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At this point, clang can be used on both 32bit and 64bit targets without issue. Make note of logic we have that will inform clang of the architecture to build for. Signed-off-by: Tom Rini <trini@konsulko.com> Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
110 lines
4.4 KiB
ReStructuredText
110 lines
4.4 KiB
ReStructuredText
Building with Clang
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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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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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Debian based
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------------
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Required packages can be installed via apt, e.g.
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.. code-block:: bash
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sudo apt-get install clang
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We make use of the CROSS_COMPILE variable to derive the build target which is
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passed as the --target parameter to clang.
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The CROSS_COMPILE variable further determines the paths to other build
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tools. As assembler we use the binary pointed to by '$(CROSS_COMPILE)as'
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instead of the LLVM integrated assembler (IAS).
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Here is an example demonstrating building U-Boot for the Raspberry Pi 2
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using clang:
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.. code-block:: bash
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make HOSTCC=clang rpi_2_defconfig
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make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang -j8
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It can also be used to compile sandbox:
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.. code-block:: bash
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make HOSTCC=clang sandbox_defconfig
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make HOSTCC=clang CC=clang -j8
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FreeBSD 11
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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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.. code-block:: bash
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ln -s /usr/local/bin/arm-gnueabi-freebsd-as /usr/bin/arm-freebsd-eabi-as
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The following commands compile U-Boot using the Clang xdev toolchain.
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**NOTE:** CROSS_COMPILE and target differ on purpose!
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.. code-block:: bash
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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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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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.. code-block:: bash
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#!/bin/sh
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exec clang -target arm-freebsd-eabi --sysroot /usr/arm-freebsd "$@"
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Known Issues
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------------
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When build U-boot for `xenguest_arm64_defconfig` target, it reports linkage
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error:
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.. code-block:: bash
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aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `do_hypervisor_callback':
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/home/leoy/Dev2/u-boot/drivers/xen/hypervisor.c:188: undefined reference to `__aarch64_swp8_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_set_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/hypervisor.o: in function `synch_test_and_clear_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_clear_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:28: undefined reference to `__aarch64_ldclr1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/events.o: in function `synch_test_and_set_bit':
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/home/leoy/Dev2/u-boot/./arch/arm/include/asm/xen/system.h:40: undefined reference to `__aarch64_ldset1_acq_rel'
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aarch64-linux-gnu-ld.bfd: drivers/xen/gnttab.o: in function `gnttab_end_access':
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/home/leoy/Dev2/u-boot/drivers/xen/gnttab.c:109: undefined reference to `__aarch64_cas2_acq_rel'
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Segmentation fault
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To fix the failure, we need to append option `-mno-outline-atomics` in Clang
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command to not generate local calls to out-of-line atomic operations:
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.. code-block:: bash
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make HOSTCC=clang xenguest_arm64_defconfig
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make HOSTCC=clang CROSS_COMPILE=aarch64-linux-gnu- \
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CC="clang -target aarch64-linux-gnueabi -mno-outline-atomics" -j8
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