[swift-dev] Swift incremental compile profiling

[George King]

Hey Sam,

One thought: if you have an app with mixed objc and swift code, then the app-bridge.h and app-swift.h files might be creating massive choke points in your dependency graph. I have no idea how optimized the bridging functionality is but it has always seemed like a potentially weak part of the dependency management. I imagine that with an objc half and a swift half of the code base, every time you make a change in a swift file you trigger recompilation of the objc half, and vice versa. I'd love to hear from the core team to what extent this is true!

George



> On Apr 7, 2016, at 5:35 PM, Samantha John via swift-dev <swift-dev at swift.org> wrote:
> 
> Thank you Jordan! This is a great starting off point.
> 
> I'm thinking about proposing a "strict import" mode in swift: A compile flag that when turned on would require you to explicitly import any file that contained a dependency you needed (like in objective-c).
> 
> I'm going to spend more time looking over the docs and the output logs to see if this would be a feasible. If anyone has opinions or insights into this I would love to hear from you. 
> 
> Sam
> 
> On Tue, Apr 5, 2016 at 9:08 PM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
> Hi, Sam. I don't think we currently have a good answer for this built into xcodebuild or xctool, and it's a reasonable idea. (Ideally all builds would be fast enough that it wouldn't matter! That's obviously not where we are.)
> 
> Since '-debug-time-function-bodies' is now public knowledge, I'll share another one of our debugging flags, '-driver-show-incremental'. You can add this to your "Other Swift Flags". The output isn't very detailed, though:
> 
> Queuing Tree.swift (initial)
> Queuing AdventureScene.swift (initial)
> Queuing AdventureScene.swift because of dependencies discovered later
> Queuing AppDelegate.swift because of dependencies discovered later
> Queuing ChaseArtificialIntelligence.swift because of dependencies discovered later
> Queuing Character.swift because of dependencies discovered later
> Queuing SpawnArtificialIntelligence.swift because of dependencies discovered later
> Queuing Goblin.swift because of dependencies discovered later
> Queuing Cave.swift because of dependencies discovered later
> Queuing AdventureSceneOSXEvents.swift because of dependencies discovered later
> Queuing HeroCharacter.swift because of dependencies discovered later
> Queuing EnemyCharacter.swift because of dependencies discovered later
> Queuing Boss.swift because of dependencies discovered later
> Queuing SharedAssetManagement.swift because of dependencies discovered later
> Queuing Warrior.swift because of dependencies discovered later
> Queuing Archer.swift because of dependencies discovered later
> Queuing Player.swift because of dependencies discovered later
> Queuing ArtificialIntelligence.swift because of dependencies discovered later
> 
> In this case, I took a version of the Adventure sample project and modified "Tree.swift"; that triggered recompilation of several other files. Unfortunately this view doesn't tell you how they're related, only which ones are actually getting rebuilt.
> 
> The next step (and moving into the territory of "working on Swift" rather than just "trying to figure out why it's repeating work") would be to look at the "swiftdeps" files stored in your DerivedData folder. These are currently just YAML files describing what Swift thinks the file depends on, as well as what will trigger rebuilding of other files. This is intended to be a conservative estimate, since not recompiling something would result in an invalid binary. (Unfortunately I say "intended" because there are known bugs; fortunately, archive builds are always clean builds anyway.)
> 
> There's a document in the Swift repo describing the logic behind Swift's dependency analysis: https://github.com/apple/swift/blob/master/docs/DependencyAnalysis.rst <https://github.com/apple/swift/blob/master/docs/DependencyAnalysis.rst>. The one thing that's not in there is the notion of changes that don't affect other files at all. This is accomplished by computing a hash of all the tokens that could affect other files, and seeing if that hash has changed.
> 
> We definitely have room for improvement here.
> 
> Jordan
> 
> 
>> On Mar 31, 2016, at 11:24 , Samantha John via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> I have a large project (308 swift files, 441 objective c, 66k lines of code) where incremental builds can be extremely slow. I'm trying to do some profiling to figure out what type of things cause large scale recompiles. The problem is that I can't find a good way of telling which files get recompiled on an incremental build and which do not. It seems like files that are not recompiled still get listed in xcode, but the compiler just passes over them really fast. 
>> 
>> Does anyone know if xctool or xcodebuild has this type of functionality? Or is there some other way to get this info?
>> 
>> Thank you,
>> Sam
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