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 5 backend support." 6 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. 11 12 NOTE: target compilation only work for _some_ ARM boards at the moment. 13 Also AArch64 is not supported currently due to a lack of private libgcc 14 support. Boards which reassign gd in c will also fail to compile, but there is 15 in no strict reason to do so in the ARM world, since crt0.S takes care of this. 16 These assignments can be avoided by changing the init calls but this is not in 17 mainline yet. 18 19 Debian (based) 20 -------------- 21 Binary packages can be installed as usual, e.g.: 22 sudo apt-get install clang 23 24 Note that we still use binutils for some tools so we must continue to set 25 CROSS_COMPILE. To compile U-Boot with clang on linux without IAS use e.g.: 26 make HOSTCC=clang rpi_2_defconfig 27 make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \ 28 CC="clang -target arm-linux-gnueabi" -j8 29 30 It can also be used to compile sandbox: 31 make HOSTCC=clang sandbox_defconfig 32 make HOSTCC=clang CC=clang -j8 33 34 FreeBSD 11 (Current): 35 -------------------- 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: 39 40 ln -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! 44 export CROSS_COMPILE=arm-gnueabi-freebsd- 45 gmake rpi_2_defconfig 46 gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -j8 47 48 Given that U-Boot will default to gcc, above commands can be 49 simplified with a simple wrapper script, listed below. 50 51 /usr/local/bin/arm-gnueabi-freebsd-gcc 52 --- 53 #!/bin/sh 54 55 exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@" 56
