Compiler design is the study of how programs written in high-level languages are converted into machine-readable instructions. Its history dates back to the 1950s when John Backus created FORTRAN and Grace Hopper developed one of the earliest compilers. This field shaped the evolution of programming, making software faster and more efficient. Today it plays a key role in areas like system development, language design, and software engineering. Because of its importance in technology, many recruiters include compiler design interview questions to test how well candidates understand both the concepts and their practical applications.
In this blog, we cover the top 25+ compiler design interview questions with clear and simple answers to help you prepare.
Fun Fact: The first commercially available compiler (FORTRAN) took around 18 person-years to develop.
Basic Compiler Design Interview Questions
Here are some basic compiler design interview questions and answers to help you understand the core concepts and prepare for interviews.
- What is a compiler?
A compiler is a software program that translates code written in a high-level language (like C, Java, or Python) into machine code or bytecode that a computer can run. It checks for errors, performs optimizations, and generates an executable program. This allows developers to write code in human-readable form while the computer executes it efficiently.
- What is compiler design?
Compiler design is the study and process of building compilers. It covers the theory, techniques, and algorithms used to convert source code into machine code. It is a core subject in computer science and vital for jobs in systems programming, software engineering, and language development.
- What are the main phases of a compiler and what does each phase do?
A compiler works in stages. Here are the main phases of compiler at a glance:
| Phase | Description | Output |
| Lexical Analysis | Breaks source code into tokens using a lexical analyzer. | Tokens |
| Syntax Analysis | Checks grammar, builds a parse tree or AST. | Parse Tree / AST |
| Semantic Analysis | Verifies meaning, types, and scope. | Annotated AST |
| Intermediate Code | Converts AST to intermediate representation (e.g., TAC). | IR (Intermediate Code) |
| Optimization | Improves code efficiency (speed, memory). | Optimized IR |
| Code Generation | Produces target machine code or bytecode. | Machine Code / Executable |
| Symbol Table Management | Stores variable, function, and type info. | Symbol Table |
| Error Handling | Detects and reports lexical, syntax, and semantic errors. | Error Messages |
- What is the difference between a compiler and an interpreter?
| Compiler | Interpreter | |
| Execution | Translates the whole program at once into machine code | Executes code line by line |
| Output | Produces an executable file | No separate executable, runs directly |
| Speed | Faster execution after compilation | Slower execution since code is parsed each time |
| Error Handling | Shows all errors after compilation | Stops immediately when an error is found |
| Feedback | Delayed, only after full compilation | Immediate, as it runs line by line |
Example:
- C and C++ are compiled languages using GCC or Clang.
- Python and JavaScript are usually interpreted line by line.
- What is a symbol table and why is it needed in a compiler?
A symbol table holds details of identifiers like variables, functions, and objects. It stores their type, scope, and memory address. Compilers use this data for semantic checks and code generation. Without it, managing identifiers would be very difficult.
Symbol Table Example:
| Variable | Type | Scope | Memory Location |
| A | Int | global | 1000 |
| B | float | local | 2004 |
- What is lexical analysis and how does a lexical analyzer work?
Lexical analysis is the first compiler step. It scans the source code and groups characters into tokens. The lexical analyzer also removes comments and whitespace. The output is a clean token stream for the parser.
Example (Lexical Tokenization):
Input:
int a = b + c;
Tokens generated:
[int] [id:a] [=] [id:b] [+] [id:c] [;]
- What is a parse tree versus an abstract syntax tree (AST)?
A parse tree shows every grammar rule applied during parsing. It is detailed but bulky. An abstract syntax tree (AST) is simpler, focusing on structure and meaning. Compilers usually rely on AST for further processing.
Visual Example for id + id * id:
Parse Tree:
Abstract Syntax Tree (AST):
As you can see, the AST is much leaner and highlights only the semantic structure, while the parse tree shows every grammar expansion.
- What is semantic analysis and what kinds of errors does it detect?
Semantic analysis checks correctness beyond grammar. It finds type mismatches, undeclared variables, wrong function calls, or scope errors. This phase makes sure the program follows language rules.
- What is intermediate code and why is it useful?
Intermediate code sits between source and machine code. It is machine-independent and makes optimization easier. It also helps retarget the compiler to different hardware without redesigning earlier stages.
Advanced Compiler Design Interview Questions
Here are some advanced compiler design questions and answers that cover complex concepts and help you get ready for challenging interview rounds.
- What is register allocation and how is it performed?
Register allocation assigns variables and temporary values to CPU registers. This reduces memory access and speeds up execution. Common methods include graph coloring, which treats allocation as a coloring problem, and linear scan, which is simpler and often used in JIT compilers.
- Explain data-flow analysis and give examples of optimizations that use it.
Data-flow analysis studies how values move across a program. It helps compilers detect which variables are live, constant, or unused. Optimizations like constant propagation, dead code elimination, and live variable analysis depend on data-flow results.
- What is loop-invariant code motion and when can it be applied?
Loop-invariant code motion moves calculations that do not change inside a loop to outside of it. This avoids repeated computation in every iteration. It is applied only when the result remains the same throughout the loop.
Example:
Before:
for (i = 0; i < n; i++) {
x = 5 * y;
a[i] = x + i;
}
After Optimization:
x = 5 * y;
for (i = 0; i < n; i++) {
a[i] = x + i;
}
- Describe how backpatching works in intermediate code generation.
Backpatching is used when the target of a jump is not known immediately. The compiler leaves placeholders during code generation and later “patches” them with actual addresses once labels are defined. It is common in translating control-flow statements.
- What is SSA (Static Single Assignment) form and why is it useful in optimization?
SSA form is an intermediate representation where every variable is assigned exactly once. This makes data-flow analysis easier and more precise. It also allows strong optimizations such as constant folding and better register allocation.
- What is interprocedural analysis and what challenges does it pose?
Interprocedural analysis examines relationships across multiple functions or modules, not just within one. It helps optimize calls, inline functions, and detect unused code. The main challenge is scalability, since large programs create complex call graphs with side effects that are hard to track.
Compiler Design Viva Questions
Here are some compiler design viva questions that are often asked in interviews and at the end of lab projects.
- Which tool is used to generate a lexical analyzer (or scanner)?
Tools like Lex or Flex are widely used to build lexical analyzers. They take regular expressions as input and generate code that recognizes tokens.
- What role does error recovery play and name one strategy used in parsing.
Error recovery allows compilation to continue even after detecting an error. A common method is panic mode recovery, where the parser skips input until it finds a safe restart point.
- What is left recursion in grammars and why is it problematic?
Left recursion happens when a grammar rule refers back to itself on the left side. It causes infinite recursion in top-down parsers like recursive descent. Removing or rewriting left recursion solves this.
- In bottom-up parsing, what is a “handle”?
A handle is a substring that matches the right-hand side of a production and can be reduced to a non-terminal. Identifying handles is key to shift-reduce parsing.
- Name and explain one method of representing intermediate code.
One common method is three-address code (TAC). Each instruction uses at most three operands, making it simple to analyze and optimize. Quadruples and triples are variations of TAC.
- Explain the concept of a one-pass compiler and its limitations.
A one-pass compiler translates code in a single scan. It is faster and uses less memory but struggles with forward references and complex optimizations. Multi-pass compilers handle those better.
Compiler Design MCQs
Here are some compiler design MCQs with answers to test your knowledge and strengthen your preparation for exams and interviews.
- Which component of a compiler recognizes tokens from characters?
a) Parser
b) Lexer
c) Code Generator
d) Optimizer
Answer: b) Lexer
The lexer scans characters and groups them into tokens.
- What does LR in “LR parser” stand for?
a) Left to Right, Rightmost derivation
b) Left Recursion
c) Left to Right, Recursive
d) Leftmost Reduction
Answer: a) Left to Right, Rightmost derivation
LR parsers scan left to right and build a rightmost derivation in reverse.
- Which of these is NOT a compiler phase:
a) Lexical analysis
b) Linking
c) Semantic analysis
d) Code generation
Answer: b) Linking
Linking happens after compilation, not during the main compiler phases.
- Which type of compiler runs on one machine and produces code for a different machine?
a) Cross compiler
b) Just-In-Time compiler
c) Single pass compiler
d) Multi-pass compiler
Answer: a) Cross compiler
A cross compiler generates code for a target machine different from the host.
- In which phase is dead code elimination typically done?
a) Lexical analysis
b) Semantic analysis
c) Code optimization
d) Code generation
Answer: c) Code optimization
Dead code elimination is part of the optimization phase.
- Which grammar class can an LL(1) parser handle?
a) Ambiguous grammar
b) Context-free grammar with left recursion
c) Context-free grammar without left recursion and ambiguity
d) Regular grammar
Answer: c) Context-free grammar without left recursion and ambiguity
LL(1) parsers need grammars that are unambiguous and left recursion free.
- What is the output of a parser generator?
a) Abstract Syntax Tree (AST)
b) Tokens
c) Parsing table and parser code
d) Machine code
Answer: c) Parsing table and parser code
Parser generators produce parsing tables or code to analyze grammar.
- Which of these methods is used for register allocation:
a) Graph coloring
b) Bubble sort
c) Binary search
d) Hashing
Answer: a) Graph coloring
Graph coloring is the standard method to allocate registers efficiently.
- What is the advantage of using intermediate code?
a) It removes the need for parsing
b) It makes compilers machine-independent
c) It directly executes code faster
d) It avoids semantic analysis
Answer: b) It makes compilers machine-independent
Intermediate code allows the same front end to target multiple machines.
- Which optimization moves invariant computations outside loops?
a) Loop unrolling
b) Loop fusion
c) Loop-invariant code motion
d) Common subexpression elimination
Answer: c) Loop-invariant code motion
This optimization reduces repeated work inside loops.
Important Compiler Design GATE Questions
Here are some of the most important compiler GATE questions that can guide your preparation and give you an idea of the exam style.
- Given a grammar, decide whether it is LL(1) or not and compute FIRST and FOLLOW sets.
To check if a grammar is LL(1), compute FIRST and FOLLOW sets. If no two productions for the same non-terminal have overlapping FIRST sets, and if epsilon is present in FIRST, then FIRST and FOLLOW must not overlap. If these conditions hold, the grammar is LL(1).
- How many passes does a typical multi-pass compiler have, and what is the advantage of multi-pass compilation?
A multi-pass compiler can have two or more passes. The advantage is better optimization and easier handling of forward references. The disadvantage is slower compilation speed compared to single-pass compilers.
- For a given control flow graph, identify basic blocks and compute dominators.
A basic block is a sequence of instructions with one entry and one exit point. Dominators are nodes that must be visited before reaching another node. Computing dominators helps in optimization tasks like code motion and SSA construction.
- Given a snippet of code, generate three-address code (TAC).
TAC expresses operations with at most three operands. For example:
x = (a + b) * c becomes:
t1 = a + b
t2 = t1 * c
x = t2
- Explain the difference between SLR, LALR, and Canonical LR parsing.
- SLR (Simple LR): Uses FOLLOW sets, less powerful, smaller tables.
- LALR (Lookahead LR): Merges states with same core items, widely used in practice (e.g., YACC).
- Canonical LR: Most powerful, handles more grammars but produces large tables.
How to Prepare for Compiler Design Interview?
Preparing for a compiler design interview takes both conceptual clarity and practical problem-solving. Here are some steps that work well:
Revise the basics first
Go over the main compiler phases – lexical, syntax, semantic, optimization, and code generation. Make sure you know what happens in each step. Ensure you go through common compiler design interview questions and answers.
Understand common algorithms
Study parsing methods like LL(1), LR, and operator precedence parsing. Learn data-flow analysis, register allocation, and optimization techniques like loop-invariant code motion and common subexpression elimination.
Work with examples
Take small code snippets and practice building tokens, parse trees, abstract syntax trees, and even three-address code. Doing it by hand helps you remember the steps.
Check past interview and GATE questions
Many interviewers pick from well-known question patterns. Going through compiler design questions and answers from recent interviews and GATE exams will give you a realistic idea of what to expect.
Practice explaining out loud
In viva or interviews, you’ll need to explain concepts clearly. Try answering questions verbally, as if you’re in the interview room.
Brush up on related tools
Tools like Lex, Flex, and YACC (or Bison) are still relevant. Knowing how they fit into compiler design can help you stand out.
Wrapping Up
So, these are the top compiler design questions and answers that can help you prepare for interviews, exams, and viva sessions. Understanding these concepts will make you more confident when facing technical discussions.
If you are looking for job roles related to compiler design, visit Hirist to find the latest job opportunities.
Also Read - Top 45+ Artificial Intelligence (AI) Interview Questions and Answers
FAQs
Compiler design questions can feel tricky because they test both theory and problem-solving. If you revise core topics like parsing, symbol tables, and optimization with examples, the questions become manageable. Most interviews start with basics before moving to advanced topics.
Leading companies hiring compiler engineers include Intel, NVIDIA, AMD, ARM, Qualcomm, Google, and Microsoft. Many startups in AI, chip design, and high-performance computing also look for compiler specialists.
The process usually has 3–4 rounds. It starts with an online coding or technical test, then moves to interviews covering compiler design concepts, data structures, and problem-solving. Some companies add system design or optimization rounds, followed by an HR interview.
Not always. GATE questions are good practice for concepts like parsing, intermediate code, and optimizations. But interviews also test applied skills, so combine theory with coding practice.
Strong knowledge of C/C++, algorithms, computer architecture, and operating systems helps a lot. Familiarity with tools like LLVM, GCC, Lex, and YACC is also valued in interviews.
