Chapter 04The Compiler's Optimization BudgetPaid chapter

Inlining — Budgets, Costs, PGO, and What Breaks It

Understand Go's inlining cost model, what causes functions to exceed the budget, how PGO changes the decision, and why inlining loss cascades into escape and allocation regressions.

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What this chapter teaches

Common belief

Extracting a helper function is free; the compiler inlines small functions anyway.

What actually happens

inlining is gated by an AST-node cost budget (80). One node over and the whole function stops inlining — which can cascade into lost escape analysis and new allocations. PGO can raise the budget for hot call sites.

Mechanisms covered

Mid-stack inlining (Go 1.19+) inlines callers of inlinable callees, but the combined cost must fit the budget.

PGO (Profile-Guided Optimization, stable since Go 1.21): with a default.pgo profile in the main package, the compiler raises the inlining budget for hot call sites identified in the profile, inlining functions it would otherwise reject.

Inlining enables further optimizations: escape analysis sees through inlined calls, BCE proves bounds across inlined code, register allocation improves.

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