Clang/Bootstrapping
Advanced users may optionally choose to bootstrap Clang by building it using the full LLVM toolchain.
This is required only if looking to move a system to be entirely GCC-free (which is currently not possible on glibc systems). This is not yet supported and is dangerous.
Bootstrapping the Clang toolchain
Mixing Clang and GCC should be fine, unless using default-libcxx or default-compiler-rt. Clang and GCC should otherwise always produce output with the same ABI.
For purity, however, some may wish to build Clang and the rest of the LLVM toolchain with itself, to prove it works and to fully dogfood using Clang.
Be warned that because of a combination of how LLVM is packaged and LLVM's internal structure, this is liable to break across upgrades of major versions.
Preparing the environment
Prepare the environment for the Clang toolchain:
/etc/portage/env/compiler-clangCOMMON_FLAGS="-march=native -O2 -pipe"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
CC="clang"
CXX="clang++"
LDFLAGS="-fuse-ld=lld -rtlib=compiler-rt -unwindlib=libunwind -Wl,--as-needed"
This example replaces not only the compiler but also the GNU linker ld.bfd with the LLVM linker lld. It is a drop-in replacement, but significantly faster than the bfd linker.
Set USE flags default-compiler-rt default-lld llvm-libunwind for Clang via /etc/portage/package.use:
/etc/portage/package.use/clangsys-devel/clang-common default-compiler-rt default-lld llvm-libunwind
Then install Clang, LLVM, compiler-rt, llvm-runtimes/libunwind, and lld with the default GCC environment:
root #emerge llvm-core/clang llvm-core/llvm llvm-runtimes/compiler-rt llvm-runtimes/libunwind llvm-core/lldIt is also possible to add the default-libcxx USE flag to use LLVM's C++ STL with clang, however this is heavily discouraged because libstdc++ and libc++ are not ABI compatible. i.e. A program built against libstdc++ will likely break when using a library built against libc++, and vice versa.
Note that llvm-runtimes/libunwind deals with linking issues that sys-libs/libunwind has, so it is preferred to use and replace the non-llvm libunwind package if installed (it builds with -lgcc_s to resolve issues with __register_frame / __deregister_frame undefined symbols).
Finalizing
Enable the Clang environment for these packages now:
/etc/portage/package.envllvm-core/clang compiler-clang
llvm-core/llvm compiler-clang
llvm-runtimes/libcxx compiler-clang
llvm-runtimes/libcxxabi compiler-clang
llvm-runtimes/compiler-rt compiler-clang
llvm-runtimes/compiler-rt-sanitizers compiler-clang
llvm-runtimes/libunwind compiler-clang
llvm-core/lld compiler-clang
Repeat the emerge step with the new environment - the toolchain will now be rebuilt with itself instead of GCC:
root #emerge llvm-core/clang llvm-core/llvm llvm-runtimes/libcxx llvm-runtimes/libcxxabi llvm-runtimes/compiler-rt llvm-runtimes/compiler-rt-sanitizers llvm-runtimes/libunwind llvm-core/lldClang may now be used with other packages!