Instruments 27 Run Comparisons & Top Functions

import Foundation
import os.signpost

// MARK: - OSSignpost setup for Instruments Points of Interest track

let signposter = OSSignposter(subsystem: "com.example.NoteApp", category: .pointsOfInterest)

// MARK: - Protocol + Types

protocol Drawable {
    func draw(into buffer: inout [UInt8])
}

struct Stroke: Drawable {
    let points: [CGPoint]
    func draw(into buffer: inout [UInt8]) {
        // Simulate per-point rendering work
        for point in points {
            let idx = Int(point.x + point.y) % buffer.count
            buffer[idx] = UInt8((point.x * 0.5).truncatingRemainder(dividingBy: 255))
        }
    }
}

// MARK: - BEFORE: existential — causes swift_project_boxed_opaque_existential
//         (visible as top hotspot in Instruments Top Functions mode)

func renderWithExistential(shapes: [any Drawable], buffer: inout [UInt8]) {
    let state = signposter.beginInterval("LassoRender", "existential path")
    for shape in shapes {
        shape.draw(into: &buffer)   // existential dispatch + boxing overhead
    }
    signposter.endInterval("LassoRender", state)
}

// MARK: - AFTER: generic — compiler specializes, eliminates boxing overhead
//         Verified via Instruments Run Comparisons: self-weight drops to ~0

func renderWithGeneric<T: Drawable>(shapes: [T], buffer: inout [UInt8]) {
    let state = signposter.beginInterval("LassoRender", "generic path")
    for shape in shapes {
        shape.draw(into: &buffer)   // direct dispatch, no existential boxing
    }
    signposter.endInterval("LassoRender", state)
}

// MARK: - Usage

var pixelBuffer = [UInt8](repeating: 0, count: 1024)
let strokes = (0..<500).map { i in
    Stroke(points: (0..<20).map { j in CGPoint(x: Double(i + j), y: Double(j)) })
}

// Baseline run (profile this with Instruments Time Profiler)
renderWithExistential(shapes: strokes, buffer: &pixelBuffer)

// Optimized run (load both .trace files into Instruments Run Comparisons)
renderWithGeneric(shapes: strokes, buffer: &pixelBuffer)