$ /Users/glider/src/asan/llvm/build/Release+Asserts/bin/clang -arch armv6 -fvisibility=hidden -DVISIBILITY_HIDDEN -Wall -Werror -O3 -fomit-frame-pointer -miphoneos-version-min=1.0 -isysroot /Users/glider/src/asan/llvm/projects/compiler-rt/SDKs/darwin -c -fPIC -o /Users/glider/src/asan/llvm/build/tools/clang/runtime/compiler-rt/clang_darwin/ios/armv6/SubDir.lib/int_util.o /Users/glider/src/asan/llvm/projects/compiler-rt/lib/int_util.c && otool -l /Users/glider/src/asan/llvm/build/tools/clang/runtime/compiler-rt/clang_darwin/ios/armv6/SubDir.lib/int_util.o 2>&1 | grep Unknown
cmd ?(0x00000029) Unknown load command

Looks like r157062 has introduced the new LCT_DataInCode load command (=0x29) which is not supported by older toolchains.

*** Bug llvm/llvm-bugzilla-archive#12950 has been marked as a duplicate of this bug. ***

You can get around this fairly easily--for certain definitions of "easy" :).

Go to opensource.apple.com and get the source to the cctools project from any Xcode version >= 4.3. We're going to build our own ar and ranlib.

After unpacking the tarball, chdir into /libstuff and build the libstuff library with make. Then, chdir to ../misc and make libtool.NEW. Execute these commands when the build finishes:

$ sudo install -c -m 0755 libtool.NEW /usr/local/bin/libtool
$ sudo ln -s libtool /usr/local/bin/ranlib # ranlib is a symlink to libtool

Finally, chdir to ../ar and make ar.NEW. When that's done, install ar.NEW to /usr/local/bin like so:

$ sudo install -c -m 0755 ar.NEW /usr/local/bin/ar

This should fix the issue you're seeing. It did for me.

grosbach: Can you comment if you intend to fix this, or is a prerelease ar now required to build clang on os x?

Yes, the data in code load command requires very recent cctools+ld64. However, why is clang trying to build anything for iOS outside of an Apple internal build environment? That shouldn't be happening (this is a good example of why).

compiler-rt is built for OS X and iOS by default. I don't know if that's intentional (ddunbar?), but I guess it makes sense since one might want to use the mac clang to build iOS apps. (I could do without that, but I don't want to use a non-default compiler-rt build config.)

For the record, I had to pass additional options in order to build ar and libtool with Xcode 4.2:

$ make LTO= TRIE= -j8 -k

(there was also a linktime error in lipo.NEW, so I had to pass -k)

FTR, the easy workaround is:
../configure --enable-optimized --enable-targets=host-only

Yes, but that breaks cross-compilation completely and doesn't just remove iOS. (I use that for the stage 1 build.)

r158466

mentioned in issue llvm/llvm-bugzilla-archive#12950