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