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I have an old computer that cannot upgrade to Lion, but I would like to use it for coding while still taking advantage of syntax such as instancetype and Objective-C literals. Has anyone tried to compile a newer Clang and LLVM to use with an older Xcode and been successful?
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Xcode also includes the LLVM GCC compiler, which uses the GCC compiler front end for maximum compatibility, and the LLVM back end, which takes advantage of LLVM's advanced code generator.
While LLVM and GCC both support a wide variety languages and libraries, they are licensed and developed differently. LLVM libraries are licensed more liberally and GCC has more restrictions for its reuse. When it comes to performance differences, GCC has been considered superior in the past.
Clang is much faster and uses far less memory than GCC. Clang aims to provide extremely clear and concise diagnostics (error and warning messages), and includes support for expressive diagnostics. GCC's warnings are sometimes acceptable, but are often confusing and it does not support expressive diagnostics.
There are various ways to use the newest LLVM/Clang version.
(1) Plugins, as described in the comments above, e.g.: http://blog.wadetregaskis.com/tot-clang-llvm-in-xcode/
As pointed out in the linked blog post, you may get errors/warnings from Xcode's real-time syntax checking as Xcode uses an older version of libclang (more on that under (3)). It is likely to compile fine, but editing source code won't be a lot of fun.
(2) Set the compiler via the CC flag. This is probably the easiest/fastest solution and the setting only affects one project. Go to the project's Build Settings, choose "Add Build Setting" -> "Add User-Defined Setting" (in Xcode 5, this is hidden in the Editor menu), name it CC and set the value to the path of your version of Clang. Worked fine for me, but as with (1), you might get conflicts with the live error reporting, especially if you want to use new syntax such as literals, or pass flags for warnings that the older version does not understand.
(3) Replace the compiler. Make sure you back up any files before replacing them. There are two files that need to be replaced: the Clang binary, and libclang.dylib. Go to /Applications/Xcode.app/ -> Option-Click -> "Show Package Contents" -> /Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr. clang is in /bin, libclang.dylib is in /lib.
Both binaries can either be compiled from source (which will give you an even newer version, of course) or just copied from the latest Xcode package. I ran into trouble using a customised version of libclang, but I suspect the problem originates from my modifications there. If in doubt, use libclang from a later Xcode version.
Note: My modified libclang binary used ARC and is therefore incompatible with Xcode 4 which runs with garbage collection. Xcode 5 itself uses ARC so the problem has vanished. If you compile libclang unmodified, you shouldn't come across the issue in either Xcode 4 or 5.
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