How to debug slow Swift compile times

Question

I have a Swift SpriteKit project with about 25 smallish files. The time to compile this project is 30-45 seconds! It's pure Swift with no ObjC.

I've watched the compile in the Report Navigator to try to find a single file that's taking the time. But, it's not really a single file. It's always the last file in the list that seems to take all the time. But that file can be different between compiles and still takes all the time.

The step right after the last file is Merge xxx.swiftmodule, which happens quickly, but I'm not sure if because it comes right after the slowness it might be related.

I've searched and tried various approaches to find the culprit. I read this post: Why is Swift compile time so slow? and tried those approaches. I've done a command line build using CTRL-\, but that does not show any useful information about the slowness.

I've gone through my project looking for places where type inference might be getting tripped up, but haven't found much (and really, if that was the case, I'd expect a single file to be the culprit).

Does anyone have any other suggestions for tracking this down? Coming from an Objective-C project that compiles nearly instantly, this is driving me crazy.

EDIT I worked on this a bit more and am including a screen shot of the build output, noting where the slowness happens. The thing is, if I comment out the code in the slow file, then the file before it in the list becomes the slow one (where it was fine before). If I comment that code out, then the one before that becomes the slow one, etc.

Build Output

Compiling or linking? Without you posting any logs, we can hardly help you, aside from telling you to a) use Swift 2.0, b) wait for Apple to actually release Swift 2.0. — Claus Jørgensen, Jul 14 '15 at 18:33
Type every array and dictionary - don't force the compiler to infer it. I use to type everything - every let and every var - to get compile speed down (on my slow MacBook Air...) — David H, Jul 14 '15 at 18:43
Thanks Claus. I'll see about grabbing some logs. By the way, I am using Swift 2.0. — coping, Jul 14 '15 at 18:57

score 6 · Answer 1 · edited May 23 '17 at 12:07

After more digging and debugging, I found the problem. It turns out it actually is type inference, as other posts have suggested. Part of the problem is that I did not notice that the build output in the Report Navigator shows an arrow for files still compiling and a checkmark for those that are done (my color blindness played a role). So I thought they were all finished compiling when one was not.

In any case, I read the article here: GM release of Xcode 6 compile and did a command line build of the entire project, which showed me the file that was the issue. Then, using the CTRL-\ approach noted above, I found the culprit.

It turns out that this line was the issue:

ourWindowCopy.position = CGPoint(x: ((26.5 + theXOffset) * self.multiplierWidth) + (4.0 * CGFloat(randomRow) * 2.0 * self.multiplierWidth), y: ((41.0 + theYOffset) * self.multiplierHeight) + (5.75 * CGFloat(randomColumn) * 2.0 * self.multiplierHeight))

I know it seems a mess - I was playing around with a bunch of different options early on and hadn't yet gone back to simplify. I replaced it with this (and will of course replace the literals as well as simply further):

let floatRandomRow = CGFloat(randomRow)
let floatRandomCol = CGFloat(randomColumn)
let pointX: CGFloat = ((26.5 + theXOffset) * self.multiplierWidth) + (4.0 * floatRandomRow * 2.0 * self.multiplierWidth)
let pointY: CGFloat = ((41.0 + theYOffset) * self.multiplierHeight) + (5.75 * floatRandomCol * 2.0 * self.multiplierHeight)

ourWindowCopy.position = CGPoint(x: pointX, y: pointY)

Now the compile speed is very fast!

I'm not sure there's really any new information here, but wanted to close this out with a solution in case someone runs across it.

score 5 · Answer 2 · answered Apr 14 '16 at 21:21

There is a hidden option in Swift compiler that prints out the exact time intervals that compiler takes to compile every single function: -Xfrontend -debug-time-function-bodies.

Simple run the following in terminal and analyze results:

xcodebuild -workspace App.xcworkspace -scheme App clean build OTHER_SWIFT_FLAGS="-Xfrontend -debug-time-function-bodies" | grep [1-9].[0-9]ms | sort -nr > culprits.txt

Awesome Brian Irace wrote brilliant article about it Profiling your Swift compilation times.

score 4 · Answer 3 · answered Jan 12 '17 at 19:26

Building on other answers, the following commands are helpful...

To see a sorted list of functions by build time:

xcodebuild -workspace [Your Workspace Name].xcworkspace -scheme [Your Build Scheme Name] clean build OTHER_SWIFT_FLAGS="-Xfrontend -debug-time-function-bodies" | grep .[0-9]ms | grep -v ^0.[0-9]ms | sort -nr > culprits.txt

To see a sorted list of functions that take over 1s to build:

xcodebuild -workspace [Your Workspace Name].xcworkspace -scheme [Your Build Scheme Name] clean build OTHER_SWIFT_FLAGS="-Xfrontend -debug-time-function-bodies" | grep [1-9].[0-9]ms | sort -nr > culprits.txt

You can get a list of scheme names like so (this command runs a clean first):

xcodebuild -workspace [Your Workspace Name].xcworkspace -list

Helpful Articles about why your build times may be long:

If you're using a project (`xcodeproj`) instead of a workspace (`xcworkspace`), then use `-project [Your Project Name].xcodeproj` instead of `-workspace [Your Workspace Name].xcworkspace` in the commands above. — Senseful, Jul 18 '20 at 22:02
Instead of the grep/sort part at the end, you can use this command which is updated for the latest output and `zsh`. It removes duplicates, and only outputs those that are 100+ ms: `grep -E '^\d{3,}.\d{2}ms' | sort -nr | uniq -u` — Senseful, Jul 20 '20 at 15:46

How to debug slow Swift compile times

3 Answers3