While we have known for months that FreeBSD 10 would use LLVM/Clang as the default compiler and deprecate GCC, this initiative is moving another step forward next month. On 4 November is when it's planned to have LLVM/Clang become the default C/Objective-C/C++ compiler in the FreeBSD world rather than the GNU Compiler Collection. This initially just for i386 and amd64 (x86_64) architectures.
As announced this week, "For the past several years we've been working towards migrating from GCC to Clang/LLVM as our default compiler.We intend to ship FreeBSD 10.0 with Clang as the default compiler on i386 and amd64 platforms. To this end, we will make WITH_CLANG_IS_CC the default on i386 and amd64 platforms on November 4th."
By becoming the default compiler for FreeBSD, cc, cpp, and c++ will now link to clang/clang++ rather than gcc/g++ for FreeBSD. The initial phase of the FreeBSD buildworld and kernel compilation will use Clang rather than GCC. For building out the ports collection, that too will happen with Clang by default.
Most of the "ports" packages are known to build and function correctly when built under Clang instead of GCC, but there's still some failures where GCC will still be relied upon. Clang also is just about C/Objective-C/C++ and not Fortran, Java, Go, or other languages that have GCC coverage (although some of these other languages do have LLVM-based compiler front-ends, but that's not part of the scope of this change). And for non-x86/amd64 systems, GCC will still be the default compiler although for ARM and other select architectures, LLVM back-ends are progressing well. Ports failing now with Clang are mentioned on the FreeBSD.org Wiki.
Among the reasons that many are interested in using Clang rather than GCC comes down to LLVM/Clang's more liberal license than the GPLv3 with GCC 4.2+, the static code analysis abilities of Clang (although this is more competitive with the new GCC), faster build times with less memory use, some generated binaries can be faster (Phoronix benchmarks still aren't definitively in favor of either compiler when on x86_64 Linux), and the modularity of LLVM/Clang.
Testing done within the FreeBSD camp has ruled that compiling with Clang is faster and uses much less system memory.
There's also been interest within the Debian camp for using LLVM/Clang although it will take much longer (if ever) before Clang becomes the default compiler in the Debian world. There's also the initiative for building the mainline Linux kernel under Clang.
Other features being planned for FreeBSD 10.0 are documented in this article.