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