doc/README.clang - uboot-imx - Git at Google

 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 "$@"
	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 "$@"