"Apple now offers a total of four compilers for Mac OS X: GCC 4.0, GCC 4.2, LLVM-GCC 4.2 (the GCC 4.2 front-end combined with an LLVM back-end), and Clang"
"GCC 4.2 is still the default compiler in Snow Leopard, but this is a transitional phase. Clang is the recommended compiler, and the focus of all of Apple's future efforts."