Compiler Design for the GATE Exam

Common failure modes

These are the patterns that cause most candidates to lose marks on this topic. Recognising them in advance is half the work.

• !Computing FIRST sets incorrectly when a production has nullable symbols
• !Building FOLLOW sets without including the eof ($) marker for the start symbol
• !Misclassifying parsing conflicts: shift-reduce vs reduce-reduce in an LR parsing table
• !Confusing LL(1) conflict conditions with LR(0) conflict conditions
• !Generating incorrect three-address code by misordering temporaries in expressions

Study tips

• 1Practise FIRST and FOLLOW set computation on at least 10 different grammars — errors here cascade into parser table construction.
• 2For LL(1) parsing, verify the LL(1) condition: no two rows for the same non-terminal have the same terminal in their selection sets.
• 3Build the canonical LR(0) item sets and SLR parsing table for a small grammar (4–5 productions) from scratch.
• 4Memorise the precedence of common code optimisation techniques: constant folding → dead-code elimination → common-subexpression elimination → loop invariant code motion.
• 5Practice syntax-directed translation: attribute grammars, S-attributed vs L-attributed, and evaluating inherited vs synthesised attributes.

Sample GATE Compiler Design questions

These sample items mirror the format and difficulty of real GATE questions. Practice with thousands more on the free Koydo question bank.

1. 1

   For the grammar S → aAb | b and A → aA | ε, FIRST(S) is:

   • A{a}
   • B{a, b}Correct
   • C{a, b, ε}
   • D{a, ε}
   Why this answer?

   (GATE CS style) From S → aAb: FIRST starts with 'a'. From S → b: FIRST includes 'b'. S cannot derive ε, so ε ∉ FIRST(S). Therefore FIRST(S) = {a, b}.

2. 2

   Which of the following is a shift-reduce conflict in an LR parser?

   • ATwo items want to reduce by different productions on the same lookahead
   • BOne item wants to shift and another wants to reduce on the same lookaheadCorrect
   • CTwo items want to shift on the same lookahead
   • DAn item set has no actions defined for a terminal
   Why this answer?

   (GATE CS style) A shift-reduce conflict occurs when, for the same lookahead symbol, the parser can either shift the symbol onto the stack or reduce by a production. This makes the grammar ambiguous for LR(0)/SLR parsing without additional lookahead.