1The biggest problem when trying to compile U-Boot with clang is that 2almost all archs rely on storing gd in a global register and clang user 3manual states: "clang does not support global register variables; this 4is unlikely to be implemented soon because it requires additional LLVM 5backend support." 6 7Since version 3.4 the ARM backend can be instructed to leave r9 alone. 8Global registers themselves are not supported so some inline assembly is 9used to get its value. This does lead to larger code then strictly 10necessary, but at least works. 11 12NOTE: target compilation only work for _some_ ARM boards at the moment. 13Also Aarch64 is not supported: Most notably boards which aren't using 14the generic board will fail to compile, but since those are expected 15to be converted this will solve itself. Boards which reassign gd in c 16will also fail to compile, but there is in no strict reason to do so 17in the ARM world, since crt0.S takes care of this. These assignments 18can be avoided by changing the init calls but this is not in mainline yet. 19 20NOTE: without the -mllvm -arm-use-movt=0 flags U-Boot will compile 21fine, but llvm might hardcode addresses in movw / movt pairs, which 22cannot be relocated and U-Boot will fail at runtime. 23 24Debian (based) 25-------------- 26Binary packages can be installed as usual, e.g.: 27sudo apt-get install clang 28 29To compile U-Boot with clang on linux without IAS use e.g.: 30export TRIPLET=arm-linux-gnueabi && export CROSS_COMPILE="$TRIPLET-" 31make HOSTCC=clang CC="clang -target $TRIPLET -mllvm -arm-use-movt=0 -no-integrated-as" rpi_defconfig 32make HOSTCC=clang CC="clang -target $TRIPLET -mllvm -arm-use-movt=0 -no-integrated-as" all V=1 -j8 33 34FreeBSD 11 (Current): 35-------------------- 36Since llvm 3.4 is currently in the base system, the integrated as is 37incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils 38is used instead. It needs a symlinks to be picked up correctly though: 39 40ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as 41 42# The following commands compile U-Boot using the clang xdev toolchain. 43# NOTE: CROSS_COMPILE and target differ on purpose! 44export CROSS_COMPILE=arm-gnueabi-freebsd- 45gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" rpi_defconfig 46gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" -j8 47 48Given that U-Boot will default to gcc, above commands can be 49simplified with a simple wrapper script, listed below. 50 51/usr/local/bin/arm-gnueabi-freebsd-gcc 52--- 53#!/bin/sh 54 55exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0 "$@" 56 57