These phases are crucial for understanding and analyzing the source code. The primary phases in the front end include:
1. Lexical Analysis (Scanning): Before constructing the AST, the source code is passed through a lexical analyzer (often generated by tools like Flex or Lex). This phase breaks the input code into tokens (e.g., keywords, identifiers, literals, operators) and discards comments and white spaces. The resulting stream of tokens is used as input for the next phase.
2. Syntax Analysis (Parsing): The stream of tokens from the lexical analysis phase is then processed by the parser (often generated by tools like Bison or Yacc). The parser checks that the token stream follows the syntactic rules of the language and constructs a parse tree or AST that represents the hierarchical structure of the code. If the code is syntactically incorrect, parsing errors are reported.
3. Semantic Analysis: After constructing the AST, the compiler performs semantic analysis. This phase checks for meaningfulness and consistency in the code. It involves type checking, symbol resolution, and other semantic checks. Errors such as type mismatches or undeclared variables are detected during this phase.
4. Intermediate Code Generation: In some compilers, an intermediate representation (IR) is generated after the AST has been constructed and analyzed. The IR is a platform-independent representation of the code that facilitates later optimizations and code generation. The IR may take the form of three-address code or another intermediate language.
5. Optimization: Compilers often perform a series of optimization passes on the intermediate representation. These optimizations aim to improve the code's performance or reduce its size. Examples of optimizations include constant folding, dead code elimination, and loop optimization.
6. Symbol Table Construction: During parsing and semantic analysis, the compiler constructs a symbol table to manage the program's symbols (e.g., variables, functions). The symbol table stores information about symbol names, types, scopes, and memory locations.
7. Error Handling: Throughout the front end, the compiler detects and reports errors. This includes lexical errors (e.g., malformed tokens), syntax errors (e.g., parse errors), and semantic errors (e.g., type errors). Error messages are generated to assist developers in identifying and fixing issues in their code.
These front-end phases collectively handle the initial stages of the compilation process, where the primary focus is on understanding and analyzing the source code to ensure that it is valid and meaningful. Once these phases are complete, the compiler can proceed to the back end, where code generation and other target-specific optimizations occur.