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A really, really dumb Xcode/LLVM/clang bug…
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README.md

A Really Dumb Xcode 11 Build Issue

Stick with Objective-C, and you'll get the fastest builds, EVER!

Yeah, maybe not.

When I build my app, Xcode 11 appears to stall out completely (dropping from full core usage down to 10-20% CPU) during the Objective-C build phases.

In contrast, my massive Swift targets will happily peg my machine with all the cores/threads getting utilized. Swift doesn't get tripped up by this bug at all.

Xcode CPU Usage

Background

This project demonstrates a horrible bug in the Xcode 11 build system (which may be happening somewhere as deep as LLVM/clang).

The bug manifested for me like this:

  1. Starting with a clean DerivedData folder, I can build my app, Capo in about 60 seconds, flat.
  2. Once I clean the build folder using command-shift-K, all subsequent builds will take anywhere from 180s to 500s (in the very worst case, which I've not seen for a while now.)

After some back & forth with Apple's engineers, I learned to turn on clang's -Rmodule-build flag to see what's going on behind the scenes. It turns out that—for every Objective-C file that is built—framework modules are being re-built unnecessarily.

Instructions

First, clone this repository:

git clone https://github.com/liscio/SubFrameworks

Command-Line Version (Preferred/Faster)

Run the included checkRebuilds.sh script at the command-line.

You will see pre-filtered output like this:

%%% Building Using Clean DerivedData %%%

SubFrameworks/SubFrameworks/SubB.h:9:9: remark: building module 'Foundation' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/Foundation-2PSPYZLNW14KI.pcm' [-Rmodule-build]
SubFrameworks/SubFrameworks/SubB.h:9:9: remark: finished building module 'Foundation' [-Rmodule-build]
SubFrameworks/MainFramework/MainE.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainE.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]

%%% Clean re-build %%%

SubFrameworks/MainFramework/MainF.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainF.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainG.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainG.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainC.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainC.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainD.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainD.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainE.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainE.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainA.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainA.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]
SubFrameworks/MainFramework/MainB.h:10:9: remark: building module 'SubFrameworks' as 'SubFrameworks/testDerivedData/ModuleCache.noindex/33MFFE8RLECJA/SubFrameworks-3U5BOV1R8307A.pcm' [-Rmodule-build]
SubFrameworks/MainFramework/MainB.h:10:9: remark: finished building module 'SubFrameworks' [-Rmodule-build]

What am I looking at here?

The script starts off by building the MainFramework target with a fresh, local testDerivedData folder.

The first block of output demonstrates that Xcode correctly builds modules for Foundation, and SubFrameworks only once. That is what the output should always look like.

The second block of output is the result of asking xcodebuild to perform a clean build action. You'll notice that Foundation is not re-built, however SubFrameworks is built *for every single .m file in the MainFramework target. I would have expected to see it only once since we are explicitly triggering its re-build.

Imagine that SubFramework contains your application's (extensive) core logic, and hundreds of source files. Now imagine that a good number of the source files in your main application #import <SubFramework/SubFramework.h>. For each of those source files, the SubFramework module is re-compiled again and again.

Crazy, right?!

Using Xcode

You can also reproduce the same result in Xcode 11.x, which is where I spend most of my time working.

Open the Xcode project, and do the following:

  1. Choose the MainFramework target
  2. Build the product. You'll see warnings in the output that show which modules are being built.
  3. Press Command-Shift-K to clear the build folder
  4. Build the product again. You'll now see warnings indicating that the SubFramework module is re-built for each source file.

Does the Xcode Version Matter?

Yes. This bug does not appear to affect Xcode 10.x. All of my installed Xcode 11.x releases (11.0 - 11.3 beta) are affected.

Hey, Apple Folks!

For anybody that cares to look further into this, this specific issue is tracked by FB7046188.

What's especially frustrating is that I've been experiencing terrible build performance since April 2018. See also FB5715747, and FB5642600.

Affected Systems

I have only been able to test this on my iMac Pro, and a 2016 MacBook Pro. Both machines have experienced this bug on Mojave and Catalina.

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