swift-runtime-performance

Swift Runtime Performance

Purpose

Use this skill to review Swift code for runtime-level performance costs without turning every abstraction into a problem. Focus on concrete costs such as allocation, ARC traffic, dispatch, specialization, copying, optimizer visibility, and unsafe memory boundaries.

This skill should help the agent distinguish real hot-path runtime costs from theoretical micro-optimizations.

When to use this skill

Use this skill when the task involves Swift runtime behavior such as:

• heap allocation, stack vs heap storage, object layout, boxed values, or closure contexts;
• ARC retain/release traffic, ownership, lifetime, weak/unowned references, or closure captures;
• direct dispatch, class dispatch, Objective-C dispatch, witness dispatch, or dynamic dispatch in hot paths;
• any Protocol, some Protocol, generics, type erasure, specialization, or unspecialized hot code;
• copy-on-write collections, large values, custom COW storage, or repeated copies;
• optimized SIL inspection, compiler optimization, inlining, devirtualization, or specialization evidence;
• unsafe Swift, pointer lifetime, memory binding, aliasing, buffer mutation, or safe wrappers around unsafe regions;
• module boundaries that affect optimizer visibility, @inlinable, @usableFromInline, @frozen, or public API resilience trade-offs.

When not to use this skill

Do not use this skill for:

• general Swift syntax or API usage questions with no runtime performance concern;
• app launch investigations where the main issue is pre-main, dyld, static initializers, SDK startup, first frame, or first interaction;
• SwiftUI performance issues where the main issue is identity, invalidation, state scope, layout, drawing, animation, or scrolling;
• Swift Concurrency issues where the main issue is task lifetime, actor isolation, MainActor responsiveness, cancellation, AsyncSequence cleanup, reentrancy, or executor behavior;
• profiling workflow questions where the main task is choosing tools, interpreting traces, designing signposts, XCTest metrics, MetricKit, or production signals;
• broad architecture questions unless there is a specific runtime cost in a hot path.