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

[Ben Asher]

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).

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). 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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