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Chapter 09The Price of AbstractionPaid chapter

Interface Dispatch — Itabs, Indirection, Escape, and Typed Nil

Understand the runtime cost of interface method calls — itab, indirect call, argument escape — when devirtualization removes it, and the interface-representation gotcha.

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What this chapter teaches

Common belief

Interfaces are just function pointers; the cost is one indirect call.

What actually happens

escaping interface boxing can allocate; local boxing can be eliminated. Unresolved method dispatch can inhibit optimization, while devirtualization can recover it. (Plus the typed-nil trap: an interface holding a nil pointer is not == nil.)

Mechanisms covered
Itabs are cached per (interface type, concrete type), so lookup is amortized. The indirect call is the real cost.
The compiler can devirtualize when it can prove the concrete type, eliminating indirection.
An interface is nil only when both its type and value words are nil; a non-nil type with a nil value pointer compares != nil.
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