Describe common compiler optimisations.
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of Compilers & Languages conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
Optimisation improves the intermediate representation while preserving observable behaviour. Common examples:
- Constant folding and propagation: compute
2 * 3at compile time and substitute known constants. - Dead code elimination: remove computations whose results are never used.
- Common subexpression elimination: reuse a repeated computation.
- Inlining: replace a call with the callee's body to remove call overhead and expose more optimisation.
- Loop-invariant code motion: hoist computations that do not change across iterations.
- Strength reduction: replace expensive operations, such as multiplying by a power of two with a shift.
- Register allocation: keep hot values in registers.
x = 4 * 8 -> x = 32
if (false) { ... } -> removed
Optimisations rely on analyses such as use-def chains, dominance and alias analysis. Correctness is paramount: an optimisation must never change output, which is why alias analysis for pointers is hard and some transformations are conservative.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.