[swift-dev] Reporting/Debugging Slow Swift Compile Time

Ben Asher benasher44 at gmail.com
Wed Oct 5 18:31:18 CDT 2016

Ah this is all super helpful. I'll do some more profiling and try to get
back with some data in the next several days.



On Wed, Oct 5, 2016 at 4:28 PM, Mark Lacey <mark.lacey at apple.com> wrote:

> On Oct 5, 2016, at 3:37 PM, Ben Asher <benasher44 at gmail.com> wrote:
> I just tried with both Xcode 8.1 beta 2 and Xcode 8.0, and 8.1b2 seems
> maybe 15s faster (to build our main huge target): 7m28s compared to 7m43s.
> It's some improvement, but I'm not exactly sure what kind of improvement
> was expected.
> The kind of benefit you might expect really depends on the specific code
> you’ve written. The changes I have in mind attempt to speed up the
> expression type checker (the thing that does the work of inferring types
> for a given expression, and the case where there is an explicit type the
> expression needs to type-check to, ensuring that it does). These
> expressions include things like array literals and dictionary literals,
> where we sometimes take a long time to type check.
> Is there any profiling/tracing you all would recommend to help find
> problem areas?
> If you’re looking for problem areas in your particular build, I have a
> couple suggestions:
> 1. Take a look at the output of -debug-time-function-bodies to see if the
> same function is getting type checked multiple times, and determine what
> the cumulative time for those functions is.
> For example below is the output I see for a simple test case. Note that
> the getter/setter that are generated from a property on line 2 of two.swift
> are type checked twice. Although in this case we type check the individual
> functions very quickly, if you have enough of this kind of redundant type
> checking happening, it can add up. This *particular* case is a known bug
> that we hope to address - the synthesized getters/setters for properties
> are type checked in each file they are referenced in.
> There may be other cases like this that we’re not already aware of, so
> it’s always good to open a bug if you find something like this.
> swiftc -c main.swift two.swift -module-name test -Xfrontend
> -debug-time-function-bodies
> 0.2ms main.swift:2:7 get {}
> 0.2ms main.swift:2:7 set {}
> 0.0ms main.swift:1:7 @objc deinit
> 0.3ms main.swift:1:13 override init()
> 0.2ms two.swift:2:14 get {}
> 0.2ms two.swift:2:14 set {}
> 0.2ms two.swift:2:14 get {}
> 0.2ms two.swift:2:14 set {}
> 0.0ms two.swift:1:14 @objc deinit
> 0.0ms two.swift:1:14 init()
> 2. Add a timer for expression type checking and see if that helps narrow
> down whether there is time being spent type checking expressions that isn’t
> accounted for in -debug-time-function-bodies. There are a few places that
> might make sense for this, but I suspect ConstraintSystem::solve() might
> be the best. This is ultimately called from a variety of places, and would
> provide the most insight into where time is being spent in the expression
> type checking. It’s possible something higher up the stack, like
> TypeChecker::solveForExpression or TypeChecker::typeCheckExpression()
> might make more sense as well. You can model this on how
> -debug-time-function-bodies is currently implemented, e.g. look
> at swift::performTypeChecking for some help on getting started. I’ll
> probably try to add this timer myself in the next few weeks if you don’t
> manage to beat me to it.
> Mark
> I don't mind building from Swift master, using someone's preferred
> profiling tools, etc. I'm not really sure where to start.
> Ben
> On Wed, Oct 5, 2016 at 1:05 PM, Ben Asher <benasher44 at gmail.com> wrote:
>> Apologies for not starting off with system info: macOS Sierra (10.12.0),
>> Xcode 8.0 (from the App Store).
>> I'll try with Xcode 8.1 beta this afternoon and report back. Ill also
>> open a ticket for improving -debug-time-function-bodies if I can confirm
>> anything.
>> Thanks!
>> Ben
>> On Wed, Oct 5, 2016 at 1:00 PM, Mark Lacey <mark.lacey at apple.com> wrote:
>>> On Oct 4, 2016, at 2:38 PM, Ben Asher via swift-dev <swift-dev at swift.org>
>>> wrote:
>>> Hello! I work with a large project (~900 .swift files and more .m
>>> files). We have a nightly job that compiles the app and calls out function
>>> bodies (using -debug-time-function-bodies) that are slower than 100ms
>>> to compile. Since upgrading to Swift 3, the number of trouble function
>>> bodies has one from > 150 to < 20, so we're really happy about that! The
>>> only issue though is that build time overall increased by ~1 min (amount of
>>> time to build all targets before automatically merging to master in our
>>> integration build).
>>> Is this using a particular release of Xcode (8.0 or an 8.1 beta?), or
>>> with one of the toolchain builds from swift.org?
>>> Xcode 8.1 beta 2 includes some type checker performance improvements
>>> which might have an impact here.
>>> To dig into this further, we've started a new nightly job that builds
>>> the app using the -debug-time-compilation flag, and using that we've found
>>> that some files take as long as 2-3 seconds to compile. But, there's no
>>> targeted output to help us get this down via the
>>> -debug-time-function-bodies flag (i.e. no function bodies that we can
>>> refactor to get compile times much faster).
>>> One thing to look out for here is that I believe there are some cases
>>> where -debug-time-function-bodies isn’t reporting type checking time. From
>>> my (potentially faulty) recollection, things like let bindings with
>>> literals or closures on the right hand side do not show up in the
>>> -debug-time-function-bodies output, and depending on the specifics of the
>>> expression these can sometimes take a long time to type check. When these
>>> appear within the body of another type declaration they can end up getting
>>> type checked multiple times during a full project build, and that time can
>>> add up.
>>> I don’t believe there is a bug open for improving
>>> -debug-time-function-bodies to help diagnose this, but opening a bug would
>>> be appreciated if you can confirm that this is the case, and of course
>>> patches to fix it are definitely welcome as well.
>>> Mark
>>> We can see that most of the time is spent in "Type checking / Semantic
>>> analysis" for these problem files, but we don't currently have any way of
>>> knowing what that means. It feels like we've exhausted the available
>>> options at this point (unless there are other flags I'm missing) in terms
>>> of existing actionable debugging/profiling/reporting, so now our question
>>> is this: what kind of reports would Swift maintainers be interested in
>>> seeing in terms of output from profiling tools, etc. to help debug/diagnose
>>> these slow compile issues? We're willing to devote time to tooling to help
>>> generate such reports and file bugs.
>>> Thanks!
>>> Ben
>>> _______________________________________________
>>> swift-dev mailing list
>>> swift-dev at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-dev
>> --
>> -Ben
> --
> -Ben

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