Using Clang

L4Re and the L4Re Microkernel both support building with Clang.

Building L4Re with Clang

Building L4Re with Clang is as easy as providing CLANG=1 to the confguration and building step:

cd $L4RE_OBJDIR
echo "CLANG = 1" >> Makeconf.local
make oldconfig
make -j9

Note

make oldconfig reconfigures the build only asking for new configuration options that are available. That step is always required when switching or upgrading the used compiler.

Hint

It is possible to use CLANG = -<version> for an installed Clang version number <version>.

Ensure that the clang and clang++ binaries are available in your $PATH. If you install your distribution provided packages this should be the case. If you download a binary toolchain in an archive be sure that the directory with the compiler binaries is included in your system search path.

export PATH=/path/to/clang/toolchain:$PATH

Cross-Compiling L4Re With Clang

Clang is capable of cross compiling without installing a specific version for the target architecture. This is why the architecture selection does not work via a prefix but by a command line argument. The L4Re build system deduces this command line argument from the provided CROSS_COMPILE variable. So again, all we have to additionally provide is the extra variable CLANG=1.

cd $L4RE_OBJDIR_ARM64
echo "CROSS_COMPILE = aarch64-linux-gnu-" >> Makeconf.local
echo "CLANG = 1" >> Makeconf.local
make oldconfig
make -j9

Note

Configuring L4Re for building with Clang will still use ld for linking. That’s why you still need a gcc cross compiler installed. To link using ld.lld set LD=ld.lld in Makeconf.local in your build directory or add it to all make invocations.

Building The L4Re Microkernel With Clang

The L4Re Microkernel’s build system gets the information which compiler to use from the kconfig configuration.

cd $KERNEL_OBJDIR
make config
# In "Compiling":
#   Set "C compiler" to "clang"
#   Set "C++ compiler" to "clang++"

make -j9

Cross-Compiling

The CROSS_COMPILE variable still has to be provided on the command line or in Makeconf.local:

cd $KERNEL_OBJDIR_ARM64
echo "CROSS_COMPILE = aarch64-linux-gnu-" >> Makeconf.local

make config
# In "Compiling":
#   Set "C compiler" to "clang"
#   Set "C++ compiler" to "clang++"

make -j9

Dedicated Toolchains

In contrast to building L4Re, when building The L4Re Microkernel, we can just switch out the configured values for our toolchains. The buildsystem deduces the cross-compilation command line parameter for Clang from the CROSS_COMPILE variable even if it is a toolchain path:

cd $KERNEL_OBJDIR_ARM64
echo "CROSS_COMPILE = /path/to/gcc/arm64/toolchain/bin/aarch64-linux-gnu-" >> Makeconf.local

make config
# In "Compiling":
#   Set "C compiler" to "path/to/clang/toolchain/bin/clang"
#   Set "C++ compiler" to "path/to/clang/toolchain/bin/clang++"

make -j9