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5f882f6ce5
Since commit b6c60f14b6 (macOS: Default to arm64 architecture on Apple
Silicon hosts, 2020-09-28, v3.19.0-rc1~63^2) we use `sysctl` to detect
that we are running on Apple Silicon in a way that pierces Rosetta.
This always sets `CMAKE_HOST_SYSTEM_PROCESSOR` to be `arm64` on such
hosts. However, macOS offers strong support for running processes under
an emulated `x86_64` architecture.
Teach CMake to select either `arm64` or `x86_64` as the host
architecture on Apple Silicon based on the architecture of its own
process. When CMake is built as a universal binary, macOS will select
whichever slice (architecture) is appropriate under the user's shell,
and `CMAKE_HOST_SYSTEM_PROCESSOR` will match.
Also offer a `CMAKE_APPLE_SILICON_PROCESSOR` variable and environment
variable to provide users with explicit control over the host
architecture selection regardless of CMake's own architecture.
Finally, if `CMAKE_OSX_ARCHITECTURES` is not set, pass explicit flags to
the toolchain to use selected host architecture instead of letting the
toolchain pick.
Fixes: #21554
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43 lines
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CMAKE_HOST_SYSTEM_PROCESSOR
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---------------------------
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The name of the CPU CMake is running on.
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Windows Platforms
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^^^^^^^^^^^^^^^^^
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On Windows, this variable is set to the value of the environment variable
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``PROCESSOR_ARCHITECTURE``.
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Unix Platforms
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^^^^^^^^^^^^^^
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On systems that support ``uname``, this variable is set to the output of:
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- ``uname -m`` on GNU, Linux, Cygwin, Android, or
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- ``arch`` on OpenBSD, or
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- on other systems,
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* ``uname -p`` if its exit code is nonzero, or
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* ``uname -m`` otherwise.
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macOS Platforms
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^^^^^^^^^^^^^^^
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The value of ``uname -m`` is used by default.
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On Apple Silicon hosts, the architecture printed by ``uname -m`` may vary
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based on CMake's own architecture and that of the invoking process tree.
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.. versionadded:: 3.19.2
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On Apple Silicon hosts:
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* The :variable:`CMAKE_APPLE_SILICON_PROCESSOR` variable or
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the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable
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may be set to specify the host architecture explicitly.
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* If :variable:`CMAKE_OSX_ARCHITECTURES` is not set, CMake adds explicit
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flags to tell the compiler to build for the host architecture so the
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toolchain does not have to guess based on the process tree's architecture.
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