Debugging and Instruments

Diagnose crashes, memory leaks, hangs, and performance bottlenecks using LLDB, Xcode Memory Graph Debugger, unified logging/signposts, and Instruments.

Scope Boundary: Route command-line .memgraph analysis to ios-memgraph-analysis and ETTrace tracing to ios-ettrace-performance.

Contents

• LLDB Debugging Workflow
• Memory Graph & Retain Cycles
• Hangs & Signpost Diagnostics
• Instruments Profiling
• Common Mistakes
• Review Checklist
• References

LLDB Debugging Workflow

Follow a structured, non-invasive triage loop:

1. Reproduce in Debug configuration; pause at the narrowest relevant line.
2. Inspect values without executing code using v, falling back to po only for formatted descriptions.
3. Traverse frames and threads to isolate invalid states.
4. Set conditional breakpoints or watchpoints to catch unintended state transitions.

(lldb) br set -f OrderViewModel.swift -l 42   # Break at file and line
(lldb) v orderState                           # Inspect variable without code execution
(lldb) po orderState                          # Print debug description
(lldb) bt all                                 # Print backtrace for all threads
(lldb) frame select 2                         # Switch execution frame
(lldb) br modify 1 -c "orderId == 404"        # Set condition on breakpoint
(lldb) w set v self.totalAmount               # Watchpoint on memory mutation

Memory Graph & Retain Cycles

Enable Malloc Stack Logging (Edit Scheme > Run > Diagnostics) to record allocation backtraces.

1. Navigate to the suspect view, perform user actions, and pop/dismiss the view.
2. Click Debug Memory Graph in the Xcode debug bar.
3. Look for purple exclamation mark warnings indicating leaked memory or retain cycles.
4. Select the instance node to inspect the strong reference graph.

Common fix: break closure cycles using [weak self] or assign delegates weakly (weak var delegate: SomeDelegate?).

Hangs & Signpost Diagnostics

Diagnose main thread stalls and measure execution intervals using unified logging and OSSignposter:

import os

let logger = Logger(subsystem: "com.example.app", category: "DataSync")
let signposter = OSSignposter(logger: logger)

func loadRecords() async throws {
    let signpostID = signposter.makeSignpostID()
    let state = signposter.beginInterval("LoadRecords", id: signpostID)
    defer { signposter.endInterval("LoadRecords", state) }

    // Intensive operation
}

View intervals and subsystem events in Instruments under the os_signpost instrument.

Instruments Profiling

Always profile in Release configuration outside the debugger (Product > Profile):

• Time Profiler: Identifies hot execution paths and CPU usage. Expand invert-call-tree and hide-system-libraries to focus on app code.
• Allocations & Leaks: Tracks heap growth, memory allocations, and leaked memory over time.
• Core ML: Measures model execution latency, neural engine offload, and memory consumption.
• Energy Log / Network: Audits battery impact and network transfer efficiency.

Common Mistakes

• Using po for simple inspection: po evaluates expressions via the Swift runtime and can cause unexpected side effects or deadlocks. Use v to read values safely.
• Profiling in Debug configuration: Debug builds disable compiler optimizations and produce distorted performance timings.
• Profiling with debugger attached: LLDB hooks introduce significant execution overhead. Use Instruments standalone.
• Ignoring purple warnings in Memory Graph: Purple warnings flag definite object leaks; resolve them before optimizing memory elsewhere.
• Blocking the main thread with synchronous work: Synchronous file I/O or network calls on @MainActor trigger main-thread hangs.

Review Checklist

• LLDB inspection favors v over po to avoid side effects
• Malloc Stack Logging enabled when capturing memory graphs
• Strong reference cycles broken with [weak self] or weak delegates
• Profiling performed in Release build without debugger attachment
• Signposts implemented around performance-critical workflows
• Time Profiler call trees inspected with system libraries hidden

References

• LLDB command reference: references/lldb-patterns.md
• Instruments template guide: references/instruments-guide.md
• Logging (unified logging system)
• Logger
• OSSignposter
• Generating log messages from your code
• Recording performance data (signposts)
• Diagnosing memory, thread, and crash issues early
• Data races
• Reducing your app's memory use
• Profiling apps using Instruments
• Improving app responsiveness
• Analyzing your app's battery use
• Analyzing the performance of your shipping app