Unfortunately, clang (and the shared llvm backend) are still fairly buggy. For example: http://blog.clickablebliss.com/2011/11/07/xcode-4-2-llvm-bui...

There's a bug in Xcode 4.2 (the latest stable release) that results in broken armv6 thumb FPU-related codegen. It's not hard to hit, either.

Since everything is based on LLVM now (including gcc-llvm), there's no way around this one (or other bugs) other than simply disabling thumb.

I'd personally rather stick to gcc4 for at least the next few clang/llvm release cycles, since the bugs are known. Unfortunately, straight GCC is no longer supported in Xcode.