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macOS: Offer control over host architecture on Apple Silicon hosts

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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
This commit is contained in:
Brad King
2020-12-08 15:20:01 -05:00
parent 0334a3c68f
commit 5f882f6ce5
8 changed files with 126 additions and 17 deletions
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Help/variable/CMAKE_APPLE_SILICON_PROCESSOR.rst
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CMAKE_APPLE_SILICON_PROCESSOR
-----------------------------
.. versionadded:: 3.19.2
On Apple Silicon hosts running macOS, set this variable to tell
CMake what architecture to use for :variable:`CMAKE_HOST_SYSTEM_PROCESSOR`.
The value must be either ``arm64`` or ``x86_64``.
The value of this variable should never be modified by project code.
It is meant to be set by a toolchain file specified by the
:variable:`CMAKE_TOOLCHAIN_FILE` variable, or as a cache entry
provided by the user, e.g. via ``-DCMAKE_APPLE_SILICON_PROCESSOR=...``.
See also the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable.
Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst
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@@ -3,13 +3,40 @@ CMAKE_HOST_SYSTEM_PROCESSOR
The name of the CPU CMake is running on.
On Windows, this variable is set to the value of the environment variable
``PROCESSOR_ARCHITECTURE``. On systems that support ``uname``, this variable is
set to the output of:
Windows Platforms
^^^^^^^^^^^^^^^^^
- ``uname -m`` on GNU, Linux, Cygwin, Darwin, Android, or
On Windows, this variable is set to the value of the environment variable
``PROCESSOR_ARCHITECTURE``.
Unix Platforms
^^^^^^^^^^^^^^
On systems that support ``uname``, this variable is set to the output of:
- ``uname -m`` on GNU, Linux, Cygwin, Android, or
- ``arch`` on OpenBSD, or
- on other systems,
* ``uname -p`` if its exit code is nonzero, or
* ``uname -m`` otherwise.
macOS Platforms
^^^^^^^^^^^^^^^
The value of ``uname -m`` is used by default.
On Apple Silicon hosts, the architecture printed by ``uname -m`` may vary
based on CMake's own architecture and that of the invoking process tree.
.. versionadded:: 3.19.2
On Apple Silicon hosts:
* The :variable:`CMAKE_APPLE_SILICON_PROCESSOR` variable or
the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable
may be set to specify the host architecture explicitly.
* If :variable:`CMAKE_OSX_ARCHITECTURES` is not set, CMake adds explicit
flags to tell the compiler to build for the host architecture so the
toolchain does not have to guess based on the process tree's architecture.