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e45fcb0e76
When build Xen target with Clang, the linker reports failure. This patch adds the related info in the documentation as a known issue and gives details for how to dismiss the building failure with Clang. Signed-off-by: Leo Yan <leo.yan@linaro.org>
113 lines
4.6 KiB
ReStructuredText
113 lines
4.6 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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**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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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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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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.. 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- \
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CC="clang -target arm-linux-gnueabi" -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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