Use LLVM Toolchain With Bazel Part 2

In the previous post, I explained how to build c++ targets using Clang instead of the default compiler gcc on Ubuntu, but failed to figure how to compile and link against libc++, libabi++ and libm. I asked the question on StackOverflow, but got no attention so I guess there isn’t a wide use of it. That post now has been removed. Now that I have figured out a solution, I might just record and share it here.

Last time I was taken down the wrong route and tried to build a custom Bazel toolchain, but that tutorial is rather confusting and prbably outdated. The right approach is to let cc_configure.bzl figure out how to set up the toolchain. To begin with, we need to add your LLVM binaries to the system PATH environment variable. In this way, Clang will be able to build with the LLVM headers when you compile with -stdlib=libc++. So in total, at this point you should have the following env variables (assuming your LLVM binaries are at /usr/lib/llvm-8/bin)

export CC=clang
export CXX=clang++
export PATH=/usr/lib/llvm-8/bin:$PATH

Note that in the previous post I set CC=clang-8 and CXX=clang++-8 while here I removed the version suffix. I have seen reports saying that if you use clang-8 there is a chance that the build won’t be able to find the headers so I would recommend create symlinks and just use clang and clang++.

Then we need to pass these environment variables onto Bazel, so in your .bazelrc file, add

build --action_env=CC
build --action_env=CXX
build --action_env=PATH
build --action_env=CXXFLAGS=-stdlib=libc++

With all these, the build should be able to use the LLVM headers and tried to link against libc++, libabi++ and libm. But the current toolchain does not know where to find them yet. We need to add the following three build settings to make it work.

build --action_env=BAZEL_CXXOPTS=-stdlib=libc++
build --action_env=BAZEL_LINKLIBS=-lc++:-lc++abi:-lm
build --host_linkopt=-fuse-ld=lld

The first two lines are used by cc_configure.bzl to locate the libraries and the last sets the linker to lld so that linkage with LLVM libraries can be done properly.

That should be it.