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CMP0123
-------
.. versionadded:: 3.21
``ARMClang`` cpu/arch compile and link flags must be set explicitly.
CMake 3.20 and lower automatically maps the :variable:`CMAKE_SYSTEM_PROCESSOR`
variable and an undocumented ``CMAKE_SYSTEM_ARCH`` to compile and link options
for ``ARMClang``. For example, the ``-mcpu=cortex-m33`` flag is added when
:variable:`CMAKE_SYSTEM_PROCESSOR` equals ``cortex-m33``. CMake requires
projects to set either variable or it raises a fatal error. However, the
project may need to additionally specify CPU features using e.g.
``-mcpu=cortex-m33+nodsp``, conflicting with the ``-mcpu=cortex-m33`` added
by CMake. This results in either link errors or unusable binaries.
CMake 3.21 and above prefer instead to not add any cpu/arch compile and link
flags automatically. Instead, projects must specify them explicitly.
This policy provides compatibility for projects that have not been updated.
The ``OLD`` behavior of this policy requires projects that use ``ARMClang``
to set either :variable:`CMAKE_SYSTEM_PROCESSOR` or ``CMAKE_SYSTEM_ARCH``
and it automatically adds a compile option ``-mcpu=`` or ``-march=`` and
a link option ``--cpu=`` based on those variables. The ``NEW`` behavior
does not add compile or link options, and projects are responsible for
setting correct options.
.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 3.21
.. |WARNS_OR_DOES_NOT_WARN| replace:: warns
.. include:: STANDARD_ADVICE.txt
.. include:: DEPRECATED.txt
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