Question

What are the main phases of a compiler?

Answer 1

A compiler is a program that translates source code written in one programming language (the source language) into another language (the target language), which is often machine code that a computer's CPU can execute directly.

This translation process is complex and is broken down into a series of distinct stages or phases. Each phase takes the output of the previous phase as its input and produces an intermediate representation of the source program.

The main phases of a compiler are typically grouped into two major parts: the Front End and the Back End.

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The Two Major Parts

1. Front End (Analysis): This part analyzes the source code to check for errors and builds an intermediate representation of the code. It is dependent on the source language but independent of the target machine.
2. Back End (Synthesis): This part takes the intermediate representation from the front end and generates the target code. It is dependent on the target machine but independent of the source language.

This separation allows for great modularity. For example, you can build a compiler that supports multiple languages and multiple target machines by creating a front end for each language and a back end for each machine, and then mixing and matching them (e.g., the LLVM compiler infrastructure).

Here are the main phases in their logical order:

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The Front End Phases

1. Lexical Analysis (or Scanning)

This is the first phase of the compiler. It reads the source code as a stream of characters and groups them into meaningful sequences called lexemes. For each lexeme, the lexical analyzer produces a token.

• Input: Raw source code string (e.g., int result = 10;).
• Output: A stream of tokens (e.g., <KEYWORD, int>, <IDENTIFIER, result>, <OPERATOR, =>, <CONSTANT, 10>, <PUNCTUATION, ;>).
• Main Task: To identify the "words" of the programming language. It also strips out comments and whitespace.
• Analogy: Breaking an English sentence into its individual words and punctuation marks.

2. Syntax Analysis (or Parsing)

The parser takes the stream of tokens from the lexical analyzer and verifies that it can be generated by the grammar of the source language. It builds a tree-like representation of the code that shows its grammatical structure.

• Input: A stream of tokens.
• Output: A parse tree or, more commonly, an Abstract Syntax Tree (AST). The AST is a condensed, hierarchical representation of the code's structure.
• Main Task: To check if the code is grammatically correct according to the language's rules.
• Analogy: Checking if a sequence of words forms a valid English sentence (e.g., Subject-Verb-Object). The sentence "runs dog the" has the right words but is syntactically incorrect.

3. Semantic Analysis

This phase checks the AST for semantic consistency with the language definition. It goes beyond syntax to check if the code "makes sense."

• Input: The Abstract Syntax Tree (AST).
• Output: An annotated AST (with type information, etc.).
• Main Task: To check for meaning and logical errors. This includes:
  • Type Checking: Verifying that operators are applied to compatible operands (e.g., you can't add a string to an integer in many languages without an explicit conversion).
  • Declaration Checking: Ensuring every variable is declared before it is used.
  • Scope Checking: Making sure variables are used only within the scope where they are visible.
• Analogy: A sentence can be grammatically correct but semantically nonsensical, like "The green idea sleeps furiously." Semantic analysis catches the equivalent errors in code.

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The "Middle" Phase

4. Intermediate Code Generation

After the front-end analysis is complete, many compilers generate an explicit, machine-independent intermediate representation (IR). This IR is easy to produce and easy to translate into the target code.

• Input: The annotated AST.
• Output: Intermediate Code (e.g., Three-Address Code, LLVM IR).
• Main Task: To create a low-level, platform-independent version of the code that is suitable for optimization and translation to machine code.
• Example: An expression like a = b + c * 10; might be translated into:
  t1 = 10 t2 = c * t1 t3 = b + t2 a = t3

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The Back End Phases

5. Code Optimization

This phase takes the intermediate code and tries to improve it to make the final program faster, smaller, or more power-efficient. This is often the most complex part of a modern compiler.

• Input: Intermediate Code.
• Output: Optimized Intermediate Code.
• Main Task: To transform the code to an equivalent but better version. Common optimizations include:
  • Constant Folding: Performing computations at compile time (e.g., changing 2 * 5 to 10).
  • Dead Code Elimination: Removing code that is never executed.
  • Common Subexpression Elimination: Avoiding re-computation of the same value.

6. Code Generation

The final phase of the compiler. It takes the optimized intermediate code and translates it into the target language, which is typically the machine code or assembly language for a specific processor.

• Input: Optimized Intermediate Code.
• Output: Target Code (e.g., Assembly or Machine Code).
• Main Task:
  • Instruction Selection: Mapping IR instructions to target machine instructions.
  • Register Allocation: Deciding which variables to store in fast CPU registers for efficient access.
  • Instruction Ordering: Arranging instructions to maximize performance.

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Cross-Cutting Components

Two other important components are active throughout the compilation process:

• Symbol Table Management: The symbol table is a data structure that stores information about all the identifiers (variable names, function names, etc.) used in the source code. It keeps track of their type, scope, and other attributes. It is used and updated by nearly all phases.
• Error Handling: If an error is found in any phase (e.g., a syntax error or a type mismatch), the error handler must report it to the user in a clear and understandable way. A good error handler will also attempt to recover and continue compiling to find more errors in the same run.

Summary Table

| Phase | Input | Output | Main Task |
| ------------------------- | --------------------------------------- | ------------------------------------- | ----------------------------------------------------------------------- |
| Front End | | | Analysis (Language-Dependent) |
| 1. Lexical Analysis | Source Code | Stream of Tokens | Group characters into "words" (tokens). |
| 2. Syntax Analysis | Stream of Tokens | Abstract Syntax Tree (AST) | Verify grammatical structure. |
| 3. Semantic Analysis | Abstract Syntax Tree (AST) | Annotated AST | Check for meaning, types, and scope. |
| Middle | | | Bridging the Gap |
| 4. Intermediate Code Gen | Annotated AST | Intermediate Representation (IR) | Create a machine-independent representation. |
| Back End | | | Synthesis (Machine-Dependent) |
| 5. Code Optimization | Intermediate Representation (IR) | Optimized IR | Improve the code's efficiency (speed, size). |
| 6. Code Generation | Optimized IR | Target Code (e.g., Assembly) | Translate IR to machine-specific instructions and allocate registers. |