated z3 code`: `_r_0 = _nums_0_len - 1` * `Updated env`: `{nums: _nums_0, l: _l_0, r: _r_0}` * **Path 4 (Line 6):** Assignment `i = 0`. * `Path`: `[{'line': 6, 'type': 'expr', 'expression': 'i = 0'}]` * `Retrieved examples`: `['assign', 'list_assign']` * `Generated z3 code`: `_i_0 = Int('_i_0'); solver.add(_i_0 == 0)` * `Updated env`: `{nums: _nums_0, l: _l_0, r: _r_0, i: _i_0}` * **Path 5 (Line 7):** Condition `i <= r`. * `Path`: `[{'line': 7, 'type': 'condition', 'expression': '(i <= r)'}]` * `Retrieved examples`: `['mod', 'list_negid']` * `Generated z3 code`: `solver.add(_i_0 <= _r_0)` * `Updated env`: `{nums: _nums_0, l: _l_0, r: _r_0, i: _i_0}` * **Path 6 (Line 8):** Condition `Not(nums[i] == 0)`. * `Path`: `[{'line': 8, 'type': 'condition', 'expression': 'Not(nums[i] == 0)'}]` * `Retrieved examples`: `['for_inlist', 'logic']` * `Generated z3 code`: `solver.add(Not(_nums_0[_i_0] == 0))` * `Updated env`: `{nums: _nums_0, l: _l_0, r: _r_0, i: _i_0}` * **Path 7 (Line 12):** Condition `Not(nums[i] == 1)`. * `Path`: `[{'line': 12, 'type': 'condition', 'expression': 'Not(nums[i] == 1)'}]` * `Retrieved examples`: `['logic', 'for_inlist']` * `Generated z3 code`: `solver.add(Not(_nums_0[_i_0] == 1))` * `Updated env`: `{nums: _nums_0, l: _l_0, r: _r_0, i: _i_0}` * **Path 8 (Line 17):** Swap `nums[i]` and `nums[r]`. * `Path`: `[{'line': 17, 'type': 'expr', 'expression': '(nums[i], nums[r]) = (nums[r], nums[i])'}]` * `Retrieved examples`: `['list_assign', 'list_append']` * `Generated z3 code`: `_nums_1 = Store(Store(_nums_0, _i_0, _nums_0[_r_0]), _r_0, _nums_0[_i_0])` **4. Final Execution Result:** ``` Code output: [_r_1 = 1, _nums_0 = Store(K(Int, 0), 0, 3), _i_1 = 1, _l_1 = 1, _i_0 = 0, _l_0 = 0, _nums_0_len = 3] ``` **5. Generated Test Case:** ```python def test_program(): solution = Solution() nums = [3, 0, 0] result = solution.sortColors(nums) assert nums == [0, 0, 3] ``` ### Key Observations * The analysis focuses on the initial steps of the `sortColors` function, likely a partial trace. * Z3 code is generated to represent the program's state and constraints, enabling symbolic execution and verification. * The test case demonstrates the expected behavior of the `sortColors` function: sorting an array containing 0s, 1s, and 2s (implicitly). ### Interpretation The diagram illustrates the process of analyzing a Python function using symbolic execution and test case generation. The `sortColors` function, which is intended to sort an array of integers (likely representing colors 0, 1, and 2), is analyzed step-by-step. The Z3 code represents the program's state in a form suitable for automated reasoning, allowing for verification of program properties and generation of test cases. The final execution result and the generated test case provide concrete examples of the function's behavior. The analysis helps to understand the function's logic and ensure its correctness. </details> Figure 9 shows an example of a success generation by LLM-Sym. We can see that during the generation process, the type predictor first correctly generates variable types for all variables in the program. In generating Z3Py code for each Python statement, the retriever first retrieves relevant templates from the knowledge base, which is extremely important for non-trivial operations. For example, in line 12, the retriever successfully returned the 'list assignment' template that helped generate the correct Z3Py statement. Meanwhile, for simple operations, the retrieval results are not so important. For example, in lines 7 and 8, the retriever returns irrelevant results, but the code generator can still generate the correct Z3Py code. An interesting finding emerges from line 12: the LLM code generator correctly generates the Z3Py code for an advanced list operation. It implements 'swap list elements" with two Array assignment operations, while similar patterns do not exist in our template knowledge base. This suggests that the LLM code generator can understand the semantics of path-to-Z3 templates (in this case, the 'list assignment' template) and flexibly compose the basic operations in these templates to implement more complex operations. From this example, we can see that the LLM code generator shows strong capability in generating Z3Py code with the retrieval and memory mechanism designed by ours. Fig. 10. An example of an incorrect generation of LLM-Sym. <details> <summary>Image 4 Details</summary>  ### Visual Description ## Program Analysis Diagram ### Overview The image presents a program analysis diagram. It shows the execution path of a Python function `removeDuplicates` along with type inference results and generated z3 code for each path. The diagram highlights the program's logic and how it's translated into constraints for a solver. The final path leads to an "UNSAT" result, indicating a potential issue or inconsistency in the program's logic. ### Components/Axes * **Program Under Test:** * `class Solution:` * `def removeDuplicates(self, nums: list[int]) -> int:` * **Type Inference Result:** * `nums: List[int]` * `i: int` * `num: int` * **Paths:** Each path is represented as a dictionary containing: * `line`: The line number in the original code. * `type`: The type of statement (e.g., 'enter', 'expr', 'condition'). * `expression`: The expression being evaluated. * `retrieved examples`: Examples related to the path. * **Generated z3 code:** The corresponding z3 code generated for each path. * **UNSAT:** Indicates that the constraints generated for a particular path are unsatisfiable. ### Detailed Analysis The diagram traces the execution of the `removeDuplicates` function. Each path shows the line number, type of statement, expression, and corresponding z3 code. * **Path 1 (Line 2):** * Type: 'enter' * Expression: 'enter: removeDuplicates(nums)' * Retrieved examples: `['list_pop', 'list_append']` * Generated z3 code: * `_nums_0 = Array('_nums_0', IntSort(), IntSort())` * `_nums_0_len = Int('_nums_0_len')` * `solver.add(_nums_0_len >= 0)` * **Path 2 (Line 3):** * Type: 'expr' * Expression: 'i = 0' * Retrieved examples: `['assign', 'list_negid']` * Generated z3 code: * `_i_0 = Int('_i_0')` * `solver.add(_i_0 == 0)` * **Path 3 (Line 5):** * Type: 'condition' * Expression: 'for num in nums the 1-th iteration' * Retrieved examples: `['for_inlist', 'for']` * Generated z3 code: * `_num_0 = Int('_num_0')` * `solver.add(_num_0 == _nums_0[0])` * **Path 4 (Line 6):** * Type: 'condition' * Expression: '((i < 2) or (num != nums[(i - 2)]))' * Retrieved examples: `['for_inlist', 'list_negid']` * Generated z3 code: * `solver.add(or(_i_0 < 2, _num_0 != _nums_0[_i_0 - 2]))` * **Path 5 (Line 7):** * Type: 'expr' * Expression: 'nums[i] = num' * Retrieved examples: `['list_append', 'list_pop']` * Generated z3 code: * `solver.add(_i_0 >= 0)` * `solver.add(_i_0 < _nums_0_len)` * `_nums_1 = Store(_nums_0, _i_0, _num_0)` * **Path 6 (Line 8):** * Type: 'expr' * Expression: 'i += 1' * Retrieved examples: `['assign', 'list_assign']` * Generated z3 code: * `_i_1 = Int('_i_1')` * `solver.add(_i_1 == _i_0 + 1)` * **Path 7 (Line 5):** * Type: 'condition' * Expression: 'for num in nums the 2-th iteration' * Retrieved examples: `['for_inlist', 'for']` * Generated z3 code: * `_num_1 = Int('_num_1')` * `solver.add(_num_1 == _nums_1[1])` * **Path 8 (Line 6):** * Type: 'condition' * Expression: '((i < 2) or (num != nums[(i - 2)]))' * Retrieved examples: `['for_inlist', 'list_negid']` * Generated z3 code: * `solver.add(or(_i_1 < 2, _num_0 != _nums_1[_i_1 - 2]))` * **Final Result:** The last path leads to "UNSAT", indicating that the constraints generated for this path are unsatisfiable. ### Key Observations * The diagram shows the step-by-step translation of the Python code into z3 constraints. * The "UNSAT" result suggests a potential issue in the program's logic or in the way the constraints are being generated. ### Interpretation The diagram provides a detailed view of how the `removeDuplicates` function is analyzed and translated into z3 constraints. The "UNSAT" result indicates that the solver could not find a solution that satisfies all the constraints generated for the program. This could be due to a bug in the program's logic, an incorrect translation of the code into z3 constraints, or limitations in the solver's ability to handle the constraints. Further investigation is needed to identify the root cause of the "UNSAT" result and fix the underlying issue. </details> Figure 10 shows a failed example of LLM-Sym. This failed generation finally lead to the generate constraints cannot be satisfied. In this example, the Z3 code generator generates correct Z3Py code until it reaches the 8th statement in the execution path: '(i < 2) or (num != nums[(i - 2)])'. Table 3. The number of passed paths solved by LLM-Sym with different numbers of templates returned by the retriever. | No. of templates | SAT | Execution pass | Path correct | |--------------------|-------|------------------|----------------| | 1 | 91 | 87 | 54 | | 2 | 97 | 94 | 65 | | 3 | 100 | 100 | 62 | | 4 | 90 | 90 | 61 | | 5 | 95 | 93 | 62 | Table 4. The recall@k of the LLM-Sym retriever in retrieving relevant templates. | Recall@1 | Recall@2 | Recall@5 | |------------|------------|------------| | 55.8% | 64.2% | 81.5% | Before this statement, the Python variable 'num' corresppond to the Z3Py SSA variable '\_num\_1', and this statement do not make new assignments to 'num', so the Z3 code generator should use variable '\_num\_1' in its generated code. However, the LLM failed to generate the correct SSA variable reference, and use '\_num\_0' instead. This will cause the generated Z3 constraints differ from the given path constraints, and eventually, in this case, fail to solve the given path when it is actually satisfiable. Answer to RQ1: LLM-Sym can generate correct Z3Py code to solve path constraints for symbolic execution. For non-trivial list operations, our retrieval mechanism allows the LLM to acquire knowledge on translating Python constraints to Z3 and generate the correct Z3Py code thereafter. However, sometimes the Z3 code generator still make mistakes, such as generating incorrect SSA variables. ## 4.4 RQ2: Submodule analysis To study the effectiveness of each module in LLM-Sym, we first investigate the impact of different settings of the retriever. Table 3 shows the results of LLM-Sym on our LeetCode dataset under different number of retrieved templates. We find that when the retriever returns two templates, LLM-Sym has the highest pass rate for execution paths, and increasing the number of retrieved samples will cause a slight drop in the passed paths. We believe that only returning one template may miss the key relevant template, while returning too many templates may mislead the LLM into generating incorrect code. We perform a case study to demonstrate how the self-refine module fixes the generated code. Figure 11 shows an example of a successful fix by our self-refine mechanism. For the input Python We further study the efficacy of the retriever by measuring their recall@k in retrieving relevant templates. The results are shown in Table 4. In this table, we exclude path constraint statements with only a simple condition (because the LLM can correctly translate them to Z3Py without external templates). On average, for over half of statements in our dataset, the retriever can return the relevant template as the most correlated retrieval result. This indicates the strong capability of the retriever. Nevertheless, there is still room for improvement in the retrieving module. Even if we return 5 retrieval templates, there are still 19% of the path constraint statements that do not successfully retrieve a relevant template. statement, the Z3 code generator first converts it into Z3Py without changing any operators. However, the Python floor division operator '//' is not supported by Z3, so when we execute the generated code, it returns a type error. The self-refine module receives the buggy code with the error message and successfully generates the fix. In the fixed code, the operator '//' is replaced by '/' because, in Z3Py, the '/' division operation on integer variables is naturally equivalent to the '//' operation in Python: it rounds the division result down to the nearest integer. <details> <summary>Image 5 Details</summary>  ### Visual Description ## Diagram: Code Generation and Error Correction ### Overview The image depicts a code generation and error correction process. It shows the initial code, the generated z3 code, an error message resulting from the execution of the z3 code, and the corrected code. Arrows indicate the flow of the process. ### Components/Axes * **Path:** `[{'line': 7, 'type': 'expr', 'expression': 'm = ((1+r) // 2)'}]` - This indicates the origin of the code, specifying line 7 and the expression `m = ((1+r) // 2)`. * **Generated z3 code:** This section shows the code generated by z3. * **Error message:** This section displays the error message encountered during execution. * **Fixed code:** This section shows the corrected code. * **Arrows:** Red arrows indicate the flow of the process. A black arrow indicates execution. ### Detailed Analysis or ### Content Details 1. **Path:** * `line`: 7 * `type`: `expr` * `expression`: `m = ((1+r) // 2)` 2. **Generated z3 code:** * `_m_2 = Int('_m_2')` * `solver.add(_m_2 == (_1_2 + _r_1) // 2)` 3. **Error message:** * `execute` (indicated by a black arrow pointing right) * `solver.add(_m_2 == (_1_2 + _r_1) // 2)` * `TypeError: unsupported operand type(s) for //: 'ArithRef' and 'int'` * A tilde line `~~~~~~~~~~~~~~>>~~` is drawn above the error message. 4. **Fixed code:** * `_m_2 = Int('_m_2')` * `solver.add(_m_2 == (_1_2 + _r_1) / 2)` ### Key Observations * The initial expression involves integer division (`//`). * The generated z3 code also uses integer division (`//`). * The error message indicates an unsupported operand type for integer division between `ArithRef` and `int`. * The fixed code replaces integer division (`//`) with standard division (`/`). ### Interpretation The diagram illustrates a scenario where the initial code containing integer division leads to a `TypeError` during execution with z3. The error arises because the z3 solver does not support integer division between `ArithRef` and `int`. The fix involves replacing the integer division operator with standard division, which resolves the type incompatibility and allows the code to execute successfully. This highlights the importance of considering type compatibility when generating code for specific solvers or environments. </details> (a) Path: [{'line': 13, 'type': 'expr', 'expression': 'nxt += (str(count) + ans[i])'}] Fig. 11. An example of the self-refine module successfully fixes a bug in generated Z3Py code. Path: [{'line': 7, 'type': 'expr', 'expression': 'm = ((l + r) // 2)'}] Generated z3 code: \_nxt\_1 = String('\_nxt\_1') solver.add(\_nxt\_1 == \_nxt\_0 + (String(str(\_count\_0)) + \_ans\_0[\_i\_0])) execute Error message: solver.add(\_nxt\_1 == \_nxt\_0 + (String(str(\_count\_0)) + \_ans\_0[\_i\_0])) z3.z3types.Z3Exception: Non-sequence passed as a sequence 3 attempts \_nxt\_1 = String('\_nxt\_1') solver.add(\_nxt\_1 == Concat(\_nxt\_0, Concat(String(str(\_count\_0)), Substring(\_ans\_0, \_i\_0, 1)))) NameError: name 'Substring' is not defined execute Figure 12 shows a failed example of the self-refine module in LLM-Sym. The input Python code converts an integer to a Python string, although such operation is theoretically supported by Z3 4 , its Z3Py API is rarely used in practice. From the example we can see that the LLM do not have the knowledge for converting an integer to a string, so it tries to generate different non-existing APIs in the three attempts, and all attempt are unsuccessful. We can conclude that when using the LLM to generate Z3Py code for non-trivial operations that are seldom seen in its training corpus, it is extremely important to provide high-quality examples to supplement it with new knowledge. Generated z3 code: \_m\_2 = Int('\_m\_2') solver.add(\_m\_2 == (\_l\_2 + \_r\_1) // 2) Error message: solver.add(\_m\_2 == (\_l\_2 + \_r\_1) // 2) ~~~~~~~~~~~~~~^^~~ TypeError: unsupported operand type(s) for //: 'ArithRef' and 'int' execute Fixed code: \_m\_2 = Int('\_m\_2') solver.add(\_m\_2 == (\_l\_2 + \_r\_1) / 2) (a) ``` ``` Fig. 12. An example of the self-refine module fails in fixing a bug in generated Z3Py code. We further compare two path chunking strategies: chunk by line (our default setting) and chunk by condition statements (discussed in Section 3.1). Table 5 shows the results of chunking by condition statements under different retrieval settings. We can observe that in all settings, chunking by condition results in fewer solved paths than chunking by line. This difference in performance is predictable because when we ask the LLM to generate Z3Py constraints for multiple Python statements, it is harder than generating for a single statement, and the LLM is more likely to make mistakes. 4 See https://microsoft.github.io/z3guide/docs/theories/Strings/#strfrom\_int-i-strto\_int-s---convert-to-and-from-nonnegative-integers for examples. , Vol. 1, No. 1, Article . Publication date: September 2018. Table 5. The number of passed paths solved by LLM-Sym with path inputs split by condition statements (and the differences to the results of chunking by line). | No. of templates | SAT | Execution pass | Path correct | |--------------------|-------|------------------|----------------| | 1 | 93 | 91 | 48(-7) | | 2 | 83 | 80 | 49(-16) | | 3 | 84 | 82 | 48(-14) | | 4 | 82 | 79 | 45(-16) | | 5 | 82 | 79 | 45(-17) | Table 6. The number and pass rates for the Z3 code generator (Z3) and the LLM solver (LLM) in solving path constraints. Notice that the sum of the solutions generated by the Z3 code generator and the LLM solver is not equal to the dataset size. This is because we do not count UNSAT generation results in the table. | No. of templates | No. generated by Z3 | No. generated by LLM | Z3 pass rate | LLM pass rate | |--------------------|-----------------------|------------------------|----------------|-----------------| | 1 | 66 | 25 | 63.6% | 48.0% | | 2 | 74 | 23 | 73.0% | 47.8% | | 3 | 76 | 24 | 69.7% | 37.5% | | 4 | 71 | 19 | 70.4% | 57.9% | | 5 | 66 | 29 | 68.2% | 58.6% | Answer to RQ2: Both the retrieval and self-refine modules play a key role in improving the correctness of the Z3 code generator. For the retriever, different numbers of retrieved examples can affect the performance of the LLM code generator. Meanwhile, adding more templates in the retrieval knowledge base can allow the code generator to support more complex operations. Considering the results on recall@k, the retriever is generally powerful but can be further improved in the future. As for the self-refine module, it can fix some of the bugs in the generated Z3Py code, but if the input Python statement is beyond the ability of the code generator itself, then the self-refine module is unlikely to fix the buggy Z3Py code. For the path chunking strategies, split by lines achieves better results than split by condition statements. ## 4.5 RQ3: comparison between Z3 code generator and LLM solver LLM-Sym adopts two approaches in solving Python path constraints. The first one is using the Z3 code generator to generate Z3Py code for solving. The second one is to use an LLM for solving when the Z3 code generator fails. It is important to analyze the performances of both components and find whether the Z3 code generator can outperform the vanilla LLM solver with basic prompts. Table 6 compares the Z3 code generator with the LLM solver in the number and pass rate of generated path solutions. For all retrieval settings, the majority of solutions are generated by the Z3 code generator, and the pass rate of solutions generated from Z3 is significantly higher than those generated by the LLM solver. For example, when the number of retrieved templates is set to 2, the Z3 code generator generates 74 solutions, which is more than three times of the solutions generated by the LLM solver. Besides, the Z3 code generator reaches a high pass rate of 73%. This shows the potential of building a symbolic execution engine fully powered by LLM to solve the constraints that are within the ability of Z3. Table 7. The results of only using the LLM solver on our dataset. | Approach | SAT | Execution pass | Path correct | Pass rate (Z3) | Pass rate (LLM) | |-------------|-------|------------------|----------------|------------------|-------------------| | gpt-4o-mini | 110 | 108 | 55 | 55.4% | 39.1% | | gpt-4o | 110 | 110 | 71 | 64.9% | 52.2 % | | LLM-Sym | 97 | 94 | 65 | 73.0 % | 47.8% | Table 8. The total cost (and cost per sample) for path constraints solving in our dataset with LLM-Sym and different LLMs. The time metrics are shown in seconds, and the money costs are shown in US dollars. | Approach | Time | Money | |--------------------|-------------|---------------| | LLM-Sym | 2241 (20.2) | 0.61 (0.005) | | gpt-4o-mini gpt-4o | 495 (4.5) | 0.04 (0.0004) | | | 1189 (10.7) | 1.96 (0.018) | We also perform an ablation to evaluate the performance of the Z3 code generator: we compare LLM-Sym with a baseline in which we use the LLM solver to solve all paths. This baseline can be seen as a variant of SymPrompt [22], where the execution path is used in prompts for test case generation. The main difference is that our baseline uses execution paths extracted from CFGs instead of ASTs in SymPrompt. Table 7 displays the results of the LLM solver on our dataset. We evaluate two proprietary models, GPT-4o-mini and GPT-4o, for the LLM solver. We record the number of successfully solved paths and the pass rate of the paths that are generated by Z3 code generator/LLM solver in LLM-Sym. We find that although directly using the LLM for constraint solving can produce more executable test cases, its successfully solved paths are less than LLM-Sym. Although GPT-4o has a higher overall pass rate than LLM-Sym, its pass rate on the subset of Z3-solved paths is still lower than LLM-Sym. Moreover, our Z3 code generator is based on the weaker and more efficient GPT-4o-mini. This indicates that using LLMs to call the SMT solver can free the LLM from complex constraint solving, and achieve better pass rates. Answer to RQ3: The Z3 code generator can generate Z3Py code that better solves path constraints than only relying on the LLM. Meanwhile, for constraints that are difficult to solve with Z3, the LLM solver is a complementation with strong potential. ## 4.6 RQ4: Time and cost efficiency The LLM-Sym is an LLM agent, and its core, the Z3 code generator, requires multiple-step generation with self-refine. So, it is important to measure its efficiency. We compare LLM-Sym with only using the LLM solver (without the Z3 code generator) in time and money costs, and the results are shown in Table 8. In this table, we record the total time of LLM-Sym in solving all 111 path constraints in our dataset and the total cost of using the OpenAI API. On average, LLM-Sym solves a single path with 20.2s, and the cost is only 0.005$. With a complicated line-by-line generation pipeline, its time overhead is less than twice that of the GPT-4o solver, and the money cost is only one-third of that of GPT-4o. While achieving similar results to GPT-4o, the price of our LLM-Sym is significantly lower, and the time efficiency is still acceptable. Answer to RQ4: Although the Z3 code generation pipeline is complicated, the time overhead of LLM-Sym is not much longer than the basic GPT-4o. The money cost of LLM-Sym is significantly lower than GPT-4o, due to the low cost of the backbone LLM GPT-4o-mini. ## 5 Threats to Validity ## 5.1 Model validity The proposed approach LLM-Sym is based on LLMs GPT-4o-mini and GPT-4o, while other models are not considered. However, we should be aware that these two models are widely used stateof-the-art LLMs, so the experiment results are representative, and the choice of these models is reasonable. Another threat is related to the capability of LLM-Sym. Currently, our approach only supports a part of the Python 'list' operation and does not explicitly support other data structures, which may be inadequate for real-world complex programs. However, because LLM-Sym supports non-trivial operations by providing Z3Py examples in the knowledge base, we can add support for other operations by adding new examples into the knowledge base. Also, the ability of the key module in LLM-Sym: the Z3 code generator, is limited by the abilities of the Z3 solver. To mitigate this threat, we introduced an LLM solver to solve constraints that Z3 cannot solve. Improvements on the LLM solver for stronger reasoning abilities are left as future work. ## 5.2 Data validity In this paper, we have only evaluated LLM-Sym on a subset of LeetCode problem solutions. As the first step of integrating LLMs with an SMT solver for symbolic execution, we should first evaluate simple, restricted programs and understand the behavior of LLM-Sym on these data before we move on to more complicated, real-world data. ## 6 Related Work ## 6.1 Symbolic execution Symbolic execution was first proposed to test whether a piece of software can violate certain properties [1]. Symbolic execution has been successful in many tasks, such as test case generation [10], inferring program invariants [14], and vulnerability detection [31]. However, symbolic execution is known to suffer from many limitations, including path explosion and difficulties in solving non-linear constraints or external API calls. To address these limitations, one of the most adopted techniques is concolic execution [25]. Instead of only executing with symbolic values, concolic execution executes the program with concrete values, collects path constraints, and explores other paths by negating constraints. Currently, many successful symbolic execution engines are based on concolic execution, including KLEE [5], SPF [20], and Triton [23]. Most symbolic execution engines use SMT solvers, such as Z3 [8], to solve path constraints. Although existing symbolic execution engines have enabled symbolic analysis in many languages (e.g., C, Java, LLVM), few of them can support Python. We assume that the lack of Python symbolic execution engines is due to the unique features of Python: dynamic typing and data structures with flexible operations. Although several previous works [2-4] tried to perform symbolic execution on Python, none of them have successfully transferred into established, well-maintained tools. For example, the Python symbolic execution tool PyExZ3 [2] only supports simple data types (no support for 'list') and has been deprecated. The most popular Python symbolic execution engine, CrossHair 5 (with 900+ Github stars), is maintained by independent developers. We did not compare 5 https://github.com/pschanely/CrossHair with CrossHair in our paper because it uses concolic execution and does not support pure-symbolic constraint solving. Another limitation of CrossHair is that it requires that the input arguments in the function under test must have explicit type annotations. ## 6.2 LLM for test case generation Recently, software testing researchers have applied LLM for test case generation in different domains [29]. One of the most discussed domains in LLM-aided software testing is unit test case generation [6, 11, 22, 24, 37], which aims to generate test cases separately for individual software components. For example, ChatUniTest [6] and ChatTester [37] aim to leverage Openai GPT models to generate correct, runnable test cases for Java methods, while TrickyBugs [15], TestEval [30] and SWT-Bench [17] built benchmark datasets to facilitate researches on generating bug-triggering or high-coverage test cases. There have also been some recent empirical studies [18, 35] which discussed the correctness, coverage, and bug-finding ability of LLM-generated test cases with different prompt designs. Symprompt [22] tried to combine LLM-based test case generation with symbolic analysis by using program execution paths to prompt LLMs. Although the overall idea of Symprompt resembles symbolic execution, it did not make additional efforts to improve the LLM's ability to solve path constraints. Instead, the execution path is provided to the LLM in one single prompt, which may hinder the LLM from solving the constraints with complex reasoning. There are also other works that tried to combine LLMs with traditional program analysis tools or software testing techniques for test case generation. For example, CodaMosa [11] improves test coverage by integrating LLMs with Pynguin [16], a search-based software testing tool. When Pynguin gets stuck in a coverage plateau, CodeMosa calls an LLM to generate new test case seeds so that Pynguin can generate new mutants on this seed and further improve test coverage. TELPA [34] leverages control flow graph analysis to extract method invocation sequences for guiding LLM-generated test cases to enter hard-to-cover branches. HITS [32] applies program slicing on Java methods under test and improves test coverage by generating test cases for different slices. ## 7 Conclusion This paper proposes LLM-Sym, a symbolic execution engine prototype which is powered by an LLM agent. It improves the solving ability of a basic symbolic execution engine by leveraging LLMs to call an SMT solver Z3 for constraint solving. Our proposed code generation approach allows LLM-Sym to solve constraints on complex data types, e.g., Python lists, using the Z3 solver. If Z3 cannot solve the constraint, it turns to LLMs for solving. LLM-Sym enables Z3 code generation for complex Python statements with a meticulously designed retrieval knowledge base and a self-refine mechanism. In this paper, we focus on supporting Python lists when building the knowledge base. Our experiments show that LLM-Sym is capable of generating correct Z3 code for long path constraints in LeetCode programs. Moreover, we find that integrating a cost-efficient LLM with Z3 can beat the most state-of-the-art LLM in constraint solving for Python symbolic execution. ## References - [1] Roberto Baldoni, Emilio Coppa, Daniele Cono D'elia, Camil Demetrescu, and Irene Finocchi. 2018. 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