LLVMFlang: Add support for targeting MSVC ABI on Windows

The compiler does not yet support everything needed to integrate well
with the MSVC ABI, in particular for runtime library selection and debug
format selection.  Document them in FIXME comments and leave this
support undocumented by CMake for now.

Fixes: #24840
Inspired-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

1 files changed, 58 insertions, 3 deletions

diff --git a/Modules/Platform/Windows-LLVMFlang-Fortran.cmake b/Modules/Platform/Windows-LLVMFlang-Fortran.cmake
index 64dc0da..3e22a6e 100644
--- a/Modules/Platform/Windows-LLVMFlang-Fortran.cmake
+++ b/Modules/Platform/Windows-LLVMFlang-Fortran.cmake
@@ -1,3 +1,58 @@
-include(Platform/Windows-GNU)
-__windows_compiler_gnu(Fortran)
-# TODO: MSVC ABI Support
+if("x${CMAKE_Fortran_SIMULATE_ID}" STREQUAL "xGNU")
+  include(Platform/Windows-GNU)
+  __windows_compiler_gnu(Fortran)
+elseif("x${CMAKE_Fortran_SIMULATE_ID}" STREQUAL "xMSVC")
+  include(Platform/Windows-MSVC)
+  __windows_compiler_msvc(Fortran)
+
+  # FIXME(LLVMFlang): It does not provides MSVC runtime library selection flags.
+  # It should be given a flag like classic Flang's `-Xclang --dependent-lib=`, or a
+  # dedicated flag to select among multiple `Fortran*.lib` runtime library variants
+  # that each depend on a different MSVC runtime library.  For now, LLVMFlang's
+  # `Fortran*.lib` runtime libraries hard-code use of msvcrt (MultiThreadedDLL),
+  # so we link to it ourselves.
+  set(_LLVMFlang_LINK_RUNTIME "-defaultlib:msvcrt")
+  set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreaded         "")
+  set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDLL      "")
+  set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDebug    "")
+  set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY_MultiThreadedDebugDLL "")
+
+  # FIXME(LLVMFlang): It does not provide all debug information format flags or predefines.
+  # It should be given a flag to enable Embedded debug information like MSVC -Z7.
+  #set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_DEBUG_INFORMATION_FORMAT_Embedded)        # not supported by LLVMFlang
+  #set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_DEBUG_INFORMATION_FORMAT_EditAndContinue) # not supported by LLVMFlang
+  set(CMAKE_Fortran_COMPILE_OPTIONS_MSVC_DEBUG_INFORMATION_FORMAT_ProgramDatabase "-g")
+
+  set(CMAKE_Fortran_COMPILE_OBJECT "<CMAKE_Fortran_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>")
+
+  if(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT_DEFAULT)
+    set(_g "")
+  else()
+    set(_g " -g")
+  endif()
+  string(APPEND CMAKE_Fortran_FLAGS_DEBUG_INIT "${_g}")
+  string(APPEND CMAKE_Fortran_FLAGS_RELEASE_INIT "")
+  string(APPEND CMAKE_Fortran_FLAGS_RELWITHDEBINFO_INIT "${_g}")
+  string(APPEND CMAKE_Fortran_FLAGS_MINSIZEREL_INIT "")
+  unset(_g)
+
+  # We link with lld-link.exe instead of the compiler driver, so explicitly
+  # pass implicit link information previously detected from the compiler.
+  set(_LLVMFlang_LINK_DIRS "${CMAKE_Fortran_IMPLICIT_LINK_DIRECTORIES}")
+  list(TRANSFORM _LLVMFlang_LINK_DIRS PREPEND "-libpath:\"")
+  list(TRANSFORM _LLVMFlang_LINK_DIRS APPEND "\"")
+  string(JOIN " " _LLVMFlang_LINK_DIRS ${_LLVMFlang_LINK_DIRS})
+  string(JOIN " " _LLVMFlang_LINK_LIBS ${CMAKE_Fortran_IMPLICIT_LINK_LIBRARIES})
+  foreach(v IN ITEMS
+      CMAKE_Fortran_LINK_EXECUTABLE
+      CMAKE_Fortran_CREATE_SHARED_LIBRARY
+      CMAKE_Fortran_CREATE_SHARED_MODULE
+      )
+    string(APPEND "${v}" " ${_LLVMFlang_LINK_DIRS} ${_LLVMFlang_LINK_LIBS} ${_LLVMFlang_LINK_RUNTIME}")
+  endforeach()
+  unset(_LLVMFlang_LINK_DIRS)
+  unset(_LLVMFlang_LINK_LIBS)
+  unset(_LLVMFlang_LINK_RUNTIME)
+else()
+  message(FATAL_ERROR "LLVMFlang target ABI unrecognized: ${CMAKE_Fortran_SIMULATE_ID}")
+endif()