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1The biggest problem when trying to compile U-Boot with clang is that
2almost all archs rely on storing gd in a global register and clang user
3manual states: "clang does not support global register variables; this
4is unlikely to be implemented soon because it requires additional LLVM
5backend support."
6
7Since version 3.4 the ARM backend can be instructed to leave r9 alone.
8Global registers themselves are not supported so some inline assembly is
9used to get its value. This does lead to larger code then strictly
10necessary, but at least works.
11
12NOTE: target compilation only work for _some_ ARM boards at the moment.
13Also Aarch64 is not supported: Most notably boards which aren't using
14the generic board will fail to compile, but since those are expected
15to be converted this will solve itself. Boards which reassign gd in c
16will also fail to compile, but there is in no strict reason to do so
17in the ARM world, since crt0.S takes care of this. These assignments
18can be avoided by changing the init calls but this is not in mainline yet.
19
20NOTE: without the -mllvm -arm-use-movt=0 flags U-Boot will compile
21fine, but llvm might hardcode addresses in movw / movt pairs, which
22cannot be relocated and U-Boot will fail at runtime.
23
24Debian (based)
25--------------
26Binary packages can be installed as usual, e.g.:
27sudo apt-get install clang
28
29Note that we still use binutils for some tools so we must continue to set
30CROSS_COMPILE. To compile U-Boot with clang on linux without IAS use e.g.:
31make HOSTCC=clang rpi_2_defconfig
32make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang -j8
33
34It can also be used to compile sandbox:
35make HOSTCC=clang sandbox_defconfig
36make HOSTCC=clang CC=clang -j8
37
38FreeBSD 11 (Current):
39--------------------
40Since llvm 3.4 is currently in the base system, the integrated as is
41incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils
42is used instead. It needs a symlinks to be picked up correctly though:
43
44ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as
45
46# The following commands compile U-Boot using the clang xdev toolchain.
47# NOTE: CROSS_COMPILE and target differ on purpose!
48export CROSS_COMPILE=arm-gnueabi-freebsd-
49gmake rpi_2_defconfig
50gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -j8
51
52Given that U-Boot will default to gcc, above commands can be
53simplified with a simple wrapper script, listed below.
54
55/usr/local/bin/arm-gnueabi-freebsd-gcc
56---
57#!/bin/sh
58
59exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"