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+Building with Clang
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+===================
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+
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+The biggest problem when trying to compile U-Boot with Clang is that almost all
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+archs rely on storing gd in a global register and the Clang 3.5 user manual
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+states: "Clang does not support global register variables; this is unlikely to
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+be implemented soon because it requires additional LLVM backend support."
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+
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+The ARM backend can be instructed not to use the r9 and x18 registers using
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+-ffixed-r9 or -ffixed-x18 respectively. As global registers themselves are not
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+supported inline assembly is needed to get and set the r9 or x18 value. This
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+leads to larger code then strictly necessary, but at least works.
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+
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+**NOTE:** target compilation only work for _some_ ARM boards at the moment.
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+Also AArch64 is not supported currently due to a lack of private libgcc
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+support. Boards which reassign gd in c will also fail to compile, but there is
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+in no strict reason to do so in the ARM world, since crt0.S takes care of this.
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+These assignments can be avoided by changing the init calls but this is not in
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+mainline yet.
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+
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+
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+Debian based
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+------------
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+
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+Required packages can be installed via apt, e.g.
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+
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+.. code-block:: bash
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+
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+ sudo apt-get install clang
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+
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+Note that we still use binutils for some tools so we must continue to set
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+CROSS_COMPILE. To compile U-Boot with Clang on Linux without IAS use e.g.
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+
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+.. code-block:: bash
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+
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+ make HOSTCC=clang rpi_2_defconfig
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+ make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \
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+ CC="clang -target arm-linux-gnueabi" -j8
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+
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+It can also be used to compile sandbox:
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+
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+.. code-block:: bash
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+
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+ make HOSTCC=clang sandbox_defconfig
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+ make HOSTCC=clang CC=clang -j8
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+
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+
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+FreeBSD 11
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+----------
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+
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+Since llvm 3.4 is currently in the base system, the integrated assembler as
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+is incapable of building U-Boot. Therefore gas from devel/arm-gnueabi-binutils
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+is used instead. It needs a symlink to be picked up correctly though:
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+
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+.. code-block:: bash
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+
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+ ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as
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+
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+The following commands compile U-Boot using the Clang xdev toolchain.
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+
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+**NOTE:** CROSS_COMPILE and target differ on purpose!
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+
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+.. code-block:: bash
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+
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+ export CROSS_COMPILE=arm-gnueabi-freebsd-
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+ gmake rpi_2_defconfig
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+ gmake CC="clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd" -j8
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+
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+Given that U-Boot will default to gcc, above commands can be
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+simplified with a simple wrapper script - saved as
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+/usr/local/bin/arm-gnueabi-freebsd-gcc - listed below:
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+
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+.. code-block:: bash
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+
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+ #!/bin/sh
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+ exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"
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