macOS: Offer control over host architecture on Apple Silicon hosts

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

1 files changed, 24 insertions, 0 deletions

diff --git a/Help/release/3.19.rst b/Help/release/3.19.rst
index 899e745..4f26058 100644
--- a/Help/release/3.19.rst
+++ b/Help/release/3.19.rst
@@ -53,6 +53,22 @@ Languages
 
 * ``CUDA`` language support now works on QNX.
 
+Platforms
+---------
+
+* Apple Silicon is now supported (since CMake 3.19.2):
+
+  * The :variable:`CMAKE_HOST_SYSTEM_PROCESSOR` is selected using ``uname -m``.
+    Since this may vary based on CMake's own architecture and that of
+    the invoking process tree, the :variable:`CMAKE_APPLE_SILICON_PROCESSOR`
+    variable or :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment
+    variable may be set to specify a host architecture explicitly.
+
+  * If :variable:`CMAKE_OSX_ARCHITECTURES` is not set, CMake adds explicit
+    flags to tell the compiler to build for the
+    :variable:`CMAKE_HOST_SYSTEM_PROCESSOR` so the toolchain does not
+    have to guess based on the process tree's architecture.
+
 File-Based API
 --------------
 
@@ -357,3 +373,11 @@ Changes made since CMake 3.19.0 include the following.
   It requires macOS 10.10 or newer.
   The package file naming pattern has been changed from
   ``cmake-$ver-Darwin-x86_64`` to ``cmake-$ver-macos-universal``.
+
+* Apple Silicon host architecture selection support was updated.
+  CMake 3.19.0 and 3.19.1 always chose ``arm64`` as the host architecture.
+  CMake 3.19.2 returns to using ``uname -m`` as CMake 3.18 and below did.
+  Since this may vary based on CMake's own architecture and that of
+  the invoking process tree, the :variable:`CMAKE_APPLE_SILICON_PROCESSOR`
+  variable or :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment
+  variable may be set to specify a host architecture explicitly.