Signed-off-by: Jeroen Hofstee jeroen@myspectrum.nl --- doc/README.clang | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) create mode 100644 doc/README.clang
diff --git a/doc/README.clang b/doc/README.clang new file mode 100644 index 0000000..6d84f37 --- /dev/null +++ b/doc/README.clang @@ -0,0 +1,56 @@ +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 with aren't using +the generic board will fail to compile, but since these 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 init call 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) +-------------- +The Debian based clang package does not have ARM support at the moment. +It must be compiled manually. + +To compile U-Boot with clang on linux without IAS use e.g.: +export CROSS_COMPILE=arm-linux-gnueabi- +make HOSTCC=clang CC="clang -target $CROSS_COMPILE -B/usr/arm-linux-gnueabi/lib/ -mllvm -arm-use-movt=0 -no-integrated-as" rpi_b_defconfig +make HOSTCC=clang CC="clang -target $CROSS_COMPILE -B/usr/arm-linux-gnueabi/lib/ -mllvm -arm-use-movt=0 -no-integrated-as" all V=1 -j8 + +FreeBSD 11 (Current): +-------------------- +Since llvm 3.4 is currently in the base system, the integrated as is +incapable of building U-Boot. Therefore gas from devel/arm-eabi-binutils +is used instead. It needs a symlinks to be picked up correctly though: + +ln -s /usr/local/bin/arm-eabi-as /usr/bin/arm-freebsd-eabi-as + +# The following commands compile U-Boot using the clang xdev toolchain. +# NOTE: CROSS_COMPILE and target differ on purpose! +export CROSS_COMPILE=arm-eabi- +gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" rpi_b_defconfig +gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0" -j8 + +Given that u-boot will default to gcc, above commands can be +simplified with a simple wrapper script, listed below. + +/usr/local/bin/arm-eabi-gcc +--- +#!/bin/sh + +exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd -no-integrated-as -mllvm -arm-use-movt=0 "$@" +