Created attachment 13634 [details]
Reduced test case.

r225048 broke ObjC on AArch64 - ld doesn't know how to handle relocations to cstring entries without labels and crashes with an assertion:

0  0x10ca67571  __assert_rtn + 144
1  0x10ca79251  mach_o::relocatable::PointerToCStringSection<arm64>::targetCString(mach_o::relocatable::Atom<arm64> const*, ld::IndirectBindingTable const&) const + 297
2  0x10ca78dcc  mach_o::relocatable::PointerToCStringSection<arm64>::contentHash(mach_o::relocatable::Atom<arm64> const*, ld::IndirectBindingTable const&) const + 58
3  0x10ca75f6e  mach_o::relocatable::Atom<arm64>::contentHash(ld::IndirectBindingTable const&) const + 42
4  0x10caac11e  std::__1::__hash_iterator<std::__1::__hash_node<std::__1::__hash_value_type<ld::Atom const*, unsigned int>, void*>*> std::__1::__hash_table<std::__1::__hash_value_type<ld::Atom const*, unsigned int>, std::__1::__unordered_map_hasher<ld::Atom const*, std::__1::__hash_value_type<ld::Atom const*, unsigned int>, ld::tool::SymbolTable::ReferencesHashFuncs, true>, std::__1::__unordered_map_equal<ld::Atom const*, std::__1::__hash_value_type<ld::Atom const*, unsigned int>, ld::tool::SymbolTable::ReferencesHashFuncs, true>, std::__1::allocator<std::__1::__hash_value_type<ld::Atom const*, unsigned int> > >::find<ld::Atom const*>(ld::Atom const* const&) + 36
5  0x10caab235  ld::tool::SymbolTable::findSlotForReferences(ld::Atom const*, ld::Atom const**) + 117
6  0x10caab141  ld::tool::SymbolTable::addByReferences(ld::Atom const&) + 25
7  0x10caab31b  ld::tool::SymbolTable::add(ld::Atom const&, bool) + 61
8  0x10cab0371  ld::tool::Resolver::doAtom(ld::Atom const&) + 893
9  0x10ca7a92c  mach_o::relocatable::File<arm64>::forEachAtom(ld::File::AtomHandler&) const + 54
10  0x10caa7b8f  ld::tool::InputFiles::forEachInitialAtom(ld::File::AtomHandler&, ld::Internal&) + 471
11  0x10cab2a72  ld::tool::Resolver::resolve() + 48
12  0x10ca680e0  main + 682
ld: Assertion failed: (atom->fixupCount() == 1), function targetCString, file src/ld/parsers/macho_relocatable_file.cpp, line 5707.

To reproduce run:

llvm-mc -arch arm64 -assemble -filetype=obj -o testcase.o testcase.s

ld -dynamic -arch arm64 -iphoneos_version_min 7.0.0 \
   -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.1.sdk \
   -o testcase testcase.o

With r225048 applied you'll see the crash. With r225048 backed out you'll just see: 'ld: dynamic main executables must link with libSystem.dylib for architecture arm64'.

Unfortunately I think this means we can't keep r225048 for the time being: There are too many linkers with this issue out in the wild. I'm going to revert on trunk momentarily.