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.

Debian based

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

sudo apt-get install clang

We make use of the CROSS_COMPILE variable to derive the build target which is passed as the –target parameter to clang.

The CROSS_COMPILE variable further determines the paths to other build tools. As assembler we use the binary pointed to by ‘$(CROSS_COMPILE)as’ instead of the LLVM integrated assembler (IAS).

Here is an example demonstrating building U-Boot for the Raspberry Pi 2 using clang:

make HOSTCC=clang rpi_2_defconfig
make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang -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:

#!/bin/sh
exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"

Known Issues

When build U-boot for xenguest_arm64_defconfig target, it reports linkage error:

aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `do_hypervisor_callback':
/home/leoy/Dev2/u-boot/drivers/xen/hypervisor.c:188: undefined reference to `__aarch64_swp8_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_clear_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_set_bit':
/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
aarch64-linux-gnu-ld.bfd: drivers/xen/gnttab.o: in function `gnttab_end_access':
/home/leoy/Dev2/u-boot/drivers/xen/gnttab.c:109: undefined reference to `__aarch64_cas2_acq_rel'
Segmentation fault

To fix the failure, we need to append option -mno-outline-atomics in Clang command to not generate local calls to out-of-line atomic operations:

make HOSTCC=clang xenguest_arm64_defconfig
make HOSTCC=clang CROSS_COMPILE=aarch64-linux-gnu- \
     CC="clang -target aarch64-linux-gnueabi -mno-outline-atomics" -j8