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 is a local proof that does not survive function calls, interface dispatch, or non-obvious index math; surviving checks insert panicIndex branches into tight loops.
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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Escape Analysis — The Compiler's Fragile Decision
Understand how the compiler decides between stack and heap allocation, why the decision is fragile, and how to verify it.
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Maps — Swiss Tables, Growth, and Permanent Memory
Understand the Go 1.24+ Swiss Table map layout, why maps don't shrink, and how to design for memory reclamation.