Guide and override compiler optimizations

Q: Guide and override compiler optimizations

This is a System Design interview question from Anthropic for Software Engineer roles. View the full question and solution on PracHub.

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Question

Guiding a Compiler for a VLIW-like Backend

You are optimizing hot loops for a VLIW-like target (e.g., DSP/AI accelerator) where the compiler sometimes mis-schedules or over-optimizes. Describe a pragmatic workflow for:

Guiding the compiler using:
- Pragmas/directives (e.g., unroll, vectorize, pipeline, ivdep).
- Intrinsics and builtins (vector ops, prefetch, assumes/alignment hints).
- Type/qualifier hints (restrict, const, alignment attributes).
- Compiler flags and function attributes (optimization levels, math/aliasing flags, inlining, target features).
When to escalate to manual transformations:
- Manual loop unrolling.
- Software pipelining (modulo scheduling by hand or directives).
- Inline assembly for specific instructions or scheduling control.
How to measure impact safely and repeatably:
- Preventing dead-code elimination, warming up, pinning, perf counters.
- Disassembly and compiler optimization reports.
- Guardrails to ensure correctness while changing flags/annotations.
Risks and mitigations:
- Undefined behavior, strict aliasing violations, alignment and math semantics.
- Portability across compilers/architectures.
- Debuggability and long-term maintainability.

State concrete tactics, trade-offs, and validation steps. Illustrate with a simple loop example if helpful.

Guide and override compiler optimizations

Guiding a Compiler for a VLIW-like Backend

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