Compiler Phases, Lexical Analysis and Semantic Processing Short Notes for GATE CS
Compiler Phases, Lexical Analysis and Semantic Processing short notes for GATE CS: 4 study cards covering concepts, formulas, shortcuts and exam traps, plus s
compiler phases lexical analysis and semantic processing short notes
Quick Revision: Compiler Phases and Symbol Table
Core Takeaways for Final Revision
Compiler Goal
Translate high-level source code to low-level target code.
Front-End (Analysis)
Lexical → Syntax → Semantic → ICG. Depends on Source Language, independent of Target Machine.
Back-End (Synthesis)
Optimization → Code Generation. Depends on Target Machine, independent of Source Language.
Symbol Table
A central, shared data structure accessed by all phases to store and retrieve identifier attributes.
Error Handling
Every phase can detect errors and report them, but the compiler may attempt to continue to find multiple errors in a single pass.
The lexer reads a lexeme (concrete string), matches it to a pattern (regex), and emits a token (abstract category).
Maximal Munch Rule
The absolute tie-breaker. When multiple patterns match the prefix of the input, the lexer must choose the one that consumes the maximum number of characters.
Regex Shorthands
r∗ = zero or more (includes ϵ). r+ = one or more (rr∗). r? = zero or one (r∣ϵ).
Identifier Rule
Always begins with a letter, followed by any combination of letter or digit. It can never begin with a digit.
Quick Revision Checklist
Quick Revision Checklist
Semantic Analysis: Checks context-sensitive rules (types, declarations, scope) using the parse tree and symbol table.
Synthesized Attribute: Computed from children. Flows bottom-up. Compatible with LR parsing.
Inherited Attribute: Computed from parent or left siblings. Flows top-down or sideways.
Dependency Graph: Must be a Directed Acyclic Graph (DAG) for valid evaluation. Topological sort gives the order.
Type Equivalence: Structural equivalence (same internal makeup) is the default assumption in compiler theory unless name equivalence is specified.
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Question 1
Level 1: Warm-up
Which compiler phase is responsible for reading the source code character stream and grouping it into meaningful units called tokens?
Question 2
Level 1: Warm-up
Which of the following errors is typically detected during the Syntax Analysis phase?
Question 3
Level 1: Warm-up
If a compiler reports an "Undefined Identifier" error, which phase is responsible for detecting it?
Question 4
Level 1: Warm-up
What is the primary output of the Intermediate Code Generator phase in a standard compiler?
Question 5
Level 1: Warm-up
Which phase of the compiler is responsible for detecting the error in the following C code snippet?
```c
int x = y + 10;
```
Assume that the variable y has not been declared anywhere in the program.
Question 6
Level 1: Warm-up
Consider a lexical analyzer that uses the Maximal Munch rule. Given the input string `intinteger`, and the following token definitions:
- Keyword `int`: pattern `int`
- Identifier `id`: pattern `[a-z]+`
How many tokens will the lexer generate for this input?
Question 7
Level 1: Warm-up
In the standard architecture of a compiler, which of the following components is strictly independent of the target machine architecture?
Question 8
Level 1: Warm-up
In the standard compiler model, which component acts as the interface between the Front-End and the Back-End?
Question 9
Level 1: Warm-up
The Back-End of a compiler is primarily dependent on which of the following?
Question 10
Level 1: Warm-up
Which of the following is a machine-independent optimization technique typically performed by the compiler?
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Compiler Phases, Lexical Analysis and Semantic Processing Short Notes for GATE CS
Compiler Phases, Lexical Analysis and Semantic Processing short notes for GATE CS: 4 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.
Quick Revision: Compiler Phases and Symbol Table
Core Takeaways for Final Revision
Compiler Goal
Translate high-level source code to low-level target code.
Front-End (Analysis)
Lexical → Syntax → Semantic → ICG. Depends on Source Language, independent of Target Machine.
Back-End (Synthesis)
Optimization → Code Generation. Depends on Target Machine, independent of Source Language.
Symbol Table
A central, shared data structure accessed by all phases to store and retrieve identifier attributes.
Error Handling
Every phase can detect errors and report them, but the compiler may attempt to continue to find multiple errors in a single pass.
The lexer reads a lexeme (concrete string), matches it to a pattern (regex), and emits a token (abstract category).
Maximal Munch Rule
The absolute tie-breaker. When multiple patterns match the prefix of the input, the lexer must choose the one that consumes the maximum number of characters.
Regex Shorthands
r∗ = zero or more (includes ϵ). r+ = one or more (rr∗). r? = zero or one (r∣ϵ).
Identifier Rule
Always begins with a letter, followed by any combination of letter or digit. It can never begin with a digit.
Quick Revision Checklist
Quick Revision Checklist
Semantic Analysis: Checks context-sensitive rules (types, declarations, scope) using the parse tree and symbol table.
Synthesized Attribute: Computed from children. Flows bottom-up. Compatible with LR parsing.
Inherited Attribute: Computed from parent or left siblings. Flows top-down or sideways.
Dependency Graph: Must be a Directed Acyclic Graph (DAG) for valid evaluation. Topological sort gives the order.
Type Equivalence: Structural equivalence (same internal makeup) is the default assumption in compiler theory unless name equivalence is specified.
Semantic Analyzer: Catches undeclared variables, type mismatches, multiple declarations, and flow-of-control violations.
Exam Rule of Thumb: If the code looks like valid English grammar but makes no logical sense, it is a Semantic error. If the grammar itself is broken, it is a Syntax error.
Compiler Phases, Lexical Analysis and Semantic Processing: Solved Questions with Step-by-Step Explanations (10 Problems)
Question 1 · Compiler DesignMCQ
Which compiler phase is responsible for reading the source code character stream and grouping it into meaningful units called tokens?
A.
Lexical Analyzer
B.
Syntax Analyzer
C.
Semantic Analyzer
D.
Intermediate Code Generator
Correct Answer:
A
Step-by-Step Solution
Key idea: This is a direct recall question about compiler phase responsibilities.
Step 1: Recall the primary function of the Lexical Analyzer. It scans the source code character by character and groups them into meaningful sequences called tokens (e.g., keywords, identifiers, operators).
Step 2: Verify other options. Syntax Analyzer builds parse trees from tokens. Semantic Analyzer checks for logical consistency and type compatibility. Intermediate Code Generator produces an abstract intermediate representation.
Answer: A
Question 2 · Compiler DesignMCQ
Which of the following errors is typically detected during the Syntax Analysis phase?
A.
Undeclared variable
B.
Missing semicolon
C.
Type mismatch
D.
Division by zero
Correct Answer:
B
Step-by-Step Solution
Key idea: Syntax Analysis checks if the token stream follows the grammatical rules of the language.
Step 1: Undeclared variable is a Scope/Semantic error.
Step 2: Missing semicolon violates the grammar structure. This is a Syntax error.
Step 3: Type mismatch is a Semantic error.
Step 4: Division by zero is often a Run-time error (or sometimes static analysis, but not syntax).
Answer: B
Question 3 · Compiler DesignMCQ
If a compiler reports an "Undefined Identifier" error, which phase is responsible for detecting it?
A.
Lexical Analysis
B.
Syntax Analysis
C.
Semantic Analysis
D.
Code Optimization
Correct Answer:
C
Step-by-Step Solution
Key idea: "Undefined Identifier" means the name has not been declared in the current scope. This requires checking the Symbol Table.
Step 1: Lexical Analysis just identifies the string as an identifier token. It doesn't know if it's defined.
Step 2: Syntax Analysis checks structure.
Step 3: Semantic Analysis checks scope and declaration. It looks up the identifier in the Symbol Table. If not found, it raises this error.
Answer: C
Question 4 · Compiler DesignMCQ
What is the primary output of the Intermediate Code Generator phase in a standard compiler?
A.
Stream of lexical tokens
B.
Concrete syntax tree
C.
Three-address code
D.
Target machine assembly
Correct Answer:
C
Step-by-Step Solution
Key idea: Each compiler phase has a specific input and output. The Intermediate Code Generator bridges the front-end and back-end.
Step 1: The Lexical Analyzer outputs a stream of tokens.
Step 2: The Syntax Analyzer outputs a concrete syntax tree (or parse tree).
Step 3: The Intermediate Code Generator takes the annotated syntax tree and produces a machine-independent representation, commonly Three-Address Code (TAC) or an Abstract Syntax Tree (AST).
Step 4: The Code Generator outputs target machine assembly.
Answer: C
Question 5 · Compiler DesignMCQ
Which phase of the compiler is responsible for detecting the error in the following C code snippet?
```c
int x = y + 10;
```
Assume that the variable y has not been declared anywhere in the program.
A.
Lexical Analysis
B.
Syntax Analysis
C.
Semantic Analysis
D.
Code Optimization
Correct Answer:
C
Step-by-Step Solution
Key idea: This is a phase-responsibility question. We must identify which compiler phase checks for identifier declarations.
Step 1: Analyze the error.
The code int x = y + 10; is syntactically correct. It follows the grammar rules of C (assignment statement). The lexical analyzer will successfully tokenize int, x, =, y, +, 10, and ;.
Step 2: Identify the missing information.
The error is that y is used but never declared. To detect this, the compiler must check the Symbol Table to see if y exists in the current scope.
Step 3: Map to the phase.
Checking the Symbol Table for identifier properties (like declaration, type, scope) is the primary job of Semantic Analysis.
Answer: C
Question 6 · Compiler DesignNAT
Consider a lexical analyzer that uses the Maximal Munch rule. Given the input string `intinteger`, and the following token definitions:
- Keyword `int`: pattern `int`
- Identifier `id`: pattern `[a-z]+`
How many tokens will the lexer generate for this input?
Correct Answer:
1
Step-by-Step Solution
Key idea: Apply the Maximal Munch (Longest Match) rule.
Step 1: Identify possible matches at the start of the string `intinteger`.
- The substring `int` matches the keyword pattern `int`.
- The substring `intinteger` matches the identifier pattern `[a-z]+`.
Step 2: Apply Maximal Munch.
The rule states that the lexer must choose the longest possible match.
Length of `int` = 3.
Length of `intinteger` = 10.
Step 3: Select the token.
The lexer selects `intinteger` as a single `id` token.
Step 4: Count tokens.
The entire string is consumed by this one token. Remaining input is empty.
Total tokens = 1.
Answer: 1
Question 7 · Compiler DesignMCQ
In the standard architecture of a compiler, which of the following components is strictly independent of the target machine architecture?
A.
Code Generator
B.
Lexical Analyzer
C.
Register Allocator
D.
Instruction Selector
Correct Answer:
B
Step-by-Step Solution
Key idea: This question tests the division of compiler responsibilities between the front-end and back-end.
Step 1: Recall that the front-end (Lexical, Syntax, Semantic Analysis, and Intermediate Code Generation) depends only on the source programming language.
Step 2: Recall that the back-end (Code Optimization and Code Generation) depends on the target machine architecture.
Step 3: Evaluate the options. Code Generator, Register Allocator, and Instruction Selector are all back-end components that must know the target CPU's registers and instruction set. The Lexical Analyzer is a front-end component and is independent of the target machine.
Answer: B
Question 8 · Compiler DesignMCQ
In the standard compiler model, which component acts as the interface between the Front-End and the Back-End?
A.
Lexical Analyzer
B.
Parser
C.
Intermediate Code Generator
D.
Code Optimizer
Correct Answer:
C
Step-by-Step Solution
Key idea: The compiler is split into Front-End (analysis) and Back-End (synthesis). The bridge between them is the Intermediate Representation (IR).
Step 1: The Front-End produces IR.
Step 2: The Back-End consumes IR.
Step 3: The phase responsible for creating this IR is the Intermediate Code Generator.
Answer: C
Question 9 · Compiler DesignMCQ
The Back-End of a compiler is primarily dependent on which of the following?
A.
Source programming language grammar
B.
Target machine architecture
C.
Lexical token definitions
D.
High-level language semantics
Correct Answer:
B
Step-by-Step Solution
Key idea: The compiler is divided into Front-End (analysis) and Back-End (synthesis) based on their dependencies.
Step 1: The Front-End depends on the source programming language (grammar, semantics, tokens).
Step 2: The Back-End takes the machine-independent Intermediate Representation and generates machine code.
Step 3: To generate correct machine code, the Back-End must know the registers, instruction set, and memory layout of the target machine.
Answer: B
Question 10 · Compiler DesignMCQ
Which of the following is a machine-independent optimization technique typically performed by the compiler?
A.
Register allocation
B.
Instruction scheduling
C.
Constant folding
D.
Peephole optimization
Correct Answer:
C
Step-by-Step Solution
Key idea: This is a direct recall question distinguishing between machine-dependent and machine-independent optimizations.
Step 1: Machine-independent optimizations improve the intermediate code without knowing the target hardware.
Step 2: Constant folding (e.g., replacing 3 + 4 with 7) depends only on the values, not the CPU architecture.
Step 3: Register allocation, instruction scheduling, and peephole optimization all require specific knowledge of the target machine's registers and instruction set, making them machine-dependent.