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:: include/STANDARD_ADVICE.rst

.. include:: include/DEPRECATED.rst
