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- Building with Clang
- ===================
- 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 the Clang 3.5 user manual
- states: "Clang does not support global register variables; this is unlikely to
- be implemented soon because it requires additional LLVM backend support."
- The ARM backend can be instructed not to use the r9 and x18 registers using
- -ffixed-r9 or -ffixed-x18 respectively. As global registers themselves are not
- supported inline assembly is needed to get and set the r9 or x18 value. This
- leads 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 currently due to a lack of private libgcc
- support. 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.
- Debian based
- ------------
- Required packages can be installed via apt, e.g.
- .. code-block:: bash
- 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.
- .. code-block:: bash
- make HOSTCC=clang rpi_2_defconfig
- make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \
- CC="clang -target arm-linux-gnueabi" -j8
- It can also be used to compile sandbox:
- .. code-block:: bash
- make HOSTCC=clang sandbox_defconfig
- make HOSTCC=clang CC=clang -j8
- FreeBSD 11
- ----------
- Since llvm 3.4 is currently in the base system, the integrated assembler as
- is incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils
- is used instead. It needs a symlink to be picked up correctly though:
- .. code-block:: bash
- 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!
- .. code-block:: bash
- 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 - saved as
- /usr/local/bin/arm-gnueabi-freebsd-gcc - listed below:
- .. code-block:: bash
- #!/bin/sh
- exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"
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