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