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 clang user manual states: "clang does not support global register variables; this is unlikely to be implemented soon because it requires additional LLVM backend support." Since version 3.4 the ARM backend can be instructed to leave r9 alone. Global registers themselves are not supported so some inline assembly is used to get its value. This does lead 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: Most notably boards which aren't using the generic board will fail to compile, but since those are expected to be converted this will solve itself. 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. NOTE: without the -mllvm -arm-use-movt=0 flags u-boot will compile fine, but llvm might hardcode addresses in movw / movt pairs, which cannot be relocated and u-boot will fail at runtime. Debian (based) -------------- Binary packages can be installed as usual, e.g.: sudo apt-get install clang To compile U-Boot with clang on linux without IAS use e.g.: export TRIPLET=arm-linux-gnueabi && export CROSS_COMPILE="$TRIPLET-" make HOSTCC=clang CC="clang -target $TRIPLET -mllvm -arm-use-movt=0 -no-integrated-as" rpi_defconfig make HOSTCC=clang CC="clang -target $TRIPLET -mllvm -arm-use-movt=0 -no-integrated-as" all V=1 -j8 FreeBSD 11 (Current): -------------------- Since llvm 3.4 is currently in the base system, the integrated as is incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils is used instead. It needs a symlinks to be picked up correctly though: ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as # The following commands compile U-Boot using the clang xdev toolchain. # NOTE: CROSS_COMPILE and target differ on purpose! export CROSS_COMPILE=arm-gnueabi-freebsd- gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" rpi_defconfig gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" -j8 Given that u-boot will default to gcc, above commands can be simplified with a simple wrapper script, listed below. /usr/local/bin/arm-gnueabi-freebsd-gcc --- #!/bin/sh exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0 "$@"