Building with Clang

The biggest problem when trying to compile U-Boot with Clang is that almost all archs rely on storing gd in a global register and the Clang 3.5 user manual states: “Clang does not support global register variables; this is unlikely to be implemented soon because it requires additional LLVM backend support.”

The ARM backend can be instructed not to use the r9 and x18 registers using -ffixed-r9 or -ffixed-x18 respectively. As global registers themselves are not supported inline assembly is needed to get and set the r9 or x18 value. This leads to larger code then strictly necessary, but at least works.

NOTE: target compilation only work for _some_ ARM boards at the moment. Also AArch64 is not supported currently due to a lack of private libgcc support. Boards which reassign gd in c will also fail to compile, but there is in no strict reason to do so in the ARM world, since crt0.S takes care of this. These assignments can be avoided by changing the init calls but this is not in mainline yet.

Debian based

Required packages can be installed via apt, e.g.

sudo apt-get install clang

Note that we still use binutils for some tools so we must continue to set CROSS_COMPILE. To compile U-Boot with Clang on Linux without IAS use e.g.

make HOSTCC=clang rpi_2_defconfig
make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \
     CC="clang -target arm-linux-gnueabi" -j8

It can also be used to compile sandbox:

make HOSTCC=clang sandbox_defconfig
make HOSTCC=clang CC=clang -j8

FreeBSD 11

Since llvm 3.4 is currently in the base system, the integrated assembler as is incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils is used instead. It needs a symlink to be picked up correctly though:

ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as

The following commands compile U-Boot using the Clang xdev toolchain.

NOTE: CROSS_COMPILE and target differ on purpose!

export CROSS_COMPILE=arm-gnueabi-freebsd-
gmake rpi_2_defconfig
gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -j8

Given that U-Boot will default to gcc, above commands can be simplified with a simple wrapper script - saved as /usr/local/bin/arm-gnueabi-freebsd-gcc - listed below:

exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"