How do I xcodebuild a static library with Bitcode enabled?

91
votes

Xcode 7 introduces Bitcode, which is some sort of LLVM intermediate binary that means Apple's servers can recompile my app for different architectures without my involvement.

At Lookback, I distribute a static archive framework with our library. It seems that when you build with anything but a "Build & Archive", bitcode is not actually emitted into my library, and anyone who links with my library in their app and tries to do a Build & Archive with Bitcode enabled will get one of two warnings:

  • ld: 'Lookback(Lookback.o)' does not contain bitcode. You must rebuild it with bitcode enabled (Xcode setting ENABLE_BITCODE), obtain an updated library from the vendor, or disable bitcode for this target. (if lib is built with Xcode 6)
  • ld: warning: full bitcode bundle could not be generated because 'Lookback(Lookback.o)' was built only with bitcode marker. The library must be generated from Xcode archive build with bitcode enabled (Xcode setting ENABLE_BITCODE) (if lib is built with Xcode 7 with a normal xcodebuild)

I have a build script that builds a device+simulator universal binary, so I can't use Build & Archive, but rather, I run xcodebuild from commandline from my script. How can I make xcodebuild generate a proper bitcode-enabled library?

4
iosxcodexcode7bitcode
HI @nevyn Im trying to get your SDK to compile in an app which uses bitcode. Is there a way? – Stoff81
@Stoff81 Sorry, I'm working on it. I need to get all my dependencies working with Bitcode first, and it's quite a bit of work. – nevyn

4 Answers

141
votes

Bitcode is a compile-time feature (not a link-time feature) which means that every .o file should contain an extra section called __bitcode when built with bitcode. You can confirm whether your binary is bitcode-compatible by running otool -l (my .o or .a file) | grep __LLVM.

When you build normally, Xcode adds the build flag -fembed-bitcode-marker to any clang invocation. This seems to be some sort of 'this is where bitcode would go, if bitcode was enabled' thing, and doesn't actually enable bitcode.

When you "Build & Archive", this flag is replaced by -fembed-bitcode, which really does build a Bitcode-enabled binary.

There seems to be two ways to make xcodebuild use -fembed-bitcode:

  • Use the 'archive' action, as in xcodebuild -target LookbackSDK archive instead of xcodebuild -target LookbackSDK build. This has the side-effect of putting binaries in your Xcode Organizer instead of the build/ folder, though you can work around that by using -exportArchive -archivePath ./build (thanks @JensAyton)
  • Force usage of the flag by adding Other C Flags with OTHER_CFLAGS="-fembed-bitcode". Your xcodebuild invocation would look something like xcodebuild OTHER_CFLAGS="-fembed-bitcode" -target LookbackSDK build.

The latter is what I chose so that I don't have to change my build system, but it will generate warnings for every file, since now both -fembed-bitcode-marker and -fembed-bitcode are sent to clang. Luckilly the latter wins, generating a Bitcode-enabled library!

Resources

42
votes

With Xcode 8, I couldn't get OTHER_CFLAGS="-fembed-bitcode" to work. I kept running into something along the lines of was built without full bitcode. All frameworks and dylibs for bitcode must be generated from Xcode Archive or Install build when I tried to create an Archive build of an app containing my static framework.

What I was really looking for was this:

BITCODE_GENERATION_MODE=bitcode

I'm actually using a Run Script inside of an aggregate target, the full xcodebuild line looks like this (just for reference):

xcodebuild BITCODE_GENERATION_MODE=bitcode OTHER_CFLAGS="-fembed-bitcode" -target "${PROJECT_NAME}" ONLY_ACTIVE_ARCH=NO -configuration ${CONFIGURATION} -sdk iphoneos BUILD_DIR="${BUILD_DIR}" BUILD_ROOT="${BUILD_ROOT}" clean build

17
votes

Once you add bitcode support for the static lib, it won't be compatible with Xcode 6. The app won't archive.

I would like to clearly mention the setting for bitcode as @nevyn's answer confused me a little.

Go to Build settings, search for "custom compiler flags". Add -fembed-bitcode. This will build your lib with bitcode.

7
votes

Select project On Build Settings -> Other C flags, set Debug to -fembed-bitcode-marker and Release to -fembed-bitcode

On Build Settings, click on the + sign at the top to add a user-defined build setting with the name BITCODE_GENERATION_MODE, and set Debug to marker, Release to bitcode

Edit schema as Release Then click the desired library. A file and get the build path. Get the library form Release folder.