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demystify-explicitly-built-modules.md

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Demystify explicitly built modules

Presenter: Michael Spencer, Compiler Engineer

Link: https://developer.apple.com/wwdc24/10171

  • New way that Swift and Clang modules built in Xcode

Modules

  • Units of code distribution
  • Multiple Swift files that together represent a module
    • In general, all the Swift files in a single target or framework are part of the same module
    • Interface is explicitly marked with access specifiers
    • Public is visible to importers
    • Modules can import other modules
  • Modules in the C interface are hand-modeled
    • Headers & module map
  • Modules share the parsing of interfaces between source files
    • In Swift, this is represented as a .swiftmodule, while in Clang, it's represented as a .pcm or precompiled module file

Using modules

  • Implicitly built modules
    • Compilers coordinate among themselves to build modules
    • This is how Swift and Clang have built modules to date
  • Xcode 16 changes to explicitly built modules
    • Takes the work of building modules and splits up the compilation of source files into three phases:
      • Scan
        • Construct a module graph across the entire project
      • Build modules
        • Explicit tasks in the build log directly provided with the compiled modules they depend on
      • Build source
        • Execution of the original compilation task after they have been modified to add the compiled modules they depend on

Benefits

  • Builds are more reliable
  • Clean build rebuilds modules
  • More efficient builds as the build system is responsible for scheduling
  • Build system can now pass modules to the debugger, speeding up startup

Module build log

  • In Xcode 16, C and Objective-C is explicitly built

  • Can be enabled for Swift

    • Select the project in the project navigator
    • Select Build Settings
    • Type "explicitly built" in filter box
    • Set the setting to Yes

  • Build log contains many scan tasks

  • New compile module tasks - build system spawns a separate compiler process for each task

    • Diagnostics attach here
    • Some modules may be built multiple times when different build settings require different versions of a module to be built

Optimize your build

  • Remove extra variants of modules that are being built to reduce/eliminate those tasks

  • Get info about module build tasks

    • Clean build folder to rebuild all modules in project
    • Go to Product -> Perform Action -> Build with Timing Summary to collect additional information about build performance
    • In the build log, go to Filter and type in "modules report"
    • Select the report - i.e. Clang modules report - and you can see variants for modules
    • Check build settings for common sources of module variants

  • Common Sources of module variants

    • Macros
    • Language mode and version
    • ARC enabling/disabling

  • In general, move build settings as broadly as possible within project to share modules between source files as much as possible