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 which aren't using
the generic board will fail to compile, but since those 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 the init calls 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)
--------------
Binary packages can be installed as usual, e.g.:
sudo apt-get install clang

Note that we still use binutils for some tools so we must continue to set
CROSS_COMPILE. To compile U-Boot with clang on linux without IAS use e.g.:
make HOSTCC=clang rpi_2_defconfig
make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang -j8

It can also be used to compile sandbox:
make HOSTCC=clang sandbox_defconfig
make HOSTCC=clang CC=clang -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-gnueabi-binutils
is used instead. It needs a symlinks to be picked up correctly though:

ln -s /usr/local/bin/arm-gnueabi-freebsd-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-gnueabi-freebsd-
gmake rpi_2_defconfig
gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -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-gnueabi-freebsd-gcc
---
#!/bin/sh

exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"