/swift-diagnostics

Build Performance Diagnostics

Systematic debugging for slow builds, Derived Data issues, and Xcode hangs. 80% of "mysterious" build issues are environment problems, not code bugs.

Diagnostic Decision Table

Mandatory First Checks

# 1. Check for zombie processes
ps aux | grep -E "xcodebuild|Simulator" | grep -v grep

# 2. Check Derived Data size (>10GB = stale)
du -sh ~/Library/Developer/Xcode/DerivedData

# 3. Check simulator states
xcrun simctl list devices | grep -E "Booted|Booting|Shutting Down"

What these tell you:

• 0 processes + small DerivedData + no stuck sims -> Environment clean
• 10+ processes OR >10GB DerivedData OR simulators stuck -> Clean first

Decision Tree

Build/performance problem?
|-- BUILD FAILED with no details?
|   |-- Clean Derived Data -> rebuild
|
|-- Build succeeds but old code executes?
|   |-- Delete Derived Data -> rebuild (2-5 min fix)
|
|-- Build intermittent (sometimes succeeds/fails)?
|   |-- Clean Derived Data -> rebuild
|
|-- Xcode hangs during build?
|   |-- Check for zombie xcodebuild processes
|   |-- killall -9 xcodebuild
|
|-- "Unable to boot simulator"?
|   |-- xcrun simctl shutdown all
|   |-- xcrun simctl erase <device-uuid>
|
|-- Tests hang indefinitely?
    |-- Check simctl list -> reboot simulator

Quick Fixes

Clean Derived Data

# Delete all Derived Data
rm -rf ~/Library/Developer/Xcode/DerivedData/*

# Also clean project-specific build folders
rm -rf .build/ build/

# Clean and rebuild
xcodebuild clean -scheme YourScheme
xcodebuild build -scheme YourScheme \
  -destination 'platform=iOS Simulator,name=iPhone 16'

Kill Zombie Processes

# Kill all xcodebuild processes
killall -9 xcodebuild

# Verify they're gone
ps aux | grep xcodebuild | grep -v grep

# Kill Simulator if stuck
killall -9 Simulator

Fix Simulator Issues

# Shutdown all simulators
xcrun simctl shutdown all

# If simctl fails, force quit first
killall -9 Simulator
xcrun simctl shutdown all

# List simulators to find problematic one
xcrun simctl list devices

# Erase specific simulator
xcrun simctl erase <device-uuid>

# Boot fresh simulator
xcrun simctl boot "iPhone 16 Pro"

Reset SPM Caches

# Clear SPM caches
rm -rf ~/Library/Caches/org.swift.swiftpm

# Reset package dependencies
xcodebuild -resolvePackageDependencies

# Full clean rebuild
rm -rf ~/Library/Developer/Xcode/DerivedData/*
xcodebuild clean build -scheme YourScheme

Identifying Build Time Hotspots

# Enable build timing
defaults write com.apple.dt.Xcode ShowBuildOperationDuration -bool YES

# Restart Xcode, build, check activity log
# Xcode > Report Navigator > Build log

Xcode Build Commands

# List available schemes
xcodebuild -list

# Show build settings
xcodebuild -showBuildSettings -scheme YourScheme

# Verbose build (more diagnostics)
xcodebuild -verbose build -scheme YourScheme

# Build for testing only (faster iteration)
xcodebuild build-for-testing -scheme YourScheme

# Run tests without rebuilding
xcodebuild test-without-building -scheme YourScheme \
  -destination 'platform=iOS Simulator,name=iPhone 16'

# Run specific test only
xcodebuild test -scheme YourScheme \
  -destination 'platform=iOS Simulator,name=iPhone 16' \
  -only-testing:YourTests/SpecificTestClass

Crash Log Analysis

# Find recent crashes
ls -lt ~/Library/Logs/DiagnosticReports/*.crash | head -5

# View crash log
cat ~/Library/Logs/DiagnosticReports/YourApp-*.crash | head -100

# Symbolicate address (if you have .dSYM)
atos -o YourApp.app.dSYM/Contents/Resources/DWARF/YourApp \
  -arch arm64 0x<address>

Environment Reset (Nuclear Option)

When nothing else works:

# 1. Quit Xcode
osascript -e 'quit app "Xcode"'

# 2. Kill all related processes
killall -9 xcodebuild Simulator

# 3. Clean all caches
rm -rf ~/Library/Developer/Xcode/DerivedData/*
rm -rf ~/Library/Caches/org.swift.swiftpm
rm -rf .build/ build/

# 4. Reset simulators
xcrun simctl shutdown all
xcrun simctl erase all

# 5. Reopen Xcode
open -a Xcode YourProject.xcodeproj

Common Error Patterns

Common Mistakes

• Debugging code before checking environment - Always run mandatory steps first
• Ignoring simulator states - "Booting" can hang 10+ minutes
• Assuming git changes caused problem - Derived Data caches old builds
• Running full test suite when one test fails - Use -only-testing

Verification Checklist

After applying fix:

• No zombie xcodebuild processes
• Derived Data under 5GB
• Simulators all in Shutdown state
• Clean build succeeds
• Correct code executes (not cached)
• Tests pass consistently