Phases of a Compiler: What You Need To Know

As you may imagine, a lot happens when you press the compile button on your favorite IDE (integrated development environment) or when you run the command on the terminal to compile your code. Behind the scenes, compilers are hard at work, translating the code you write into machine language that your device can understand and execute. But how exactly do compilers do this? What are the different compiler phases involved throughout the process? 

Such are the questions every new programming or software development student ponders. We’ll be exploring the phases of a compiler in this post, from lexical analysis to code generation, so you can gain a deeper understanding of this essential process of software development. 

Main Phases of a Compiler

Compilers typically have two main phases: the analysis phase and the synthesis phase. In the analysis phase, the compiler reads and analyzes the source code to identify its structure and semantics. In the synthesis phase, the compiler generates machine code from the analyzed source code.

To perform these tasks, compilers are often divided into two modules – the front-end and the back-end. The front-end module handles the analysis phase and is responsible for analyzing the source code’s syntax and semantics. The back-end module handles the synthesis phase and is responsible for generating the machine code.

All the six phases of a compiler that we’ll look at are split between the two modules of the compiler. The front-end module handles the first three phases (lexical analysis, syntax analysis, and semantic analysis), while the back-end module handles the last three phases (intermediate code generation, code optimization, and code generation).

With the basics out of the way, let’s now dig deeper into the workings of each of the six phases.

Lexical Analysis

The first step in the analysis phase is lexical analysis. It’s the phase where the compiler scans the source code and breaks it down into tokens, like chopping up the code into bite-sized pieces for easier analysis. 

Tokens can be words or symbols and they have a specific meaning in the programming language. They are the basic building blocks of the code, such as keywords, identifiers, and operators. For example, in the C programming language, the semicolon (;) is a token that denotes the end of a statement.

The lexical analyzer scans the source code character by character and groups them into tokens based on predefined rules. For example, if the lexical analyzer encounters the characters “int x = 5;”, it would group them into the tokens “int”, “x”, “=”, “5”, and “;”.

Lexical analysis also performs some initial checks, such as removing comments and whitespace and identifying any invalid characters or keywords.

Syntax Analysis

After lexical analysis, the next step is syntax analysis. Here, the compiler checks for grammar and syntax errors while creating parse trees in readiness for the parsing process. Parse trees represent the structure of the code and show how the different tokens in the code are related to each other.

The syntax analyzer checks if the tokens generated by the lexical analyzer conform to the grammar rules of the programming language. If it encounters a syntax error, it generates an error message and stops the compilation process, and provides feedback to the programmer.

Syntax analysis also identifies any syntax errors in the code and provides feedback to the developer. For example, if a developer forgets a semicolon at the end of a statement, the compiler will flag this as a syntax error and notify the developer. If there are no syntax errors, it goes on to create a parse tree that represents the structure of the source code.

Semantic Analysis

Semantic analysis is the third phase and it is all about checking for semantic errors and generating an intermediate representation of the source program. By semantic errors, we mean those errors that occur when the meaning of the source code is incorrect, even though the syntax is correct.

The intermediate representation is called an annotated syntax tree. It is a data structure that represents the code in a more abstract form than the original source code. The semantic analyzer checks the parse tree generated by the syntax analyzer for semantic errors. 

It also checks for undeclared variables and performs type checking. The latter ensures that the types of variables and expressions used in the source code are compatible with each other.

Intermediate Code Generation

After ensuring that no semantic errors exist, the compiler generates an intermediate representation of the source program. The code generator uses this intermediate code representation in the next phase. Intermediate code is a low-level representation of the source program that is easier to analyze, optimize, and manipulate than the source code itself.

The intermediate code generator first takes the parse tree previously generated by the semantic analyzer. It then uses it to generate the intermediate code. There are different types of intermediate codes, such as three-address codes (TAC), which represent each instruction using at most three operands. Postfix notation is another common form of intermediate code, which represents expressions in a way that is easier to evaluate using a stack-based algorithm.

After generating the intermediate code, the compiler can further optimize it to enhance the efficiency and performance of the resulting machine code. 

Code Optimization

The purpose of code optimization is to improve the performance of the machine code generated by the compiler. It’s more or less like like fine-tuning the code to make it run faster and more efficiently. 

Code optimization is thus an important phase of the compiler since it could significantly improve the performance of the compiled code. The optimization process can range from simple techniques, such as constant folding and algebraic simplification, to complex algorithms that restructure the entire program.

During the code optimization phase, the compiler analyzes the intermediate code to identify patterns and redundancies to minimize the number of instructions and eliminate redundant operations. The compiler then applies transformations to the intermediate code to improve its performance while preserving the program’s functionality. The optimizations performed by the compiler will vary based on the target platform and the type of program under compilation.

For example, if the program contains a loop that iterates over an array, the compiler can optimize the loop by unrolling it. This means that it generates code that performs multiple iterations of the loop in a single step. The compiler can also perform constant folding, which evaluates expressions with constant values at compile-time instead of run-time. Overall, code optimization can reduce the overhead of the code and improve the performance of the program.

Code Generation

The final phase of the compiler is code generation. Here the compiler translates the optimized intermediate code into machine language. During the code generation phase, the compiler generates machine instructions that correspond to the intermediate code. 

The code generation process involves allocating memory, managing registers, and generating assembly code. The code generation phase of the compiler produces the executable code that the target platform can run, ensuring that it runs as efficiently as possible.

Phases of a Compiler: Wrapping Up

The process of compiling source code into machine language is a fundamental step in the creation of software applications. While compilers may seem like complex tools, they actually aren’t in the real sense. Breaking down the compilation process into its individual phases of the compiler can help us understand it easily. From lexical analysis to code generation, each phase plays a critical role in ensuring that the final executable code is efficient and free from errors.

There are many different types of compilers, each with its own unique features and capabilities. Some compilers are designed for specific programming languages, while others are optimized for particular hardware architectures. For instance, the GCC (GNU Compiler Collection) is a compiler system for several programming languages, including C, C++, and Fortran. 

Other languages such as Java and Swift have their respective compilers. The Java compiler compiles Java source code into bytecode. Bytecode can run on any platform that has a Java Virtual Machine. Swift, a relatively new programming language developed by Apple, has its own compiler. It converts Swift code into optimized machine code that can run on iOS, macOS, and other Apple platforms.

All in all, by understanding the various phases of a compiler, you can gain a deeper appreciation of how it works and how you could probably write more efficient code that would result in fewer compilation errors.