FindOpenMP: Add option to control OpenMP runtime with MSVC

The MSVC compiler's `-openmp` flag accepts `:{experimental,llvm}`
values.  Add an option to specify one.

Closes: #25570

Help/release/dev/FindOpenMP-runtime-msvc.rst

@@ -0,0 +1,5 @@
+FindOpenMP-runtime-msvc
+-----------------------
+
+* The :module:`FindOpenMP` module gained a ``OpenMP_RUNTIME_MSVC``
+  option to control the OpenMP runtime used with MSVC.

Modules/FindOpenMP.cmake

@@ -16,8 +16,22 @@ flag to support OpenMP.
 .. versionadded:: 3.5
   Clang support.
 
-Variables
+Input Variables
-^^^^^^^^^
+^^^^^^^^^^^^^^^
+
+The following variables may be set to influence this module's behavior:
+
+``OpenMP_RUNTIME_MSVC``
+  .. versionadded:: 3.30
+
+  Specify the `OpenMP Runtime <msvc-openmp_>`_ when compiling with MSVC.
+  If set to a non-empty value, such as ``experimental`` or ``llvm``, it
+  will be passed as the value of the ``-openmp:`` flag.
+
+.. _`msvc-openmp`: https://learn.microsoft.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support
+
+Result Variables
+^^^^^^^^^^^^^^^^
 
 .. versionadded:: 3.10
   The module exposes the components ``C``, ``CXX``, and ``Fortran``.
@@ -121,7 +135,11 @@ function(_OPENMP_FLAG_CANDIDATES LANG)
     else()
       set(OMP_FLAG_IntelLLVM "-fiopenmp")
     endif()
-    set(OMP_FLAG_MSVC "-openmp")
+    if(OpenMP_RUNTIME_MSVC)
+      set(OMP_FLAG_MSVC "-openmp:${OpenMP_RUNTIME_MSVC}")
+    else()
+      set(OMP_FLAG_MSVC "-openmp")
+    endif()
     set(OMP_FLAG_PathScale "-openmp")
     set(OMP_FLAG_NAG "-openmp")
     set(OMP_FLAG_Absoft "-openmp")

Tests/FindOpenMP/Test/CMakeLists.txt

@@ -26,8 +26,21 @@ foreach(c C CXX Fortran)
   endif()
 endforeach()
 
+if(CMAKE_C_COMPILER_ID STREQUAL "MSVC"
+    AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 19.30
+    AND NOT CMAKE_C_COMPILER_ARCHITECTURE_ID STREQUAL "ARM64")
+  set(test_msvc_runtime 1)
+  set(OpenMP_RUNTIME_MSVC "llvm")
+endif()
+
 find_package(OpenMP REQUIRED)
 
+if(test_msvc_runtime)
+  if(NOT OpenMP_C_FLAGS STREQUAL "-openmp:llvm")
+    message(FATAL_ERROR "OpenMP_RUNTIME_MSVC='${OpenMP_RUNTIME_MSVC}' not honored: '${OpenMP_C_FLAGS}'")
+  endif()
+endif()
+
 foreach(c C CXX Fortran)
   if(NOT "${OpenMP_TEST_${c}}")
     continue()

Utilities/Sphinx/conf.py.in

@@ -98,6 +98,7 @@ linkcheck_allowed_redirects = {
     r'https://cdash\.org': r'https://www\.cdash\.org/',
     r'https://cmake.org/get-involved/': r'https://cmake.org/documentation/',
     r'https://docs\.nvidia\.com/cuda/': r'https://docs\.nvidia\.com/cuda/index\.html',
+    r'https://learn\.microsoft\.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support': r'https://learn\.microsoft\.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support\?.*',
     r'https://learn\.microsoft\.com/en-us/cpp/c-language/parsing-c-command-line-arguments': r'https://learn\.microsoft\.com/en-us/cpp/c-language/parsing-c-command-line-arguments\?.*',
     r'https://openjdk\.java\.net/jeps/313': r'https://openjdk\.org:443/jeps/313',
     r'https://www\.sphinx-doc\.org': r'https://www\.sphinx-doc\.org/en/master/',