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The compile options `--march=<arch>` and `--mcpu=<cpu>` and the
link option `--cpu=<cpu>` are automatically added by CMake based
on `CMAKE_SYSTEM_PROCESSOR` or `CMAKE_SYSTEM_ARCH`. But this is not
sufficient, because armclang also supports enabling or disabling
features using `+<feature>`:
-mcpu=<name>[+[no]<feature>+...]
For example:
-mcpu=cortex-a57+nocrypto+nofp+nosimd+crc
(Reference: https://developer.arm.com/documentation/dui0774/k/Compiler-Command-line-Options/-mcpu?lang=en)
The problem is, even if a project adds a flag with features it needs,
CMake still adds flags, resulting in code that is compiled with wrong
CPU features and unable to run.
Add policy `CMP0123` to not automatically add compile or link options,
and let projects set them instead.
Co-Author: Brad King <brad.king@kitware.com>
Fixes: #21173
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33 lines
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CMP0123
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.. versionadded:: 3.21
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``ARMClang`` cpu/arch compile and link flags must be set explicitly.
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CMake 3.20 and lower automatically maps the :variable:`CMAKE_SYSTEM_PROCESSOR`
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variable and an undocumented ``CMAKE_SYSTEM_ARCH`` to compile and link options
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for ``ARMClang``. For example, the ``-mcpu=cortex-m33`` flag is added when
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:variable:`CMAKE_SYSTEM_PROCESSOR` equals ``cortex-m33``. CMake requires
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projects to set either variable or it raises a fatal error. However, the
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project may need to additionally specify CPU features using e.g.
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``-mcpu=cortex-m33+nodsp``, conflicting with the ``-mcpu=cortex-m33`` added
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by CMake. This results in either link errors or unusable binaries.
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CMake 3.21 and above prefer instead to not add any cpu/arch compile and link
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flags automatically. Instead, projects must specify them explicitly.
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This policy provides compatibility for projects that have not been updated.
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The ``OLD`` behavior of this policy requires projects that use ``ARMClang``
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to set either :variable:`CMAKE_SYSTEM_PROCESSOR` or ``CMAKE_SYSTEM_ARCH``
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and it automatically adds a compile option ``-mcpu=`` or ``-march=`` and
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a link option ``--cpu=`` based on those variables. The ``NEW`` behavior
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does not add compile or link options, and projects are responsible for
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setting correct options.
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This policy was introduced in CMake version 3.21. CMake version |release|
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warns when the policy is not set and uses ``OLD`` behavior. Use the
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:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
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.. include:: DEPRECATED.txt
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