Optimize a compute kernel with a simulator

Quick Overview

This question evaluates performance engineering and low-level kernel optimization competencies, including profiling, data-layout and instruction-level techniques, correctness verification, and benchmark-driven measurement within the System Design domain.

Optimize a compute kernel with a simulator

Company: Anthropic

Role: Software Engineer

Category: System Design

Difficulty: hard

Interview Round: Onsite

You're given a compute kernel and a cycle-accurate simulator that verifies functional correctness and reports runtime. Within two weeks, achieve the largest speedup without changing outputs. Describe your end-to-end plan: establish a baseline, profile to find bottlenecks, form hypotheses, choose data-layout transformations, use bitwise operations and hashing where helpful, and apply VLIW-style instruction scheduling to exploit instruction-level parallelism. Explain how you validate correctness after each change, avoid overfitting to the simulator, and quantify speedup improvements. Prioritize the first three optimizations you would try and justify them.

Quick Answer: This question evaluates performance engineering and low-level kernel optimization competencies, including profiling, data-layout and instruction-level techniques, correctness verification, and benchmark-driven measurement within the System Design domain.

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Anthropic
Sep 6, 2025, 12:00 AM
hardSoftware EngineerOnsiteSystem Design
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Performance Optimization Plan for a Compute Kernel

Context

You are given:

  • A compute kernel (single critical function or set of loops) to optimize.
  • A cycle-accurate simulator that both verifies functional correctness and reports runtime/cycle counts.

Goal: Within two weeks, achieve the largest possible speedup without changing the kernel's outputs.

Task

Describe your end-to-end plan to:

  1. Establish a reproducible baseline.
  2. Profile to find bottlenecks and form hypotheses.
  3. Select and apply optimizations, including:
    • Data-layout transformations.
    • Strength reductions via bitwise operations.
    • Hashing where helpful.
    • VLIW-style instruction scheduling (manual ILP and software pipelining).
  4. Validate correctness after each change.
  5. Avoid overfitting to the simulator.
  6. Quantify speedup improvements.

Requirements

  • Outputs must be identical to baseline.
  • Prioritize the first three optimizations you would try and justify them.
  • Explain how you will measure and report gains after each change.

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