WWDC 2017: App Startup Time: Past, Present, and Future

Terminology:
Startup time = everything before main()
Launch closure = all info necessary to launch an app

dyld history

dyld 1.0 (1996-2004)

• Shipped in NeXTStep 3.3
• Predated dlopen() becoming POSIX-standardized
• Prebinding added in Mac OS X Cheetah 1.0 - would patch your binaries on every launch to have the correct address of the libraries, so didn't have to re-bind in most cases on next launch

dyld 2.0 (2004-2007)

• Shipped in Mac OS X Tiger 10.4
• Complete rewrite w/ full native dlopen()/dlsym() semantics
• Was much faster, so didn't need to do prebinding hack anymore

dyld 2.x (2007-2017)

• More architectures and platforms: x86, arm64, iOS, tvOS
• Improved security: code signing, ASLR, bounds checking
• Prebinding completely replaced by shared cache

Today: dyld 3 (2017)

Complete rethink of dynamic linking
Starting with Apple's system apps, will eventually be used for 3rd party apps too

Most of dyld is now a daemon, so easier for the dyld team to test (previously, was very hard to test since XCTest itself relied on dyld)

3 components:

• Out-of-process Mach-O compiler and parser and writes launch closure
  • Resolves all searchpaths, @rpaths, env vars
  • Creates and writes launch closure with results
• In-process loading engine (i.e. embedded in your app)
  • Validates launch closure
  • Maps in all dylibs
  • Applies fixups and runs initializers
  • Jumps to main()
• Launch closure caching service (most launches go through this path)
  • Built into shared cache
  • 3rd party app launch closures built during install

Preparing for dyld 3:

• Unaligned pointers in __DATA
  • Can happen if you use #pragma pack(1) and __attribute__((__packed__, aligned(1)))
  • ...but this is impossible in Swift! So use Swift
• Previously used lazy symbol resolution, so if you used unknown symbol would crash on first-use
  Now does eager symbol resolution for better perf, so now app will crash on launch if uses unknown symbols