[llvm] [runtimes][CMake] Move Fortran support code from flang-rt (PR #171610)

[Petr Hosek via llvm-commits]

================
@@ -0,0 +1,233 @@
+# Check whether the build environment supports building Fortran modules
+# flang-rt and openmp are the only runtimes that contain Fortran modules.
+#
+# Sets:
+#  * RUNTIMES_FLANG_MODULES_ENABLED Whether .mod files can be created
+#  * CMAKE_Fortran_*                CMake Fortran toolchain info for older versions of CMake
+
+
+# Check whether the Fortran compiler already has access to builtin modules. Sets
+# HAVE_FORTRAN_INTRINSIC_MODS when returning.
+#
+# This must be wrapped in a function because
+# cmake_push_check_state/cmake_pop_check_state is insufficient to isolate
+# a compiler introspection environment, see
+# https://gitlab.kitware.com/cmake/cmake/-/issues/27419
+function (check_fortran_builtins_available)
+  if (CMAKE_Fortran_COMPILER_FORCED AND CMAKE_Fortran_COMPILER_ID STREQUAL "LLVMFlang")
+    # CMake's check_fortran_source_compiles/try_compile does not take a
+    # user-defined CMAKE_Fortran_PREPROCESS_SOURCE into account. Instead of
+    # test-compiling, ask Flang directly for the builtin module files.
+    # CMAKE_Fortran_PREPROCESS_SOURCE is defined for CMake < 3.24 because it
+    # does not natively recognize Flang (see below). Once we bump the required
+    # CMake version, and because setting CMAKE_Fortran_PREPROCESS_SOURCE has
+    # been deprecated by CMake, this workaround can be removed.
+    if (NOT DEFINED FORTRAN_HAS_ISO_C_BINDING_MOD)
+      message(STATUS "Performing Test ISO_C_BINDING_PATH")
+      execute_process(
+        COMMAND ${CMAKE_Fortran_COMPILER} ${CMAKE_Fortran_FLAGS} "-print-file-name=iso_c_binding.mod"
+        OUTPUT_VARIABLE ISO_C_BINDING_PATH
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+        ERROR_QUIET
+      )
+      set(FORTRAN_HAS_ISO_C_BINDING_MOD "")
+      if (EXISTS "${ISO_C_BINDING_PATH}")
+        message(STATUS "Performing Test ISO_C_BINDING_PATH -- Success")
+        set(FORTRAN_HAS_ISO_C_BINDING_MOD TRUE CACHE INTERNAL "Existence result of ${CMAKE_Fortran_COMPILER} -print-file-name=iso_c_binding.mod")
+      else ()
+        message(STATUS "Performing Test ISO_C_BINDING_PATH -- Failed")
+        set(FORTRAN_HAS_ISO_C_BINDING_MOD FALSE CACHE INTERNAL "Existence result of ${CMAKE_Fortran_COMPILER} -print-file-name=iso_c_binding.mod")
+      endif ()
+    endif ()
+  else ()
+    cmake_push_check_state(RESET)
+    set(CMAKE_TRY_COMPILE_TARGET_TYPE "STATIC_LIBRARY")
+    check_fortran_source_compiles("
+      subroutine testroutine
+        use iso_c_binding
+      end subroutine
+      " FORTRAN_HAS_ISO_C_BINDING_MOD SRC_EXT F90)
+    cmake_pop_check_state()
+  endif ()
+  set(HAVE_FORTRAN_INTRINSIC_MODS "${FORTRAN_HAS_ISO_C_BINDING_MOD}" PARENT_SCOPE)
+endfunction ()
+
+
+set(FORTRAN_MODULE_DEPS "")
+if (CMAKE_Fortran_COMPILER AND ("flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES OR "openmp" IN_LIST LLVM_ENABLE_RUNTIMES))
+  cmake_path(GET CMAKE_Fortran_COMPILER STEM _Fortran_COMPILER_STEM)
+
+  if (_Fortran_COMPILER_STEM STREQUAL "flang-new" OR _Fortran_COMPILER_STEM STREQUAL "flang")
+    # CMake 3.24 is the first version of CMake that directly recognizes Flang.
+    # LLVM's requirement is only CMake 3.20, teach CMake 3.20-3.23 how to use Flang, if used.
+    if (CMAKE_VERSION VERSION_LESS "3.24")
+      include(CMakeForceCompiler)
+      CMAKE_FORCE_Fortran_COMPILER("${CMAKE_Fortran_COMPILER}" "LLVMFlang")
+
+      set(CMAKE_Fortran_COMPILER_ID "LLVMFlang")
+      set(CMAKE_Fortran_COMPILER_VERSION "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}")
+
+      set(CMAKE_Fortran_SUBMODULE_SEP "-")
+      set(CMAKE_Fortran_SUBMODULE_EXT ".mod")
+
+      set(CMAKE_Fortran_PREPROCESS_SOURCE
+          "<CMAKE_Fortran_COMPILER> -cpp <DEFINES> <INCLUDES> <FLAGS> -E <SOURCE> > <PREPROCESSED_SOURCE>")
+
+      set(CMAKE_Fortran_FORMAT_FIXED_FLAG "-ffixed-form")
+      set(CMAKE_Fortran_FORMAT_FREE_FLAG "-ffree-form")
+
+      set(CMAKE_Fortran_MODDIR_FLAG "-J")
+
+      set(CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_ON "-cpp")
+      set(CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_OFF "-nocpp")
+      set(CMAKE_Fortran_POSTPROCESS_FLAG "-ffixed-line-length-72")
+
+      set(CMAKE_Fortran_LINKER_WRAPPER_FLAG "-Wl,")
+      set(CMAKE_Fortran_LINKER_WRAPPER_FLAG_SEP ",")
+
+      set(CMAKE_Fortran_VERBOSE_FLAG "-v")
+
+      set(CMAKE_Fortran_LINK_MODE DRIVER)
+    endif ()
+
+    # Optimization flags are only passed after CMake 3.27.4
+    # https://gitlab.kitware.com/cmake/cmake/-/commit/1140087adea98bd8d8974e4c18979f4949b52c34
+    if (CMAKE_VERSION VERSION_LESS "3.27.4")
+      string(APPEND CMAKE_Fortran_FLAGS_DEBUG_INIT " -O0 -g")
+      string(APPEND CMAKE_Fortran_FLAGS_RELWITHDEBINFO_INIT " -O2 -g")
+      string(APPEND CMAKE_Fortran_FLAGS_RELEASE_INIT " -O3")
+    endif ()
+
+    # Only CMake 3.28+ pass --target= to Flang. But for cross-compiling, including
+    # to nvptx amd amdgpu targets, passing the target triple is essential.
+    # https://gitlab.kitware.com/cmake/cmake/-/commit/e9af7b968756e72553296ecdcde6f36606a0babf
+    if (CMAKE_VERSION VERSION_LESS "3.28")
+      set(CMAKE_Fortran_COMPILE_OPTIONS_TARGET "--target=")
+    endif ()
+  endif ()
+
+  include(CheckFortranSourceCompiles)
+  include(CheckLanguage)
+
+  set(RUNTIMES_FLANG_MODULES_ENABLED_default OFF)
+  check_language(Fortran)
+  if (CMAKE_Fortran_COMPILER)
+    enable_language(Fortran)
+
+    if (CMAKE_Fortran_COMPILER_ID STREQUAL "LLVMFlang" AND "flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES)
+      # In a bootstrapping build (or any runtimes-build that includes flang-rt),
+      # the intrinsic modules are not built yet. Targets can depend on
+      # flang-rt-mod to ensure that flang-rt's modules are built first.
+      set(FORTRAN_MODULE_DEPS flang-rt-mod)
+      set(RUNTIMES_FLANG_MODULES_ENABLED_default ON)
+    else ()
+      # Check whether building modules works, avoid causing the entire build to
+      # fail because of Fortran. The primary situation we want to support here
+      # is Flang, or its intrinsic modules were built separately in a
+      # non-bootstrapping build.
+      check_fortran_builtins_available()
+      if (HAVE_FORTRAN_INTRINSIC_MODS)
+        set(RUNTIMES_FLANG_MODULES_ENABLED_default ON)
+        message(STATUS "${LLVM_SUBPROJECT_TITLE}: Non-bootstrapping Fortran modules build (${CMAKE_Fortran_COMPILER_ID} located at ${CMAKE_Fortran_COMPILER})")
+      else ()
+        message(STATUS "Not compiling Flang modules: Not passing smoke check")
+      endif ()
+    endif ()
+  endif ()
+
+  option(RUNTIMES_FLANG_MODULES_ENABLED "Build Fortran modules" "${RUNTIMES_FLANG_MODULES_ENABLED_default}")
+else ()
+  set(RUNTIMES_FLANG_MODULES_ENABLED NO)
+endif ()
+
+
+# Determine the paths for Fortran .mod files.
+#
+# Sets:
+#  * RUNTIMES_OUTPUT_RESOURCE_MOD_DIR   Path for .mod files in build dir
+#  * RUNTIMES_INSTALL_RESOURCE_MOD_PATH Path for .mod files in install dir, relative to CMAKE_INSTALL_PREFIX
+macro (fortran_output_and_install_paths)
----------------
petrhosek wrote:

This doesn't need to be in a macro, I'd move it to a top level in which case it'll be evaluated when this module is `include`d.

https://github.com/llvm/llvm-project/pull/171610