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

Mark Lacey mark.lacey at apple.com
Wed Oct 5 15:00:15 CDT 2016

> 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 <http://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.


> 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
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