So I just installed Xcode as I registered to dev program so do I actually have to change the compiler from somewhere in Xcode or is LLVM on default? Thanks in advance!
"LLVM GCC 4.2" is the default in the initial 4 release it seems. (i.e.: The LLVM back-end with the GCC 4.2 parser.)
You can verify this by creating a new project, selecting the main project target (the bit at the top of the left hand pane) and then selecting the "Build Settings" option in the main area. If you then select "All" (the default is "Basic") and scroll down, you'll see "LLVM GCC 4.2" selected as the default.
You can of course change this to GCC 4.2 (the Xcode 3.x default) or pure LLVM.
There's a bit more information on the various settings (they're effectively identical to Xcode 3.2.x) in the existing Xcode 3.2.1 GCC CLANG and LLVM demystification question.