25_Compiler Front-end Design

# Compiler Front-end Design

The front-end of the compiler is responsible for understanding the syntax and semantics of the source code. It includes the following stages: lexical analysis, syntax analysis, semantic analysis, and intermediate code generation.

## Lexical Analysis

The lexical analyzer (or lexer) splits the source code into a sequence of tokens. These tokens are the smallest meaningful units of the code, such as keywords, identifiers, operators, and punctuation.

Consider the following C++ code:

```c++
int main() {
  int number = 10;
  return 0;
}
```
The lexer would tokenize it into: `int`, `main`, `(`, `)`, `{`, `int`, `number`, `=`, `10`, `;`, `return`, `0`, `;`, `}`.

## Syntax Analysis

The syntax analyzer (or parser) takes the tokens produced by the lexer and organizes them into a parse tree or abstract syntax tree (AST). The AST represents the grammatical structure of the code.

For the C++ code above, a simplified version of its AST might look like this:

```
FunctionDefinition
  - FunctionDeclarator: "main"
  - CompoundStatement
    - Declaration
      - Declarator: "number"
      - Initializer: "10"
    - ReturnStatement: "0"
```

## Semantic Analysis

The semantic analyzer checks the AST for semantic errors, such as type mismatches or undefined variables. It also performs some necessary transformations on the AST for later stages of the compiler.

For example, if the C++ code tried to assign a string to `number`:

```c++
int number = "ten";
```

The semantic analyzer would report a type mismatch error.

## Intermediate Code Generation

The final stage of the front-end is to translate the AST into an intermediate representation (IR) that the back-end can work with. The IR is a lower-level representation of the code that is easier to optimize and translate into machine code.

For the C++ code, a simple IR might look like this:

```
define i32 @main() {
entry:
  %number = alloca i32
  store i32 10, i32* %number
  ret i32 0
}
```

This is a basic overview of compiler front-end design. Each stage involves complex algorithms and data structures, and there is a lot of room for optimization and customization.