Random Crashes with Xcode 4, LLVM 2.0, and armv6

After recently deploying a simple update to Boxing Timer, I started to get support requests about the app crashing. I couldn’t understand what would cause the crashes as the update was simply adding French localisation. Some quick smoke testing revealed no issues on an iPhone 4 or iPad. However, reverting back to an iPhone 3G or iPod touch G2, I was able to reproduce the crash. What was particularly strange was that the crash could only be reproduced by exercising a very specific code path. In general though, the app seemed to work fine. However, when the app did crash the logs looked something like this:

Exception Type: EXC_BAD_ACCESS (SIGBUS)
Exception Codes: KERN_PROTECTION_FAILURE at 0x00000002
Crashed Thread: 0

Thread 0 Crashed:
0 ??? 0x00000002 0 + 2

Thread 1:
0 libSystem.B.dylib 0x000014b8 mach_msg_trap + 20
1 libSystem.B.dylib 0x00004094 mach_msg + 60
2 CoreFoundation 0x00057002 CFRunLoopRunSpecific + 982
3 CoreFoundation 0x00056c18 CFRunLoopRunInMode + 44
4 WebCore 0x000846f0 RunWebThread(void*) + 412
5 libSystem.B.dylib 0x0002b7b0 _pthread_body + 20

Thread 0 crashed with ARM Thread State:
r0: 0x00000000 r1: 0x00000000 r2: 0x380ab7fc r3: 0x00138a50
r4: 0x00138a50 r5: 0x00019dd0 r6: 0x00138a50 r7: 0x3817b1b4
r8: 0x00170f80 r9: 0x001fc098 r10: 0x00000004 r11: 0x38179ff4
ip: 0x3810a810 sp: 0x2ffff300 lr: 0x30b336c4 pc: 0x00000002

As it turned out the LLVM 2.0 compiler that shipped with Xcode 4.0 and 4.0.1 was responsible for these crashes. Fortunately Xcode 4.0.2 has fixed this issue. I was able to rebuild my app and get an expedited review, which equals happy customers again!

In a nutshell if you’ve been using Xcode 4.0 or 4.0.1 to build your apps with LLVM 2.0, they will most likely experience crashes on armv6 devices. So make sure you’ve upgraded to Xcode 4.0.2.

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