A two-pass compiler is a type of compiler that analyzes and translates the source code into target code in two passes. In the first pass, the compiler builds a symbol table, which is a data structure that stores information about the identifiers in the program, such as their types and scopes. The compiler also performs type checking in the first pass, which ensures that the program is well-typed. In the second pass, the compiler generates the target code, which is typically machine code or bytecode.
Front End & Back End Phases in Compiler
In a two-pass compiler, the front-end and back-end stages are generally divided across the two passes, each focusing on different aspects of the compilation process.
First Pass: Front-End
The first pass, often referred to as the front-end, is primarily responsible for the analysis of the source code.
- Lexical Analysis: This phase breaks down the source code into tokens or lexemes.
- Syntax Analysis: It involves parsing the structure of the code to ensure it adheres to the grammar of the programming language.
- Semantic Analysis: The front-end performs checks on the code to identify any semantic errors, type checking, and building of data structures like symbol tables. It essentially determines the meaning of the code and builds an intermediate representation (IR) that represents the code’s logic and structure.
- Intermediate Code Generation: Based on the AST and other information collected during the analysis phases, the compiler creates an intermediate representation that’s often closer to the target machine code but still abstracted from machine-specific details.
Second Pass: Back-End
The second pass, or the back-end, focuses on the synthesis or generation of the target output (e.g., machine code or an intermediary form).
- Optimization: This phase involves various transformations and optimizations to enhance the efficiency and performance of the code. The information gathered during the front-end analysis is utilized to optimize the code in this phase.
- Code Generation: The back-end generates the actual target code based on the optimized intermediate representation produced by the front-end. This could involve creating assembly code or machine code that can be executed by the target hardware.
Advantages & Disadvantages of Two Pass Compiler
Advantages of a two-pass compiler:
- It can optimize the code better, because it has a complete understanding of the program.
- It can support more complex languages, because it does not have to worry about forward declarations.
- It is more modular, because each pass can be implemented independently.
Disadvantages of a two-pass compiler:
- It is slower than a one-pass compiler, because it has to scan the source code twice.
- It requires more memory, because it has to build and maintain a symbol table.
Overall, two-pass compilers are a good choice for compiling high-level languages, such as C and Java. These languages are often complex and require type checking and optimization.
Examples of two-pass compilers
- Pascal P-Code compiler
- COBOL compiler
Two-pass compilers are also used in some embedded systems, such as the ARM Compiler and the MSP430 Compiler.