How does a compiler front-end analyze source code?

A compiler front-end analyses source code by converting it into an intermediate representation for further processing.

The compiler front-end is the first phase of the compilation process, and it's responsible for understanding the syntax and semantics of the source code. It performs lexical analysis, syntax analysis, semantic analysis, and some parts of optimisation. The output of this phase is an intermediate representation (IR) of the source code, which is used by the compiler back-end for further processing.

The first step in the front-end process is lexical analysis, also known as scanning. The lexical analyser reads the source code character by character and groups them into meaningful sequences called lexemes. Each lexeme corresponds to a token, which is a pair consisting of a token name and an optional attribute value. For example, a token could be a keyword, an identifier, a constant, or a symbol.

Next comes syntax analysis, or parsing. The parser takes the tokens produced by the lexical analyser and arranges them in a way that represents the grammatical structure of the program. This structure is usually a parse tree or a syntax tree. The parser checks if the expressions in the program are formed correctly according to the rules of the programming language.

Semantic analysis is the third step. The semantic analyser uses the syntax tree and the symbol table (a data structure created by the compiler that contains information about identifiers) to check the source code for semantic consistency with the language definition. It checks for type mismatches, undeclared variables, and other errors that are not related to the syntax of the program.

Some compilers also perform a part of optimisation in the front-end phase. This can include constant folding (replacing constant expressions with their values), dead code elimination (removing code that does not affect the program's output), and other optimisations that can be performed without knowing the target architecture.

The output of the compiler front-end is an intermediate representation of the source code. This IR is a lower-level representation of the program that retains all the information needed to produce the final machine code. The IR is passed to the compiler back-end, which performs further optimisations and generates the final machine code.