Meta·Software Engineer·Online Assessment (OA)·Senior

Senior

Apr 2026

Summary

Meta SWE coding round with an AI-assisted twist. The task was to take source code or an IR representation of a small compiled language and apply compiler optimizations to cut memory and runtime. Felt more like a compilers course exam than a typical coding interview, which I did not fully anticipate.

Questions Asked (1)

Given the source code or intermediate representation of a small compiled language, implement compiler optimizations to reduce both memory usage and execution time. Sub-tasks include constant folding and propagation, dead-code elimination, common subexpression elimination, variable reuse, and reordering of independent operations. Pass as many test cases as possible within the time limit. An AI coding assistant is allowed but not required.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

I leaned on the AI assistant more than I should have for the dead-code elimination part and it gave me something that passed maybe two thirds of the cases but missed edge cases around loops.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the input format (source code or IR), the available time, and the test harness. Then prioritize optimizations by impact and implementation complexity: begin with constant folding/propagation and dead-code elimination, followed by common subexpression elimination, variable reuse, and operation reordering. Implement each pass as a separate module, test incrementally, and use profiling to guide further optimizations.

Pro tip: Focus on correctness first: an optimization that breaks semantics will fail more tests than it passes. Use a pass manager with dependency tracking and validate after each pass to catch regressions early.

1. Understand the Problem and Constraints

Clarify the input format (source or IR), the test harness, time limit, and allowed tools. Identify the language features and optimization opportunities.

2. Design a Modular Optimization Pipeline

Plan a sequence of independent passes (constant folding/propagation, dead-code elimination, CSE, variable reuse, reordering) with clear interfaces and a pass manager to handle dependencies.

3. Implement and Test Incrementally

Start with the simplest high-impact passes (constant folding, dead-code elimination). Write unit tests for each pass and run the full test suite after each addition to ensure correctness.

4. Optimize for Performance and Memory

Profile the compiler and generated code. Use efficient data structures (e.g., hash consing for CSE, liveness analysis for variable reuse) and consider trade-offs between optimization aggressiveness and compile time.

5. Validate and Iterate

Run the provided test cases, measure improvements, and iterate on the most impactful optimizations. Ensure all passes preserve program semantics.

Key Points to Mention

Constant folding and propagation: evaluate constant expressions at compile time and replace variables with known constants.
Dead-code elimination: remove instructions whose results are never used or code that is unreachable.
Common subexpression elimination: identify and reuse repeated computations, using available expression analysis.
Variable reuse: perform register allocation or reuse variables when their live ranges do not overlap, reducing memory usage.
Reordering of independent operations: schedule instructions to improve locality and parallelism without violating data dependencies.
Trade-offs: balance optimization effectiveness against compile-time overhead and implementation complexity; prioritize passes with highest impact.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.