Open Radar

 
Summary:
In Xcode 6.3.1, incremental builds spend more than 20 seconds in MergeSwiftModule.


Steps to Reproduce:
1. Compile a Swift program consisting of multiple files I guess? I don’t really know what all the necessary preconditions are.
2. Pick a file which, when changed, will not require other files to be rebuilt.
3. Add a character to that file, delete it again, and save.
4. Build.


Expected Results:
I expected the build to be so quick as to be unnoticeable.


Actual Results:
Rebuilding the file incrementally: ultraspeedy. But then upwards of 20 seconds spent in “MergeSwiftModule”:

    MergeSwiftModule normal x86_64 /Users/rob/Library/Developer/Xcode/DerivedData/Tesseract-ayeelaykavrrpeemymskjterqjzv/Build/Intermediates/Tesseract.build/Debug/Tesseract.build/Objects-normal/x86_64/Tesseract.swiftmodule


Regression:
This didn’t happen in 6.3, or any other previous version of Xcode.


Notes:
I’ve attached a sample of the swift process which is doing this merge.

Enabling Whole Module Optimization does not improve matters, it makes it worse. Of course it does; it throws away incremental builds altogether! (cf https://twitter.com/satefan/status/586235706395258881 for context)

Please, if there is a workaround, I’d love to hear it. This delay is making it very difficult for me to continue working on things that make me happy.