LLVMFlang: Add support for LLVM Flang

LLVM Flang (https://github.com/llvm/llvm-project/tree/main/flang) is an LLVM
Fortran compiler that shares the same name as Flang (also known as Classic
Flang). Classic Flang is in active development and is already identified by
CMake as Flang. As such, LLVM Flang will be identified as `LLVMFlang`.

Fixes: #22387

Help/release/dev/LLVMFlang-compiler.rst

@@ -0,0 +1,6 @@
+LLVMFlang-compiler
+------------------
+
+* LLVM's `flang`_ Fortran compiler is now supported, with compiler id ``LLVMFlang``.
+
+.. _`flang`: https://github.com/llvm/llvm-project/tree/main/flang

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -18,7 +18,8 @@ include:
   Clang = LLVM Clang (clang.llvm.org)
   Cray = Cray Compiler (cray.com)
   Embarcadero, Borland = Embarcadero (embarcadero.com)
-  Flang = Flang LLVM Fortran Compiler
+  Flang = Classic Flang Fortran Compiler (https://github.com/flang-compiler/flang)
+  LLVMFlang = LLVM Flang Fortran Compiler (https://github.com/llvm/llvm-project/tree/main/flang)
   Fujitsu = Fujitsu HPC compiler (Trad mode)
   FujitsuClang = Fujitsu HPC compiler (Clang mode)
   G95 = G95 Fortran (g95.org)

Modules/CMakeDetermineFortranCompiler.cmake

@@ -91,6 +91,7 @@ else()
       set(_Fortran_COMPILER_NAMES_Absoft    af95 af90 af77)
       set(_Fortran_COMPILER_NAMES_PGI       pgf95 pgfortran pgf90 pgf77)
       set(_Fortran_COMPILER_NAMES_Flang     flang)
+      set(_Fortran_COMPILER_NAMES_LLVMFlang flang)
       set(_Fortran_COMPILER_NAMES_PathScale pathf2003 pathf95 pathf90)
       set(_Fortran_COMPILER_NAMES_XL        xlf)
       set(_Fortran_COMPILER_NAMES_VisualAge xlf95 xlf90 xlf)

Modules/CMakeFortranCompilerId.F.in

@@ -154,6 +154,13 @@
 # if defined(__FLANG_PATCHLEVEL__)
 #  define COMPILER_VERSION_PATCH DEC(__FLANG_PATCHLEVEL__)
 # endif
+#elif defined(__flang__)
+        PRINT *, 'INFO:compiler[LLVMFlang]'
+# define COMPILER_VERSION_MAJOR DEC(__flang_major__)
+# define COMPILER_VERSION_MINOR DEC(__flang_minor__)
+# if defined(__flang_patchlevel__)
+#  define COMPILER_VERSION_PATCH DEC(__flang_patchlevel__)
+# endif
 #elif defined(_AIX) || defined(__AIX) || defined(__AIX__) || defined(__aix) || defined(__aix__)
         PRINT *, 'INFO:compiler[VisualAge]'
 #elif defined(__hpux) || defined(__hpux__)

Modules/Compiler/LLVMFlang-Fortran.cmake

@@ -0,0 +1,13 @@
+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 "-module-dir")
+
+set(CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_ON "-cpp")
+set(CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_OFF "-nocpp")