Chapter 05The Compiler's Optimization BudgetPaid chapter

Bounds Check Elimination — What the Compiler Can Prove

Understand how the compiler eliminates array/slice bounds checks and what code patterns defeat the proof.

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What this chapter teaches

Common belief

Bounds checks are negligible / the compiler removes them all.

What actually happens

BCE uses compiler range and control-flow proofs. Opaque results can hide useful bounds, but calls do not universally invalidate existing proofs; inspect each surviving check.

Mechanisms covered
The _ = s[n-1] idiom gives the compiler a proof point for all indices 0..n-1, eliminating per-iteration checks at zero runtime cost.
Indexing through interface methods or opaque function returns defeats BCE because the compiler cannot see the length.
compile
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