[build] Compile iOS static library as Relocatable Object File

[friedbunny]

This is an experimental change, PR’d for discussion.

Changes

Sets the iOS static library’s Mach-O Type to Relocatable Object File (mh_object), where it was previously the default, Static Library (staticlib). Also disables dead code stripping on the static library, as this is not compatible with mh_object.

Effect

This reduces the uncompressed size of a release-build, symbol-stripped static library from 280 MB to 110 MB. See the footnote in #9022 (comment) for raw numbers. This has no effect on the ultimate size of an application.

Mechanism

Per this post about the ld docs, enabling relocation: “Merges object files to produce another mach-o object file with file type MH_OBJECT.” And per this post this has the effect of “helping” mark symbols as private, which probably accounts for some of the size reduction.

Questions

Does this still qualify as a sufficiently static library?

Is it compatible with our current static library? So far, the answer is yes: inserting this library into an existing app works as expected. Have tested with Swift, will need to test with Obj-C.

Does this affect debugging in client apps?

Have not tested yet.

Does this break anything? 😬

So far, no, but our static library testing infrastructure is weak. Our symbol visibility script does not indicate any missing symbols.

Related issues

#8089, #9022

/cc @boundsj @kkaefer @1ec5

[friedbunny]

This does bring some new warnings, such as these for every dylib we link:

▸ Linking libMapbox.a
⚠️ ld: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator10.3.sdk/usr/lib/libz.dylib, ignoring unexpected dylib file

▸ Linking libMapbox.a
⚠️ ld: ignoring file /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.3.sdk/usr/lib/libz.tbd, file was built for unsupported file format ( 0x2D 0x2D 0x2D 0x20 0x21 0x74 0x61 0x70 0x69 0x2D 0x74 0x62 0x64 0x2D 0x76 0x32 ) which is not the architecture being linked (armv7): /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.3.sdk/usr/lib/libz.tbd

[1ec5]

There’s scant official documentation on mh_object. The little bits I can find do indicate that it’s normal for a static library to have this type. However, neither this documentation nor the Stack Overflow posts you’ve linked to are particularly recent. Is there anything special about iOS that we should consider, such as how Bitcode might be affected?

[friedbunny]

Research seems to point to this type of static library being poorly supported and buggy. Being that we generally do not encourage the use of the static framework anyway, I’m going to abandon this speculative change.

• http://www.openradar.me/35781555
• http://landonf.org/code/ios/Radar_15800975_iOS_Frameworks.20140112.html