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Anthropic·Software Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

Interviewed for a software engineering role at Anthropic and got a pretty deep technical question about compiler behavior on VLIW-style backends. Not the kind of thing you can fake your way through with buzzwords.

Questions Asked (1)

Q1

A modern compiler is mis-scheduling or over-optimizing code for a VLIW-like backend. Walk through how you'd guide it using pragmas, intrinsics, restrict/const qualifiers, and compiler flags. When would you resort to manual loop unrolling, software pipelining, or inline assembly, and how would you measure the impact without introducing new bugs? Also cover the risks around undefined behavior, aliasing, and portability.

Technical Trade-offsSystem DesignRoot Cause Analysis
Author's notes

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AI HintsAI Generated

Suggested Approach

Start by diagnosing the mis-scheduling with compiler reports and profiling, then apply high-level hints like restrict/const and pragmas before resorting to intrinsics or manual transformations. Emphasize a measure-driven approach: benchmark, inspect assembly, and validate correctness with sanitizers to avoid undefined behavior and portability pitfalls.

Pro tip: Always verify the generated assembly and use compiler optimization reports (e.g., -Rpass, -fopt-info) to confirm that your hints actually took effect; many pragmas are silently ignored if the compiler can't prove safety.

1. Diagnose the issue

Use profiling and compiler optimization reports to identify where scheduling or optimization is failing, and inspect the generated assembly to understand the compiler's decisions.

2. Apply high-level hints

Introduce restrict/const qualifiers to clarify aliasing, and use pragmas (e.g., #pragma unroll, #pragma ivdep) or compiler flags (e.g., -O3, -march) to guide optimization without changing semantics.

3. Escalate to intrinsics or manual transformations

If high-level hints are insufficient, use intrinsics for specific operations, or manually unroll loops or software pipeline critical sections, ensuring correctness is maintained.

4. Measure and validate

Benchmark performance changes, verify correctness with tests and sanitizers (e.g., ASan, UBSan), and check portability across target architectures.

5. Consider inline assembly as last resort

Only use inline assembly when absolutely necessary, and isolate it in small, well-documented functions with clear interfaces to minimize maintenance and portability risks.

Key Points to Mention

  • Use restrict and const to help the compiler disambiguate pointers and enable better scheduling.
  • Pragmas like #pragma unroll and #pragma ivdep can guide loop optimizations but may be ignored if dependencies are unclear.
  • Compiler flags such as -O3, -march, -mtune, and -fno-alias affect optimization and scheduling decisions.
  • Manual unrolling and software pipelining can improve performance but increase code size and risk of bugs; measure with benchmarks and verify with sanitizers.
  • Undefined behavior (e.g., signed overflow, strict aliasing violations) can cause miscompilation; use -fno-strict-aliasing cautiously and rely on UBSan.
  • Portability: intrinsics and inline assembly are architecture-specific; abstract them behind macros or provide fallbacks.

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